Heater blows hot and cold
Q. Our forced-air furnace is behaving strangely. Instead of heating the house in a normal manner, the gas burners turn on and off repeatedly, while the air continues to blow. Does this sound like a serious problem, and could it be dangerous?
A. The first thing to check is your return air filter. If it is congested with dust, restricted airflow could cause the furnace to overheat. That could explain why the burners turn off, while the blower remains on.
Your furnace operates like an air-cooled engine. Heat is produced by gas burners inside of a metal firebox, known as a heat exchanger. Circulating air flows against the outside of the firebox, performing two simultaneous functions: The air keeps the heat exchanger from overheating, and the heated air then blows into the rooms of your house.
If your air filter is congested with dust, restricted airflow causes the firebox to overheat. This activates the temperature limit switch, which turns off the burners. The blower continues to operate until the firebox is sufficiently cooled, whereupon the burners reignite.
Operating a forced-air furnace with a dusty filter reduces the efficiency of the heating system, which means higher utility bills. Worse yet, overheating the heat exchanger can cause cracks or holes to develop, which permits dangerous exhaust fumes, such as carbon monoxide, to vent into your home.
Your return air filter should be changed at least twice per year, or as often as it appears dusty. Maintaining a clean filter will increase operational efficiency, prolong the life of the system, and enhance its functional safety.
If the problem with your furnace is not caused by a dusty air filter, you could have a faulty temperature limit switch. Either way, the system should be serviced by a licensed HVAC contractor.
Q. The exhaust from my clothes dryer blows directly under my house and is causing a huge buildup of lint. The distance from my dryer to the nearest outside wall is about 22 feet and I've heard that long dryer vents can increase the drying time for clothes. What do you recommend?
A. Venting a clothes dryer into a crawl space is a common defect that is prohibited in the Uniform Mechanical Code. There are two reasons for this prohibition:
• Lint build up in the subfloor area is a fire hazard because the lint is highly flammable.
• Moisture condensation in the crawl space can cause fungus damage to wood framing.
To vent your dryer properly, 4-inch diameter metal ducting should be used. The fittings should be secured with tape, not screws, because lint build up on the ends of the screws can restrict airflow inside the duct. The end of the duct should also have a vent hood to prevent back-drafting.
For most dryers, the length of the duct should not exceed 14 feet. If the duct has more than two 90-degree turns, the length should be decreased by 2 feet for each additional turn. A longer duct will lengthen the drying time. It can also cause your dryer to overheat, which is a fire hazard. In your case, a 14-foot run to the exterior is not possible. However, a longer duct is permissible if an assist blower is installed in the line.
• To write to Barry Stone, visit him on the web at www.housedetective.com, or write AMG, 1776 Jami Lee Court, Suite 218, San Luis Obispo, CA 94301.
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