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No hot water with tankless + circulator

So as you may know, temps are at all time lows. My pool pump has been running 24/7 for a few days now, and all looks well with that equipment. 

However, in my garage I’ve got a tankless water heater (Eternal 2015 model) that’s connected to a water circulator. Unfortunately, I forgot to turn off the timer function, which typically shuts off the circulator at midnight and starts again at 7am daily. And overnight the temps dropped like crazy, so yes—I have no hot water. 

So now how to fix this? The circulator is whirring noticeably louder, presumably bc its running dry? The tankless heater itself isn’t showing any error codes, but the “burner” light isn’t turning on either. According to the manual, the burner won’t fire if it’s not receiving water. My guess is that the intake is frozen, except it doesn’t feel frozen at the tankless heater connection site. And there are no pipes on the other side of the wall that are exposed, so I guess the pipes runs
 behind the drywall. 

Any solutions here? Also note that cold water runs like normal in most locations in the home, except a few faucets where there’s both no cold nor hot water. 

Thanks I’m advance!


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,037
    edited February 2021
    When you say "no Hot Water" do you mean the water flows cold... OR ... no water flow at all?

    Are all the faucets and taps affected?

    Since you are experiencing very cold conditions, you may have a frozen pipe causing a blockage of flow. It is difficult to find a frozen pipe however you can start by leaving some faucets open and using a heat gun or hair blowdryer look at obvious locations in a crawlspace or other unconditioned space where pipes may be exposed to the extremely low temperature.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Jamie Hall
    Jamie Hall Member Posts: 23,419
    Oh boy. First question -- what material is the pipes?

    First comment -- I'd turn off the tankless until you get definite flow out of your hot water faucets. The pump will run dry -- for a while -- but it really isn't going to be happy about it.

    Second comment -- the faucets where you have neither hot nor cold... you may have the cold lines frozen to those.

    So back to the first question. If the piping is PEX, it usually won't be damaged by freezing. Once in a while a fitting may leak, but even there, if they're well done they shouldn't. On the other hand, the only way to thaw the stuff is to warm it up from outside. Do I take it that the feed or the outlet to the tankless is in a wall? Is that an exterior wall? Can you get into the wall at all? Because if you could, you could direct the air from an electric heater -- barn or milk house heaters are cheap -- and get at least that much of the piping warm (it may take hours to thaw... patience). On the other hand, if the piping is copper, it can -- in principle -- be thawed with a welder (or a special machine designed for that). This is not something I'd recommend as a do it yourself project, though, as you really have to be super careful about hooking the thing up (the piping may be bonded and grounded -- actually, should be -- and an improper hookup can be ... interesting.) and you have to be very aware of how warm things are getting. Just as important, the water should be turned off while doing it, as copper sometimes splits when it freezes, and you don't want Niagara Falls in some odd spot.

    Or wait for spring...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,324
    Hi @ssktx , One approach would be to drain down all of the water in the Eternal to prevent freeze damage and unplug the pump, so it doesn't self destruct. Do you have a way to test to see if water can flow into and through the heater? Another approach would be to try and warm the garage so you know there is no ice blockage in the lines and no freeze damage can happen to the equipment. From there things can be troubleshot as needed.
    Also, just to be clear, when you say "water circulator", do you mean a pump on a pipe that recirculates hot water, so you get quick hot at the taps?

    Yours, Larry
    ps, I'm sure you know that Eternal is out of business, so if anything in the unit is damaged by this, that heater is likely unrepairable and of no further use.