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Friday, October 30, 2009

Furnace Cycling

Last weekend we had to use our furnace overnight. We noticed that while it ran and heated well, after a few hours, it would go off and then on again; over and over until I had to manually turn it off. Later it would run fine. It did this both nights that we had to use it. Is there an easy answer to this?
Kerry, (St. Petersburg, FL)


Kerry, it's quite possible the furnace was cycling on limit. What that means is that the limit switch, a safety device internal to the furnace housing, was sensing a higher than normal temperature and was shutting down the furnace. The question is what caused the high limit situation? Especially if it has worked fine prior to this excursion. The most common reason for creating higher than normal internal temperatures is by closing off too many floor registers or by collapsing or kinking a delivery duct. After the furnace cools, the limit switch snaps closed and the furnace starts back up; until the high temperature limit is exceeded once again. And the cycle continues.

Open up all the air ducts in the coach and look for the possibility of a kinked, collapsed or otherwise blocked duct. It happens easily if an exposed duct is installed in or near a storage area where you may have recently placed goods or gear. Additionally, if all the ducts are clean and free of restrictions, possibly the limit switch itself can be the culprit. A limit switch can be checked with an Ohmmeter. It will have but two terminals and is a normally closed switch. The limit switch is actually a bi-metal disc that can become fatigued over time and simply may have lost its integrity. A new one should be readily available at any well-stocked RV appliance repair facility.

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