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Preheating tankless WH or combi with solid fuel boiler

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GroundUp
GroundUp Member Posts: 1,909
in Domestic Hot Water
Super dumb question: will a tankless or combi boiler recognize the hot inlet water and resist firing if the water was preheated with a plate HX in a wood boiler loop? I do this all the time with heat-only boilers, with the TT interrupted by an aquastat on the OWB supply line, but with a tankless or combi operated by a flow switch I'm not sure what the proper channel is. Has anyone done this? The application in question is a Lochinvar Noble 110, but I can't imagine there would be much difference with any other tankless. TIA!

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  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
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    Why wouldn’t you just dump the hot water from the boiler into a storage tank?

    There was an error rendering this rich post.

  • GroundUp
    GroundUp Member Posts: 1,909
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    Because he doesn't want a storage tank, he wants a combi.
  • hot_rod
    hot_rod Member Posts: 22,158
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    I feed my Cadet and Nobel pre-heated water, either from the solar or wood boiler. If the pre-heat drops below 110°F the Nobel fires up.
    The flow switch always "makes" when it sees .5 gpm or more, but the burner will stay off on temperatures.
    If you feed it say 90°, the combi kicks on low fire to make the difference to 120° or set point.

    I have heard of excessively high temperature from a wood boiler to the mod con, boiler side, depending on piping tripping a high limit. See if yours has an automatic, or manual reset high limit.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GroundUp
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,304
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    Hi, Tankless gas heaters don't often do well with preheated water if they have thermostatic controls. Electric tankless can do better. A problem happens when the supply water is close to the desired temperature because tankless gas cannot heat it just a little. Something with a big burner and a small tank (like the HTP Crossover) would probably do better.

    Yours, Larry
    STEVEusaPAGroundUp

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