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Size boiler by Measured heat load, "Manual J" or Designed Heat Load?

gmcinnesgmcinnes Member Posts: 11
I'm considering a new boiler. I'm trying to come to come to an agreement with myself on the right size.

I did a heat load based on our fuel use over January and some of February last year using the method here. That came to about 137MBH ( Details of calculation)

Then I did a heat load based on the size of the cast-iron radiators in the house, and that came to about 288 MBH (Details of calculation)

The heating system has 4 zones and there's an indirect domestic hot water. One zone is the garage, and one is spare rooms over the garage. Both of those are rarely heated over 40F. Is it crazy to size a new boiler based on measured heat usage, even though it's significantly lower than the design load implied by the size of the radiators?

FWIW, the current boiler is 413 MBH DOE. I'm pretty sure that's too big in any case!

Also, a guy from a supply house came to do a "Manual J" for me. I put that Manual J in quotes, because I don't get to see the results, just the sales recommendation. But he recommended a Burnham MPO-IQ231 - 203K BTU.

Obviously I'll be getting a tech to work with to do this right, but I like to be able to have an informed discussion when I'm getting quotes, so any advice would be appreciated.

Comments

  • Jamie HallJamie Hall Member Posts: 7,760
    Is this steam? Possibly not, as you mention zones -- unusual with steam -- and your spread sheet doesn't use the steam conversion factor for EDR to BTUh.

    What conversion factor, then, did you use? And on what basis?

    Manual J is a little casual, but usually works pretty well. There are better heat loss estimators (Slant Fin has one), but they take more time.

    So... if I assume that you are talking about hot water, then the correct heat loss and thus boiler size would be based on the manual J or a more sophisticated heat loss calculation. The choice of boiler, however, based on that will be governed by your radiation. You need to look at the relationship between your radiation, the design day heat loss, and the resulting feed water temperature and return water temperatures. That will tell you whether you can benefit much from a condensing boiler or not.

    Now. If you have a steam heating system, then the required boiler size is set by the EDR of the installed radiation. No compromises. Don't oversize -- but don't undersize either.

    Actual heat loss measured by fuel use is attractive. It's also not always a good approach. First, unless you correct the fuel use in practice to the design day conditions, you will almost certainly be undersize -- possibly by a substantial margin. You appear to have done that correction, however. Based on the discrepancy between the Manual J and that, it seems to me that your best bet at this point is to go to a more detailed heat loss calculation for your structure, and work from that.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • gmcinnesgmcinnes Member Posts: 11
    @Jamie Hall 'tis a hot water system.
  • gmcinnesgmcinnes Member Posts: 11
    @Jamie Hall Also, whoosh this just went way over my head:
    the correct heat loss and thus boiler size would be based on the manual J or a more sophisticated heat loss calculation. The choice of boiler, however, based on that will be governed by your radiation. You need to look at the relationship between your radiation, the design day heat loss, and the resulting feed water temperature and return water temperatures


    Any book, or other recommendations where I can learn a bit more about this?
  • Jamie HallJamie Hall Member Posts: 7,760
    There are some books out there -- take a look at the store, on this site -- but the basic principle is fairly straightforward: a radiator will put out a certain amount of heat, based on the effective area of the radiator (the EDR) and the temperature -- modified to a certain extent by paint colour and enclosure, but the area and temperature are the key factors. For steam, that relationship is 240 BTUh per square foot of effective area. For hot water, it varies depending on the water temperature dropping more or less linearly with dropping temperature.

    So -- you have a certain amount of radiation in your various spaces. The best way to run a hot water system is to have the water being fed to the radiation set at just the right temperature to give the amount of heat needed, so that the circulation never stops. This gives you lovely even constant heat.

    Now if there is enough radiation, you can run at a low enough temperature so that the boiler can condense. This raises the efficiency about 10%, so it's great if you can do it. However, if you don't have enough radiation to be able to run your water that cool, there is no point in spending the extra money -- which is significant -- on a boiler which can condense.

    That help?
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • gmcinnesgmcinnes Member Posts: 11
    Yes. That helps a lot. Thank you.

    I'm skeptical whether we'll get to a condensing boiler. We have:
    • cast iron radiators, with unknown amounts of heat exchanger clogging sediment
    • hard well water, full of boiler soiling minerals
    • concrete and iron septic components that will be unhappy with imperfectly treated condensate
    • local boiler techs that haven't been able to reliably service the low tech monster that I have
    The extra complexity seems like a bad risk / reward trade-off, in my situation, for that last 10% :)

    Having said that, I see you have a 2.75 gph Carlin on 7200 sq. ft. I have a Carlin firing at >3GPH on a 3500 sq. ft. house, half of which I barely heat, so there's got to be room for improvement without getting all the way into condensing systems.


  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 3,498
    @gmcinnes , you should be able to get your old boiler fixed without sooting. The Smith/Carlin combination is a good one even though it may be oversized.

    Carlin is in New Haven and Smith is in Westfield. Why not call then and see if they can refer a qualified technician before you give up on this boiler.

    You can put something new in and not be any better off if the installer doesn't know what he is doing
  • gmcinnesgmcinnes Member Posts: 11
    @EBEBRATT-Ed Great idea. I'll call Carlin and Smith and see where I get.

    Just out of curiosity, given that the boiler is rated at 413MBH and all the methods of assessing heat load show 200MBH or less, is there any way to figure out how much extra that's costing me in fuel?

    I'm using approx. 100 gal / week in oil at the moment, with the tstat set at 65, 55, 40 and 40 in each of the 4 zones respectively.

    If I can save $2K a year in oil a new boiler pays itself back pretty quickly.

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