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The best Delta T for a mod-con?

JeffGuyJeffGuy Member Posts: 70
My Alpine is installed and working, thanks to the great plumbers I ended up with who did a great job.

There is one thing that bothers me a bit. The pump that was included with my Alpine 210 boiler for use in primary loop is a Taco011. My installation is almost exactly as shown in the manual, with less than 10 feet of 1-1/4" copper pipe in the primary loop (plus 4 elbows, two T's, pump with integrated flow check plus ball valve flanges ) - in other words there is very little resistance to flow. When I figure the head loss for the primary loop and where that crosses the Taco011 pump curve, it indicates I should expect a 20 deg delta T. And indeed this is what my system is seeing - 20 degree delta T.

The boiler is specified in the manual to run from 20 to 35 degree delta T. So it is within spec and I'm not having any trouble. But wouldn't my system be running more efficiently if I put in a smaller primary pump so that I got an increased delta T? I can tell by using my hand that even when all zones are calling for heat that there is positive flow through the primary loop T's.

In general I guess I'm asking whether it wouldn't be better for a mod-con to operate at a higher delta T? Doesn't that increase the amount of time in condensing mode and improve efficiency?


  • ChrisChris Member Posts: 3,056
    edited January 2012

    In my manual the boiler pump is a 0014 not a 0011. The 0011 is actually more pump then you need. On a 25 delta flow rate is 15.5gpm @9.8' head. That's why the recommended boiler pump is a 0014. Manual gives you the flow rates and head for different delta's. The wider the better. The lower the return boiler water temp the better chances of staying in condensing mode all the time. Just don't exceed the 35 delta.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Tim McElwainTim McElwain Member Posts: 3,895
    I love to see

    40 degrees if the system will allow it this helps to stay in the condensing mode much longer.
  • ChrisChris Member Posts: 3,056
    Hi Tim

    Agree on that 40. Size all Vitodens boilers pumps for it. Unfortunately the Alpine manual has a big NOTICE disclaimer not to run more then the 35 in the manual.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • JeffGuyJeffGuy Member Posts: 70
    They changed it

    The manual that came with my boiler still says the Taco014, but they included a supplement to the manual along with the Taco0011. I think I would have been better off with the 014, but even that would have only given me a 25 degree delta T if my numbers are correct. I *really* wish they had included the Grundfos so I could try the different speeds, but unfortunately they didn't.  By the way, the manual is also incorrect about the Grundfos setting - it should be at speed 2 and not speed 1 to get 25 degree delta T.
  • furnacefigher15furnacefigher15 Member Posts: 502
    Do not Change the pump

    Installing a smaller pump will be of no benefit on the primary.

    Slowing down the boiler pump will do the following:

    1- Increase time water is in the boiler causing a higher temp RISE. (not lower return)

    2- Increase the amount of boiler cycling on target temp.

    3- Increase stack temperature (the heat has to go somewhere)

    If you want more condensate, then you need lower return water temperature, at the design temp rise of the boiler.
  • edited January 2012
    Tim is right....

    The higher the delta tee, the longer the boiler will stay in condensing mode.    As the flow drops the supply temp will rise so the delta tee will be higher, however, the delta tee on system emitters will also rise, giving you a lower return temp.  IE.  If you need an average of 120F water in the emitters to heat your home at a specific outdoor temp you could supply 130F water and have a 110 F return (20 degree delta tee) or supply 140F water and have a 100F return (40degree delta tee).    Both scenarios will give you the same heat output from the emitters, but since the return temp is lower for the latter, the boiler will condense more.  In addition, you will only need a fraction of the amount of electricity to power the pump at 40F delta tee vs. 20delta tee.   IIRC you will need only 1/4 the power.  These combined reasons are why nearly the whole world has standardized on a delta tee of around 30 to 35F (except the US).  Just remember, bot the primary and secondary loops need to be  designed for a high Delta tee for the whole thing to work best. 
    The Steam Whisperer (Formerly Boilerpro)

    Chicago's Steam Heating Expert

    Noisy Radiators are a Cry for Help
  • furnacefigher15furnacefigher15 Member Posts: 502
    Primary pump (boiler)

    Has no effect over what happens in the secondary (system) as far as system delta.

    I agree increasing system supply temp will increase heat output from the radiators, or baseboard, etc...

    The problem is you just increase the delta on the system, by reducing water flow.

    The delta on the system will increase, but at a cost of reduced btu flow.

    Reduced water flow equals reduced btu flow. No way around it. The next problem is the reduced system flow will result in more boiler supply to return bypass, increasing the return temp.

    In order to increase delta correctly, there needs to be more heat removed from the boiler and the system.

    For example: Existing high temp boiler (180) is replace in existing home.

    For ease of math I'll use simple numbers.

    Original boiler output 100,000 btu designed at 180 supply and 20 degree delta (170 av)

    In order to emit that 100,000 btu the systems has 667 square feet of edr @ 70 degree space temp.

    New boiler output 100,000 btu designed at 140 supply and 40 degree delta (120 av)

    would require 2000 square feet of edr. @ a 70 degree space temp (more than triple)

    So you can't just take an existing home and only change the boiler and reduce flow.

    My point is flow is reduced flow is not the answer, increased heat emitter would be.

    The key here is system design.
  • AlHeatingAlHeating Member Posts: 17
    edited January 2012
    Big mistake! More flow = BETTER !!!

    Thinking that the DeltaT of the boiler will keep you in condensing mode longer is the most common mistake we see when designing mod/con systems.  The best option is a SMALLER DeltaT.  Here's why :


    No matter what is the boiler DeltaT, the system supply remains the same.  Same amount of BTUs.  Less flow with hotter water, or more flow with coolest water from the boiler. If you reduce the boiler flow, more return water will bypass the boiler.  After you mixed with the return from the boiler, your mix will be at the exact same temperature. 

    It's absolutely NOT the boiler pump flow that will makes you condensate, but the SYSTEM pump.  If you increase the boiler delta T from 20 to 40, and your system pump remains the same, the mix after the hot pipe from the boiler is thrown in the system will be the exact same.  200 MBH is 200 MBH.  Basic principle of a primary secondary configuration and basic thermodynamics rules.

    To make the boiler modulates better, you should always put a system sensor, which reduces cycling and tells you what the REAL supply temperature is.

     1) That being said, with no flow variation in the SYSTEM, the return temp will be the same.  So, if you have a return temperature of 110F, and you have a 40F deltaT in the boiler, the average temperature of the heat exchanger surface will be 130F.  If you have a 20F deltaT, the average temperature in the exchanger itself will be 120F.  The supply temperature, after mixing with the system flow that won't go in the boiler, is the exact same.  Which option is better??  LESS DELTA T !!!

    2) In both options, the flow in the boiler will be turbulent in those restrictive exchangers, so there's no difference on that side.

    3) Most stainless steel exchangers have small passes, especially the ones with the Gianonni heat exchangers.  Have a greater flow rate will keep you from blocking if there's ever a leak bringing fresh water in the system, or a lot of sediments in the system.  We've seen a couple Lochinvar, Laars and NTI exchangers having overheating issues caused by 40F deltaT.  Option?  GREATER FLOW RATE

    4) The energy cost of running a bigger pump is so little that we should even not talk about it.  With 3 speeds pumps, having a pump adjusted to a superior speed draws 0.2 amps more.  No big deal!!!

    The REAL option to make the system more efficient?  System pumps controlled with the outdoor temperature.  Your maximum system flow is required only at the outdoor design temperature.  So if you decrease the flow rate when the outdoor temperature is not too low, you'll extend the condensation range of the boiler by reducing the return temp.  When it's colding down, the system flow rates increases, and then you fall off the condensation range of the boiler.  (you would have anyways, probably...). 

    As someone said before, most system pumps are WAY too big.  This is real bad for the MOD/CON boiler's efficiencies.  However, the flow in the boiler itself should better be HIGHER since the return temp is invariable and the goal is to keep the biggest heat exchanger surface below the condensation point.

    Cheers from a chilly -8F Montreal tonight.
  • AlHeatingAlHeating Member Posts: 17
    No, he's unfortunately not!

    Be careful not to mix the boiler DeltaT with the system DeltaT.

    In a primary secondary systemp, your system DeltaT won't go to 40F because you increased the boiler DeltaT to 40F.  The system deltaT and supply temperature will stay the exact same if you just increase the boiler's DeltaT.   And the DeltaT in the emitter won't change if you don't move the system flow rate up. 

    Example (round numbers for clarity only...) :  System at 30 GPM.  Return at 100F.  200 MBH boiler.

    Option #1:  20 DeltaT.  20GPM in the boiler.  The flow splits in 2 : 10 GPM (30-20) will by pass the boiler, at return temperature of 100F.  20 GPM will be heated to 120F in the boiler.  After you mixed it :  (10 GPM @ 100F)  +  (20GPM @ 120F) = 30 GPM @ 133.333F

    Option #2 : 40 DeltaT. 10GPM in the boiler.  The flow splits in 2 : 20 GPM (30-10) will by pass the boiler, at return temperature of 100F.   10 GPM will be heated to 140F in the boiler.  After you mixed it :  (20 GPM @ 100F)  +  (10GPM @ 140F) = 30 GPM @ 133.333F !!!!!  (EXACT SAME!!)

    By moving boiler DeltaT ALONE up or down, it changes nothing in the rest of the system.  This was the purpose of the question.

    But if you look at the two options,  in which one the average exchanger (and contact surface) temperature was the coldest??  Option #1!!  Which one is more efficient?? ......
  • AlHeatingAlHeating Member Posts: 17
    You got it!

    You are absolutely right.  Some of our clients did a LOT of savings by reducing the system flow in the mid seasons, when the max flow is not required.  All this with a safe and efficient 20F deltaT in the boiler.  See my post below... you'll see that it's even better to have a lower deltaT!!
  • edited January 2012
    Reduced water flow does not equal reduced btu flow....

    What changes BTU flow is the change in average temperature in the emitters, not the flow rate.    The formula is BTU/hr / (8.3 x 60 min x delta tee) = gpm. 

    100,000 btu/hr / (8.3 x 60 x 20) = 10.04 gpm

    100,000 btu/hr / (8.3 x 60 x 40) = 5.02 gpm

    Same btu's transfered, but 1/2 the flow. 

    In your example above, your first design runs at 180sup with a 20Delta tee, giving you and average water temp in the emitters of  170F.  If you ran a 40 delta tee  then your supply temp would need to be 190 F (return at 150) but again the emitters are at an average of 170F.   Delta tee changes with reduced water flow, but emitters output is the same because the average emitter temperature is the same.

    In warmer weather , the supply could be reduced with outdoor reset and very quickly the return temp will drop into condensing mode, even with emitter sized for 170F water on the coldest day.
    The Steam Whisperer (Formerly Boilerpro)

    Chicago's Steam Heating Expert

    Noisy Radiators are a Cry for Help
  • JeffGuyJeffGuy Member Posts: 70
    edited January 2012
    Wow, thanks for all the replies

    I'm sorry I didn't post more info about my situation, since several posts made reasonable assumptions that aren't true in my case. I'm sorry if I led you astray, but let me try to set the record straight and make sense of everyone's replies. Please don't be offended if I get it wrong - and if I do please let me know!!

    I think everyone agrees that increasing the system delta T will improve the mod-con efficiency. System delta T here would be the temp that the boiler sends out versus the temp that returns to the boiler. Boilerpro quoted the equation that makes sense to me and that shows why this is true - namely that when flow decreases, delta-T increases since they are inversely proportional.

    The question is how to achieve increasing system Delta T  in a primary-secondary piped system.

    Let me start by saying that my mod-con came with an outdoor reset sensor that is connected since that was suggested in a couple posts. And it is with this sensor in operation that I'm seeing the 20 degree delta T. I'm not sure what a "system sensor" is that AlHeating referred to - was this another name for the outdoor sensor or is this what is called a "header sensor" in the Alpine manual? The Alpine manual doesn't say why or when to use a "header sensor" - is this really useful for increasing system delta T (which is my goal) and if so how would it be connected?

    One other suggestion was to increase my emitter EDR - but my house has old cast iron radiators that have more than enough EDR and I'm not realistically able to change that.

    So the purpose of my post was to suggest that decreasing the GPM in the primary loop would be another way to accomplish this. First of all, note that I am currently seeing a 20 degree delta T with the circulator that was supplied with my boiler. But the Taco011 circulator is larger than the manual led me to believe it would be (since they changed it after printing the manual) - and I think it is larger than it should be. The reason I think this is true, is that the GPM through my primary loop is higher than the GPM through my secondary loops, even when both zones are calling for heat. (Note that I am NOT in the situation AlHeating assumed of having large secondary circulators). So there is always positive flow through the closely spaced T's. If I am able to reduce this positive flow through the T's by decreasing the GPM through the primary loop then there will be less water freshly out of the boiler that immediately returns back to the boiler, and more of this freshly heated water that will instead go through my secondary loops. Therefore my return temperature should be lower and therefore my delta T should be higher.

    I hear the concern about making the primary loop circulator too small, and therefore making the delta T too large. The Alpine manual states that a delta T between 20 and 35 is fine. Would it be reasonable to try different circulators until I find one that will get me to a 35 degree delta T? Or should I be cautious of this - do I need to measure actual flow or is using measured delta T sufficient - or is there some other way to accomplish this? (I'm not crazy about the idea of removing a brand new and expensive circulator from my system to replace it with another brand new and expensive circulator if there is some other option).

    Thanks again for all the input
  • How I'd approach it...

    Since it sounds like the flow through the system is higher than the boiler, I would begin by throttling down the flow through the system.  This would start to increase the delta tee on the system.  Most gravity converted systems need only little pumps.  One for a 2,000 sq ft home probalby only needs a little NRF-9 or equivalent.  You could also use a throttling valve on the existing pumps, but this will use more electricity.    As you increase the system delta tee, you will need to ramp up the boiler supply temperature to achieve the same average temperature in the emitters as you had with a lower delta tee.  With the lower flows in the system  main loops, no you can throttle back the flow through the boiler.  Again a smaller pump or throttling valves.   With the reduced flows through both the boiler and system, you should see the coldest return temps  you can get.  Just make sure you maintain adequate minimum flow through the boiler.

    With the low system flows you may need to balance the flow through your radiators in order to get even flow distribution and even heating of all the radiators.
    The Steam Whisperer (Formerly Boilerpro)

    Chicago's Steam Heating Expert

    Noisy Radiators are a Cry for Help
  • JeffGuyJeffGuy Member Posts: 70
    edited January 2012
    Actually the reverse

    I think you missed this in what I wrote above: "The GPM through my primary loop is higher than the GPM through my secondary loops, even when both zones are calling for heat. (Note that I am NOT in the situation AlHeating  assumed of having large secondary circulators). So there is always positive flow through the closely spaced T's."

    I have a very large house and *lots* of radiation - about 1200 sq feet of radiation in my radiator zone. My secondary circulator for this zone is quite a bit smaller than suggested in the article here that discusses using circulators in converted gravity systems:

    Which is why I keep coming back to reducing the size of my primary circulator.

    You said: "Just make sure you maintain adequate minimum flow through the boiler." How do I know this? Is it sufficient to check the delta temperature? If not, how do I measure the flow I am getting?
  • furnacefigher15furnacefigher15 Member Posts: 502
    Don't reduce boiler flow.

    I'm not 100 percent certain of how the alpines algorithm decides how to modulate the burner, but I'm assuming it is like every on else and only looks at the supply temp to reach the target, and the return to see what the system is doing / track progress. They should base the modulation on the differential between the boilers supply and its return, but they don't.

    So the trouble with reducing boiler flow is that the algorithm which governs the burners firing rate will drive the burner up the firing rate too quickly to reach its target, and since the water flow is reduced and with it the btu transfer to the water and out of the boiler to the system is reduced, the boiler will reach its target, and shut down the burner. Then once the temps fall again it will re-fire.

    Lets say you follow your gut and reduce the flow in the boiler.

    The supply water target at outdoor design is 180

    System pump sized to 20 degree delta (180 supply to160 return)

    Boiler pump sized to 40 degree delta ( 160 return to 200) you will cycle on limit!!!!

    But at an outdoor temp at say 50, and the reset supply target now at 130 and that same desired 40 degree spread.

    System pump sized to 20 degree delta (130 supply to 110 return)

    Boiler pump sized to 40 degree delta (110 to 150) you will cycle on reset target achievement.

    I see no situation to where reducing boiler flow will aid in any situation.

    Leave it alone. The saying "if it ain't broke, don't fix it." exists for a reason, and this is it.

    If you want less supply to return bypass in the boiler loop, then increase the size of the system pump.

    The manufacture specked the pump they did for the boiler because flow through the boiler is critical.
  • Paul48Paul48 Member Posts: 4,229

    The manufacturer doesn't provide the equivalent head of the boiler, so that is difficult but could be calculated if you knew the pressure before and after the existing pump.By the way, and not that it matters, your boiler is on the secondary side.The loop off the closely spaced tee's is secondary.
  • AlHeatingAlHeating Member Posts: 17
    Thanks for the input!


    Thanks for the update.  If you have a greater flow in the boiler than in the system, it would not be a bad idea to reduce the boiler flow a bit so you have no recirculation of hot water coming back straight to the inlet of the boiler.  This unlikely situation is the best way to kill the efficiency in the mid seasons because your return will always be higher than the real return from the system.   It's good that your system pump is not oversized, but in your case, don't try reducing your system flow to save more money;  it would make your "positive flow" even worst.

    It was probably more than clear in my previous post than I'm not a big fan of reducing the boiler flow.  But if you recircle water "backward" and your inlet water in the furnace is more hot than your system return, I would think about reducing the boiler flow just a bit.  35F deltaT will always look too high to me, but 30F could be a good compromise.  Do it if you want to have the perfect system, but as many others said : don't touch it if it works. 

    To mesure the flow rates, you can use the deltaT at 100% rate.  It gives you a precise enough mesure.

    Finally, I was less than clear when talking about the system sensor!  I meant a  "remote manifold sensor".  I wasn't clear at all!!  Ahah  Some manufacturers (Lochinvar is one of them) includes this sensor and a well in the box.  This sensor, on the supply pipe, may help to prevent short cycling.  When this system sensor is not present, the boiler uses its own outlet temperature to match the setpoint.  This sensor being pretty close from the heat source, it sometimes can react too fast for the boiler.  Putting a system sensor on the manifold gives a better idea of the actual temperature thrown in the system since it considers the amount of water that "by pass" the boiler when it reads the temperature.

    Have a great day  :-)
  • confusion.....

    Different people call the  different loops primary and secondary.  Sounds like your boiler loop has the most flow, so throttle it first.  Many boilers have both supply and return sensors that can be read from the internal display of the boiler.  To check for flow, the boiler and system has to be running at full output to achieve maximum delta tee.  The delta tee drops at lighter loads since the flow stays the same, but the btu's moved drops.  You may be able to throttle the systems loops too if your delta tee is still below the 30 to 35F range at full load.

    Also, lets see a picture of your boiler piping, your desciption makes me suspect that the P/S tees may be installed incorrectly.
    The Steam Whisperer (Formerly Boilerpro)

    Chicago's Steam Heating Expert

    Noisy Radiators are a Cry for Help
  • JasonJason Member Posts: 293
    Even another opinion

    I run a lower delta-T to stay in condensing mode for a longer period of time. If you know the return temperature and add the delta T, that is your supply temp. We know that the cooler the water the more efficient the boiler is. We condense when any part of the boiler is below 130f. More of the boiler operating below 130f the more we condense and take advantage of latent heat.

    If we operate with 110f return and work on a 35f delta T the supply is 145f. Most of the boiler will not be condensing.

    Same return temperature 110f but a 20f delta T the supply is 130f Still condensing.

    If the boiler is over sized reduce the input which means we can reduce the flow through the boiler but still maintain the 20 - 25f delta T.

    Flow in a heating system does affect output of the radiation. Look at the charts. At the same water temp at 1 or 4 gpm the output is different. Also the delta T is lower as water becomes cooler. If you have a 20f delta T at 180f water and drop the water to a 110f water you will have much less delta T with fixed speed pumps and increase the resistance to flow. Cooler water has a higher resistance to flow that hotter water

    As we drop water temperature with ODR I am concerned with doing a higher system delta T due to less heat at the radiation at the end of the loop. It can cause very uneven heating. Variable speed pumps can help in this situation.

    The primary pipe is not always the boiler piping. The tees are always mounted on the primary pipe and the secondary is off the branch of the tees. Primary or secondary has nothing to do with the boiler but how it's piped. If the boiler is piped off the branches the boiler would be secondary piping.
  • wrxz24wrxz24 Member Posts: 285
    For my set up

    I have been trying to see what speed I should set my boiler pump to. I have a tt 110 piped pri/sec. I find that the more flow I can get through the hx, the firing rate modulates better with a smaller delta t. My system delta t for all of my pumped zones are in the 20 range.

    In the beginning, I thought that setting the 15-58 to speed one was going to get the best results and wide delta t, i found the boiler would high fire and hit set point and shut down. As soon as I switched to speed 3, the boilers firing rate would reduce due to a smaller delta t.

    With my system, bb and high mass radiant, my mixed returns are more than a 20 degree delta t. So with the boiler pump on speed 1, the boiler spent most of the time on high fire due to the high delta t.

    The problem I am having is with my tstat and outdoor reset working against eachother. I am starting to not like the tekmar 519 with pulse width modulation. The boiler is humming along and the tstat shuts it down. I wonder If I should scratch this tstat and go constant circulation?
  • SWEISWEI Member Posts: 7,356
    Try it and see

    Set the stat for 85ºF and leave it there for a few weeks.  Adjust the ODR curve and balance your emitter flows until your indoor temp remains stable across the full range of outdoor temps.

    The posts above contain some misinformation.
  • sunlight33sunlight33 Member Posts: 120
    Pump speed

    Low loss header, one pump on the primary flow, another pump on the secondary flow. To achieve the highest efficiency, should the two pumps (3-speed) be set at the same flow rate or different?
  • ChrisChris Member Posts: 3,056
    edited February 2014
    Not Necessarily

    It's all going to depend on type of condensing boiler and the application. What I mean is what is the heat loss of structure, type of condensing boiler and each zone's heat loss. Boiler pumps tend to be fixed speed as is any 3 speed pump.

    That pump is moving a fixed gpm flow rate every time it turns on. If you have a small zone let's say that only needs 2gpm but your boiler pump is moving 6gpm all the time the 4gpm left over has no place else to go other then to b-line right into the boiler return. This is sending 4gpm of nice hot boiler water back into the return thus elevating the boiler return temp and in most cases it takes the boiler out of condensing mode.

    There are a few post above with some wrong math.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • sunlight33sunlight33 Member Posts: 120

    If the boiler pump is set to a lower GPM than the system pump, that would keep the return temp low, but is there a drawback to this?
  • TonySTonyS Member Posts: 849
    edited February 2014
    Delta t is set by existing radiation

    Lets just say we have a one room house with one radiator. We do a heat load on the home and on a 30 degree day it needs 10000 btus to maintain what is being lost.

    We look at our radiator, we measure it, we open the book and it says that piece of radiation will need an average water temp of 170 degrees to emit 10000 btus an hour. We look at our outside temp and it is 30 degrees...perfect. Ok so we figure if we put in 180 to the rad and we adjust the flow that we get 160 out...20 degree delta t, 170 average, we are emitting 10000 btus and are going to replace just what is being lost..that day and time.

    We do have options, we could put in 175 degree water and adjust the flow that we have 165 coming out..again we have our 170 degree radiator, not too much not too little.

    or we could put in 190 degree water and adjust flow that we get 150 out...average temp...170 and again were right on the money... you can do what you want, as long as your average temp of the radiator is 170, anything else is too much or too little for that day. It doesnt matter what you do back at the boiler, pri sec, straight through, whatever.

    In a very short time the system will reach equilibrium and the boiler will modulate down to exactly what is being consumed...10000 btus.
  • GordanGordan Member Posts: 891
    Increases the average water temp across heat exchanger

    So, you could expect that, on paper, the boiler would be less thermally efficient. In practice, however, this may not be significant. And with some boilers in some systems, there are very good reasons for boiler flow to be significantly less than system flow (the heat exchanger simply can't flow as much.)
  • GordanGordan Member Posts: 891

    The notion that a lower supply temp, rather than a lower return temp, would lead to more condensing, is one of those.
  • wrxz24wrxz24 Member Posts: 285
    The drawback

    For me was the boiler not modulating its fire. A wide delta t forces the boiler to high fire for my set up due to the mixed high temp and low temp water return. If the target is lets say 150 and i have 130 coming back from my baseboard zone but only 90 degree water from my 3 slab zones, the mixed temp of all returning water forces the boiler to high fire. Condensing will always happen if all my zones are calling.
  • GordanGordan Member Posts: 891
    I doubt that the boiler was high-firing

    But there may have been insufficient flow for low modulation, causing the boiler to reach setpoint with the water it has in its heat exchanger while there's plenty of much cooler water in the rest of the system. I have seen similar behavior but the boiler was not on high fire. It starts at 50% modulation and it modulates down pretty quickly, but sometimes not quickly enough.
  • JasonJason Member Posts: 293
    edited February 2014

    Seems everyone is forgetting the delta-T of system  radiation changes with water temperature on fixed speed pumps. 20f delta-T happens at design temperature which is anything we designed at. Most existing systems if designed was normally 20f. If that system is run at 150f the 20f delta-T system is now about 14f due to non-lineal heat output as water temperature changes.

    With all the variables ever changing in heating systems like delta-T, water flow even changes with water temperature there cannot be exact answers. Get things close and go with it.

    BTW the Alpine boiler manual states no IFC pumps just under the pump sizing charts.
  • EastmanEastman Member Posts: 783
    edited February 2014
    the old delta t debate...

    i'm curious what impact unnecessary delta has on heat exchanger efficiency, assuming identical return temp.

    a true counter flow, i would think, would be dependent on mainly return temp or modulation output as the primary variables governing boiler efficiency when the system is at equilibrium.

    but a lot of these modcon heat exchangers don't look like counter flow. i would think, therefore, that such designs would suffer a more significant penalty from unnecessary high supply temp output, but i haven't seen any manufacturers' data that would suggest this to be true
  • TonySTonyS Member Posts: 849

    Thats one of the reasons our curves are not linear. We have to move them to compensate for these variables. Look what happens to convection after you get to 120.

    The point is..the btu loss of the house is predetermined...The average temp across the existing radiation is also predetermined by the loss of the house.. Therefore the output of the boiler is also predetermined.... so if we are adding x amount of btus to raise the temp of x amount of # of water the whole thing will see equilibrium in very short order.

    Piping it this way or that way means nothing( from a btu and temperature standpoint) of course some boilers have to be piped differently because of pressure drop issues.
  • ChrisChris Member Posts: 3,056
    edited February 2014
    I Think

    Everyone is missing one key point...BOILER PUMPS are FIXED Speed. Boiler delta-t is always changing based on the systems flow demand and return water temp..You're selecting a boiler pump based on the flow rate wanted at design temp to get the full btu/hr out of the boiler.

    If you are sizing a boiler pump for a 20 degree delta-t you will only get a 20 degree boiler delta-t on design day when the system side can pull out the entire flow rate of the primary/boiler side into the secondary and the secondary's delta-t is in full match and harmony..
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • GordanGordan Member Posts: 891
    Is everyone missing that key point?

    Gosh, good that we have you to bring us all to our senses. ;-)

    Yes, delta T changes all the time, but with a lower fixed speed - output held equal - it will be higher than with a higher fixed speed. Which translates to somewhat higher boiler supply and somewhat lower boiler return. Which is what people have been discussing the pros and cons of.
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