Dear Jim: Our old central air conditioner still works, but our house often seems too humid indoors. We set the thermostat lower, but it does not help a lot. How can we reduce the humidity and improve comfort? — Michael T.
Dear Michael: High indoor humidity levels can make people uncomfortable. Damp, cool indoor air creates a muggy atmosphere that often feels much worse than warmer, humid air from open windows. This is particularly true for allergy-sufferers because many allergens thrive in damp conditions.
Excessively humid indoor air also can drive up air-conditioning costs. As you mentioned, you sometimes set the thermostat lower to compensate for the high humidity and discomfort. Each degree you lower the thermostat setting increases your electric bill.
First, try to reduce the humidity you’re already producing. The kitchen and bathrooms are the greatest contributors to high humidity levels. Make sure your stove’s exhaust hood is ducted outside, not into the attic — recirculating stove hoods are ineffective at controlling odor and moisture — and run the fan when cooking, especially while boiling water. Also consider using small countertop cooking appliances outdoors on a patio or deck.
As in the kitchen, run the bathroom vent fan whenever showering or bathing. Don’t turn it off as soon as you are done because there is residual moisture in the air. Some of the new, quiet bathroom vent fans have humidity level sensors to run long enough to exhaust the moisture but not too long to waste electricity and conditioned indoor air. You can also try a simple countdown timer as the wall switch — set it for 30 minutes, and the fan turns itself off.
If you can get the indoor humidity level low enough, it often is possible for your family to get by at a much higher thermostat setting and using ceiling fans.
The air movement from a fan increases evaporation and creates a “wind-chill” effect for added comfort. Make sure the ceiling fan rotates to blow the air downward during summer and upward on low speed during winter.
Proper sizing of a central air-conditioning system is critical for low humidity and comfortably cool indoor air. An HVAC contractor generally sizes the cooling system properly for a new house. Over the years, you may have made energy-efficiency improvements to your house such as more insulation and new windows or doors.
With these improvements, the cooling requirements for your house may have dropped. A unit that’s too large for the space will operate inefficiently and could even cause mold problems because of humidity. A licensed professional should size your central air-conditioning system using a mathematical code or an automatic computer program.
If you plan to install a new high-efficiency air conditioner or heat pump and humidity is an issue for your family, consider a two-stage or variable-output model with a variable-speed blower motor. With the matching smart thermostat, these models are designed for efficiency and humidity control. You can set both the desired temperature and humidity settings. The air conditioner will run as normal to cool the air to the desired temperature. Once that temperature is met, the blower speed slows down to provide more dehumidification and less cooling.
Installing a whole-house ERV (energy recovering ventilation) system is an efficient way to exhaust stale, humid indoor air and bring in fresh outdoor air. Both heat and humidity are exchanged in the system to minimize energy loss. These systems are often controlled by a humidity sensor.
If you don’t want to upgrade to a new, efficient air conditioner, a contractor may be able to change some settings to slow the blower motor on your current unit. This will dehumidify more but will likely somewhat reduce the unit’s efficiency. If the lower humidity level allows you to set the thermostat higher and still be comfortable, you should save electricity overall.
There may be certain rooms in your house where you find the humidity level to be more uncomfortable than others due to the activity level. A portable air conditioner that can be moved from room to room and vents outdoors through a window can provide spot-cooling and dehumidification.
Don’t forget to fix leaky gutters and downspouts. If rainwater leaks, saturating the ground around your house, some of that moisture will eventually migrate indoors.