A heat pump dehumidifier uses a fan to draw indoor air over a heat exchange coil. The coil is almost freezing. The water in the air condenses on the coil and is drained. A second heat exchange coil reheats the air, which the dehumidifier exhausts into the room.

A heat pump dehumidifier dumps heat lost from the compressor and fan motors into the air. It returns to the indoor air the heat generated by the dehumidifier turning water vapour to liquid.

Chemical adsorbent dehumidifiers

This type of dehumidifier is designed for hot, humid climates and is not really suited for use in Canada.

Chemical adsorbent dehumidifiers absorb moisture from the air with a desiccant drying agent such as silica gel. The desiccant is on a heat exchange wheel. A separate air loop dries the wheel and exhausts the hot, damp air outdoors through special ducting.

A chemical adsorbent dehumidifier uses more energy than a heat pump dehumidifier. It is only cost-effective when it uses natural gas for heat exchangeand then only if natural gas is available at a low summer rate.

Homemade dehumidifier

Some inventive people use bags of road de-icing salt to absorb moisture from the air. The wet salt solution drips into a pan or floor drain, drying the air. Because the salt runs off with the water removed from the air, it must be replaced. The system has no moving parts.

If you try this homemade dehumidifier, remember that salt is highly corrosive to metals and quite hard on the skin.

Dehumidifying ventilators

This type of dehumidifier has a sensor-controller and exhaust fan. You set the sensor-controller to run when humidity reaches a set level. A dehumidifying ventilator is particularly effective if the humidity source is in your basement.

Dehumidifying ventilators don't recover heat but they use less electricity than heat pump dehumidifiers. They are not effective in hot, muggy weather, as they bring more outside air into the house. They can be effective in cold weather.

A dehumidifying ventilator depressurizes your basement, which can cause combustion gas spillage. Make sure your heating contractor checks the venting for your gas furnace, water heater and wood-burning appliances. Consider using a carbon monoxide warning device if you install a dehumidifying ventilator.

Choosing a Dehumidifier

Dehumidifier purchase costs

Heat pump dehumidifiers are complex, low-production machines and are not low-cost items. Spring seems to be the best time to buy: in the last few years dealers have held sales early in the spring with savings of $50 to $100.

The more water a dehumidifier holds which is measured in litres or U.S. pints a daythe more it costs. You pay more for special features.

In 2000, you can expect to pay between $200 and $250 for a unit rated at 10 L (21 U.S. pints or 2.1 imperial gal.) a daysuitable for a small house with a moderate moisture problem. The price rises to between $250 and $350 for a unit with a capacity of 20 L (42 U.S. pints or 4.3 imperial gal.) a daysuitable for a large house with more severe problems.

Running costs for both units are about the same if they both have the same Efficiency Factor. The next section explains the Efficiency Factor.

Don't try to save money by buying a unit that does not have automatic defrost. If a room is too cool, a dehumidifier can frost over and stop removing moisture. A frosted-over dehumidifier left running for a long time will break down.

Get a unit with a two-speed fan. They are quieter at lower humidity, which reduces the nuisance factor. Test all units in the store for noise level, especially if you plan to use one in or near a bedroom.