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We are considering installing a tankless propane on-demand water heater in a residential setting, where multiple tenants use a shared bathing facility (presently served satisfactorily by a 40 gal electric tank heater). A big concern about installing a propane tankless heater, is the possibility of a cold air down draft in the chimney into the unit and having the core freeze (and break). We experience wintertime lows outside temps of -40°F to -50°F and colder.

In this setting there might be early AM use and then, possibly, a long period through the day with no demand until dinner time. (i.e. natural use will not necessarily have the unit cycling on and off).

Does anyone know of units where there are dampers in the flue that positively (as opposed to passively) close that would protect against such a freeze up. Anyone with direct experience?

  • where the hell do you live that gets -50F ? WOW. BTW, an on demand water heater doesn't need to use the chimney. I usually has a direct vent/exhaust out the side of the building. – shirlock homes May 6 '13 at 21:43
  • Wisconsin, Minnesota, Michigan Upper Peninsula? – Fiasco Labs Dec 21 '13 at 3:37
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Where is the tankless water heater to be located? If it is in a space that is heated, then freezing will not likely be a concern as there will be plenty of heat gain through the equipment enclosure.

If freezing is still a concern, look for a tankless unit that has a electric element for freeze protection. This will use a lot of energy if the unit is located in a cold garage, so best practice would be to locate the unit inside a heated space.

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What you are asking for is called a "non-return vale" you should verify what type is acceptable for your unit.

However there are several ways of avoiding freezing from your tankless heater.

  1. Consider buying a 'combi' unit instead of just a domestic hot water heater. As the temperature decreases, your residential heat demands will increase and which will keep water flowing though the unit and the burner on.
  2. Buy a model that has built in freeze protection. In my recent search I noticed Westinghouse / HTP have modes with ceramic heaters built in.
  3. Use a recirculating pump (some models have one built in) to create a hot water loop flowing though the unit. Conceivably, one could link the pumps to outside temperature data, having it kick at a low flow rate (below the rate that would trigger your heater) at a set outside temperature, and couple that with an inline temperature sensor that increases your flow rate to trigger heating anytime the water in your lines drops below 36°F. If you have plumbing running up against outside walls, you likely might want to do this anyway, as there are setups that can include the cold water line in the loop.
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I had a propane tankless freeze, then the copper line broke and flooded the utility room. I ordered the same unit so install would go quick. Then put a 40 watt bulb inside the unit with a makeshift wire from an old lamp. It worked good. 40 watt heavy duty bulb can run steady for about 3 months. On really cold windy days that went below zero, I ran a trickle of water to be sure it wouldn't freeze.

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