Q: Dear Jim: Although we use several room air cleaners, my family still has allergies. We are considering installing a central air cleaner for our heat pump. What is the most effective and efficient design to buy? - Judy T.

A: Dear Judy: A central air cleaner is probably a good idea, but don't totally discontinue using a room cleaner or two. A combination of a room air cleaner and a central furnace/air-conditioner mounted air cleaner is a good defense against allergies.

Some room air cleaners, especially those using HEPA filter elements, have fairly powerful motors. Running several of these continuously will consume quite a bit of electricity. In contrast, most central air cleaners, even the electronic ones, use a relatively small amount of electricity or none at all.

The effectiveness of air cleaners is often compared by dust arrestance ratings.

The effectiveness of various air cleaners differs with the type of particle (allergen) you need to remove from the air. Mold spores, dust mite feces and pollen are relatively large airborne particles that are relatively easy for many filter materials to remove.

Tobacco and fireplace smoke, bacteria and viruses are very small particles that require different types of filtration methods.

Another consideration is the amount of resistance the air cleaner creates in the duct system. A thick, densely packed filter may be effective at catching almost all the airborne particles, but it can cause excessive air flow resistance. This higher resistance to air flow can make your furnace/air conditioner operate inefficiently and increase your utility bills.

To avoid excessive air flow resistance and still have a super filtration, several designs of bypass HEPA air cleaners are available with their own blowers. HEPA filter media is often used in hospital operating rooms and is probably the most effective filtration method for both large and small particles in the air.

These models are attached to the main return duct. They draw air out of the duct, clean it and blow it back into the duct again.

If you have a very airtight house that gets stuffy, select a HEPA model that introduces filtered fresh outdoor air into your duct system. A lack of fresh air is often as much of a problem as dirty indoor air. In extremely hot or cold climates, choose a model with a heat recovery unit to save energy.

The most common types of in-line (uses furnace blower) air cleaners are electronic, self-charging electrostatic and pleated media. Electronic air cleaners use very little electricity. They create a high-voltage static charge that causes particles in the air to stick to washable cells. They are particularly effective for removing the smallest particles from the air. Most models will have to be professionally installed and require ductwork modifications.

Regularly clean the filter cells to maintain their high efficiency and to minimize the production of ozone gas.

Most washable self-charging electrostatic air cleaners slip into the existing filter slot in your furnace. The air blowing over the special combination of plastic materials creates a static charge. These are very effective for removing larger particles from the air. Each month, remove it from its slot and rinse it off.

Pleated media filters are dense filters that are very effective for removing larger particles. To purify air in addition to cleaning it, install an ultraviolet (UV) light purifier in the return duct. The UV light kills virus, bacteria, mold spores and other germs.

Write for (instantly download - www.dulley.com) Utility Bills Update No. 950 - buyer's guide of 11 central air cleaners manufacturers (35 models) listing filtration methods, dust arrestance ratings, features, prices, and an allergen/design selector chart. Please include $3 and a business-size SASE. James Dulley, Illinois Country Living, 6906 Royalgreen Dr., Cincinnati, OH 45244.

James Dulley is a mechanical engineer who writes on a wide variety of energy and utility topics. His column appears in a large number of daily newspapers.

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