@rogerch where do you live? In the US, AC commonly refers to cooling. There may be people who use it to refer to heating but they are using it mistakenly, because the complete term HVAC is heating, ventilation, and air conditioning.
Since becoming a piano owner, I have lived in Japan and in the U.S. Midwest (including the northern Midwest, central Midwest, and southern Midwest) and in the U.S. Southeast. In Japan, humidity control was challenging but made do-able by the very efficient and capable mini-splits that are so common in most houses.
In US, in the last house we owned, we replaced the HVAC. And in the last rental house we lived in, we had a hell of a time controlling the RH. So Iāve spent a lot of time reading about HVACs, monitoring my own systems, and also talking to HVAC installers.
So my understanding is specific to the regions Iāve lived in of course, but with that caveat in mind, in my understanding, itās both common knowledge as well as my direct experience that AC, as in cooling, lowers the RH.
Unless thereās something wrong with the system or the house. Which is not a minor issue, and not that uncommon. (Our last rental house fits in this category and is one of the reasons I was in a hurry to move and one of the reasons Iām so happy that the house we just bought has such a good HVAC).
So with regard to the system, itās also common knowledge among HVAC installers that an oversized system can often result in the AC portion not working as intended, such that when the AC is on, the indoor RH actually increases.
So AC (cooling) resulting in an increase in humidity (an increase to problematic levels, that is) usually means either that the system is not well-suited to the house, or else that thereās some moisture problem in the house. In the first house we owned, we lived in the southeast where it was hot and humid in the summer and cold and damp in the winter. And the house was about 60 years old. So after I upgraded from an acoustic upright to a grand, I replaced the HVAC, had the crawl space encapsulated and installed a whole house dehumidifier. I had hygrometers all over the house and monitored it closely and there was a big improvement.
In our last rental house (which we just moved out of two months ago), there were problems with the house and the system. The system was oversized, and Iām pretty sure that the crawl space had an unmitigated water problem so it was always damp. Because it wasnāt my house, I couldnāt do anything about either of those problems, but if I were the home owner, I would have done what I did in our last house ā replaced the HVAC and had the crawl space encapsulated. I would bet that this would solve the problem, but if it didnāt, I would have then had a whole house dehumidifier installed.
In our new house, which is only about 30 years old, the previous owner had the crawl space encapsulated, and the basement is treated (i.e. in the finished parts of the basement, there are HVAC ducts and vents and returns). Because itās a newer house then previous house i owned and the previous rentals i lived in, the windows are better and the insulation is better, and the HVAC is well-suited to the house. So on the main level, humidity control has not been a problem, and when the AC is running, the RH drops in all rooms where I have hygrometers.
Ok, sorry, Iāve gone on much too long here! š
Itās just that i have spent a lot of time and effort ā and money āworking on humidity control ā¦ š
So, back to you @rogerch what area of the world are you in? Iām guessing if itās the U.S., maybe youāre in the southwest? Also I notice that you wrote that your RH goes up to maybe 40 or 50, I donāt think thatās a problematic increase and so I wouldnāt say it means your hvac is oversized or something. In the last rental we lived in, the one with the problems, with the AC on, the RH would jump up to 75% or 80% ā so I started using a free-standing dehumidifier. It was awful! (And we donāt even have to talk about how unpleasant it is to have a free-standing dehumidifier going 24 hours a day in your living roomā¦ nothing like the noise of a jet engine to make your house cozyā¦)