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Boiler size for indirect water heater

wklopfwklopf Member Posts: 36
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: 2,424
    Yes. It would. I'd match those two pieces with confidence.
    For installations, troubleshooting, and private consulting services, find John "JohnNY" Cataneo here at :
    "72°F Mechanical, LLC"
    Or email John at [email protected]
    John is the Boilers and Hydronic Heating Systems Course Instructor at NYC's Mechanics Institute, a professional Master Plumber, licensed by The Department of Buildings of The City of New York, and works extensively in NYC while consulting for clients in and out of state.
    John also oversees mechanical installations and maintenance for metro-area clients with his family's company, Gateway Plumbing and Heating along with his brother/business partner.
  • gennadygennady Member Posts: 776
    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: 238
    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: 479
    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!
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  • ZmanZman Member Posts: 5,623
    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: 12,958
    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
  • JellsJells Member Posts: 182
    Little late here, but something I found increased my indirect recovery time dramatically was to use a full port zone valve. I was thinking I had to replumb the whole manifold to 1", but the valve did it. My supplier had sold me a priority controller that cut off the other zones when the tank was calling, but it took so long the house was getting cold! I also use a programmable thermostat with a 15 degree spread so in summer the passive loss is low enough the boiler stays off all day when no one's using hot water.
  • My little 50,000 BTU Munchkin takes about 20 minutes to recharge our Viessmann 50 gallon indirect. But there are only two of us in the house with one bathroom. That calculus changes when there are more bathrooms and more people.

    If everyone wants to shower at close intervals, you have to get a larger indirect. Or just get an on-demand water heater that can handle peak flow.
    Often wrong, never in doubt.

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  • cnmdesigncnmdesign Member Posts: 83
    Too bad doubling up on showers does not double your hot water. :p
    Combat Veteran owned, Final Salute LLC on FaceBook & Twitter.
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