How to Keep Your Furnace Breathing Easy

How long has it been since you last replaced the filter on your heating or air conditioning system? Chances are that if you are like most homeowners and renters, your filter is long overdue to be changed.

Experts suggest that to keep a heating system operating at peak efficiency, and to keep heat exchangers, ducts and room registers as clean as possible, the filter must be replaced at least once a year for heating systems and twice a year for combination heating/air conditioning systems.

In especially dusty areas, filters require even more frequent changes.

The best times of the year to replace a filter is the fall and spring, before your climate control system is really put to the task during the winter and summer.

Replacing the filter is an easy job that any homeowner can do within minutes. The first step is to figure out where the filter is located.

If your central furnace/air conditioner system is an “upright” design in a closet or basement with ducts attached at the top, chances are the filter will be found at the bottom of the unit behind the lower panel.

To reach the filter, raise the lower panel about half an inch, then pull it out from the bottom. On some units the upper panel must be taken out before the lower panel can be lifted.

With the front panel(s) removed, the filter will usually be visible lying flat at the bottom of the inside of the machine. It looks like a small, rectangular pad and normally measures about 18 by 24 inches. Most systems have a disposable filter in a cardboard frame.

To remove the filter, simply slide it out of the brackets on either side of the filter compartment.

Because of space constraints, some heating and heating/cooling units are placed horizontally instead of vertically. These units are normally located in the attic, on the roof or under the house.

The filter for horizontal systems will be found either behind a “return air” grill inside the house on a wall or ceiling, or behind a four-to-six-inch-wide “slide-up” panel on the large sheet-metal duct that enters the furnace/air conditioning unit from the side.

To determine how your filter is located, first look around your walls and ceilings for a metal grill that looks like it has hinges on one side. Such a grill will either snap open when you pull on it, or there will be small tabs on the side opposite the hinge that turn to release it.

Once you get the grill open, the filter should be just behind it.

If you do not see a return air grill inside the house, go to the furnace/air conditioning unit itself. Coming into it from one side will be a large sheet-metal duct. On the duct, look for a vertical panel that slides upward.

Pull up on the top edge of the panel with pliers to lift it up and out of the way. The filter stands vertically inside and is removed by sliding it out of the brackets at the top and bottom of the filter slot. To match the filter with a replacement, take its measurements, or take the filter itself, to any large hardware store or home center. There you will find an assortment of new filters in many sizes.

Some systems have a reusable filter that can be cleaned. This filter looks like a thick straw mat and is usually green. It will not have the cardboard frame characteristic of disposables, but should be found in the same locations.

To clean, shake off as much dirt as possible, then spray with a high-pressure nozzle on your garden hose. Hit it from both sides until all the dust and debris is gone. Dry it in the sun before re-installing.

A new disposable or newly cleaned reusable filter goes back into the furnace/air conditioning unit or behind the return air grill exactly as the old one came out. Be sure that the arrows on a disposable filter point toward the center of the unit (in the direction of air flow).

Also, be sure that the removable panels on the unit are reinserted into their frames securely. Some systems have a safety switch that will prevent operation of the system if the panels are not properly seated.

Finally, post the date of the filter change near the furnace to help you make the next change on time.