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Can tankless combi condensing boiler function if water main is turned off?

I have an NTI Tx Combi condensing boiler (propane fueled) that serves my in-slab radiant heat system and domestic hot water. (My house is more of a cabin and is only 700sf) Since I will have to leave it periodically for extended periods (a week or so at a time in winters) and have no back-up power yet (except a woodstove when I am here), I want to minimize any damage from frozen pipes above the floor serving our domestic water until we get a generator here. I am assuming (but not positive) that the heating portion is a closed loop and separate from the domestic hot water. Is this correct?

If I turn off the water main and the breaker to my well pump (and drain piping to the sink, etc) will my boiler continue to function to provide heat (assuming it still has power)? Would there be any reason for the boiler to turn itself off as a result of this? (pressure drop or something)? I don't want to potentially damage my boiler but am trying to minimize damage.

Related to that...my radiant piping (PEX with oxygen barrier) is in 5" of concrete, with 6" of insulation under the slab and 3" around the perimeter. Upon loss of power, how long do you think I have before the in-slab pipes would freeze? Is it typical for the radiant pipes to have glycol or something in them? (The system is 6 years old and I don't remember what occurred when the plumber installed it and he has since retired). I should be able to get back here in within a day or so to at least get the woodstove going which heats the whole place up pretty quick. Plus I have a lot of solar gain in winter (assuming the sun is out).

Thanks for your help!


  • Jamie Hall
    Jamie Hall Member Posts: 23,436
    Some radiant floor (and, for that matter, hot water systems in general) do have glycol. Some don't. A good heating professional should be able to test that for you pretty easily -- and, if you haven't lately, it should be, and the boiler checked and cleaned as needed...

    Now to the will it freeze part. I think it is safe to assume that if you have power that the boiler will run. They are pretty reliable. And, if the boiler is running, the structure will be warm enough to keep the pipes from freezing -- so there is no need to drain on that account. I don't know for sure about your unit, but if for some reason it fires with no water in the domestic water heat exchanger I think that will be the end of it. Which also argues against draining the plumbing. Further, most hot water systems have a provision for automatic water feed in the event of a leak on the heating side; without your well running, you wouldn't have that -- and you'd have to hope that the boiler would shut down on overheat if it got low on water.

    Bottom line. I'd leave things running, and find a friendly and more or less competent local to come around from time (daily, if it's cold) and see how things are.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,256
    Yes you can run it with water to the potable side turned off. It is generally a plate heat exchanger with a flow switch to trigger the boiler to heat DHW.

    It really depends on temperatures around the building, wind, solar gains, etc as far as how long it would hold heat. I'd guess that much slab mass would go a number of days before freezing.

    Glycol is an option, or constant circulation in the radiant to prevent freezing. One method requires power, the other does not .

    A few followers here have added computer back up power sources with a beefed up batteries to run the boiler and circulator for extended power outages.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream