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Sizing a boiler w/indirect heater

Casey Jones
Casey Jones Member Posts: 2
I have spoken with many differernt people on sizing a boiler with an indirect heater. My question is do I, or do I not figure the indirect to my calculations?

Example :
A house with a loss of 45,000 btu's and a 40 gal. indirect.

Would a boiler with a net out of 80,000 be enough or should you go to the next size?


  • Keep the boiler

    at 80,000 BTU's; any higher and you're going to short-cycle when heating the house. Odds are the heat exchanger in the 40 gal. indirect max.'s out near 80,000 BTU's anyway.

    Make sure your piping is primary/secondary. It will keep your boiler happy, especially if you have more than 2 or 3 zones.

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  • Joe Brix
    Joe Brix Member Posts: 626
    Check the recovery rates of the indirect

    Make sure it will generate the amount of HW you want in a given period. You'll also want to put the tank on priority so it doesn't fight for BTU's with the heat zones.
  • PS
    PS Member Posts: 49

    It's been my understanding that unless you need extraordinary capacity or expect to have continuous DHW demand on design days - that you size the boiler for your heat loss? Maybe add a little so as to get all the hot water you need on a design day (more a function of the indirects capacity) - but other than that it shouldn't really factor in?
  • oil-2-4-6-gas
    oil-2-4-6-gas Member Posts: 641

    size for heat-loss and wire for priority
  • Paul Pollets
    Paul Pollets Member Posts: 3,656
    BTU's for DHW recovery

    I always like to size for at least 60K btu for tank coil recovery for a 53g tank and 90K BTU's for an 80g indirect. The heat loss is always less, but downsizing the boiler past a certain point doesn't work well. DHW tanks need plenty of BTU's to recover in a reasonable time. Modulating boilers will ramp up to high temp to satisfy the tank, when it calls for recovery. Good control systems and proper design will prevent boiler short cycling and recover the DHW tank quickly. BTW, a 40g indirect works for a 11/2 bath home without teenage daughters. I'd recommend the 50g or 60g tanks for greater drawdown and more efiiciency.

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