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Boiler size based on gas consumption

Proceed with extreme caution...

This winter someone here had a post where the heat couldn't keep up in nasty weather and they'd used consumption and btus/degree day to size their boiler as well. Fortunately it was the installers own home. Believe he'd changed from a conventional boiler to a mod-con (Munchkin) but won't swear at that. It might have been another conventional boiler.

Depending on whose comments you listen to, he'd either achieved "perfect sizing" or had undersized...

The wife will probably get used to it (especially if the fuel bills are reduced considerably) and the kids don't have much say, but tenants can be <I>much</I> less tolerant and <I>do</I> have something to say.

Domestic hot water can be an utter wild card, especially in a multi-unit building. Unless DHW uses a different fuel or separate meter at best you'll only be able to factor in an average based on the non space heating months. Use too many averages and you won't be able to handle the peaks.

Does the boiler currently supply DHW or is it from a separate source?


  • Dr. Z
    Dr. Z Member Posts: 6
    Question on boiler sizing based on gas usage

    I have a 2-story 3200 sf 4 unit apartment bldg, supplied by a 200,000 BTU/hr input, 160,000 BTU/hr output Bryant boiler, with 4 zone pumps. The boiler is about 20 years old. My gas usage is 27,000 BTU/degree day.

    I calculate that based on a 0 degree design basis, I would use 78,750 BTU/hr in order to keep up.

    Am I safe replacing the boiler with a unit sized based on this calculation? Also, how much added efficiency should I expect to pick up by going to a smaller unit? Is it worth doing or should I forgo the capital input and just keep the old unit?

    I'm in Buffalo NY, so winters can be nasty at times.
  • ALH_4
    ALH_4 Member Posts: 1,790

    Those numbers make sense assuming standard stick frame construction. I would verify with Hydronic Explorer.

    I think a modulating boiler on outdoor reset is really the way to go to save fuel. Essentially you are limiting how hot your tenants can get their apartments by limiting the fluid temperature. No more 90°F apartments with the windows open. ;-) This could pay off very quickly depending on how your tenants are currently using their heat.
  • ALH_4
    ALH_4 Member Posts: 1,790

    And if it is an indirect water heater fired off the boiler, what size and model is it?

  • Boilerpro_5
    Boilerpro_5 Member Posts: 407
    Be careful

    As was already posted, one installer discovered that he left something out of the calculation. What he left out is the fact that cold air infiltration accelerates as it gets colder outside in a non-linear fashion. This can be seen in the basic ASHRAE fundamental heat loss formulas. If you figure the heat loss at 20F outdoor he may have a .7 air changes per hour air leakage. At -5 you may have a 1.2 air changes per hour air leakage rate. So if you find out how much heat you are using at 20F and just multiple it to compensate for a colder outdoor temp, you will end up short. Another aspect of apartments is that they have rather high interior gains, so the boiler doesn't need to run as much, as long as all the units are occupied. What I would suggest is to find out what your typical heat load is for a typical winter day, size a boiler for optimum efficiency for that day, and then keep the other boiler to pick up the few days that are really cold. This "base loading" technique is used by governmental units and utilities as a way to optimize system efficiencies and cut costs. Outdoor reset is a real plus when tenants abuse your heat bill by leaving storm windows open and windows open.


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  • Constantin
    Constantin Member Posts: 3,796
    Proceed with Caution?

    Mike, I think we need to recall some of the details of that installation.

    IIRC, John Ketterman not only used his old boiler as a yardstick, he also assumed that the new boiler would be 30% more efficient (look over the thread for the rest of the info). Therein lay the problem: On a design-day, even old-dog boilers are going to perform better than they would the rest of the year. On the coldest days of the year, he was 6,000 BTU's short of what he needed to maintain 68° throughout the house with that Munchkin 80M.

    All in all, John came remarkably close even if shaving off 30% DOE output "just because it's a mod-con" is drastic. I would have stuck to differences in the AFUE rating, since design-day conditions come as close as you get to AFUE conditions. Had John stuck to AFUE differences, he would have been safe.

    Plus, averaging gas consumption over a month is inherently less accurate than clocking a boiler during design conditions. I would stick to clocking the boiler under ideal conditions (i.e. at design temp, with a 15MPH wind, and no sun to help the place) and calculate a heat loss based on the measured AFUE of the boiler under those conditions.

    FWIW, my heat loss calculation and the boiler runtime here (even if Ms. Vitola is oversized, non-condensing, etc.) coincided to within a few percent. So, shaving off 30% of Ms. Vitolas clocked design-day input would surely cause a cold house down the road...

    To summarize: if Dr. Z clocks his/her boiler on a design day, and then only applies appropriate differences in the AFUE rating between the new and old boiler plant, then this approach may work very well unless there are nearly constant IDWH loads that have to be covered concurrently. If that is the case, simply add the DHW load to the house load, or install a dedicated water heater, etc.

    I definitely would use heat explorer or some other heat loss program to verify the calcukated heat loss results, plus how to size the plant for truly extreme conditions. Heat loss calculators also help sizing the radiators, performing "what if" scenarios (i.e. upgrading windows), etc. I was very happy with HVAC-Calc in this regard.

    Things to look for: Design conditions vs. extreme design conditions (2% vs. 1% ASHRAE conditions, for example). Here in Boston, I'd size to 0°F, since we have come close a couple of times for sustained periods of time. Also, if choosing a mod-con, pay attention to DOE output as a function of return water temperature.
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