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Hydronic system buffer tank effect on modulating boiler

TexTRex
TexTRex Member Posts: 5
Background:



Located in the Dallas TX area, in a 130+ year old home (completely renovated/insulated/etc) that has had a totally hydronic geothermal system (radiant floor and water coils) in place and functioning wonderfully for 10 years. Due to reasons to complicated to explain in a post we have lost our water source for the system. While exploring options for restoring the water source, I am considering adding a gas boiler to the system which would provide heat for this winter giving us more time to deal with the water source issue and if restored options (gas or electric geothemal) for future heating.



Pertinent facts :



approx 130K BTUs required

past water temp for heating 120 degrees

buffer tank 120 gal

all pumps and tekmar dual point controls already in place

if installed the boiler will also replace gas hot water (will add indirect)



Question:



Because the buffer tank is already in place, won't it negate the efficiency difference between a modulating boiler and a "Bang - Bang" ? Won't the buffer tank and proper control set points actually perform the "modulating"? In this scenario is there any value in spending more in initial cost and on going maintenance without the efficiency payback?

Comments

  • CMadatMe
    CMadatMe Member Posts: 3,085
    Scratch, ScratchIng

    My head trying to figure out a home in that climate needs 137k. Home has to be all of 7,000 sqft.



    Buffer tank will keep the boiler from short cycling.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • TexTRex
    TexTRex Member Posts: 5
    Failing memory

    Wasn't remembering correctly....went back and looked at the calculations ...total heat loss is 86,794. Will be adding indirect DHW.



    House is 4000 sq ft but is asymtrical victorian (4 wings as opposed to a box).



    Also the buffer tank is only 80 gal
  • Greg Maxwell
    Greg Maxwell Member Posts: 212
    Buffer Tank

    I dont know about a geothermal system, but an 80 gallon buffer on an 86,000btu system, is a LOT of water, too much in my opinion for any boiler.
  • Ironman
    Ironman Member Posts: 6,581
    It Depends...

    More info will have to be provided to make a correct determination:

    1. What is the minimum load that the system must operate (smallest zone in btu's)? At what design temp?



    2. Are you going to use a mod/con or fixed rate boiler?



    3. What will be the minimum firing rate of the boiler?



    4. If you choose a cast iron boiler (other than Buderus), do you realize that boiler return water will have to be kept at 140* minimum? That will involve the necessary controls to mix the water temp down to the system.



    5. What type of heat emitters are you using? I'm assuming radiant floor. Slab, in joist, or both?



    An 80 gal tank does seem large for a load of 86K but if it's well insulated, it shouldn't be an issue. Remember, you're gonna need it if you go back to geo. It will also be beneficial if you use a fixed rate boiler to prevent short cycling.



    You should size the boiler for a minimum of 10 min. run time. If the boiler can't turn down to the minimum load, a buffer tank is the solution to prevent short cycling.



    The following is the formula for sizing a buffer tank:



    Desired run time X (Min Boiler Output - Min System Load) / System Delta T X 8.33 X 60.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    What are the units for that?

    "Desired run time X (Min Boiler Output - Min System Load) / System Delta T X 8.33 X 60."



    For my system, when running only my smallest zone when it is quite warm out,



    10 minutes * (16000BTU/hr - 3000BTU/hr)/1F * 8.33 * 60

    comes out to

    10*13000/1*8.33*60 = 64,974,000 gallons?



    That is 1/2 the size of the olympic swimming pool where I used to go to college.



    Now if the output and loads are in units of 1000 BTU/hr this would come to 64,974 gallons which is still a lottle bit high.



    Should that formula be



    "Desired run time X (Min Boiler Output - Min System Load) /  (System Delta T X 8.33 X 60)?"



    With both of these changes, I would get

    10*13/(1*8.33*60) = 0.260 gallons, or about a quart.

    That seems low.



    So maybe it should be units of BTU/hr after all like this



    10*13000/(1*8.33*60) = 260 gallons?



    I could believe that.
  • Ironman
    Ironman Member Posts: 6,581
    1* Delta T?

    JDB,

    As you noted, I left the ( ) out. My error. I'm still not used to stating math formulas with a key board.



    However, you're using a 1* Delta T in your numbers. If you're referring to a radiant floor, shouldn't that be a 10* Delta T? That would give you a 26 gal tank. If it's a baseboard or rad loop, the Delta T should be figured at 20* and that would mean a 13 gal tank.



    8.33 is the weight of a gallon of water. 1 Btu is the heat required to raise one pound of water one degree. Firing and loads rates are calculated in btu's per hour. 60 is for minutes in an hour.



    Thanks for catching my error :)
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    1F delta T

    "However, you're using a 1* Delta T in your numbers. If you're referring

    to a radiant floor, shouldn't that be a 10* Delta T? That would give you

    a 26 gal tank. If it's a baseboard or rad loop, the Delta T should be

    figured at 20* and that would mean a 13 gal tank."



    I was using the numbers I actually get. In warm weather the delta T is about 1F for both my large radiant loops in concrete slab at grade. Similarly, it is about 1F in the baseboard zone that has very low heat load. I cannot really say what the design temperatures were. The slab was there before I bought the house, and I do not know the spacing of the tubes. The baseboard zone I put in 14 feet of Slant/Fin 2000 in each of the two rooms. That was all I could fit in there. I wanted to run that at as low a temperature as I could, and it runs at about 130F supply at design temp of 14F. I do not believe I ever get over 10F delta T in either zone. I have the reset run as low as possible consistent with getting enough heat.



    So the radiant zone runs from 75F when it is warm out to 112F on design day and the baseboard zone runs from 110F to 130F on design day. On a warm day with 110F going into those oversized baseboards, I am not surprised that they put out so little heat. But it is enough. I have fixed speed Taco 007 IFC circulators in there that may be a little large. But if I were to slow them down to get 10 or 20F delta T, the heat would be extremely uneven. Upstairs, the basboard stuff is a single serial loop, so the second room would be cold or the first would be hot. In the radiant zone, perhaps I could slow the circulator down a little, but I do not see why I should. In any case, I do not get short cycling when it is very cold out, when I get higher delta Ts; only when it is warm out and I get very low delta Ts.
  • Ironman
    Ironman Member Posts: 6,581
    edited August 2011
    Delta Tee's

    JDB,



    10* and 20* Delta T are design and not necessarily actual numbers. As you pointed out, the actual Delta T will increase as you get closer to design temp outside, but it's not unusual that the actual never reaches the design number.



    If the discrepancy between them is too drastic at design temp, then the loop (or system) is over-pumped. In that case, I would recommend looking at an ECM circ.







    P.S.



    8.33 X 60 is a constant that can be replaced with 500. As in the universal hydronics formula: BTU's = Delta T X 500 X GPM. With a 20* Delta T, you get 10K Btu's per gal. circulated.



    I gave the full formula above, but a simple rule of thumb is for every 1K Btu's difference between min. output vs. min. load, you need 1 gal. of buffer capacity at 20* Delta T, based on 10 min. minimum run time. :)
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • TexTRex
    TexTRex Member Posts: 5
    Ironman - answer to your questions

    1. What is the minimum load that the system must operate (smallest zone in btu's)? At what design temp?



    Smallest zones would be downstairs radiant - divided into 2 manifolds with 4 - zones each: manifold 1 is 25,489 BTUs, manifold 2 is 13,898 BTUs and the smallest zone on the manifold is 1,905 BTUs. Dallas design temp is 100 summer and 22 winter.



    2. Are you going to use a mod/con or fixed rate boiler?



    That is question I'm trying to resolve. Is there a sufficient payback to warrant the additional cost of a mod/con when I already have a large buffer tank and control system to allow for a large Delta T in the buffer tank?



    3. What will be the minimum firing rate of the boiler?



    To be determined when a decision as to which type boiler will be purchased.



    4. If you choose a cast iron boiler (other than Buderus), do you realize that boiler return water will have to be kept at 140* minimum? That will involve the necessary controls to mix the water temp down to the system.



    Will not be using a cast iron boiler because of weight. No basement and utility room is on the second floor.



    5. What type of heat emitters are you using? I'm assuming radiant floor. Slab, in joist, or both?



    Radiant sandwiched between plywood above joist with sand filler for thermal mass and Unico high velocity air hotwater coils.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    edited August 2011
    If the discrepancy between them is too drastic

    Let's look at this for a bit.



    Say the lowest my boiler will go is 16,000 BTU/hour and my heat load in my smallest zone on the warmest day when a thermostat there would call for heat is 1600 BTU/hour. At that point, say my reset supplies 110F water for this and that is just enough to provide the heat required. But since there is a Taco 007-IFC in there, that the actual delta-T experienced is 1F. It might be even less, but I can read the temperatures only to 1F.



    I happen to have about 70 feet of 1/2 inch copper tubing to supply that space, 28 feet of 3/4 inch of Slant/Fin, and 24 feet of 3/4 inch plain copper tubing in that circuit. If I remember correctly, that calculates out to about 2.8 gallons/minute. I am not sure if I counted all the 90-degree elbows because some of them are hidden beneath the floor and I would have to rip up some of that to count them accurately. But I suppose that would only reduce the flow a little bit. In any case, I very much doubt the flow is over 4 feet per second or I would hear cavitation. On the other hand, I am not sure I know what cavitation sounds like, but I suspect it sounds something like air in the system sounds like, but not identical.



    I assume the ECM circulator you suggest is hooked up delta-T mode, not delta-P or constant speed. Do you really think I should hook up the baseboard zone (the one where the cycling rate is "too high (less than 10 minutes sometimes) to get 20F delta T? If I am putting 110F water in there, just how slow would that circulator have go to get the return water down to 90F? And if it did that, would I get enough heat in the second room? Because if I did that, the supply to the second room would be only 100F. Right now, with the present circulator, calculates out to 109.5F  I have not calculated what would happen with the lower temperature water in the second room, but that room would surely be colder than it is now, and that might be too cold. One could say I should install more baseboard in that room to make up for this, but there is really no room for that because of obstructions. If there were room for that, I would have already put more baseboard in them to get the return water temperature down.



    As far as I can tell, as long as I do not get cavitation up there now, the only advantages to the ECM pump would be the lower cost of electrictiy to run it, and the lower cycling rates on warm days. And the disadvantages of it are the costs of replacing the existing circulator with an ECM one (parts and labor) and the risk of unacceptably uneven temperatures between the two rooms.



    Just how slow can a delta-T ECM circulator go without stalling out? Lets say one that could deliver 2 gpm into whatever head I have at full output.



    There is nothing magic about that 110F minimum water temperature I run up there. I initially set it to about 80F and that gave enough heat, but then the delta T was so low (I could not measure it) that it cycled very rapidly. So I raised the minimum temperature to increase the heat dumped into those rooms. That reduced the cycling rate somewhat. They would heat up faster, but since I do not use setback, that was of no concern.



    My old (oil) boiler did not have reset. It would cycle on for about 30 seconds and off for about 45 seconds on warm days and only a little slower on very cold days. Outdoor reset is really a great thing on a modulating boiler. On a cold day when heating my radiant (large) zone, It switches on and just runs for up to 18 hours or so a day without cycling at all. It is just when only the small baseboard is asking for heat that the cycling is "too high" and I have diddled the temperatures and firing rate to reduce it some. If my mod-con had a 50:1 modulating range instead of 5:1, I would be OK. Or even if I could have gotten one 1/2 the size of the one I have, I would be somewhat better off.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    sand filler for thermal mass

    Why do you want increased thermal mass?



    I have a radiant zone that is a concrete slab at grade. It has thermal mass. With the non-modulating oversized boiler I used to have, it was impossible to achieve a high level of comfort because of the delay  between when the thermostat called for heat until the slab heated up enough to make a difference (4 to 8 hours or more), and symmetrically, from when the thermostat was satisfied and when the slab stopped dumping heat into the rooms. The fact that the new boiler is a modulating one with outdoor reset greatly increased my comfort, and allows me to run the thermostat 1F cooler than before, alone was worth the cost of the new mod-con to me. The fuel savings was considerable, but I might have gotten a lot of that even with a new conventional boiler. The old one was almost 60 years old.
  • TexTRex
    TexTRex Member Posts: 5
    @JBD

    I am not looking to add thermal mass, I was answering the question as to what heat emitters I'm using.



    The house foundation is beams on blocks, the soil content is extremely expansive clay. Pouring lightweight concrete to house the pex was not a option because of the soil movement. 10 years ago grove panels with aluminum plates weren't readily available. So I created a "flexible" floor system by sandwiching the pex between to sheets of plywood with nail strips, filling the space around the pex with sand, The thermal mass of the sand spreads the heat away from the pipe producing evenly heated floors.
  • TexTRex
    TexTRex Member Posts: 5
    edited August 2011
    Back to the question at hand

    The 1st question is: Would using a Mod/con running 120 degree temp constantly at 40K BTU be more efficient (beyond the AFUE difference) than a bang bang running at 80K BTU for half the time?



    For sake of aurgument, assume the mod/con system is circ pumped/zone valved with adequate controls and heatload/outside conditions are in the range that it would run constantanly at 120 degrees in half modulation (40K). Further assume that the bang bang system is set up with mixing valve controls and a buffer tank to maintain a significant enough Delta T that it would run 12 hours per day at 80K BTU.



    Given that it seems to me that both systems are producing 960K BTU and so the only difference is the AFUE factor, so if the mod/con is 10% more efficient then it is saving 120K BTU (960/.95-960/.85=120). Gas produces roughly 1000 BTUs per cu ft of gas. BTU savings of 120K is equal to 120 cu ft of gas. Gas cost in my area is roughly $9 per 1000 cu ft. Savings equals $1.08 (120/1000*9=1.08)



    So that savings of $1.08 per day would take 2778 days (of the system running at its most efficient setting 24 hours per day) to recover the cost of the $3000 difference between a 95% eff mod/con and a 85% eff bang bang. Understanding that is not a real world situation and that the value of a mod/con is in its ability to adjust to the actual need to maximize eff, are my calculations correct or have I missed something in my lab controlled environment example?



    More questions to come after validation or rejection of these basic assumptions.
This discussion has been closed.