u/-jbrs

[TL;DR at bottom]

So humidity in my rental has been high since I moved in, and I've been paying attention to it because I've had mold problems before, but in the last few weeks it has been 65%, 70%, even up to 78%. It's been crazy.

Me, my landlord, two different A/C technicians have all been wracking our brains trying to fix it for over a week to no avail. I've been especially worried because it doesn't take much time above 60, 65% for mold to really start growing.

I have also been having symptoms and wondering how on earth this could be causing them. Night sweats, heart palpitations, facial nerve inflammation - all things that I do not have pretty much ever! Even my dog seems to be affected, and he's been more lethargic and refusing foods he normally loves.

We tried many things that might be creating an issue:

• Checked the furnace vents and replaced filters. I only moved in a few months ago, but my dog had shed a lot of hair that was picked up by those and they were pretty clogged up and the system was clearly having to work pretty hard to draw air. We were hopeful that that was the fix, but no impact on the humidity. (It can create issues though - Filters should be replaced every 2-3 months, more often if e.g. you have a dog who sheds.)
• Short cycling. So after an A/C tech came out he mentioned to go outside for some time and record how long the system is running. A/C systems run the hot air indoors through a cold chamber that pulls heat out and then returns it back into the home. But importantly it also has coils that condense moisture out of the air, and this is how it pulls excess humidity out of your indoor air. The system however has to run long enough for the moisture to condense, and sometimes systems are so efficient that they cool the home before enough condensation happens, leading to excess indoor humidity. (This often happens with oversized systems.) I went outside and recorded the on/off cycle times and my system was short cycling! It was running for max ~10 minutes at a time, when systems generally need to run about 15+ mins at least for each cycle. Finally we had an answer! Or so I thought - I let the temp come up the next day so that it could run a long time bringing it down; it ran for 3 hours, and still the issue with humidity.

The system would bring humidity down while it ran, but as soon as it stopped actively cooling, humidity would start to creep up. I'd start the day at 50% and then it would creep up to the mid 60s by the afternoon, and possibly into the 70%+ range if i did not take steps to control it (by opening windows or running the heat). We even got a large dehumidifier rated for a home the size of my rental, but even that was not enough.

I noticed that the air would smell kind of odd once the system stopped running and it was just blowing normal air out of the vents. I started to think that the system was just pulling humidity out and then reevaporating it once it was done, leading to the weird smell and the high moisture indoors. I mentioned it to my landlord, but we still couldn't figure it out.

I was actually getting ready to move out and relocate - extremely overwhelmed by all of it - when we finally figured it out, totally by a fluke.

It was the fan setting.

The fan was set to "ON" which means always on. I thought this was a good thing, because I thought more circulation would help prevent mold. But no. Those coils that I mentioned earlier, the ones that condense the moisture out, only drain the moisture out if the fan is not running. Otherwise the air moves over the coils and the moisture just reevaporates before it can pool and drain.

This was also why I was getting sick - the rental is newly constructed, and there is a fair amount of construction dust in the vents. The moisture would absorb dust (and maybe even metal from the coils) before reevaporating, leading to pretty significant indoor air pollution. Luckily I had some air filters running, but even with them it was still enough to create significant symptoms.

Anyway, it is a great relief to have finally figured this out. Humidity in my place is now solidly under 50%, and my symptoms are improving rapidly. I don't know why they even have an ON setting on these things, or why nobody tells you not to turn it on, or how we all missed it, but at least we know now. I hope this can be helpful to others.

tl;dr the fan setting on your thermostat needs to be set to "auto" instead of "on" because otherwise moisture will not be able to condense and drain while the system is not actively cooling.

P.S. I recommend getting one or two of these and putting them in different areas around your house or apartment. They're <$10 and work very well, and can save you a lot of trouble - not sure I'd have caught this in time without them. Honestly if I were a landlord I'd just give them to every tenant when they moved in.