We have a central heating and air conditioning system in a single family home (in Southern California), where there is a large intake vent in the house that goes directly to the central house fan and furnace (which is located in the garage), and which works in conjunction with an A/C compressor on the side of the house. In the end of 2019, we had the whole system (i.e. the fan, furnace, and outdoor compressor) replaced.
That winter we used the heater, and then we transitioned to using both and then only the A/C into the summer of 2020. And then we switched back to the heater for the 2020/2021 winter.
However, in the Fall of 2020 I noticed something, that after the heater had been used and then we needed the A/C again for a hot day, the air smelled, I think, like mold or mildew. After the A/C ran for about 10 minutes it seemed to clear out though. Then we later used the heater, and on the next hot day we ran the A/C, it again smelled. Except this time it seemed to me that the smell didn't dissipate completely even after it was running for a while. We haven't turned on the A/C since then, but that has mostly not been an issue given the winter weather.
Now the weather is heating up again, and I'm afraid about using the A/C. The thing is, I don't understand what the problem could be. The A/C when running didn't smell, and then the heater when running didn't smell, and then the A/C when running after that did smell. What could cause something like that?
Of course, we could try turning the A/C on and hoping the smell isn't there or dissipates, but it still leaves me worried about mold. If the smell goes away completely maybe I would be mostly at ease, but even then I'd wonder if there is something of concern here.
Another detail which might be relevant, when the system was brand new, it smelled quite bad, which we were told was on account of the soldering done on installation. Eventually the smell went away by running the central fan for a long time, but the smell came back initially the first times we used the heater shortly thereafter, and it may have had the soldering smell as well at first when running the A/C in the middle of 2020. But I don't think it was the same sort of smell as it had in the Fall of 2020.
Best Answer
Smells in AC systems are somewhat common.
When the heat is used the drip pans dry out and any gunkies (a professional term for growth in the drip pan).
When the AC is used the dormant gunkies get some moisture and come to life. The moisture is normal when the AC is on.
Ways to reduce or eliminate this growth add condensate pan tablets or strips that kill the gunkies without stinking the house up, bleach will work diluted but it stinks. My normal mold treatment is a no no (hydrogen peroxide) DO NOT use any acids as they will eat the galvanized treatment off the drip tray and etch the aluminum parts. You really only need to use these during cooling seasons not during heating because when heating there is no moisture in the pan. The tablets, strips or whatever use the moisture to kill the active fungus in the gunkies.
The gunkies are dust with mold / fungus growing and they can build up and block the condensate drains, produce bad odors and create health hazards like legionnaires (first found in commercial AC systems). Simple sanitizing tablets or strips prevent the growth and can be purchased at most home stores.