When the heating season over, homeowners tend to forget about their fireplaces. But that could be a mistake - particularly if you can't remember how or when the chimney was last cleaned.

It's best to clean fireplace chimneys right after the heating season is over, when the unwanted layer of creosote that coats flue linings is relatively soft and easy to remove. Chimney creosote, which is a fire hazard, is produced when acids in the smoke of a wood fire combine with the moisture from the wood, according to the National Fire Protection Association.

Summer heat dries and hardens the creosote. So by next fall, there may be a tenacious crust of creosote in the flue that will be difficult to remove. In addition, the acids in the accumulated creosote may be attacking and weakening chimney mortar. It can also help set your house ablaze.

When creosote builds up to a thickness of one-half inch or more, it's primed for fire. Typically, these flare up when flames from the hearth reach the smoke shelf and touch off creosote chips. The blazing chips, in turn, ignite creosote stuck to the flue and make a roaring torch of your chimney. With luck, chimney fires simply serve to burn off the creosote crust.

But chimney fires are so hot they're capable of burning through mortar to flammable house structures. They send sparks shooting out the chimney that can set fire to the roof. And the heat from a chimney fire can weaken flue linings.

Creosote buildup can also narrow or distort flue openings, causing smoky fires, backdrafts and other problems. Somky, poorly burning fires are prime producers of creosote. So, like Pinocchio's nose, the crust and your fireplace problems keep growing at an accelerating rate if you don't remove the creosote crust at an early stage.

It's prudent to have your fireplace flue cleaned periodically. If you've begun to have problems with a fire place that previously functioned well, investigate for creosote buildup.

You can make a cursory inspection of the flue yourself. OPen the damper wide and, with a flashlight, look over to the smoke shelf just beyond the damper if you can. If you see an accumulation of dark chips on the shelf, chances are it's creosote that has flaked off the flue, an indication that there is a substantial buildup of creosote.

If yours is the kind of damper that blocks your view to the smoke shelf, try reaching up and with your fingers, scraping the side of the flue. Usually, if there's creosote it will come loose.

Although you can do it yourself, few homeowners want to concern themselves with the drudgery, dirt and danger connected with chimney sweeping.

But someone must sweep if the creosote is to be removed. Caked-on creosote can only be cleaned out by scrubbing flue linings with stiff wire brushes the old-fashioned way. When this doesn't work, your chimney sweep should be prepared to carefully chip away the crust with long-handled tools.

Vacuum cleaning equipment, no matter how powerful, won't get rid of any but the loose creosote. Accordingly, when you select a service, be sure they sweep and are prepared to chip if necessary. It's surprising how many chimney cleaning services only vacuum.

It's virtually impossible to keep a fire from producing some creosote. But you can minimeze the problems by burning only dry, well-seasoned hardwoods. Avoid wet wood or softwoods. And keep the fire burning as briskly as you can.