Gas furnace owners: I want you to go find your furnace and check a few things out. First, find the flue pipe that comes out of the furnace. The flue is the furnace’s “exhaust pipe.” It carries the hotand possibly toxiccombustion products from the furnace to the outside of the house.
If you’ve got a modern, high-efficiency furnace, the flue might be white plastic pipe, just like your plumbing drains. If you’ve got that kind of flue, the rest of this article doesn’t apply to you. But wait: If you’ve got a gas water heater, read on. The problems we’re going to address here apply to gas furnaces and to gas water heaters.
If you’ve got a metal flue, you’ll recognize it as the gray pipe that’s about 4 inches across; it’s also hot to the touch when the furnace is running.
Eyeball the whole flue pipe, from the point where it leaves the furnace or water heater to the point where it goes out of your house. Make sure there’s at least a 1-inch gap between the flue and anything that will burn. (Inside tip: That silvery insulation on the ductwork is not metal. It’s paper, and it’ll burn like a Pinto.) I’ve actually had contractors and homeowners argue with me about this. Apparently, a whole lot of people believe that all silver things are fireproof, just like a lot of people think that all gold things are valuable.
If any part of the flue is less than 1 inch from anything flammable, you’ve got a fire hazard. I know this because the flue pipe that’s used for most gas flues has these words embossed right on it: “One inch minimum clearance to combustible materials.” These words have been there about as long as first base has been 90 feet from home plate. Even so, people tell me every day that this is news to them. And I believe ’em, because I find a flue pipe stuck up against paper, wood, or ceiling board just about every day. (Inside tip No. 2: Wall and ceiling board have paper facing on both sides, so they’re flammable. Don’t let anybody tell you different.)
Every day, I have to tell people that they need to do as much cutting, snipping, and tweaking as is necessary to get 1-inch clearance around their gas flue. And every day, somebody asks me how they ought to go about doing it.
Well, it’s a simple job in concept, but it’s devilishly hard in practice. Cutting holes in finished floors, ceilings, and roofs is tough. Sometimes, there’s no practical way to make the holes bigger. You just have to install a new flue, running it along a new route that’s not surrounded by flammable things.
If you live in an old house, you might have anotherand more dangerousflue problem. Until recent years, it was common practice to run furnace and water-heater flues to an old brick chimney. Some buckethead contractors still do it, contrary to manufacturers’ specifications and common sense.
Often, these old chimneys have no lining and no cap. Over the years, the chimney can fill up with chimney-swift nests, starling and squirrel corpses, leaves, twigs, and berries. These things can block the flow of combustion gases. If this happens, your house can fill up with toxic combustion products like carbon monoxide, which can cause you to wake up dead.
Another other problem with furnace flues connected to old brick chimneys: When you replace an old, low-efficiency furnace with a new, higher-efficiency model, the flue gases will be cooler and might not make it out of the chimney. Once or twice a year, I’ll hear from somebody who has an unexplained water leak around a chimney. Usually, they’ve had some poor roofer looking for roof leaks, and they don’t believe him when he says there’s nothing wrong with the roof. I end up proving the roofer right by pointing out that the problem is water vapor (a combustion by-product, y’know) condensing inside the chimney.
If your flue runs up an old brick chimney, hire a heat-and-air contractor (not a plumber, even if it is water-heater flue) to make sure the chimney’s clean, and check the flue draft. If there’s no cap on the chimney, put one on, to keep the flora and fauna out. If you want the job done right, get the flue lined, all the way to the top.
Visit Walter’s Web site at http://www.nashscene.com/~housesense/. Or e-mail him at firstname.lastname@example.org