My wife and I have a summer home down south.
The house sits locked up tight for months before we get down to it.
The first week we are back is always the worst week of our trip. The house is always a total mess. The surfaces are all dusty and there are dead bugs everywhere. The toilets and shower all get moldy due to the water sitting stationary. The HVAC device also needs some work. The air filter should be changed due to the dust and stale air. The inside of the heating and cooling device could use some cleaning too. You never know what has crawled and died inside of the HVAC. The worst is checking out the ductwork attached. The outdoor vent is in the garden. There could be dirt, leaves or dead animals in the outside vent. Then if we turn on the HVAC, the air blows over that debris and pollutes the air quality. It is always a scary moment checking the vent to see what got into the HVAC ducts. My wife and I take turns looking inside of it. The best is when it is a little bit of dirt. We just stick a long pole with a towel attached to it through the air ducts. The worst was when a squirrel got into the HVAC duct and died. We got a professional ductwork cleaning business into the house to take care of that one. It was super stinky and gross. Now the two of us forever worry about it happening again.