How to Troubleshoot a Duo Therm Furnace


In the 1990s, Duo-Therm was one of the biggest manufacturers of furnaces for recreational vehicles. Today, many older recreational vehicles still use a Duo-Therm furnace to heat their interior. However, these heaters can sometimes malfunction. Common problems can be encountered in areas such as the furnace's air filter and intake valves. Learn how to troubleshoot your Duo-Therm device so you can enjoy your RV even in the coldest of winters.

Things You'll Need

  • Duo-Therm 66000-series furnace heater
  • Damp rag
  • Hand-held vacuum cleaner
  • Make sure you the furnace is set up properly if you cannot turn it on. Turn the furnace's thermostat to the "OFF" position, found on the top of the furnace box. Slide out the heater's front grill and locate the gas controls, found on the inside left edge of the furnace. Turn the knob to the "OFF" position and wait for 10 minutes before turning the knob back on. This helps restart the internal gas flow. Slide the furnace's front grill back in and turn the thermostat back to your desired temperature. The internal ignition should ignite the gas flow and the furnace should start. Due to the Duo-Therm furnace's old age, you may need to perform this step multiple times to get the gas flowing through the ignition chamber.

  • Vacuum the combustion chamber, found on the back of the inside of the Duo-Therm furnace, if you continue to experience problems with igniting the gas flow. Over time, a buildup of soot and ash may have formed in the combustion chamber. This limits its capacity to burn at a high rate. Vacuuming once a month is typically sufficient to keep the combustion chamber burning efficiently.

  • Wipe down the Duo-Therm furnace's air filter, found on the back of the outside of the furnace, if you find the furnace's output unsatisfactory. Dust, debris and other matter can clog the filter and limit its hot air output. Use a damp rag and wipe thoroughly. Exercise caution if cleaning the furnace shortly after it was turned off, as its metal surface may be hot.

  • Clean the external intake valve, found on the outside of your recreational vehicle, if you notice that the furnace's air output is extremely low or if the furnace shuts down unexpectedly. The furnace sucks fresh air from the outside to both heat and distribute, as well as to provide oxygen to its combustion chamber. A clogged intake reduces both, which may cause the combustion chamber to sputter out. Clean the valve with either a vacuum cleaner or a rag.

Tips & Warnings

  • Never open the Duo-Therm furnace while it is running, as this increases the risk of burns and an open fire in your vehicle.


  • Photo Credit Neil Gould:
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