Why does the furnace fan/condensor box, run outside my house in the winter?
It may be a heat pump. If you’re not familiar with the terminology, imagine a refrigerator working in reverse. That is, it’s “cooling” the outside by extracting latent heat from the atmosphere (same as your refrigerator extracts latent heat from the contents you put inside) and pumping that heat into your home (just like your refrigerator already does — feel the warm draft behind it or underneath it when it’s running). Even on a “cold” day (down to some practical limit; you’d have to look at the specs on yours to know what its operating temperature range is) there is ‘heat’ available in the atmosphere that can be transferred indoors via the heat pump. It won’t work on very cold days, which is why heat pumps are only useful in late fall, early spring and other times when it’s not especially cold — or in climates where it never gets much below freezing.
