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boiler w/ indirect storage tank or tankless or both

I would like to get some opinions on these three options for providing hot water for hydronic (radiator) heat and potable hot water: - boiler with tankless - or - - boiler with indirect storage tank - or- - boiler with indirect storage tank AND tankless (Boiler and tankless would be NG-fired.)

Which system is cheapest to install? Which system is cheapest/most efficient to operate? I don't have a heat-calc completed yet, but have a contractor's preliminary estimate of 70,000btus.

We are in South Carolina, so our heating season is about 6 months long. One heating contractor has suggested that should have a boiler plus indirect tank plus a tankless so we can turn off the boiler loop when not in the heating season.

If we do have an indirect tank and a tankless, how would the indirect tank be sized? (Essentially it's just for the radiators.)



  • Robert O'Brien
    Robert O'Brien Member Posts: 3,450

    you mean a tankless coil in boiler or an instantaneous(tankless) water heater?

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  • maryrenov8s
    maryrenov8s Member Posts: 3
    I'm referring to a tankless instantaneous

    water heater when I say "tankless."

  • Constantin
    Constantin Member Posts: 3,782

    Coils in tanks tend to be very inefficient as the boilers have to maintain a very hot interior temperature all year long, waiting for a water call. A coil in the boiler is probably the cheapest option.

    A boiler with an indirect gives you the opportunity to decouple the water demands somewhat and simply let the boiler "top off" the indirect as needed. The larger the tank, the more buffer you have... which is beneficial if you may encounter large temporary demands but only have to cover a small heat load. In those case, a large buffer affords you the opportunity to still size the boiler for the heat loss, not the continuous hot water demand.

    In theory, a tankless unit will consume the least amount of gas for a given hot water load - there are little to no standby losses. My main concerns with such units is the HX (may scale easily), low flow-rate response (i.e. what happens if you only open one faucet), and the fact that you now have two appliances to service and maintain. Tankless heaters are also quite expensive but they do make a lot of sense for some applications.

    Anyway, that's why I think that most homeowners with hydronic heat would be best served with an indirect water heater.

    If the hot water demands of the home are such that they eclipse the heating portion (i.e. large tubs, multiple endless showers, etc.) then the heating plant has to be sized to meet that need, not the heat loss. Once you're in this category of user, I hope that budgets are no longer a concern...
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