Working as an energy auditor, I am often in homes that require dehumidification. Controlling the humidity level within a home is very important for indoor air quality and the durability of the home. In this blog I will be discussing how a dehumidifier works, what they cost to operate, recommended humidity levels and the sources of moisture that can add to the humidity of the home.
How a dehumidifier works. When moisture levels within a home are above 50%, dehumidifiers become an effective way to reduce this humidity, (basement humidity levels may be above 60% during the summer). A dehumidifier 
works by drawing moist air into the unit with a fan. The fan moves the humid air across a coil that is cooled by a refrigerant. When the warm air comes in contact with the very cold metal coil, the moisture in the air condensates on the metal and drips into a bucket at the bottom of the machine. The air, which has been removed of it’s moisture, then is pushed across a small heater which returns the air to it’s original temperature and is distributed back into the room. This is the same principle used by refrigerators and air conditioners, of course without the heater.
How are dehumidifiers sized? Dehumidifiers are rated by the amount of water that is removed from the air in a 24-hour period of continuous use. Common sizes are 25 pints, 30 pints and 40 pints. A large unit can remove 70 pints per day. Units that remove more moisture, of course, cost more to operate. The chart below was produced by the Association of Home Appliance Manufacturers.
How much do they cost to operate? Cost of operation varies depending on the size of the dehumidifier, the run time of the unit and your cost of power. Some dehumidifiers have an option to drain removed moisture into a hose, which is then dumped into a household drain. Many times, these owners will run the dehumidifier continuously. Using a power recording meter (such as a Kill-O-Watt meter) to measure the actual power consumption or finding the rated power consumption from the name tag that is located somewhere on the unit is the first information that will be needed. Secondly, you need to know how much power costs in your area. Lastly you need to know the run time of the dehumidifier. The formula to use is:
Kilowatt x cost of power x time = cost of operation
An example is a dehumidifier that consumes 500 watts of power and operates 12 hours per day. We will use a cost of power of $.10 per Kilowatt. 500 watts is .5 Kilowatts (one Kilowatt is equal to 1000 watts.) .5 Kilowatts x $.10 = $.05 x 12 hours run time per day = $.60 per day. Multiply the $.60 per day time 30 days in the month and you get $18 per month.
Tips for purchasing and maintaining a dehumidifier. Be sure to purchase an Energy Star rated unit. An Energy Star rated model will have more efficient refrigeration coils, compressors and fans using less energy to remove moisture. Read the owner’s manual and follow the manufacturers instructions on maintaining and cleaning the unit. Many dehumidifiers have filters to clean the air moving through the unit. These filters and the coils should be cleaned at least once a year depending on use.
Moisture sources. The cost for removing moisture within the home is usually going to be higher than eliminating or reducing the source of the moisture. I’ve been in many homes through the years with crawl spaces or portions of basements that have uncovered dirt floors. Bare soil can be a big source of moisture within a building. Simply covering the soil with plastic sheeting (polyethylene) and sealing it to the foundation walls and around any penetrations will reduce this moisture source.
Use your bath and kitchen exhaust fans. Showering and cooking can also raise humidity levels within the home. These fans are there to control moisture (and odors) and are cheaper to operate than dehumidifiers.
Whole house ventilation units. A heat recovery ventilator (HRV) or energy recovery ventilator (ERV) are balanced whole house ventilators designed to bring in fresh air and exhaust an equal amount of stale air. Balanced ventilation is required on all new home built in Minnesota, and these units are awesome at removing moisture during the winter months. Operating them during the summer can increase humidity levels by bringing the moist air outside into the home. My suggestion is turn on the HRV or ERV in October, turn them off in April. Open the windows during the summer to let fresh air in when possible. I have a future blog planned on discussing HRV and ERV use.
Moving exterior bulk water away from the home will help keep basements dryer. Sloping the soils around the home away from the foundation and the addition of gutters are two common methods. Designing large eaves on new homes can also be effective.
Dripping metal water lines, usually located in basements, are acting like dehumidifiers, but the water is just cycling from vapor to liquid and back to vapor. The moisture is never removed, just changes form. I’ve seen puddles on basement floors caused by water dripping from metal water lines. Wrapping these pipes with a foam insulation is a cheap fix.
Other sources of moisture might be coming from the construction materials and techniques used to build the home. Improper exterior flashing details or errors made while constructing the home might cause bulk water to enter the home during rain events. Depending on the drying potential of the building, this moisture may end up inside the home. These construction deficiencies often aren’t detected until it’s too late. Mold is often the first indication there is a problem and the remedy is usually expensive.
The ideal humidity level. One of the first questions I ask when conducting an energy audit is what is the humidity level in your home? Most homeowners are not sure. Moisture or frost on the windows during the winter months is often a clue. Using a dehumidifier during the summer months a may also be an indication. My suggestion is during the winter, the humidity should be as low as you can tolerate, typically around 30%, but never above 40%. At 40%, you will probably have quite a bit of water or frost on the bottom of your windows. Summer months I recommend levels below 60%. 70% is when there is a risk of mold growth. The only way to know what the humidity level is inside a home is with a gauge called a hygrometer. I suggest having at least one in your home.