Hot attic cause furnace blower to activate

Posted1/19/2020 6:00 AM

Q: Last year we replaced our rock roof with asphalt shingles, and the strangest thing has been happening ever since. Whenever the weather is hot, 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, and the roofing contractor says it's not his fault. How can we solve this annoying problem?

A: The new shingles on your roof may be absorbing more solar heat than the old roofing. If so, additional attic vents are probably needed.


Insufficient venting can cause an attic to overheat during hot summer months, and this can activate the temperature limit switch in your forced air furnace, causing the blower to turn on. To eliminate this problem, there are two conditions that need to be addressed. First, have a heating contractor check the temperature limit switch in the furnace to be sure it is properly adjusted. The switch may be set at a lower-than-normal temperature, or it may simply be defective and in need of replacement.

Next, place a thermometer in your attic on a hot day to determine the level of heat gain. If the temperature is nearing 150 degrees, that would account for the unexpected activation of the furnace blower.

The most effective ways to vent an attic, without installing mechanical fans, are ridge vents and turbine vents. A roofing contractor or general building contractor can advise you regarding the best approach, given the design of your roof and attic.

Q: My home has a masonry fireplace that was converted for use with gas logs instead of wood. Now that I'm selling the property, the buyers' home inspector advised removing the damper before we close escrow. I explained that this would be impractical because an open chimney flue would allow warm air to escape from the home during winter months. The inspector agreed but insisted that dampers are unsafe when fireplaces have gas-log burners. This seems like a conflict of priorities. Is the home inspector right about removing the damper?

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A: The home inspector is right about the damper. Safety requirements for fireplace dampers were upgraded in 1991. According to these standards, dampers must either be removed or permanently secured in the open position when gas logs are installed. The purpose for this requirement is to prevent noxious combustion gases from venting into the living area. If the gas burner is ignited when the damper is closed, combustion exhaust, including carbon monoxide, could vent into the home.

This is particularly hazardous with gas burners that can be turned on by means of a switch, because someone could ignite the burner without considering whether the damper is open or closed.

Fortunately, removing the damper is not necessary for compliance. Instead, the damper can be disabled by installing a special clamp, available wherever fireplace equipment is sold. If heat loss from the dwelling is a concern, the buyers can install glass doors on the front of the fireplace after they take possession of the home.

• To write to Barry Stone, visit him on the web at, or write AMG, 1776 Jami Lee Court, Suite 218, San Luis Obispo, CA 94301.

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