BumbleBee question

Eastman

What happens if you flip the supply and return sensors?  How does it handle a negative deltaT?

SWEI

Is there a 'cooling' mode?

That ought to invert the feedback loop.

Gordan

DON'T DO IT!!!!

This is how the singularity happens! If you do this, the circulator's brain will decide that, since it's clearly smarter than the humans installing it, it might as well oust them and rule the world. Can you imagine what it might be like to have a ruler whose agenda consists of making you run around in circles?



Come to think of it... maybe that's not THAT different from our current bosses. Go ahead.

Gordan

Not really invert the loop

If it measures the delta T as an absolute value, it should work in either scenario. More flow still leads to less delta T, regardless of the direction in which the transfer medium is conveying heat.

heatpro02920

I wouldn't do it

I believe it says something in the instructions about interrupting the space/time continuum, then once it runs backwards you will run into the entire butterfly effect thing, and get nose bleeds and turn into Ashton Kitcher or Ben Affleck I can't tell them apart...

hot_rod

seems

as many installers want reverse ∆T as standard. Good way to double the sales of a product, give the installers what they need.

Eastman

I wish it could do a 0 degree delta.

Want 0 degree delta too.

Paul48

Hmmm

If you were to do that on the primary of P/S wouldn't you overheat the HX? The instructions are clear that sensor placement is very important.

Jean-David Beyer

sensor placement is very important.

On my W-M Ultra, the supply and return sensors are on the secondary (system) loop, not the primary (boiler) loop. I infer that they let the primary loop take care of itself. The controller has other sensors on the supply and return of the heat exchanger, but these are mainly for emergency use (over temperature, too rapid temperature rise, etc.) and not operating devices.



It seems some people interchange the supply and return sensors (by mistake, I suppose) and get failure indications. When return is hotter than supply for more than a few seconds, the controller faults to give you time to fix it.

Paul48

And

How about a cast-iron boiler, and not a mod/con? Would the results be an overheating HX?

Eastman

On the WM Ultra...

are we talking about a variable speed pump?  Or just setpoint control?

Jean-David Beyer

variable speed pump? Or just setpoint control?

Setpoint control. The controller deals with 2 or three different outdoor reset curves, and they use the reset curve to determine the temperature that the modulation tries for. By default it can wander +|- 5F about the set point. But the temperature they measure is in the secondary (system) loop, not the primary (boiler) loop.



The boiler loop runs with a fixed speed pump supplied by W-M; for my sized boiler, it is a Taco 007 painted black. In the secondary loop, the installing contractor used two Taco 007-IFC circulators, one for each heating zone. At least one of those is too large. If I were designing the system, I would have used a delta-P ECM circulator and zone valves, It would be tricky to use a delta-T circulator because one zone uses a supply temperature of 110F to 135F, and the other zone uses a supply temperature of 75F to 120F, and it is not a bit clear I want to run a constant delta-T when the outdoor reset is diddling things. I do not want the feedback system in the controller to be fighting the delta-T function in the circulator and making the whole thing unstable.

Eastman

I'm with you...

I don't see the advantage of a fixed delta.  I feel it should be variable.  And the flow should be variable, etc..

Steve Thompson (Taco)

Reverse Delta T

Guys, I get the reverse setpoint.



Stupid questions (yes, I am known for these):  When would you need a reverse delta T (pump speeds up as system delta T goes down).  Also, can you let me know when you would NOT use a fixed delta T?

Eastman

Steve

"as system delta goes down"



Are you referring to the absolute value of the delta, or the simple subtraction result?



For example, going from a delta of -5 to -4 to -3, that I would consider going up.

CMadatMe

Advantage of a Fixed Delta

How about you know your emitter is absorbing the required btu/hr to heat the space instead of just sending made btu/hr for a ride around the roller coaster to end up back in the boiler return.



You like those small boiler rises and can't figure that out for the life of me. If you keep that boiler return elevated it will take you forever to get into the condensing mode. In JBD's case just zone with two bees.



If you have control of the delta you have control over btu/hr delivery. The two constants in a condensing boiler are the head across the HX based on the flow of the selected boiler pump. They never change. What changes it your system side flow rate and if you can control the system side delta you have a much better chance of the boiler return seeing the coldest water possible. That is why mod/con boiler pumps should be sized for the highest available rise and the system side on a 20 delta. Just have to make sure that the mixed temp at design conditions is going to get the job done. If not find the rise that get's it done. That would take the heat loss broken out zone by zone and emitter measurement for capable btu/hr output at that's zones flow and needed water temp.



Now to get everyone off those 20 Rise boiler pumps and those huge boiler flow rates. Rarely is the system side taking them. That's another subject for debate.



Would love to see JB do a blog for sizing mod/con boiler pumps.

Paul48

Eastman

Why would you want a variable delta-t ? I seem to get more confused lately....It's gotta be an age thing.

Eastman

Hi Chris!

It's not that I prefer a high or low delta.  Every situation has different requirements.  For example, perhaps during peak loads a 30 degree emitter delta is acceptable, but otherwise the system is expected to maintain 15.  (Or perhaps the opposite.)  In low temperature systems, sometimes ODR setpoint temperatures can collide with fixed high delta T settings.

Paul48

Poor Steve!

They gave us something to set a perfectly designed delta-t on the system side, and now you want to vary it! LOL

Eastman

Hypothetical example...

Let's say you're using a delta-P zoning strategy on the secondary loop.  The temperature delta across the manifold is variable.  The ideal primary loop pump solution would be a variable delta T pump that matched the secondary delta over the allowable flow range of the boiler.

Paul48

I Disagree

I feel the boiler would be chasing modulation all over the place.

Eastman

Why do you think so Paul?

?

Eastman

Maybe you'll like this hypothetical better...

Suppose you have a mod/con with a fixed speed primary pump connected to a home run TRV panel rad system.  The boiler delta is variable.  It varies with the load and the setpoint.  The optimal secondary pump solution would be a variable delta that matched the primary delta.

Paul48

You'd

be constantly altering flow and distribution of btus to the secondary side, which would cause the boiler to modulate based on nothing other than changes in flow through itself. The neurotic dog, endlessly chasing its tail. Just discussion.

gennady

bumble bee

I actually did it by mistake. Is much as I can recall it just stack at 6.8 GPM. It is minimum capacity in dE mode.

Paul48

Honestly

I don't like the variable delta-t idea. It goes to my other post about boiler controls. More intuitive controls could and should learn. I don't think we need to vary the DT.I think we need smarter controls that will recognize a "glitch" in the curve and adapt to it. That's just my opinion, and you know what they say....

Jean-David Beyer

emitter is absorbing the required btu/hr to heat the space

I know the emitter is absorbing the required btu/hr to heat the space, because if it were not, the temperature would drop (if it is too low) or rise (if it is too high). But it can do that whether I know the delta-T or not. It happens I do know it and in my small, baseboard zone with oversized baseboards (14 feet in each room instead of about 6 feet if I put 180F water in them) run with about a 1 degree temperature drop around the loop in warm weather, when 110F water is supplied to the baseboards, and maybe 8F when 131F water is supplied. Now if I lower the flow rate into those baseboards when the boiler is supplying 110F water until I get a delta-T of 20F, my guess is that, since the baseboards are in series, the first room will heat pretty well, and the second almost not at all. It seems to me that basically, if I did that, I would be putting out less heat altogether than if I pump it too fast (as it is, now), with little temperature drop. Slowing that pump down a little might be almost OK. But already the system cycles a bit too much on warm days because the boiler will not modulate down far enough even though it is the smallest W-M Ultra boiler they make. Pumping it too fast makes the average temperature of the loop about the same from end-to-end, and since the rate of heat delivery is proportional to the difference between the water temperature inside the baseboard and the temperature of the air outside, a smaller delta-T delivers more heat to the rooms than a larger one, since it is the second of the two baseboards that runs at the lower temperature. Ever look at fin-tubed baseboards as the water temperature decreases? The BTU/hour/foot goes down faster than the water temperature does. Imagine, for example, that the water gets below the room temperature. No heat delivered at all. Or worse.



"If you keep that boiler return elevated it will take you forever to get into the condensing mode."



Well, that is not the case. In my large, radiant slab zone, it always runs in condensing mode, sometimes very condensing (when supply is 76F, for example, and less on the return, though sometimes only slightly less). In the small baseboard zone it almost always runs in condensing mode too (when supply is 110F. Even when it gets up to 135F (that it never does, but the reset curve goes up that high) it should be condensing slightly. The reason it always runs in condensing mode is that the supply temperature never gets over 135F, so it always condenses. When the reset lowers the temperatures, it condenses more. With 76F supply, the return cannot be over 76F, obviously. Even on Design day, the supply to the radiant zone is only 112F, so it should condense quite well. The baseboard zone on design day will have a supply of 131 and a return somewhere around 125, so some condensing will be taking place.





"If you have control of the delta you have control over btu/hr delivery."



Yes, in a manner of speaking. But even with a constant flow circulator, you have control over the btu/hr delivery by adjusting the boiler output temperature. That is what I do, since I have control over the outdoor reset curves. I have them set to deliver the minimum amount of heat required to heat the space. I deliver a little too much heat in really warm days because the boiler will not modulate down far enough. Then it switches to on-off mode, but even then, ON is modulated down as far as the boiler will go. And if the emitters do not dump the heat, slowing the circulator will not cause it to dump more heat than it does now. The return temperatures may be less, but the heat delivered will not be less since the average temperature of the emitters will be less, not more.

Eastman

Practicaly speaking...

I don't know if variable delta T control will pan out.  But purely from a thermodynamic efficiency perspective, the flows in the primary and secondary should be synchronized as much as each system allows.  And this happens when the deltas are equal.

hot_rod

suppose

you have a mod con running on ODR, as required in many places in Europe. You have radiant or low temperature panel rads, also very timely. How does a fixed ∆T circ work when supply temperatures ramp down to 90F or lower?



What's wrong with the ∆T moving, as it does to reach thermal equilbrium and maximizing condensing efficiency and limiting short cycling?

tom3holer

Sorry, but am a bit confused

Hi,

As someone trying to learn hydronic systems for the past several months I am a little confused, after reading some of the posts here, by the relationship between flow and BTU extraction. As I understood it decreasing the flow in a hydronic heating loop would allow the emitters to extract more heat/BTUs from the water thus lowering the returned water temp. I do understand there are  limitations when we get into low water temps as BB does not do well with low temps. It just seems that returning water with a  20* D/T  is got to be more efficient that returning 5* D/T water because we haven't actually used many of the available BTUs in that water to do what it is supposed to do, dump the heat into the rooms.

 My guess is that 20* was chosen as a sorta standard because much higher would cause considerable differences in heat output from the beginning to end of a loop.

I look forward to any thoughts or comments.



Tom

Paul48

Thoughts

Once the mod/con has modulated as low as it can based on the information it recieved from the system, changing the flow will not bring you any closer to thermal equilibrium.

Jean-David Beyer

As I understood it decreasing the flow in a hydronic heating loop would allow the emitters to extract more heat/BTUs from the water thus lowering the returned water temp.

I can parse the words, but my understanding is different,



1.) Reducing the flow rate will result in a lower return water temperature, but



2.) It will not result in more heat being emitted: in fact it will result in less heat being emitted because the average temperature of the baseboard will be less..



Consider (assume baseboard because it is easier for me to think about);

If the flow rate (in the extreme) results in no measurable temperature drop through the entire length of the baseboard, the delta T will appear to be zero. But that will deliver more heat to the surrounding than if I put the same supply temperature in and get 20F less on the return, because the average temperature of the baseboard will be 10F less than with the high flow rate.



Example #1: high flow rate (considered too high by most):

Supply temperature: 120F, return temperature 119F; average temperature 119.5F.

Heat emitted (depending on make and model) 200 BTU/hr/foot.



Example #2A: rate low enough to get a delta T of 20F

Supply temperature 120F, return temperature 100F, average temperature 110F.

Heat emitted: 150 BTU/hr/ft.



Example #2B: rate low enough to get a delta T of 20F

Supply temperature 130F, return temperature 110F, average temperature 120F.

Heat emitted: 200 BTU/hr/ft,. But you now have to run your boiler 10F hotter to get the same heat output delivered to the load, and it will now be uneven (in my case) because one room will run at 125F average temperature and the other at 115F average temperature. And without lengthening one baseboard and shortening the other, there will be nothing I can do about it.



Running the boiler at 130F instead of 120F will surely result in more heat up the chimney and out of the boiler into my otherwise unheated garage where the heat is not needed. If the goal is solely to get low return water temperatures, I could install some snow melting tubing under my driveway to cool the return water down even more. But my goal is to heat the house, and returning low water temperature is just one element in the process. I chose to do it by using much longer baseboards, but that puts lower temperature water into them, and as a result the return is lower also, but the delta T is low also. There are about 75 feet of 1/2 inch copper tubing, 24 feet of 3/4 inch copper tubing, and 28 feet of 3/4 inch Slant/Fin  Base/Line 2000 baseboard in there, and an awful lot of 90 degree elbows. The flow rate is whatever comes out of a Taco 007-IFC that I estimate to be around 2.8 gpm. I should get around 6F delta T when the outside temperature is 0F, but design temperature is only 14F, so I never get that much on that zone.

Paul48

J-D B

New Math? Using the sum, to calculate the sum? If the emitter, regardless of its length emits 20*, that's it. And in the boardroom,failures to meet expected earnings are not losses.

Jean-David Beyer

New Math? Using the sum, to calculate the sum?

I do not understand your post.



If I have a piece of baseboard that is 120F and one end and 100F at the other end, it is surely emitting less heat than if it is 120F all the way along. What has math, new or old, got to do with this?

Paul48

J-D B

The emitter outputs are rated at a given temperature and FLOW.The more you increase the flow, the more you diminish the output.

gennady

Median temperature

Median temperature of the emitter affects emitter outlput. If you increase delta T you decreasing output. To compensate you need bigger radiation surface. This is the reason I always check existing emitters to room loads. Then I can determine maximum deltaT system can safely Handle.

Eastman

@Tom

Higher deltas directly imply that more usable energy is being transported on a per gallon basis.  You could say that a system with high deltas has good pumping efficiency.  An engineer may take advantage of this at the design stage to allow him/her to use smaller diameter piping, reduce electrical consumption, reduce labor, or possibly reduce other distribution related inefficiencies and delivery problems.  The disadvantage may be increased difficulty in matching final heat delivery to heat losses.



A high dT system does not directly imply that return temperatures are lower.  For example, let's say you have a series fintube loop that spans three rooms.  At current conditions, let's further assume that the last fintube unit in the last room at the end of the circuit needs about 140 to keep up with its heat loss.  If you were using a fixed delta T of 20 degrees on this circuit, than we must supply the circuit with 160 water, right?  Now let's increase the dT to 30 and lower the return temperatures --the system fails since the last room now no longer meets temp requirements.  One would be forced to raise the supply temperature to 170 to restore the 140 degrees required at the end of the circuit.  What is better?  170/140 or 160/140  If the rooms at the beginning of the circuit are overheating, than 160/140 is better.  If the rooms are too cool, than 170/140 is better.



System deltas go hand in hand with system temperatures.  A high dT is not a direct indication that the total thermal system

efficiency is better.  It is just one variable of many.  In other words,

it doesn't necessarily imply you are going to save any money. 

Jean-David Beyer

emitter outputs are rated at a given temperature and FLOW.

"The emitter outputs are rated at a given temperature and FLOW."



Absolutely!



 "The more you increase the flow, the more you diminish the output."



Not according to Slant/Fin, who ought to know.



Here is part of their table for Base/Line 2000 residential baseboard that I have:

BTU/HR per linear foot.

Temperature   1GPM   4 GPM

110F                 150       160

120F                 200       210

130F                 250       260

140F                 300      320

150F                  360      380

160F                  430      450

170F                   500     530

180F                   570      600

190F                   630     670



Looks like the more you diminish the flow, the more you diminish the output.

CMadatMe

Read this

http://jbblog.flopro.taco-hvac.com/

Gordy

There's your answer

Educational read Chris!



Diminishing returns for sure. As Chris has said bank on the boiler delta it's a constant, and where returns are maximized.

CMadatMe

Your Welcome

I don't think I said boiler delta was a constant. Boiler flow is a constant. Your boiler delta in a condensing boiler piped pri/sec or LLH is a moving target and continually changes. I just feel that boiler pumps need to be sized for a 40 Rise flow not a 20 or 25 flow. I want the least amount of flow on the boiler side so I can scrub it out to the system side which I would size using a 20 delta-t for my flow rate.