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My hot water tank cannot heat up water in extrem cold weather, like -25 degree celsius. I called my plumber who replaced the hot water tank for me three years ago when this happened the same year he replaced it for me, he suggested me to turn up my temprature gauge on the tank. But it does not help. Usually I am not too worried because when the weather comes back, the hot water comes back, too. This has been happening once per year in the past three years.

However, it happened twice in recent two weeks, because the extrem cold hit us twice. This second time has been three days, and I expect it to last for another two/three days, looking at the weather broadcasting.

Why is the cold weather affecting the tank? The tank is in my basement, where is pretty warm - 18 degrees celsius.

My tank uses natural gas, I believe.

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    Perhaps the hot water is going through pipes in a wall that is really cold and you are losing heat that way. Pipe insulation may fix the problem if you can get to the pipes. Jan 7 at 5:33
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    Is it possible that your gas service doesn't have enough capacity to run this water heater plus your furnace?
    – jwh20
    Jan 7 at 10:35
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    When the heater is not working, you should go outside to where the vent pipes terminate (probably a wall near the water heater, or sometimes they go through the roof). Check if there is any ice built up in either of the pipes. If there is, that could be your problem. Jan 7 at 14:44
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    Turn on a hot water valve somewhere and then check the temperature on the hot water pipe just above the tank. It should be very hot to the touch. If it is not, then the tank is not working right. If it is hot and the water coming out the tap in the house is not, then there is a problem with poor insulation of the pipe. Do you have a recirculation loop? Jan 7 at 16:51
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    What do you have for fresh air intake? If both the tank and the heater are fighting for the fresh air, one could shut off. I had issues with my boiler when I plugged the PVC pipe the internet came in because wasps were nesting in the basement, and it prevented the boiler from operating for more than a few seconds at a time because there was no other fresh air source.
    – rtaft
    Jan 7 at 18:36

2 Answers 2

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Make sure you have adequate fresh air intake. There is typically a sensor on gas boilers, water heaters, and furnaces that will shut off the flame if it detects low pressure from blowing the exhaust out. If both the furnace and hot water tank are fighting over a limited air intake, it could cause one or both to not operate properly. It could also be that the hot water tank's sensor is a bit more sensitive than the furnace to the lack of fresh air and shuts off while the other does not. Also make sure the intake is not plugged.

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  • A tip for the readers: the tricky part for me was I had to put my hand inside the intake pipe to find the snow/ice. Looking only the outside did not help as the outside was very clean
    – sofname
    Jan 7 at 19:54
  • So does this arrangement have separate PVC pipes for intake (combustion) air and for combustion products (water vapor, CO2 + traces of noxious gasses*)? It looks like you have a very high quality installation. This appears to be a high efficiency "condensing" water heater which requires forced air to expel the exhaust gas because the exhaust is not hot enough to draw exhaust out by convection. This is essential for high efficiency, but IIUC if you lose electric power the water heater will not heat water. Possibly these have a limp mode where they heat at a fraction of the normal rate. Jan 7 at 22:04
  • Some tankless water heaters use concentric pipes for intake and exhaust. Do any "condensing" tank water heaters use concentric pipes for intake and exhaust? Jan 7 at 22:09
  • @JimStewart Yes this syste has separate pipes for intake air and outcoming products. It does say high efficiency somewhere and I believe it has some exhausting moter/fan.
    – sofname
    Jan 8 at 8:31
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This screams poor pipe insulation. I bought a new house many years ago and when we would have severe cold snaps that went well below freezing, I would have one of my bathroom taps freeze up. I learned that leaving the cabinet doors open helped some, but the line was too close to the wall and would still freeze in some extreme circumstances. It relied too heavily on the wall insulation and needed a direct insulation.

As a simple "start here" you might want to insulate the parts of the hot water line in your basement that are not copper (most code does not allow pipe insulation down the the tank itself). That would at least get you some thermal efficiency back for minimal cost and effort. Be sure to use closed-cell foam and not the cheaper stuff. You want as much R-value on that pipe as possible.

You could see about getting a thermal camera and try to trace where the pipes are losing too much heat (in extreme cold it should be easy to find on the outside of your house). If that location is accessible, you might be able to open a small section of the wall from the inside, add some insulation, and close it back up with minimal effort.

Failing that, you might be able to get a small point-of-use tank into places where the water never gets warm for the winter months (not cheap, but it would work)

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  • Turn on a hot water valve somewhere and then check the temperature on the hot water pipe just above the tank. It should be very hot to the touch. If it is not, then the tank is not working right. If it is hot and the water coming out the tap in the house is not, then there is a problem with poor insulation of the pipe. Do you have a recirculation loop? Jan 7 at 16:45
  • @JimStewart Might want to post this under the question
    – Machavity
    Jan 7 at 16:47
  • Thanks for the idea of insulation. However, from the test suggested by Jim Stewart, the problem should be the tank itself. I think I will add some insulation later, for different reason though - in the summer, I don't want the pipes heat up my house.
    – sofname
    Jan 7 at 18:16

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