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Questionable combisystem

greenplumber Member Posts: 1
Have a home owner who wants to put a solar DHW system in a smaller home. Is is it possible to use a dual coil storage tank for the solar DHW, using the upper coil to preheat water prior to boiler for space heating. Any design ideas or precautions. The home is passive solar and will require little heat load from the boiler as main heat source will be wood burning stove. Local authorities want a back up heater, the home owners want to harvest the excess heat from solar storage tank.


  • TonyS
    TonyS Member Posts: 849
    Dual coil Lochinvar SDT solar tank

    for the solar system use the tank as a prefeed to a Navien Combi unit. This will take care of any standby loss other than what came from the sun and serve as a condensing backup boiler. The upper coil in the solar tank can be used to bleed heat into the hydronic system when the domestic load is taken care of.


  • Karl_Northwind
    Karl_Northwind Member Posts: 139
    tank option.

    The biggest downside to the upper coil is the limited size.  that's the major downside to them.  they're designed to move heat from 180 F boiler water to 120 F DHW,

    it will deliver a tiny bit of heat, as long as you and the customer know it's not optimal.

    for that sort of application I'm considering using the SME 80 or 120, mainly because they have the largest heat transfer area available in single tank system sort of things.

  • hot_rod
    hot_rod Member Posts: 22,266

    if you have enough collector "horsepower" to make it worth the expense, as Karl mentioned. Your solar "harvest" drops considerably in cold winter months in most areas. Consult with a solar design program or use the free NASA solar radiation data to see what your area offers.

    The other side of that is a large enough array to do the heating load will be oversized for summertime DHW only, loads. Unless you have a pool or large DHW demand.

    Most solar controllers have 3 or 4 options built into them to do this combi system.

    Some use dual coil tanks, some triple coil tanks, and some use the tank capacity as the heating fluid. Here are a couple examples.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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