VERY humid AC air

Did an mould inspection yesterday for people with a very strong musty smell in the house, on their clothes etc, interior doors that stick, a black growth they noticed on some of the register covers among other things. This place has had an extended period of high humidity.

During the inspection I discovered the AC air coming out of the register was about 13 degrees celcius, 80-85% RH. They had a Geothermal unit installed a few years ago. Indoor unit is in a dirt crawlspace with a poor vapour barrier. There was no return and ductwork was sealed pretty good so I don’t think it’s drawing in much air from the crawl. Duct work was not insulated and had a little condensation on the outside.

This thing is basically working like a swamp cooler adding cool moist air to the house, but our summers here are very humid… not a good combination.

Would there be condensation on the inside of the ductwork adding to the problem or is the Geo Thermal unit just not woring right? It’s still under warranty so they are going to have it serviced.

The supply air fan has to take a suction from somewhere so there has to be a return from somewhere maybe not from the right place but from somewhere. If it is drawing from the crawl that is totally wrong.

All duct work supply and return is required to be insulation in non-conditioned spaces

Sorry, I meant there was no return in the crawlspace which was good. It did have a supply in the crawl, but it was closed. Returns were on the main level. Isulating the ductwork is on my list of recommendations.

Sounds like the AC unit is too big and not running long enough to remove the moisture???

I think so … Roy …

A…simple fix add a dehumidifier but not very cost efficient .

No problem just trying to get the big picture. Humidity at 80%-85% is almost like raining inside the home the A/C system can only remove a limited amount of moisture depending on the run time of the unit and if mold is growing on the registers you have a serious problem, was the A/C system over sized for the home what was the cycle time between on and off. I would very seriously check the return air side of the system to ensure no external air was being introduced into the system

Thanks guys. Sizing might be the issue. Will recommend further review on that. I mentioned adding a dehumidifier as a bandaid, but would be a costly battle with that air.

No, that is not correct.

I have no time to teach you the laws of thermodynamics and psyhcrometrics at this time, all I can say is that you are so far out in left field, your out of the park!

I recommend that you get someone (other than an HVAC Contractor) to work with you on this.

David, what specifically is not correct?


Alot of “duct leakage” is actually leakage through the air handler cabinet. I have seen studies where as much as 505 of the total “duct leakage” is actually in the air handler. I would guess that the house returns are few and undersized so the system must get return air (to balance off the supply air flow) from somewhere! That somewhere would be through the air handler leaks in the crawlspace without a moisture barrier. The moist air in the crawlspace is in fact entering the home’s air flow.

If there is high humidity in the crawlspace, there is more of a problem than just the air conditioning.

#1 All registers condense in humid conditions.
Is indoor 24C / 50% rh OK for you?
If so, it will sweat when the a/c turns off.

#2 What is “Wrong” with 13C / 85%rh? You tell me.

I’ll tell you this;
13C 85% changes when mixed with indoor air.

16C will become 70%
18C will become 60%
21C will become 50%
24C will become 40%

#3 There is no way in hell it is working like a swamp cooler (period).

#4 Condensation inside duct? No, it will not.
As the air moves downstream from the evap coil (which is even colder) it warms up. The evap coil is at “saturation”. ANY Rise in air temp will lower the rh below dew point. Unless there is something in the house that is colder than the evap coil, it can not condense.

What happens on the outside of the register is not the same thing.
When you stop the air flow, the register comes in more contact with the indoor ambient air which has more moisture than that which comes out of the duct.

85% rh is not a moisture quantity measurement.
Figurer that out and all things will become clear.

Thanks Sheldon. You are reading too much into my comments. My wife keeps telling me our kids don’t get my sarcasm, I guess it doesn’t translate well online either.
Not worried about that now, going to be too busy helping rebuild our town after it was badly hit by a tornado.