BumbleBee question

CMadatMe

I'm Not Feeling It.

What is the number one reason why boilers short cycle? They don't reach steady state efficiency because they cannot run long enough. When you have small system side delta's you cannot scrub the boilers made btu/hr out of the system. You send it around in a circle and the boiler reaches it's set point to fast. Be it a standard cast iron boiler or a condensing boiler.



Every single condensing boiler that is piped pri/sec or LLH in baseboard, hydro applications and utilizes the pump design around the 20 or 25 Boiler Rise is effected by elevated boiler return water temp for the same reason.



I think we tend to get too caught up in supply water temp thinking that magic 150 supply number will get us to condense and that is just not the case.

Paul48

Chris

I have to say...I probably annoyed you the most regarding the high primary DT, but I finally get it. I think my big hang-up was getting stuck on the imbalanced DTs and not considering trying to comes as close as possible on the flows.Anyway.....thanks for being patient.

                Paul

CMadatMe

Finally! Thanks Paul

You saw the light. Maybe we can get a following going. If you don't move the flow of newly created btu/hr out it just goes right back to where it was created. Not a good thing..

Paul48

The Secret

Never mention the DT. Just tell them to come as they can to matching the flows primary to secondary, while still meeting the minimum flow for the heat exchanger?

CMadatMe

I Like That

But minimum flow is just the flow needed to get the full out put of the boiler. Say your heat loss was 80,000 btu/hr. You boiler choice because you are stuck in the middle was 110,000 btu/hr input, couldn't your minimum flow be:



80,000/ (40 x 500) = 4gpm or 80,000 / (35 x 500) = 4.5



So couldn't you pull the HX pressure drop chart and size your pump for these flows? You don't need 110,000 btu/hr. See, I think that it is the pump that over sizes a mod/con not that mod/cons btu/hr input or output.. In many cases HX's across different size inputs are the same.



Look at Viessmann. The Vitodens 100, 26 and 35 is the same HX. The Vitodens 200, 19,26,35 are the same, as goes the 45,60 being the same and the 80,105 being the same. The difference is in the control logic. Could I not limit upper end modulation rate just by sizing by pump correctly?

Paul48

OK

Isn't that minimum flow at high fire? Can you ensure 80000 as max?

CMadatMe

Well Yes

My heat loss told me that. Personally I would adjust the control to limit the high end modulation rate. I don't need it. I could turn that 110,000 btu/hr boiler into an 80,000 btu/hr boiler. So what happens when you reduce modulation rate? You pick up efficiency.

Paul48

Yep

And reduce the short cycling associated with an over-sized boiler.Very interesting...you down-fire the mod/con

CMadatMe

You Got It

The math never lies. Just have to be brave enough and willing to do it. I hate the install manuals for mod/cons. They don't explain what those flow rate charts really mean. Plus they stick you with a pump way over sized in the majority of applications out there.



In the Viessmann Vitodens 200 world it's a simple percentage change in the control and you can have your heating modulation rate independent of DHW. So I could still size my domestic pump for the full flow and output. In other words my domestic pump could be sized to move the full flow rate at a different HX pressure drop.



So I just made two boilers out of one..DHW would have to be set on priority when doing this.

tom3holer

I got it

Finally I got it.. More flow, more BTUs but lower D/Ts.  Thank you all or the time and effort put into this discussion. I will have the new 80 installed on this Sat and have a Bumblebee to put on one of the zones. Am anxious to see how well it will maintain the D/T.



I don't see how downsizing the top end of the mod rate will help with short cycling. Unless its caused by the boiler ramping up too fast and overshooting the set point by more than the overshoot limit. Short cycling is caused by the low end being too high for the load at the time, and that is not adjustable, low end is fixed.



Tom

CMadatMe

Well, You Almost Go It

If I size my boiler pump to deliver the flow I need off my heat loss and it is less than the full out put of the boiler then I need to cap off my high end modulation rate so I don't flash the water moving across the HX to steam.



You are installing an Alpine 80 with a heat loss of 30K, piped pri/sec. The boiler is going to come with a boiler pump for a flow rate you don't need? Why couldn't you size a boiler pump to move 3gpm across the HX instead of 7.3gpm if you cap off the modulation rate? What causes condensing boilers to short cycle when piped pri/sec or LLH is the boiler pump not the boiler itself.



It's a question, not telling you to do it. Based on this with 150 degree supply water temp and the system moving 3gpm. I would be condensing with 130 degree return water temp. Your return boiler temp would be 142. Not even close to condensing.



So is it the boiler reducing your efficiency or the boiler pump reducing your efficiency?

Jean-David Beyer

I would be condensing with 130 degree return water temp.

You just start condensing with 130F return water temperature. Generally speaking, you do not get any condensing above that temperature. But if you look at the graph in John Seigenthaler's book, "Modern Hydronic Heating" Third Edition, page 60, Figure 3-11, you will see that at that point you can expect a boiler efficiency of about 87%. If you get the return temperature down to about 110F, you can expect an efficiency of around 93%, and to get 98%, you need to get the return water temperature down to about 70F.



P.S.: That book is available at this web site. Click the "Shop" item near the top of this page.

Jean-David Beyer

I don't see how downsizing the top end of the mod rate will help with short cycling.

"I don't see how downsizing the top end of the mod rate will help with

short cycling. Unless its caused by the boiler ramping up too fast and

overshooting the set point by more than the overshoot limit. Short

cycling is caused by the low end being too high for the load at the

time, and that is not adjustable, low end is fixed."



I do set the maximum firing rate for one of my heating zones down to help reduce short cycling. Whether it helps with other controllers, I would not know. It would depend on the damping ratio of the control system, and that is not adjustable on my controller (and a good thing too: if you do not know feedback control systems, you will confuse yourself beyond belief).



That is what happens with my W-M Ultra when running my small baseboard zone. That zone requires 6500 BTU/hour when it is 0F outside, and design temperature around here is 14 F, so you can guess how often that zone needs even 6500 BTU/hour. And the least (other than none) my boiler will put out* is 16,000 BTU/hour. So it will cycle on the limits because it will not modulate down far enough.



I have been able to adjust the boiler so it cycles only about six times an hour, when this is happening, which I suppose will have to be good enough. To do this, I have had to make three changes from the defaults:



1.) I set the minimum supply temperature to 110F to the baseboards, where I would prefer to set it to about 80F. Setting it higher will allow the system to dump the heat more rapidly so the boiler takes a little longer to get up to the upper limit.



2.) I set the upper and lower limits to be 15F apart instead of 10F (the default) apart, so it takes longer to get from one limit to the other.



3.) I set the maximum firing rate (for this zone) to be 55% instead of 94% (the default maximum firing rate). Leaving it at the default would mean the boiler temperature increases so fast that the controller does not have time to reduce the firing rate until it is right at the upper limit. With it firing at 55%, it gives the controller time to reduce the firing rate to minimum. Now minimum is still too high, but it is better that something higher.



As I say, I am willing to live with 6 cycles per hour on that zone.



The radiant slab zone does not have that problem and cycles only once or twice a day except on very warm days. That zone can sink 20,000 to 25,000 BTU/hour if it is cold enough out.



_____

* Actually, I do not mean "put out": that is the input to the boiler, not the output. But that does not help me much.

CMadatMe

Not True

See attached.

CMadatMe

So you Max Flow Rate Is

5 gpm for the radiant 25k/ 5000 = 5gpm

.65 for the board so the max flow rate you need

5.65 but you said design is 14 so .65 is not truly

it's flow rate.



Any way. What according to The manual is your

Boiler pump moving across the HX?

Jean-David Beyer

See attached.

I have seen that .pdf and am puzzled at the apparent difference between the two. It is very difficult for me to believe that John Siegenthaler is mistaken on this point, especially since it was the same graph in his second edition (I have never seen the first edition). Surely, if it is incorrect, someone would have brought it to his attention and he would have corrected it by now.



And since I am just a homeowner, not a heating professional, I am also reluctant to say that Veissmann is mistaken.



So how to reconcile these two graphs -- so they can both be right?



It is not a case of flat-out contradiction. I.e., Veissmann's graph is not the same as Siegenthaler's graph, but with different numbers on it. It is not even just that Viessmann's graph has a different shape.



Siegenthaler's graph is just a plot of boiler efficiency vs. return water temperature.

Viessmann's graph is more complex (not in appearance, bit in meaning). It is a plot of boiler efficiency as a function of the design (not actual operating) supply and return temperatures and the load factor at any given moment.



Now that I look more closely at Veissmann's graphs, it seems to me they are incorrect. I look at the top curve and it says if the system is designed for a delta T of 18F, and is running at 100% load that the boiler efficiency is about 95%. It seems to me that if the load is sinking 100% of the offered heat production, that should result in the lowest return water temperatures. Yet as the load factor goes down, they say that the boiler efficiency goes UP. That seems counterintuitive to me. If the load is sinking less heat, the return temperatures would go up and the boiler efficiency would go down. Clearly, something is missing from their graphs. I am not saying their graph is wrong; I am saying their graph has information missing from which it is impossible to decide what is going on with their system using only the information in their graph.



In the meantime, I go with John Seigenthaler.

Jean-David Beyer

What according to The manual is your Boiler pump moving across the HX?

If you mean the installation manual, I have not the slightest idea. It does not tell me the head of the heat exchanger. I suppose I could figure out the head loss through the Flow-Check valve, the closely-spaced Ts, and two other Ts and two other 90 degree elbows. It is a Taco 007, supplied by W-M as being appropriate for this boiler. The piping is almost entirely 1 1/4 inches except internal to the boiler where it is 1 inch.



Let me guess: between 6 and 7 gpm in the boiler loop. And that if just the radiant zone, the baseboard zone, or both are running at the same time. And the same if both heating zones are off and the indirect is running. This guess is based on what they say will go through their indirect when using the a Taco 007 circulator, that is what they recommend.

CMadatMe

I Think I Figured It Out JD

Yeah, that Weil manual doesn't tell you much on the HX but it does give you the gpm and head arcoss the plus 40 tank. So, I went to Triangle's site and it has 1' of head.



In the manual it says you need to move 6.5gpm @ 8.5' of head across the indirect. So let's subtract the 1' for the indirect and we get 7.5' of head across the hx.



The 80 has an output of 71,000 so let's plug in the numbers for a 20 Rise.



71,000/10,000 = 7.1gpm. Pull the Taco 007 curve and we find that at 7.5' of head the 007 will move 8gpm. Know our head may be a tad off so your little Taco 007 is moving 7.1gpm every time the boiler comes on when there is a call for heat. You can't change it.



You system side is not pulling out 7.1gpm EVER! It can't, your heat loss tells us that. I picked 5gpm for the radiant based off it running on a 10 degree delta-t. If the delta-t is larget that flow rate sinks.



Now since you can't pull 7.1gpm out to the system, the difference only has one place to go and that's to head back to the boiler return. That's why you short cycle not the low end of the modulation rate. Primary Boiler side flow is nothing more then a conveyor belt moving btu/hr to the system side and if you can't take it YOUR BOILER PUMP IS TOO BIG!



We wouldn't be having this conversation is we could have variable speed boilers pumps controlled by the boilers logic. It would do it for us based on how we set the boiler up.

Jean-David Beyer

I Think I Figured It Out JD

I am glad you figured it out without using the Veissmann chart. ;-)



My guess and your calculations have it pretty much the same, I think.



"Yeah, that Weil manual doesn't tell you much on the HX but it does give

you the gpm and head arcoss the plus 40 tank. So, I went to Triangle's

site and it has 1' of head."



Right, and my indirect says Weil-McLain on it but we both know it is Triangle Tube; at least they were both made in Belgium, and how many tank-within-a-tank indirects could be made there with the same specifications?



"In the manual it says you need to move 6.5gpm @ 8.5' of head across the

indirect. So let's subtract the 1' for the indirect and we get 7.5' of

head across the hx.







"The 80 has an output of 71,000 so let's plug in the numbers for a 20 Rise."



Where did you get the 20 degree Rise number from? If I watch the supply and return readings, I do not think I ever so one so high. when it is very warm out, I see 0 to 1 degree rise (if I wait for steady-state conditions to occur), and as it gets colder, I have once in a while seen (for the large, radiant slab zone) almost 10 degrees. But this may not matter if we do not fool ourselves into thinking we have more accuracy than we have. I think we are both more interested in the principles that the fine points of accuracy in my specific case.



"71,000/10,000 = 7.1gpm. Pull the Taco 007 curve and we find that at 7.5'

of head the 007 will move 8gpm. Know our head may be a tad off so your

little Taco 007 is moving 7.1gpm every time the boiler comes on when

there is a call for heat. You can't change it."



Well I could change it by putting in a smaller pump, or by partially closing a ball valve. But I do not think that a good idea. I imagine W-M want me to run that much flow through the heat exchanger to keep it from getting too hot. On the other hand, if there were no flow in the system loop, the heat loss in the boiler loop (excluding the boiler) would be extremely low and not much help.



"You system side is not pulling out 7.1gpm EVER! It can't, your heat loss

tells us that. I picked 5gpm for the radiant based off it running on a

10 degree delta-t. If the delta-t is larget that flow rate sinks."



I do not think I understand you. My system side has two Taco 007-IFC pumps, so if the resistance of those two zones is comparable to that of the indirect, I could pull double that. Now that assumption is probably way too high. I calculated that I run about 2.8 gallons/minute in the small baseboard zone and about 7.8 gallons/minute in the radiant slab zone. The slab zone numbers can be way off because I do not know the lengths of the pipe down there, or even the exact pipe sizes. There are 5 1/2-inch copper tubing entering the slab (and 5 rooms downstairs), and one 1-inch copper tube coming out. Each 1/2 inch tube has a ball valve on it and three are wide open, one is partly closed, and the last is almost closed.



I do not know if I ever saw a delta T of 10 degrees on the radiant zone. (I know I certainly never saw that on the small baseboard zone.) On warm days it would be tough to get that because when it is over 50F outside, I put 76F water into the slab and the downstairs thermostat is set at 69F, so the greatest delta-T I might get would be 7F, but when it is running like that, the delta-T is about 1F. On design day, I put 112F into the slab and the return is about 105F, so delta-T is around 7. This delta-T is not very accurate because my memory is not that great. It is very difficult to get a steady state reading because it takes quite a while from when the zone turns on until it gets into the steady state. Like 4 hours or more (that slab has a lot of thermal inertia), and while that is going on, the indirect that has more priority will shut it off and run for a while, and then the little zone can turn on for a while and change the load and so on. So do not look for a lot of accuracy in my numbers. I know the supply temperatures accurately, and the instantaneous return numbers, but the return numbers are not steady state.



"Now since you can't pull 7.1gpm out to the system, the difference only

has one place to go and that's to head back to the boiler return. That's

why you short cycle not the low end of the modulation rate. Primary

Boiler side flow is nothing more then a conveyor belt moving btu/hr to

the system side and if you can't take it YOUR BOILER PUMP IS TOO BIG!"



I do not think this is correct. If instead of considering the water flow rates, we consider the BTU flow rates, you are partly right. But if I watch the thing run, especially the small zone that is the one that rapidly cycles, I can watch the modulation rate, and the supply and return temperatures. And what happens is that when the small zone calls for heat, the boiler circulator and the zone circulator start. 20 seconds later the boiler fires and starts to heat up. It fires at 50% firing rate for a minute, and then goes up to the maximum firing rate. When the supply temperature closes in on the upper limit set by the reset curve, the firing rate drops eventually all the way to the minimum firing rate that is 16,000 BTU/hour input. Way too much. So eventually the supply temperature hits the high limit and firing stops. after a while (about 5 minutes)  things cool down to the low limit and the boiler fires again, this time not at the maximum rate, And it keeps doing that because there is nothing else it can do. It is not the water flow rate that is too slow (the low delta-T I get from that zone indicates to me that it is already too high), it is the BTU flow rate from the closely-spaced Ts up the the baseboards. And it is not a pumping problem, the problem is that even my 28 feet of baseboard will not sink 16,000 BTU/hour at the temperatures I run at. And I would not want to increase the amount of baseboard more than it is already. Those rooms do not need more heat. If it were not for the fact that the baseboard needs different temperatures and heat at different times from the radiant zone, it would make more sense to have only one zone. But one zone is on a slab with radiant downstairs, and the other zone is upstairs with baseboard. Originally, there was only one zone and the upstairs was always 5 degrees or more colder than downstairs. So much for the theory that heat rises. Well, I can tell it does, but not fast enough.



As far as the boiler pump being too big, W-M seem pretty worked up about using the pump they supply. They even paint it black instead of Taco Green so I do not mix it up with the others. I thought that was to keep from overheating that aluminum heat exchanger. They sure do not want the water to flash to steam inside there. So even if I replaced it with a Taco 006 or 003, I do not think it would make any difference. But if I melted the pins off the fire side of that aluminum heat exchanger, I am sure I would regret it.

CMadatMe

I Don't Think You Understand What Minimum Flow Rate

Means in the charts. It means that you need to move x gpm @ a particular rise to get the full btu/hr out put of the boiler. That's the pump boiler mfgs supply and they all base that pretty much off a 20 Degree Rise. So if the boiler pump is sized to move 7.1gpm @ 7.5' of head. It always moves it. IT'S A FIXED SPEED PUMP. How else could you get the full btu/hr out put of the boiler?



This is how I got the Rise: The Universal Hydronic Formula except I reversed it.



gpm = btu/hr / (delta-t x 500)



71,000 (Out put of the boiler) /500/7.1 = 20 Temp Rise. So the only time that boiler will make 71,000 btu/hr is when the difference between boiler supply and return is 20 degrees. Rise is continually changing in a condensing boiler. You've said it yourself.

Jean-David Beyer

I Don't Think You Understand What Minimum Flow Rate Means in the charts.

I think I do.



"It means that you need to move x gpm @ a particular rise to get the full btu/hr out put of the boiler."



That is just what I think it means.



However, it seems to me I could also get the same btu/hr out of the boiler by pumping more gpm and, therefore, getting less rise. It may not be the smart way (more energy to run the pump, more erosion of the pipes). It seems to me that in my system, that boiler pump is going to be big enough because first of all, the boiler is about twice the size I need, and because the system size is too small to sink all the heat the boiler can produce (which may just be another way of saying the same thing). I sure wish they made smaller mod-con boilers, or ones that turned down the firing rate a lot more, like to 2,500 BTU/hr at the low end.



"71,000 (Out put of the boiler) /500/7.1 = 20 Temp Rise. So the only time

that boiler will make 71,000 btu/hr is when the difference between

boiler supply and return is 20 degrees."



As I said, I can get it all out by pumping faster and with less temperature rise. I suppose the boiler would actually be a little more efficient if I did this because the temperature drop from the fire side of the heat exchanger to the water side would be more at higher pumping speeds, but this may be a small change, and if I pump too fast, cavitation and erosion start to become a problem.



"Rise is continually changing in

a condensing boiler. You've said it yourself."



Not exactly. It is continually changing with a modulating boiler; condensing has little to do with it. I suppose it is also true with a conventional boiler. What really controls the rise is the firing rate and the water flow rate and the temperature of the return water. In a conventional boiler, the firing rate does not change, but the water flow rate could change (e.g., as zone valves open and close) and the return water temperature could certainly change (as the difference between the supply temperature and the room temperature change).

CMadatMe

You Can't Pump

More out of the boiler. The mfg provided you a pump that will only move 7.1gpm across the HX. Whether you want to believe it or not. The full out put of the boiler will only happen when you get a 20 degree difference between the boiler supply outlet and the boiler return outlet. If you don't get it you won't get the full out put of the boiler.



You cannot increase the boiler flow to get more out of the boiler. A btu is a btu is a btu. Increase flow lowers rise less btu/hr. Increase rise more btu/hr.



The constant is the boiler flow rate It cannot be changed with a fixed speed pump. It is your boiler rise that is dictating btu/hr output. It''s the aiming device the control logic uses to modulate! Now you have an older version and at start it may go to 50% and then decide what it wants to do but didn't you say you limited the mod rate in a previous post. You cannot get around the Universal Hydronic Formula no matter what.



7.1 x 7x 500 = 24,850 btu/hr boiler output. You do understand we are not talking about your zone delta-t we are talking about the temp at the boiler return as it enters the boiler right?

Eastman

Nuts

I missed all the action.

CMadatMe

Well Get Back In The Game

And out of the clubhouse. You eating fried chicken and sucking down beers in there?

Eastman

I got into a car accident.

Everyone is ok thankfully.

CMadatMe

Sorry to Hear That

Glad to hear everyone is ok.

Jean-David Beyer

You Can't Pump More out of the boiler.

If you mean that I cannot pump more heat from the boiler than the burner puts in, I completely agree with you. And the amount of heat that goes in is limited by how much the controller, blower, and gas valve puts in.



For the sake of arguement (and I do not even disagree with you), if you put in whatever heat goes in from the burner is 71,000 BTU/hour, then I agree that I cannot get anymore out than that, no matter what. But I do not think I agree that the only way to get that heat out is to set the flow to 7.1gpm getting a delta T of 20F. That is certainly one way to do it, but I see nothing inherently sacrosanct about the 20F figure.. I will accept that the flow you specify gets that temperature rise through the boiler. Now I am not sure that the math is linear, but to simplify I assume that it is. In that case, it seems to me I could run 14.2 gpm through the boiler, get a delta T of 10F and still get that heat out of there.



"It is your boiler rise that is dictating btu/hr output."



I disagree. I believe it is the product of the boiler rise times the flow rate that determines the output. If I had enough helpers, I could empty a small swimming pool with a few large buckets or a lot of small buckets in the same amount of time.



"It''s the aiming device the control logic uses to modulate!"



As far as I can tell, the only thing the boiler control uses to modulate  the firing rate is how far away from the reset curve the supply temperature in the secondary (system) loop is. It has the return temperature in the secondary loop available, but as far as I can tell, it does not use it as an operational value. (It does use it to be sure the sensors are not wired backwards, to help in low water detection, and other malfunctions that cause the boiler water temperature to go up too fast, and things like that.)



"Now you have an older version and at start it may go to 50% and then

decide what it wants to do but didn't you say you limited the mod rate

in a previous post."



I do not know what the current version of the U-control does. What mine does in the initial startup is to run just the blower to purge any residual gas in the burner so the thing does not blow up. It then turns on the gas at 50% and turns on the electrozapper to start the fire. It then stops zapping and uses it as a flame detector. I assume it picks 50% to be sure there is enough gas-air mixture to be sure of starting the fire and not having it blow out. After a minute, it lets the feedback control system set the firing rate, and it does not change the rate at a very high rate. It can take what seems like a minute or so to go from maximum to minimum firing rate, but this is just my impression. I did not time it with a stopwatch.



When running the little zone, I did set the maximum firing rate to 55%. I would have had it even lower, but I was afraid to make it lower that the initial starting rate, so I picked 55% in case things did not run that exactly.



"You cannot get around the Universal Hydronic Formula no matter what."



I did not think I said you could. If I gave that impression, I am sorry I mislead you. This whole discussion, I bet, could have been resolved in less than 1/2 hour of face-to-face discussion with a blackboard and some chalk, The Internet stuff really slows things down.



"7.1 x 7x 500 = 24,850 btu/hr boiler output. You do understand we are not

talking about your zone delta-t we are talking about the temp at the

boiler return as it enters the boiler right?"



I can understand that if you like. Trouble is, that is not what my boiler measures. Except when heating the indirect, it does not measure what the temperatures at the boiler inlet and outlet are (what I would call the boiler return and supply, respectively, in case there is any misunderstanding); it measures the supply temperature at the output end of the closely spaced Ts, that is probably very close to the boiler supply temperature, and it measures the return temperature near the input end of the closely spaced Ts. This is probably pretty close to the boiler return temperature when the slab zone is running, or when both the slab zone and the baseboard zone are running at once. It is probably somewhat far off (too low) if only the baseboard zone is running because so much of the supply water will go backwards through the Ts and into the boiler return. So I am willing to talk about the boiler supply and return temperatures, but we must be clear that all I can measure is the temperatures of the system loop on either side of the closely spaced Ts, and they can be somewhat different.



I would really like to have thermometers installed where I bet you wish I had them, but even if I could afford that, there is really no room for them. I can measure the boiler supply temperature with the tridicator, but it is so small it is difficult to read it with any useful accuracy. I can probably do it to 5F, or maybe a little better, but that is too crude to be useful.

CMadatMe

Your Missing The Whole Point

The circulator that is supplied with the boiler, in your case a Taco 007 is a fixed speed pump. They choose that circ to move a particular gpm across the HX at a specified ft of head. You cannot change that without changing the circulator PERIOD!



The boiler flow rate is going to be 7.1gpm no matter low fire, high fire, mid fire, etc. The reason boiler mfg want pri/sec piping is that they know you cannot move that flow rate out into the system at all times. So, what cannot be moved out goes back to the boiler return.



That flow is chosen based on gpm = btu/hr / (delta-t x 500). So at the mfg knows you need to move 7.1gpm on a 20 degree rise to get the full output of the boiler. Forget the system side we are not talking about that.



The btu/hr out put of a condensing boiler is dictated by its Temp Rise meaning the difference between the boilers not the systems difference between it's supply out to the secondary loop and its return at the boiler not the system side.



You can never change the formula of gpm = btu/hr / (delta-t x 500) and that is the logic the boiler uses in it's modulation rate. You can disagree with it all you want, and I respect you but you still need to plug in the delta or temp rise into the formula to get what the boiler is actually making in btu/hr.



The burner is set to fire at particular ranges and caps off on a high and a low. Your boiler flow rate needs to meet that low so you don't flash to steam. You can't turn off the btu/hr being produced by the burner so you need to make sure the flow being moved across the HX will take it away. On the high end it does not matter as long as you can cap off the high end modulation rate of the boiler. When you can do that you can now size your pump to move less flow to accommodate your need at the coldest day of the year.



This is what they do in Europe via the control. This is the reason why Kurt (SWEI) and I want control logic to be able to control the pump. We want the pumps flow to meet the needed flow on the system side so we can scrub out the btu/hr the boiler makes to the emitters so they can keep you and your wife comfy and warm.



You need to wrap yourself around the fact that boiler pumps are over sized and the universal hydronics formula. The MATH NEVER LIES!



Your running small deltas because your emitters cannot absorb the btu/hr the boiler is making. Nor more no less, period. Don't believe me. I'm willing to supply you with 3 Caleffi Quick Setters. One for your boiler out, one for your zone 1 out and one for your zone 2 out. Bad time of year to do it but willing just because I want you to understand. I'm in NJ too and will hand deliver them..

SWEI

Boiler logic

Is actually pretty simple.  The modulating burner is driven to maintain supply setpoint (whether fixed or ODR controlled) via a PI or PID loop.  Boiler return temp is a product of emitter capacity and volume of water pumped.  The rise varies as the boiler modulates up and down maintaining that setpoint.



We can add sensors and controls (or buy those packaged as a ∆T pump) to improve this but we'll never know as much as the boiler does (plus the cost can really add up, particularly on smaller jobs.)

Jean-David Beyer

I must be Missing The Whole Point.

I am not being sarcastic. We are clearly not talking about the same thing, but I am pretty clear I am not talking about what you are talking about (whatever it is), and I have my doubts you are talking about what I am talking about (as far as I can tell). I am not accusing you of bad faith or anything. But we seem to be talking across one another instead of with each other. I wonder how to straighten this out.



What is the whole point, as you see it? I guess I really do not know.



I think my original point was that I could not see how a (fixed) delta-T pump could work in a modulating condensing boiler system with outdoor reset. I based my confusion on the fact that my radiant zone runs a nominal supply temperature of 76F when it is above 50F outside, and works its way up to 120F when it gets down to 6F outside, and that I have never seen it get down to 6F here where the design temperature is 14F. It could, but it would be very seldom and not for long each time. Those are the nominal values of the reset curve for that zone. Actually, the hysteresis is + and - 5F, so the temperatures actually run between 71F and 125F.



Let us pretend I have a delta-T circulator in my radiant slab zone. I run my thermostat in that zone at 69F. So what delta-T value should I use. It better not be more than 2F because if it were, how would I get a delta-T of more than 2F from that zone. And even at the other end of the hysteresis, where it is supplying 81F, I notice that I get a delta T of only a couple of degrees. So the delta-T circulator would have to stop and still fail to get a sufficient delta-T. If it stops, I might as well change the reset curve to have it stop instead and use a normal pump.



Similarly, at the high end, what delta T do I want there? Let us say it is 6F outside, so I am running 115 to 125F supply to the slab, and say I get a 6F delta T with the fixed speed pump. And that is what it takes to heat the zone. Assuming the pump I have in there is sized exactly right (I am not kidding myself: I do not think this assumption is right, but I do not even know which way it is off). If I pump slower, I will get more temperature drop through the loop(s), but will I get more heat into the zone? I do not think so because the loop(s) will now be colder at the far ends due to the slower flow rates. And even if I would accept the uneven floor temperatures, to get the average temperature back up, I would have to raise the supply temperature so the average temperature in the loop(s) is the same as it was when using a faster pump.



And everything we both have said since has not clarified the situation. As I said before, I am not accusing you of ignorance: you are clearly intelligent and well informed. And you have not clarified my thinking either, and I think I am intelligent and fairly well informed. So we should be able to do this. But I do not know how.



"Your running small deltas because your emitters cannot absorb the btu/hr the boiler is making."



I know that. I have known that from the beginning. I have an 80,000 BTU/hour (input) boiler and a house that needs about 30,000 BTU/hour when it is 14F below design temperature outside. But it was the smallest W-M mod-con in the product line. And my contractor even wanted to put in the next larger size, but I was lucky enough to refuse that (because I did a heat loss and he did not).



So I think my question is, with my circumstances, how would a delta-T circulator make running my system work any better, say by reducing rapid cycling in the small baseboard zone, or using less gas to heat the house?



It seems to me a delta-T pump that raises the delta T above what I get now with fixed speed pumps will put even less heat into the zones that are not taking enough out of the boiler as it is.



I still do not see how running lower flow through a baseboard (or a slab) will increase the heat emitted, while at the same time lowering the average temperature of the emitter. And nothing you have said has helped me understand that.

CMadatMe

Agreed

Jean I'm not taking you to be sarcastic at all. I like reading your posts and for someone not in our industry you provided nice information to those looking for help.



I agree with you we are talking about different things. Your system is piped pri/sec. So you have in essence three separate systems. A boiler system that makes energy, a supply system (Zone Piping) that moves energy and the delivery system (the emitters) that sends the created energy out to the living space.



Let's concentrate on the boiler system first. Your boiler output is 71,000 btu/hr. They supply a Taco 007 as a boiler pump. After some detective work we figured the head loss in the HX is 7.5'. I pulled the 007 pump curve and that pump is moving a tad over 7gpm. So using the formula gpm = btu/hr /(delta-t x 500) I am able to find the boiler rise they are using. Boiler rise means, to get the full 71,000 btu/hr the temp difference between the boilers supply and return must be 20 degrees.



71,000/500/7 = 20 Degree Temp Rise



So we know your Taco 007 Boiler Circulator is moving 7.1gpm across the boiler at all times. Now, how many btu/hr being carried by that 7.1gpm is going to be dictated by the boilers temp difference between it's supply and return water temp. It has to. Your piped pri/sec.



Your heat loss is 30,000 btu/hr. Using the boilers known 7.1 flow rate and that formula again we can see when that boiler would be over sized. Mind you this is if all zones are open.



30,000 /500/7.1 = 8.45 Boiler Temp Rise



So anytime the difference between your boiler supply and return is over 8.45 degrees your boiler is over sized. This is why having control over your boiler delta-t (Temp Rise) is critical to overall efficiency. Your case is a little different because your supply water temps are always in the condensing mode. In the majority of applications out there they are not. Please see attached for what happens when just your radiant calls. It's not exact. I used the radiant running on a 10 degree delta-t and your posted heat loss of 25,000. Boiler supply 125. Notice the difference between boiler supply and return is 7 degrees.



7 x 7.1 x 500 = 24,850 btu/hr being produced by the boiler at that given time.



If 71,000 is 100% (high fire) and 16,000 is 20% (low fire) the firing rate @ 25K is 65%



-----------------------------------------------------------------------------------------------------------------



Now we can move to your system (delivery) side..



In reading your post I'm getting the feeling your talking Delta-t using the room set point. We are talking about the temp difference between the zones supply and it's return. In radiant slab we design for 10 degrees. So 125 in 115 out (10 Degree Delta-T). I'm not sure what your radiant return temps are, you haven't disclosed that. So using this 10 degree delta-t and the heat loss we can determine that zones flow rate.



I believe you said 25,000 btu/hr at design. So using that UHF again we find our flow rate.



25,000 / (10 x 500) = 5gpm



You cannot move any more btu/hr with 5gpm than you can with 8gpm by increasing flow. Why, you will decrease the delta-t. Decrease the delta-t to 6.



25,000/ (6 x500) = 8.33gpm to get the same result. Yes you increased flow but you give up the same amount of btu/hr as the 10. The space can only take 25,000 btu/hr right? All you did was move a larger volume of water faster but the emitter is only going to take what it needs. That's why the smaller delta-t. Moving to fast and the emitter can't absorb any more energy.



The reason we want control over Delta-t is to use this aiming device to move the required btu/hr period.

Jean-David Beyer

Mind you this is if all zones are open.

"Your heat loss is 30,000 btu/hr. Using the boilers known 7.1 flow rate

and that formula again we can see when that boiler would be over sized.

Mind you this is if all zones are open."



Agreed. and it is even more oversized when only one zone or the other, is running.



"So anytime the difference between your boiler supply and return is over 8.45 degrees your boiler is over sized."



I do not understand this at all. It seems to me that if the delta-T is higher, I can infer that more heat is being put into the load and that at that point, the boiler is less oversized. In my little zone, it is difficult to get more than a degree or so of difference (as measured in the system loop) between the supply and return, and that is obviously severely oversized. The lowest the boiler can do is 16,000 BTU/hour (input) and the most that zone can take is 6500 BTU/hour, and usually needs much less. I suppose I could get a greater delta T up there by doubling or tripling the amount of baseboard there (not really practical), and then it would sink the boiler's heat faster. But would it? The reason for the question is that I would then diddle the reset curve to run still lower temperatures up there. One objective I had, though it is not on stone tablets, is to keep the temperatures as low as possible to get more condensing, and if I get the reset curve right, the thing would run all the time. I cannot actually do that because the infiltration depends on the wind and if I have enough heat to cover the case where it is very cold outside and high winds, I need a little more heat than otherwise, and since it will deliver that little more heat when it is not necessary, the thermostat will shut it off. So I would lower the temperature if I had more baseboard, and (without calculating it), I would expect it to break even.



On the other hand, the radiant slab zone has a greater delta T, though still not a whole lot. So it seldom cycles. On some days the thermostat calls for heat and the call stays on for 12 to 18 hours, and the boiler just runs, never hitting the high or low limit. So it is surely not oversized in this situation..



It seems to me that the low delta-Ts I get in the system loops shows me only that my circulators in that loop are, by normal standards, running too fast. I recognize that running them slower (I would have to replace them: Taco 007-IFC) would increase the delta-T. But it would reduce the amount of heat delivered, and make it more uneven besides. I do not think the low delta-Ts show me anything about whether my boiler is oversized or not. I know it is at least double the size it needs to be, but not from these data.



"This is why having control over your boiler delta-t (Temp Rise) is critical to overall efficiency."



I do have control over my boiler's delta T, but only indirectly. I.e., I cannot set it from the control panel. But the controller observes the system supply temperature and compares it to what the reset curve asks for and adjusts the firing rate accordingly. It is not linear, though. If it is near the reset curve point, it varies the firiing rate, slowing it down as it approaches the set point, and slowing it down even more if it passes the set point. But if it gets 5 degrees below the set point, it tends to fire at maximum rate, and if it goes 5 degrees over the set point, it stops firing entirely. This is not exactly true because as far as I can tell (W-M do not say in the manual) it is proportional plus integral for sure and probably differential as well. (I used to design feedback control systems, but not for hydronic heating. For automatic airplane landing computers, helicopter stationkeeping systems, and stuff like that. But that was 50 years ago or thereabouts, and I do not remember all that stuff anymore.) Anyhow, they do not control the differential except indirectly: higher supply temperatures will increase the amount delivered to the zone, so I expect the differential to increase. Not directly proportional to the supply temperature; proportional to the temperature difference between the air in the zone and the water in the emitters.



"Please see attached for what happens when just your radiant calls."



Oops! Nothing seems to be attached.



"In reading your post I'm getting the feeling you [are] talking Delta-t using

the room set point."



Well, there is a room set point of 69F, but that is not directly related to the delta T I think we are talking about.



" We are talking about the temp difference between the

zones supply and it's return. In radiant slab we design for 10 degrees."



Maybe you design it for that. Maybe all present-day professionals do the same thing. But is that what the builder of my house did in 1950? Perhaps, but I very much doubt it. He bought a boiler from GE. The one on page 11 and 12  here:



http://www.heatinghelp.com/files/articles/1025/177.pdf



Using the page numbers at the top of the pages. I do not know the heat output, but when the burner quit, the heating contractor replaced the burner with a Beckett with a 1/2 gallon per hour one, and it supplied enough heat. It was over sized too. That would have been 70,000 BTU/hour input. I think that GE was the smallest one they made. And heating oil was probably something like $0.20/ gallon in those days. If I remember correctly, it was around $0.40/ gallon in 1976 around here. It was almost $4.00/ gallon when I switched to natural gas.



I suspect that system was not designed at all. As far as I can tell, the slab is uninsulated. He must have used good concrete for it, though, because the copper tubing 63 years later does not seem to be leaking. I cannot tell what the tubing lengths are (each room has a separate circuit at the supply end, but they are all tied together at the other end somewhere inside the slab itself). Each room  upstairs had a 3-foot finned tube emitter in it. It was aways cold up there. The only way to get more heat up there was to throttle down the loops downstairs to the point where they did not heat well. As I say, I do not think it was designed at all.



Anyhow, if I increased the supply temperatures to the slab to get a 10 degree delta T between supply and return, I would be providing way more heat than I usually need, and get less condensing too.  Alternatively, I could slow the zone circulator down to get a greater delta T, but then the heat would be uneven.



"So 125 in 115 out (10 Degree Delta-T). I'm not sure what your radiant

return temps are, you haven't disclosed that. So using this 10 degree

delta-t and the heat loss we can determine that zones flow rate. "



I am not being cagey about the radiant return temperatures. It is difficult to get them because it takes so long to get they system running in the steady state. So if the zone has been off for 8 to 12 hours because the sun came out, or it got warmer outside, the slab will have cooled down a lot. So when that zone calls for heat it can take at least 4 hours to achieve what looks like a steady-state reading, but realistically, it can take 24 hours to really achieve it. Not conducive to getting a lot of useful readings. I do not recall ever getting 10F delta T. I do see 5F frequently in that zone though. So if either zone starts up after being idle for a while, I get high delta-T readings, but they do not mean much. And if I wait, they go down. I usually lack the patience to see what they are. In the baseboard zone, they never really hit steady state except when it is very cold out. Downstairs they often do, provided the outdoor temperature does not change too much. But it does. The sun comes out in the morning, so it heats up quite a bit, and it sets at night and cools down a lot. The thermal mass of that slab makes analysis difficult, and outdoor reset goes a long way in increasing comfort by eliminating severe over and under shooting of temperture.



If I were designing a house with an hydronic heating system, I think I would put radiant in the ceilings and only enough heat in the insulated slab to take the chill off, but not to provide a much of the heat loss. Then everything would work better. And I would buy a mod-con with a maximum firing rate of, say, 34,500 BTU/hour for this house. Or, more likely, a ground source heat pump. Since the water table is only 6 feet down, maybe I would put the pipe field all the way down into the water.



If someone else is paying for it, anyway. ;-)



"You cannot move any more btu/hr with 5gpm than you can with 8gpm by

increasing flow. Why, you will decrease the delta-t. Decrease the

delta-t to 6."



I do not understand you. I know that decreasing the flow will decrease the delivered BTU/hour, though it will decrease the return temperature as well. Probably not in linear proportion. But as a result of a longer time the hot water is in the emitter. In other words, slowing down the flow will increase the delta T and reduce the actual heat delivered.



I think I am finally coming to understand what our disagreement is. On most items, we agree.



"Yes you increased flow but you give up the same amount of btu/hr as the

10. The space can only take 25,000 btu/hr right? All you did was move a

larger volume of water faster but the emitter is only going to take what

it needs. That's why the smaller delta-t. Moving to fast and the

emitter can't absorb any more energy."



See where we disagree? If I pump fast enough, the emitter will be at the same temperature from end to end. And it will not deliver more heat than that no matter what you do to the pumping rate.



 If I pump slower, it will be cooler at the return end. Another way of saying this is that if you pump slower, the average temperature of the emitter is reduced, and so the amount of heat delivered is less.

CMadatMe

I'm Giving Up

With this. I respect your opinion but not going to get you to see the light. If the temp across the emitter is the same and you have virtually no delta-t ie, 125 out 124 back you transferred just about the equiv of 2' of baseboard running at 180 degrees through 28' of existing board.



2.8 ( your posted flow in the BB loop) x 1 x 500 = 1,400 btu/hr and this is why your boiler short cycles so much. 1,400/ 28 = 50 btu/hr sqft. You might as well get rid of the zone and light a candle in each room.



Your not pulling anything out of the zone loop and the difference between your boiler (NOT YOUR SYSTEM) supply and return is nil. I reattached what I sent early. This is your radiant zone. Now plug in the BB zones numbers into the formula and your boiler supply and return temps actually end up the same.

Jean-David Beyer

I'm Giving Up With this.

I guess that makes sense. This must be something we cannot do over the Internet.



"You might as well get rid of the zone and light a candle in each room."



I tried that in my bedroom when the power was off for almost a week after storm Sandy. I actually used 8 candles, and one had three wicks. It did raise the room temperature from 58F to 60F, and I did not think that was enough. It did set off my sensitive CO detector that will alarm at 7 PPM.



"If the temp across the emitter is the same and you have virtually no

delta-t ie, 125 out 124 back you transferred just about the equiv of 2'

of baseboard running at 180 degrees through 28' of existing board."



That seems right to me.



But it seems to me that if I have a piece of finned tube in my zone that has (essentially) the same temperature at both ends; e.g, 120 out, 119 back, that it just has to put out more heat than the same piece of finned tube that has 120 out and 110 back. And so far you have not helped me see that that is false.

SWEI

putting out more heat

would have to mean that the water is cooler at the end of the emitter.  Period.



Where else would the heat come from?

CMadatMe

Sure I Did

gpm = btu/hr / (delta-t x 500)



2.8 x (10x500) = 14,000 btu/hr - Same flow rate on a 10 degree Delta-T

2.8 x ( 6 x 500) = 8,400 btu/hr - Same flow rate on a 6

2.8 x (4 x 500) = 5,600 btu/hr - Same flow rate on a 4

2.8 x (1 x 500) = 1,400 btu/hr - Same flow rate on a 1



So how can shrinking the delta-t give you more btu/hr in the emitter? It leaves more btu/hr in the pipe meaning what was made by the boiler just went around in a circle.



What would happen with the Bumblee Bee is that your flow rate would go to where it should be providing you select the right delta-t so:



6,500 btu/hr / 500 / 13 = 1 gpm. So I would run a 1gpm flow rate @ a 13 degree delta-t. For that board and yes it will heat the space. It has to..



6,500/28' = 232 btu/hr sqft. What water temp do you need on your design day? 125 Degrees if you look at the charts. So how does more flow get more you more btu/hr?



Your stuck on water temp.

Jean-David Beyer

putting out more heat would have to mean that the water is cooler at the end of the emitter.

Of course the temperature would go down to put out some heat. But if it drops only one degree, I cannot measure it.. The only way I can think of to increase the output from a piece of baseboard is to increase the average temperature of the thing. And slowing it down with any kind of pump, throttling valve, or whatever, will lower the average temperature of the thing, so it will reduce the heat output. Besides, what delta T do you really want to run on a mod-con that is already running very low loop temperatures? Whenever the boiler changes its firing rate, the supply temperature will change. So at some really cold outdoor temperature, I might be able to get 10F deltaT because the supply might be 120F, but when it is only 50F or more outside, I cannot possibly get that delta T because the boiler is putting out 76F water and the house thermostat is asking for 69, so I could not possibly get more than 7F, and would be astounded if I could get that because the heat output from a finned-tube emitter goes way down as the water temperature approaches the temperature of the return air to it.



I am afraid that Chris is right to give up on this thread. I do not see how to resolve it in this medium.

Jean-David Beyer

As you can tell, I am just not getting it.

Assuming I am wrong about all this, I wonder just what my mental block is. I just do not see how a piece of baseboard that is a nearly constant temperature from end to end could put out less heat than one that has the same temperature at the supply end, but a lower temperature at the return end, since its average temperature would be less than in the first case. That is where my mental block must be, but I do not see how to get around it.



"6,500 btu/hr / 500 / 13 = 1 gpm. So I would run a 1gpm flow rate @ a 13

degree delta-t. For that board and yes it will heat the space. It has

to.."



Well, to heat that baseboard zone at 14F design temperature to 69F I run 131F (that is the reset setting; the controller runs in bang-bang mode due to the oversized boiler and the limits are 15F apart to reduce the cycling rate) water through that baseboard. Let us say I get a degree or two delta T. The temperature wanders around because of the cycling so I can never actually measure the delta T in any meaningful way. I mean I can write it down, but if the loop is cold because the boiler was running at the lower limit (or not at all), and I start putting water at the high limit in, it takes a while before the matching water comes out, and by then the boiler has stopped firing and sometimes the return water is hotter than the supply, and those temperatures, although actually measured, are not meaningful.

Paul48

The Goal

            Get the heat to the room and leave it there. All systems are designed that way.Pay no attention to Boiler Delta-Ts. What has been said......The boiler is shipped with a circ designed for a certain flow at max firing rate. If you limit the max firing rate, you don't need that much flow. This serves a couple purposes....It will bring the flow in the primary circuit down, closer to the flow of the secondary. More of the produced btus will go out to the secondary, as opposed to just going back to the boiler.

             Now, for arguments sake, I'll concede that your approach of minimum DT on the secondary side gives you the most heat. But, that's not the only goal. It's to provide heat with low return temps, so we slow the circulation down, take an appropriate DT on the secondary side, and return much cooler water. All the heat that's made is used, efficiently.

             I apologize for dummying it down gentlemen. Chris.....Is that what you said? Less eloquently?

SWEI

a nearly constant temperature from end to end

happens when the fluid temp approaches the air temp, at which time heat transfer comes almost to a stop.