Hello Forum!

I just had a new high efficiency Carrier AC & Downflow Gas Furnace installed. It's doing a great job keeping my house cool.

When the AC compressor is on ... the air coming out of the vents is around 66F with a relative humidity (RH) of around 45%, which is great.

As soon as the compressor goes off and in FAN ONLY on, in about 1 minute the RH of the air vent air jumps to around 75% ... and stays high until the compressor comes on again knocking the RH down in the air vent air to about 45% again.

I did an experiment shutting off the compressor completely when this high humidity problem occurred ... and it took about 2 to 4 hours for the air vent air to return to normal (homeostatic with the ambient house temp and RH)

Is this rapid rise in the RH (from 45% to 75%) of the air vent air right after the compressor goes off normal?

I wouldnt have even thought to measure the RH except that I noticed whenever my AC compressor would go off ... the air coming out of the air vents began to smell "musty" ... like an old castle.

As soon as the compressor comes back on, knocking the RH down again yo 45%, the air vent air smells great.

Note that I never noticed this smell problem in my old 20 year old carrier air conditioner -- although I never measured the RH in the vents before.


My AC company told me this was normal. Is it? They said it was caused when the house air started flowing over the wet coils putting the moisture back into the house. One thing that is different is my old Carrier AC had a very high FAN ONLY speed, while the new Carrier AC has a very slow FAN ONLY speed.

I am thinking about having my AC company increase the FAN ONLY speed to very high (my thinking is that the fast air would tend to dry out the coils and drip pan faster)

Note that I run 3 dehumifiers in my 1800 sq ft house ... So my house RH is kept at a beautiful 45% all summer long ... so the air running into the AC intakes (recirculating through the coil) is also a nice 75F and 45% RH ... the only moist air in my house drifts out of my air vents just after the compressor goes off.

Question1: Is this rapid rise humidity of air vent air from 45% to 75% after the compressor cycles offf normal? (Even when my house air is kept at a constant 75F and 45%)

Question2: If this is normal, would it be better for me to:

a.) have the FAN set to OFF in the summer season (would that cause the water on the coils and drip pan to mold) ...

dark + wet + no wind = mold

or

b.) would it be better to set the FAN speed higher so the moisture on the coils and drip pan dries out faster blowing the moisture in my coil and drip pan through my air vents so my dehumidifiers could wring it out.

Thanks for any advise.