How to Troubleshoot a Gravity Furnace


A gravity furnace is typically a wall model with a direct venting system. These furnaces are meant to warm individual rooms or spaces, not an entire home. Since the gravity furnace is gas heated and the furnace surface may warm to very hot levels, children and pets should always be supervised when the furnace is in use. Only adults should operate or troubleshoot the furnace if problems occur.

Things You'll Need

  • Can of compressed air
  • Clean cloths
  • Turn on the gas supply valve, counterclockwise, if the gravity furnace is not heating. Rotate the gas control dial past “Pilot” to “On” and set the furnace thermostat to the desired temperature.

  • Bleed air out of the gas line if the pilot light isn't lit but the gas supply valve is on. Turn the brass screw, called the minimum rate screw, located slightly above and to the right of the gas control knob. Unscrew the minimum rate screw and remove the screw completely from the furnace using a flat-head screwdriver. Adjust the heater control knob to “Pilot Position” and push down on the knob until the scent of gas exits the screw hole. Immediately replace the minimum rate screw once you smell gas, then turn the screw clockwise until it's firmly closed. Wait at least five minutes before relighting the furnace pilot light.

  • Shut off the furnace and the gas supply valve if the pilot light still doesn't ignite after bleeding the furnace. Open the main access panel and use a can of compressed air to blow air through the pilot orifice to remove dirt and debris that may be obstructing the pilot light. Close the access panel and relight the pilot. Increase the thermostat setting if the pilot light will ignite but doesn't stay lit.

  • Light the pilot light after troubleshooting or a power failure by turning the furnace thermostat down to the lowest setting, then pushing and turning the gas control knob to “Pilot” and pressing the red button next to the control knob. Continue to hold the gas control knob in for 30 seconds after the pilot light is lit, then release the knob, turn it to “On” and set the thermostat.

  • Examine the area near the thermostat if the thermostat setting doesn't match the furnace's heat output. Look for direct sources of heat or drafts, such as open windows or doors, lamps or televisions, and move or resolve the extra temperature source if possible. Note that the furnace may not be large enough for the desired heating space if the thermostat and furnace both operate correctly but the furnace doesn't generate the desired amount of heat.

  • Locate the outer vent cap on the gravity furnace and clean the cap if the pilot light ignites but doesn't stay lit even after increasing the thermostat temperature. Don't block the furnace vents, as this obstructs air flow and proper function. Clean the air inlet tube if it's obstructed as the same problem may result if the air inlet is blocked.

  • Clean the thermocouple or the connections at the gas valve, then tighten the thermocouple if the pilot light doesn't ignite or doesn't stay lit.

Tips & Warnings

  • Contact the furnace manufacturer or another qualified furnace service professional for an annual inspection, a cleaning and any necessary repairs at least once a year.
  • If you detect a gas scent at any time other than during the bleeding process, shut off the furnace and leave the home immediately prior to calling local emergency services or your gas company.

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