I have received several inquiries recently from readers about the use of dehumidifiers inside the home. Most of the inquiries occurred during the extended rainy spell in August. I even encountered one in use at a home during our energy efficiency makeover project. Just so happened, it was during that rainy period. The unit was located in a half-basement, and the homeowner explained it was necessary as a comfort solution in order to keep the humidity down. Furthermore, the portable dehumidifier ran 24/7, and the drain tube was positioned where it could drain into a sink. Upon further investigation, I discovered something that piqued my interest, and I decided to share it with you.
A common theme from each inquiry was that the home was uncomfortable. The air felt sticky, there was an earthy smell and the homeowners were worried about mold forming. Some homes were built on slabs while others were on crawlspaces or half-basements. They also noted that their air conditioning systems were not running very much. Many of us experienced this scenario because of the below-normal temperatures during that rainy period and the absence of the sun’s solar heat gain.
As a reminder, heat is always trying to move into cooler areas. Also, moisture is trying to move to drier areas.
When a home has an inadequate thermal barrier (insulation) and a high amount of unwanted air infiltration, our laws of nature react even faster. Furthermore, those who have adequately insulated homes, especially using foam insulation and low amounts of air infiltration, can also experience the same problem. While super-efficient homes may take longer to become uncomfortable, these homeowners must be aware that moisture can become trapped inside if they are not mitigating the moisture created by cooking, showering, bathing, etc. Toss in humidity from Mother Nature, and all homes can have moisture challenges if not mitigated.
The main reason many experienced uncomfortable homes this summer was that air conditioners were not running much because the indoor and outdoor temperatures were almost identical. This lack of runtime creates a stuffy, sticky and uncomfortable environment. To some, the solution was to purchase a dehumidifier, which is an effective way to remedy the situation. Although this is a reasonable idea, it comes with some unintended consequences. First, portable residential dehumidifiers generally cost between $150 and $300. They require electricity to operate and regular dumping of the water collection pan or a place to accommodate a drain hose. And here’s what really caught my attention — the significant amount of heat discharged from the dehumidifier. And what’s the last thing you want inside your home on a summer day? More heat.
To determine just how much heat the dehumidifier creates, I acquired a typical residential dehumidifier and put it to the test with my infrared camera and a Kill-A-Watt power usage meter. Let’s start with this fact: a dehumidifier and air conditioner are quite similar. Both lower the humidity, discharge heat in the process and use electricity for operation. But the air conditioner moves its heat to the outside. The humidifier distributes its heat into the home. As shown in the above infrared image, the test unit discharges heat. Also, the Kill-A-Watt meter revealed the test unit consumed about 10 kilowatt-hours per 24-hour day, or $1 daily and $30 monthly. Plus, the air conditioner must run even more to remove the heat created from another appliance.
My recommendation for these occasional weather periods is to allow your air conditioner to do its job. When properly designed (and not oversized), it will both cool and dehumidify your entire home. My general rule is set and forget the thermostat. However, in the case of a humid rainy period you may have to drop the thermostat a degree or two and force the unit to run until the sun shines again. Then return the thermostat to your previous comfort setting.
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