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chapx 09-23-2009 09:38 PM

Humidity issues
We bought a house (new construction) and moved in mid July. The house was an inventory home finished in Nov '08 and served as an open house until we bought it. Within the first week, we noticed something was not right with the AC. The house felt stuffy/clammy and vent air flow was minimal. When the AC tech came to check it out he discovered all the dampers were installed in the wrong direction. The techs flipped the dampers to the correct direction and adjusted to get good vent air flow. I purchased a thermometer/humidistat (takes indoor and outdoor readings) because the house still feels humid and clammy. The thermostat is set to 75 deg F. The relative humidity inside the house averages 58% throughout the day, even when humidity outdoors drops to the 30%s. We live in a humid area, 30 mins from the Texas coast, and are told (by AC company and builder) that's the reason why inside humidity is such and that that level is normal. I took my thermometer/humidistat to the builder's office, which is a model home, and the relative humidity read 48% (and probably would have been lower had I stayed a longer time) - their thermostat is also set at 75 deg F. If I set the thermostat at 71 deg F, I have noticed a drop in humidity to about 52%, but rises back up when thermostat is set to 75degF. I don't want to run the AC that much (high energy bill!) to lower the humidity. The builder also loaned us a dehumidifier, which brought the humidity level down to 49-50% with the thermostat set at 74-75 deg F, but as soon as we turned if off, the humidity rose back up. Any ideas as to what can be causing this problem. I have done some research and have found that ideal humidity is between 40-50%. above that, mold can start forming. Any help is appreciated. By the way, we have a two story, with separate AC units, and humidity tends to be 2-3% higher upstairs.

KAVF 09-23-2009 09:43 PM

What is the fan setting on the thermostat set for?

Auto or On?

chapx 09-23-2009 09:47 PM

Auto. Also, the A/C's fan speed was slowed down.

KAVF 09-23-2009 09:50 PM

On a 90+ deg day how long is the A/C on cycle? for upstairs and downstairs units if you know?

chapx 09-23-2009 10:06 PM

Hmm, not sure. I hear it come on and later shuts off, but I haven't paid close attention to the length it stays on.

KAVF 09-23-2009 10:11 PM

On a hot day like 90+ the A/C should run around 15 mins on, to run long enough to remove enough humidity. Your home should be around 47% to 52% if all is working properly.

Only set fan switch to Auto as On will increase humidity level in space when A/C is used.

beenthere 09-24-2009 05:43 AM

Oversized A/Cs.
Leaky duct system.
Leaky house.

chapx 09-24-2009 09:22 PM


Originally Posted by beenthere (Post 331777)
Oversized A/Cs.
Leaky duct system.
Leaky house.

Don't think A/C units are oversized:
Upstairs is 1165 sq ft with 2.5 ton Carrier 13 seer unit
Downstairs is 1860 sq ft with 3.5 ton Carrier 13 seer unit

What are the signs of a leaky duct system/leaky house or who can I have check for such?
update: Today, humidity read 61% upstairs, 58% downstairs. Temperature outside was in the low 70's so A/C didn't run much during the day (thermostat set to 73 deg F).

beenthere 09-25-2009 04:11 AM

Look for Energy Auditor companies in the yellow pages, or search on line for a local one. Electric companies sometimes can help you find one.

Leaky duct systems can have troubles maintaining temp when its close to design. Or, they won't remove moisture as well as the should.

The symptoms are silmular between over size and a leaky duct system.

Good chance your systems are over sized. Since they were sized by rule of thumb.
In your case it looks like they were sized by 500 sq ft to the ton.
1165+1860=3025/500=6 tons.

On your hottest days. Do they run 24/7. Or do they cycle on and off.

Over sized will cycle on and off.

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