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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?

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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

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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