By Barry Stone
Dear Barry,
We are selling our home that has a brand new asphalt shingle roof. We have noticed that 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 at the electric panel. We called a local home inspector who advised us to install an attic fan. We took his advice, but the new fan runs continually during warm weather, and this hasn't stopped the furnace from blowing also. How can we solve this annoying problem before we list the house? -- Doug
Dear Doug,
Your problem may be the temperature limit switch in your forced air furnace. This is a built-in thermostat, which lets your furnace blower know when to turn on. Limit switches in furnaces are typically set to activate at about 150 degrees, and overheating of the attic during hot weather could be causing it to turn on. What is surprising, however, is that the problem has continued after installing a ventilation fan.
There are two conditions which may be causing unwanted operation of your furnace blower: The temperature limit switch may be turning on when temperatures are less than 150 degrees. If that's the case, the switch could need repair or replacement. If, however, the attic is actually heating to 150 degrees, then additional ventilation is needed. Therefore, my advice is two-fold:
First, have your heating contractor check the limit switch to be sure proper adjustment and operability. The switch may be set at a lower-than-normal temperature, or it may simply be defective.
If the limit switch is OK, place a thermometer in your attic on a hot day to determine just how hot your attic is becoming. If the temperature is nearing 150 degrees, then installing a set of turban vents along the roof ridges should adequately lower the ambient attic temperature during hot weather.
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