Many Seattle / Tacoma area homeowners unwittingly caused their furnaces to freeze – and had to endure cold homes without heat during the last round of cold temperatures.
Today’s high efficiency furnaces generate condensate discharge as a normal part of operation. Sometimes furnaces are located in unconditioned space such as an attic or crawl space. Condensate harmlessly drains away under normal operating conditions even when outside temperatures go well below freezing. However, if the furnace has been operating, has condensate present and is then shut off in freezing conditions it is possible for the furnace’s condensate system to “freeze” – preventing the furnace from running. The problem can occur when going to bed at night and turning the set point way down. Since the house is warm when going to bed there may not be a call for heat for several hours while the home cools down. If the furnace is located in a freezing environment (unconditioned attic or crawl) it could freeze over night. Of course the problem resolves itself when temperatures rise above freezing and things thaw out.
Keep in mind this is only a concern with furnaces located in unconditioned spaces subject to freezing. Garage located furnaces typically do not freeze and are not a concern.