I just had 2 new fan coil units installed in our house, before I just had window units. I'm noticing major difference between the upstairs and downstairs humidity levels.

We have a 2-ton unit serving the upstairs and attic (900sft), and a 2-ton unit serving the downstairs (700sft). The system is maintaining temperature fine, but humidity downstairs hovers around 40%-45%, while upstairs is 60%-65%.

We closed all the storm windows and taped off the windows to see if that was having an effect, but so far no difference.

I'm not sure how else to troubleshoot the issue. We've had a tech from the installer look at it, but they sort of handwaved the issue as typical in an old house and that upstairs is always more humid due to stack effect, they did just increase the fan speed from low to high, so I guess we'll see if that does anything.

Basically want to find out if this is due to some issue with the install and try to get the installers to rectify, or if this is typical and I need to do additional work like add a dehumidifier to the upstairs or something.

1 Answer 1


I'm not an expert by any means, but how was the equipment sized for your home? Reviewing some of the recommendations from Energy Star a 2 ton/24,000 BTU unit may be too large for the spaces to dehumidify them properly. The guidance they provide is:

Room air conditioner sizing is all about matching the room size to the air conditioner size. A unit that is too big will be less effective and offer reduced air conditioner efficiency. Air conditioners remove both heat and humidity from the air. If the unit is too large, it will cool the room, before it has a chance to remove the humidity. The result will be a room that's not very comfortable to spend time in That cool moist air will make the room feel damp and clammy.

An oversized unit will cool effectively but won't be able to de-humidify a space properly, although I'd assume the downstairs would be the more humid space given it is smaller.

Hope this helps!

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