Originally Posted by
teddy bear
Bingo!
Try this for a theory.
The duct board exterior surfaces in the attic are below the dew point of the air in the home during evening hours. During the cold evenings, the attic and duct surfaces cool, moisture from the air in the home condenses inside the ducts on the cold surfaces of the duct board in the attic. While heating, the ducts warmed, the condensed moisture is evaporated and blown into the home. This raises the indoor humidity rapidly.
I suggest operating the fan in the "on" mode enough to avoid the ducts from cooling below the dew point of the air in the home is currently +50^F. With ducts in a cold attic, you need to keep the duct surface above the dew point of the air in the home. The fact remains that you are not getting not getting enough fresh air into your home, but may not beable to avoid some condensation in attic ducts during cold weather. Operating you blower in "on" mode low speed on the coldest evenings will eliminate the considensation on the inside of the exterior duct surface of the ducts in the cold space.
A clue to the amount of fresh air naturallly infiltrating your home. Four occupants in the 2,200 sqft. home should get 80 cfm of fresh air when the home is occupied at a minimum. This will purge indoor pollutants, renew oxygen, and keep the indoor dew +10^F above the outdoor dew point. Currently, your area is 50^F outside dew point. Expect the indoor dew point to be 60^F. At 75^F inside, expect 60%RH. The dehu will run a little to maintain <50%RH. When the outdoor dew point drops below 40^F, the dehu should not run if you are getting 80 cfm of fresh air passing through your home.
Operate you fan "on" mode for a couple days to test "the condensation in the ducts" theroy.
Keep us posted. Operating a bath exhaust 24/7 will also increase ventilation to reduce winter moisture problems.
Most important is to keep the fresh air moving through the home during the calm warm times of the year to purge the indoor pollutants and renew oxygen. Unfortunately, you will need more dehumidification when the outdoor dew points are high. It is a small cost to pay for good indoor air quality.
This is an excellent learning opportunity. Keep us posted.
Regards TB