Heat Pumps and Electric Heating - Heat Pump Smell ????

When the weather drops below 35 degrees or so and our heat pump turns on, it sometimes gives out this "sour" smell. It doesn't happen when the temp is above 35-40 degrees, and I noticed this morn the little green light lit up on the thermostat when it happned (my husband said the heat strips came on). Anyways, last year when it was happening the heat pump people couldn't get it to happen when they were there (of course, but it wasn't cold enough) and they said the coil looked good (but they would replace it or clean it) and that we could try one of those UV lights . . . any suggestions, since it's only sporadic and only when it's cold enough for the aux heat/heat strips turn on? :confused:

Dont know this---"sour" smell
Ill go two ways .You sure that the AC trap drain is full. No sewer gas can get back out of it??? Then next is air filter is it good no lint can get by it and on to the electric elements. This sound like what you have. Around that temp when the unit goes into defrost the elements will come on.

maybe what's been referred to as "dirty sock" smell . . . just a stale smell. not all the time, though. I try to use a decent air filter and change it once a month. we were told last year that our return air vent downstairs wasn't large enough, and we even had all that up-sized to supply more air to the unit.