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January seems like a good time to talk about fireplaces. You probably used yours around the holidays. Any problems? In home inspections, I find quite a few fireplaces that need maintenance or modifications.

First, let me get my prejudice out in the open. I dislike fireplaces on principle. Burning wood inside a building is a dirty, dangerous, inefficient way to generate heat. Yes, it's pretty, but I just can't feel romantic about something that's cranking out hundreds of times more air pollution than my van. Now on with the newsletter.

A few definitions are in order, mainly because I hear lots of people misusing the terminology and confusing each other. A FLUE is a hollow passage up through the middle of a chimney. It's not a piece of hardware, it's an empty space. The DAMPER is a door-like fitting, usually just above the fireplace, used to stop airflow up the flue. You must open it before starting the fire, and close it later to minimize energy loss. The LINER is the inside surface of the flue. Beginning in the 1940's, it became standard to line brick chimneys with fired clay tubes. These days, metal liners are often used because they are easier to install and have higher temperature tolerance. The FIREBOX is the lower part of the fireplace, where you pile up the logs. The SMOKE CHAMBER is the space above the damper, before the beginning of the flue liner.

So what happens when you build a fire? The carbon and hydrogen in the wood combine with the oxygen in the air, heat gets released, and the byproducts go up the chimney, along with most of the heat. What's so bad about that? Well, the carbon isn't in a nice clean hydrogen compound like natural gas. So the stuff going up the chimney contains many compounds, some of which are poisonous, (like carbon monoxide), and others that are just messy, (like tars and resins). This sticky stuff is in a vapor state when it leaves the flame, but when it touches the cooler flue liner, it condenses back to goo. Flammable goo. If you only have a few fires per year, it may take many years for this stuff (called creosote when it's in the chimney) to build up. When it gets to be a thick layer, it can start a chimney fire, which can be really spectacular. Imagine a jet engine inside your chimney! If you have an old-fashioned clay tile liner, it will crack when the fire gets it up past 1,700 deg F. (Temperatures over 2,300 are common in chimney fires.) The flames then shoot out through the cracks,and often spread to the rest of the structure. Hmm... flashing lights, sirens, roaring diesels, flames, water, paramedics... feeling romantic yet?

The more softwood and scrap lumber you burn, the faster the creosote will build up. Hardwoods are better in this regard, but I feel strange recommending the burning of such a precious resource. Have you priced hardwood lately?

What else is going on when you have that fire going? Well, what else is being used? Oxygen, lots of it. The draft that carries the smoke up the chimney represents a lot of air from inside your house. Air that you've paid to heat! That air has to be replaced, and usually this is done through leaks around your doors and windows. As your toes are getting toasted, your bedrooms are freezing. In a modern, tight, house there may not be enough leakage to supply the fire, and then the fireplace will back-smoke. The other flue, connected to the furnace and water heater, may also start flowing backward, but you probably won't notice this till your monoxide alarm goes off. The immediate cure for all this is to open a window. The long term fix is to have a combustion air supply duct installed. You may have seen one of these on a newer home. It looks like a small grille on the outdoor surface of the chimney. Air flows in through the grille and directly up into the firebox, feeding the flame without affecting the house air supply. Your bedrooms stay warm! A chimney mason can usually retrofit such a duct in an older home.

As the fire dies, some people want to close the damper so they can go to bed. BAD IDEA! That glowing bed of coals is a mass-production monoxide factory! Don't close that damper until the ashes are COLD.

So what kind of maintenance schedule should you have for your fireplace? It depends a lot on how frequently you use it. A good sweep of the chimney every 100 or so fires is a good idea. That means a professional sweep, preferably by a member of the National Chimney Sweeps Guild. Many of these people now use video cameras, so they can examine every inch of the flue. A hairline crack in a liner is impossible to see without the camera in many cases, but it indicates a chimney fire in the past, and the likelihood of problems in the future.

Other maintenance should include an occasional examination of all visible portions of the brickwork, inside and out. The special light-colored brick in the firebox is laid using a high-temperature clay instead of mortar, and this product wears away over time. Repointing the firebrick may be needed. If you don't feel that handy, ask the sweep for the name of a good chimney mason; not every brickie is competent to deal with fireplaces and chimneys.

If you find a crack where the face brick and firebrick meet, this may indicate a support problem below the hearth. Repoint the crack, and look again in a few weeks or months. If there is a new crack, something is settling or weak. Call a good home inspector. ;-)

In some older homes, the hearth itself is sometimes too small. You need a non-combustible surface extending 16-20 inches in front of the firebox. If this surface has been covered with carpet or whatever, don't use the fireplace.

The upper end of the chimney belongs more with roof maintenance than fireplaces, so it will be covered another day. For now, just be sure you have a screen of some sort up there to keep out the beasties.

The montly nag for January? Hmm, just test those smoke alarms. And think Spring!