I have heard that there is a minimum temperature at which you should have your house in the winter to protect the heat exchanger on a high-efficiency furnace. I've heard stories of people going on vacation and leaving their thermostat temperature set to 45 degrees or something, and it does damage to the heat exchanger. Is this true, and if so, what is the recommended minimum temperature that I should set my thermostat to ?
I have been installing these for 15 years and never heard any such thing, but it does bring up one issue. If the furnace is used some condensate may remain in the secondary heat exchanger, therefore freezing this condensate may cause the heat exchanger to expand and crack. Interesting thought. I would also assume that the condensate is not pure water, but filled with other constituents due to the process of burning natural gas or propane and would therefore have a lower freezing temperature than pure water. I can't see any temperature above freezing causing any damage.
I have installed and serviced many 90+ furnaces. It clearly states in the installation book you must have a min return air temperature of no less than 60 degrees for continuous running and 55 degrees for intermittent. Also you don't want the return air to be over 80 degrees. If the return air is too cold in a 90+ furnace the combustion air will condensate where its not suppose to, namely the unprotected first heat exchanger, and not just in the second, lined, heat exchanger. Which rots out the first heat exchanger. I would not set my stat lower than 60 just to be safe.
The only truth to this is if you have an outside fresh air tied into your ductwork. In this case the outside air is sucked in through your return duct system and heated with the rest of your air. If you are sucking in very cold air and only mixing it with 45 degree air it could possibly have very cold air hitting your heat exchanger which YES is very bad for a heat exchanger. Imagine taking a ice cold pan out of the freezer and running it under hot water - the pop you hear is the same pressure you would be putting on your heat exchanger. My advise would be that if you have a fresh air pipe tied into your return system (most likely a 4 pipe) to just plug it while you are away and all will be fine.
The stories are not true. The only reason for keeping min. temps in a house is to keep water pipes from freezing.
