July 23, 2014

Lower Humidity, Lower Electric Bill

Q: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?

A: 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. Excessive humidity also increases air-conditioning costs because each degree that you lower the thermostat to compensate for the discomfort increases your electric bill. But there are things you can do to improve the situation.

First, try to reduce the humidity you’re already producing. The kitchen and bathrooms are the greatest contributors to high humidity. 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. In the summer you can 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 and let it run a little while after you are done because there is much residual moisture in the air. Some of the new, quiet bathroom vent fans have humidity level sensors that run long enough to exhaust the moisture, but not too long to waste electricity and conditioned indoor air. You can also try installing a simple countdown timer as the wall switch—set it for 30 minutes, and the fan turns itself off.

If you can get the humidity level low enough, it is often possible to get by with a much higher thermostat setting and using ceiling fans.

The air from a fan increases evaporation and creates a “wind chill” effect for added comfort. Make sure the ceiling fan blows the air downward during summer and upward on low speed during winter.

Proper sizing of a central air-conditioning system is also critical for low humidity and comfortably cool indoor air. An HVAC contractor generally sizes the cooling system properly for a new house, but over the years, you may have made home energy efficiency improvements such as more insulation and new windows or doors. With such improvements, the home’s cooling requirements may have dropped from, for example, 3.5 tons (12,000 Btuh/ton) to 3.0 tons. A unit that’s too large for the space will operate inefficiently and may even cause mold problems because of the excess humidity. A licensed professional should size your central air-conditioning system using a mathematical code or automatic computer program.

If you plan to install a new high-efficiency air conditioner or heat pump and humidity is an issue, 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, and 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 this system to minimize energy loss, and are often controlled by a humidity sensor.

If you don’t want to upgrade to a new unit, a contractor may be able to change some settings to slow the blower motor on your current one. This will dehumidify more, but will likely reduce its efficiency somewhat. 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 where you find the humidity to be more uncomfortable than others due to the activity level. A portable air conditioner, which can be moved from room to room and vents outdoors through a window, can provide spot cooling and dehumidification.

Further, don’t forget to fix leaky gutters and downspouts. If rainwater leaks out and saturates the ground around your house, some of that moisture will eventually migrate indoors. Send inquiries to James Dulley, Michigan Country Lines, 6906 Royalgreen Dr., Cincinnati, OH 45244 or visit dulley.com.

-James Dulley is a nationally recognized mechanical engineer writing about home energy issues for the National Rural Electric Cooperaive Association.