When Vitas Gerulaitis, the tennis star, died of carbon-monoxide poisoning in September, the gas was described as having come from a faulty heating unit. After hearing the news, many homeowners became concerned about possible threats in their homes.
Periodic furnace inspections can help keep the unit running properly, but they may not eliminate the hazards.The furnace, water heater and fireplace all need oxygen to support combustion. They draw it from the air in the house. The air-fuel mixture is burned and converted into heat and noxious gases. Carbon monoxide is one of those. These gases are drawn out through the chimney. As the air is burned and expelled, new air has to be introduced to replace it. The new air enters through cracks and gaps around doors, windows and joints.
Older houses usually have more cracks and gaps - and also more drafts - than newer, "tight" houses. In some of the older houses all the air might be exhausted and replaced six times an hour. The air is fresh, but also cold, and it has to be heated six times an hour, a problem that can result in high heating bills.
After the energy crises of the 1970s homeowners sought ways to cut fuel and heating costs. Architects and builders responded by designing and building tighter houses. As a result, the air in those newer, tighter houses is replaced two times an hour. Less fresh air enters the house. Usually that would be ample for the needs of the inhabitants and the furnace.
But most modern houses also have appliances that "suck" air. Clothes dryers, range hoods, central vacuums and exhaust fans in attics, bathrooms, basements and workshops all have powerful electric fans that pull air from the house and force it outside.
Those units can seriously upset the air balance in a house. A modern stove with a top grill has a fan that vents downward; it can draw all the air from an average-size house in less than 25 minutes.
With some or all of these fans running simultaneously the air is being pulled out faster than it can enter. That is because modern houses do not have intake fans to replace exhausted air. Replacement air has to come from someplace, but where? Like Santa Claus, it comes down the chimney and into the house. That also carries the noxious combustion gases from the furnace or fireplace with it. This reverse airflow is called backdrafting.
Not all houses have back-drafting problems. But because of the potential hazards it is important to conduct tests that are fortunately neither difficult nor expensive.
First, close all the doors and windows. Make sure that the fireplace damper is completely closed. Next, turn on any exhaust fans and also the clothes dryer. Start the furnace - by turning up the thermostat - and turn on the hot-water heater. Opening a hot-water faucet should start the heater.
Go to the furnace and wait a minute for the flue, the large pipe emerging from the top of the furnace, to heat up. The heated flue should draw all the combustion gases up the chimney.
To make the test hold a lighted match or a smoking incense stick near the draft diverter - the funnel-like collar around the flue pipe. On some furnaces the draft diverter may be a rectangular grill on the front panel. Check the hot-water heater the same way.
If the smoke from the match is drawn into the diverter, then the draft is functioning properly. If the smoke is blown away from the diverter, there is a backdraft. You should repeat that test every time while making home improvements. Adding insulation, recaulking, changing windows or installing an exhaust fan can all affect air flow.
For a more permanent solution consult a professional, who may suggest installing an "induced draft" unit in the flue. That is essentially an electric fan that pulls the combustion gases from the furnace, forcing them outside through the chimney.
Although that is an improvement, it may not be sufficient because the furnace still has to draw air for combustion from the house. A more effective solution involves installing a duct that supplies fresh air from outside directly to the furnace.
Another duct is installed to expel the combustion gases. That system is called a closed or sealed combustion unit. Many building codes now require them in new houses.
A house might also need additional ducts to bring fresh air into the rest of the house. They should be equipped with heat exchangers to preheat the incoming air. Heat exchangers operate with ambient house heat and do not add to the cost of home heating.
Even new houses with top-of-the-line heating units can experience malfunctions. That is why it is important to check for back-drafting and to install carbon-monoxide detectors.