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Boiler size for indirect water heater

wklopfwklopf Member Posts: 20
I'm looking to replace my boiler and water heater. My heating needs can be satisfied by a boiler of 75,000 to 80,000 BTUs. The 45 gallon indirect heater which I'm looking at lists a boiler requirement of 155,000 BTUs. My guess is that hat size boiler would provide the maximum hot water from this system, and a smaller boiler would result in a slower recovery. But, shouldn't a 45 gallon indirect with an 80,000 BTU boiler still deliver a lot more hot water than a 40 gallon water heater with 34,000 BTU input? Thanks.


  • JohnNYJohnNY Member Posts: 1,824
    Yes. It would. I'd match those two pieces with confidence.
    For private consulting services, find John "JohnNY" Cataneo here at :
    "72°F, LLC"
    Or email John at [email protected]
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  • gennadygennady Member Posts: 680
    Do you have mixing valve installed on your tank? if yes, than 45 gallons of 140F will produce 45/1.25x140/125= 40 gallons 125F water. As example, 2.5 gallons per minute shower, 40/2.5 x125/100 = 20 minutes of shower. To re-heat tank from 45F cold water back to 140F, you will need (140-45)x 8.33x40=31,654 btu. boiler with input 80.000 btu hr, will output 80.000x.8=64,000 btu hr. so it will take about 30 minutes to reheat the tank. If you need to cut recovery time you will need higher output boiler. 155.000 btuhr boiler will recover tank in 15 minutes, if tank will allow it. In simple terms, that's my 2 cents.
    Gennady Tsakh

    Absolute Mechanical Co. Inc.
  • flat_twinflat_twin Member Posts: 58
    Recovery time with 80k boiler and 45 gallon indirect will be fine, just a little slower than the larger boiler specs they give.
    Our 40 gal indirect w/ 110k boiler set at 150 degrees when making DHW recovers in 12-15 minutes after a large demand like two showers and breakfast cleanup.
  • 4Johnpipe4Johnpipe Member Posts: 476
    Another option and money savings is go with a 30 gallon indirect and store high and mix down. it will give you more hot water then a 40 standard tank all day long. What is your hot water demands?
    Considerate People, Considerate Service, Consider It Done!
    email: [email protected]
  • ZmanZman Member Posts: 3,517
    The only concern I would have with this is if the boiler is non condensing and you have no control over the flow from the boiler, you could cause the boiler to condense.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot rodhot rod Member Posts: 7,094
    Higher tank temperatures, around 150F will also assure no bacteria can grow. Higher operating temperatures does shorten the tank life however. More mineral precipitation as temperatures increase and thermal expansion and contraction of the tank at higher ∆ operation.

    Public buildings, and possibly residential will require above 140F recirculated once a day to protect against legionella growth. I think the VA recently added that requirement in their buildings.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
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