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Barry Stone, New roof affects forced-air system

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DEAR BARRY: Last year we replaced our tar-and-gravel roof with asphalt shingles, and the strangest thing happened after that. Whenever we have hot weather, the forced-air furnace in our attic turns on. The only way we can get it to stop running is to shut off the circuit breaker in the electrical panel. Our home inspector found nothing wrong with the furnace, so we’re perplexed. How can we solve this annoying problem?

— Doug

DEAR DOUG: When the new shingle roofing was installed, changes may have been made that are causing your attic to become hotter than before. If the new roofing is a darker color than the old roofing, there could be increased heat absorption from the sun. It is also possible that the roofing contractor blocked or eliminated some of the attic vents.

Whatever the cause, excessive heat in the attic could be affecting the temperature limit switch in your forced-air furnace. The purpose of the temperature limit switch is to turn the blower on or off when the interior of the furnace is above or below 150 F. Excessive heat in the attic could be activating this switch.

It is also possible that the switch is faulty or out of adjustment. This means that it could be turning on when the attic is at temperatures below 150 F. If so, the switch may need adjustment or replacement. This should be determined by a licensed HVAC contractor. By the way, since you are now on your second roof, this may be a very old furnace. So be sure to have the entire system evaluated for function and safety.

If overheating of the attic is the problem, increased ventilation is advised. There are several good ways to vent an attic, but the most effective means is a combination of eave and ridge vents. A competent roofing contractor can advise you accordingly.

DEAR BARRY: Our home has a 20-year-old furnace that works fine in every way except one. When the blower is running, the rush of air at the intake grill is terribly loud. According to our home inspector, the blower velocity can be changed from high to low speed by making some adjustments, but we’re wondering if there are any negative side effects to this solution.

— Ron

DEAR RON: The high-speed setting on a furnace blower is generally intended for air-conditioning purposes. When the system is operating in heat mode, slower air circulation is recommended. Therefore, it is unlikely that you will experience any adverse consequences if you reduce the blower speed of your forced-air system.

If you check the manufacturer's rating plate on your furnace, you will observe a listing of temperature ranges for each speed setting. For low blower speeds, the range is usually between 40 and 70 F. This means that the temperature of the heated air blowing from the room registers should be 40 to 70 F hotter than the air that is drawn through the intake grill. This is something you can check with a thermometer.

To be sure that this is done correctly, have the system checked by a licensed HVAC contractor before proceeding with any changes.

ACTION COAST PUBLISHING

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