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BTU's's needed for heat AND domestic hot water?

Hello, I am a new member so please forgive me if this question has been covered. It is time to replace my 29 year old weil mclain model 68 oil fired boiler w/ domestic hot water coil as it has leaking at the cast sections near the coil. I have used many online BTU calculators to find my needed size but none seem to take into account heating domestic hot water. I have a 2 story, 2 zone baseboard radiator 1100 square foot cape, very well insulated with 8' ceilings. just 2 occupants with single bath. The btu calculators are only saying between 48,000 and 60,000 BTU's for this size house but I'm guessing DHW requires more? My woodstove does most of our heating anyway. I am very concerned that I will be upsold on too much boiler. We have had no issues over the years with heat or hot water with our current boiler. Thanks for reading and Happy new year!


    EBEBRATT-Ed Member Posts: 15,613
    Calculate the heating load only. Do not add for dhw
  • Gordy
    Gordy Member Posts: 9,546
    If you think about it a stand alone 40gal gas water heater only has 36k input. For two people that’s plenty.

    As for the ci boiler you won’t find much lower inputs, but I’ll bet 48k for 1100 sf ch is high.
  • bob eck
    bob eck Member Posts: 930
    Are you staying with oil or going to natural gas or LP gas.
    If staying with oil I would go with a good oil boiler like the Trio
    Oil boiler



    And use a indirect water heater for domestic hot water.

    If you install the Trio oil boiler and for what ever reason in the future you want to switch to natural gas or LP gas the Trio boiler can run on all three types of fuel. Just need to switch burners.

    Can install a conventional natural gas or LP gas boiler and a indirect water heater or if you can benefit from a condensing boiler or condensing combi boiler take a look at those boilers.

    A good heating contractor can give you many options and he can ask you a few questions and you can go from there.

  • bob eck
    bob eck Member Posts: 930
    With a boiler (oil or gas) and a indirect water heater you can give the indirect water heater priority over the heating zones and thus no extra BTU needed for the heating of domestic hot water with a indirect water heater.
    Size the boiler for the heating BTU load only.
  • giceman1337
    giceman1337 Member Posts: 41
    I'm currently in a very similar position. I'm replacing my existing DHW water heater, at the same time I am adding radiant floor heating. From other help I am seeing to look at the 2 systems basically independently and size to the largest one.

    For DHW getting an indirect water heater (e.g. storage tank) will make the BTU needs from DHW nearly moot, as whatever will supply the radiators will almost certainly exceed whatever the DHW would need.

    For the boiler, get a modulating and condensing boiler. The modulating will allow the boiler to decrease the BTUs based upon the needs at that time. What I've read is that it is key to prevent the boiler from short-cycling. If you to buy an overkill boiler, and it doesn't modulate you will be throwing your money away both on the up-front cost, and the continual operation, and on the replacement costs when it breaks down.
  • Jamie Hall
    Jamie Hall Member Posts: 23,426
    If you're staying with oil...

    Am I the only one left in the world with an oil fired hot water heater? The thing is wonderful -- very fast recovery (should be; it's fired at 75K BTUh) and good storage and very good efficiency.

    And it means that Cedric doesn't have to run in the summer, which is when the place tends to have guests.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • scrook_2
    scrook_2 Member Posts: 610
    > @Gordy said:
    > If you think about it a stand alone 40gal gas water heater only has 36k input. For two people that’s plenty.

    And a 4500W electric water heater is only 15k, and as DHW use is intermittent, the BTU requirement from the boiler becomes trivial.