I bought a 25-year-old house last year and replaced the central HVAC system. The heat is gas. This year I've had to switch from AC to heat and then back again twice because of the hot - then cold - then hot - then cold again weather cycles. Each time I've run the heat for the first time, the humidity has shot up from around 45% to 60%. Then, after a while, it starts coming back down again.

That seems to be telling me that there's water left inside the system from the AC and the heat evaporates it, raising the humidity until it's all evaporated, correct?

I live in Arkansas and here the AC primary and secondary drains are both to the outside. When running the AC, the primary drain is draining water, so appears to be working fine.

My concerns are that I've never had that happen in any house I've lived in before. Is it normal and I've missed it before? If it's not normal, is it something to be concerned about? I'm concerned that, if there is water in the system somewhere, it may be rusting or corroding something it shouldn't.

Any thoughts or advice are appreciated.

I have a heat pump and the same issue. I chalk it up to the coil still retaining moisture that takes a bit to dry out. What I learned to do is to run the fan only (in addition to my dehumidifier) and then once the humidity starts dropping again I turn the unit to heating. Doing it this way the humidity goes from 45% to 55% and then back down.

When I would turn the heater on right away, it would go from 45% up to 70-75% before finally dropping again

Thanks. Good info.

Humidity is not an issue when running the AC or, after the first day or so, running the heat.

It's only that first day of running the heat.

TTT for the HVAC guys on here, hopefully.

One more TTP to hopefully get the HVAC guys' attention.

El duderino:

I've done a bit of googling on this problem and found a couple of guys online who had the same problem. Here's a guy who finally figured it out, at least for his situation, and it was fixed by Trane and his dealer (note that the link is to page 3 of a 3 page discussion):

https://hvac-talk.com/vbb/threads/2234567-Trane-HVAC-Strange-Humidity-Problem-Switching-from-AC-to-Furnace/page3

Interesting. And I have a Trane as well so that's probably the issue with that coil design, but I just had mine replaced (original developed a leak within 3-4 years) so I doubt I can swing that for warranty work. Good to know for the future though.

And I have an American Standard which, as I understand it, is made by Trane (same parts are used in both). So that may explain my problem as well.