Spill Switch Inspection

by Nick Gromicko, CMI®

A spill switch is a device designed to sense and correct unsafe venting of combustion gases from a gas-fired appliance into a building. This “spilling” of carbon monoxide (CO) or extreme heat is detected through a sensor that shuts off gas to the furnace, boiler or water heater before enough CO is emitted to put the building's occupants at risk.

CO spillage may occur for any of the following reasons:

  • insufficient supply of combustion air to the heating appliance, as would be the case if it was located in a small, well-sealed room that lacks an outside air supply;
  • building depressurization, which happens when the chimney’s natural draft is overcome by the action of fireplaces or fans elsewhere in the building;
  • flue or vent blockage by leaves, snow, or bees’ or birds’ nests;
  • improper chimney height;
  • improper chimney installation, such as where a small heating appliance vents into a large flue. The appliance might not develop adequate draft, especially in cold weather; and
  • the heating appliance vents into the room for several minutes after it is turned on while the chimney warms up. This problem typically fixes itself before the spill switch detects any spillage.

Spill Switch Testing

Spill switches should be tested periodically. Remember to provide adequate ventilation while testing, or CO vapors might become overwhelming and cause personal injury. To do this, first turn off the appliance.  Then, remove the vent from the flue and block the vent with sheet metal, plywood, rags or some other material. Turn the appliance on and wait; the switch should trip within 10 minutes, according to the American National Standards Institute, although many manufacturers have shorter time requirements. The switch should be replaced if it doesn’t open within the time specified by the manufacturer. To reset the switch, press the button between the switch’s wires.

Spill Switch Inspection

InterNACHI inspectors can test for the following spill switch defects:

  • The spill switch was removed. Spill switches are sometimes removed by homeowners or technicians who don’t understand the importance of these devices. If a gas-heating appliance isn’t equipped with a spill switch, look for it lying loose on or near the equipment. Also check for holes drilled at the edge of the draft hood, where the switch may have been originally installed.

  • The spill switch was never installed. Spill switches can be installed on older systems that were never equipped with the device. The gas control valve will need to be replaced if it is not compatible with the new spill switch, but this may prove prohibitively expensive.

  • The spill switch is installed in the wrong location. The switch needs to be installed where it will encounter combustion gases, which is typically at the edge of a gas-fired appliance’s draft hood next to the flue pipe as it leaves the unit to enter the chimney or wall. Some appliances may also have a spill switch installed at the gas burner's opening itself.

Other Tips

  • Spill switches can fail when down-drafting cold air mixes with improperly vented combustion gases, effectively diluting and disguising the CO gases from the spill switch’s sensor. To prevent such "false positives" that fail to trip the sensor, CO detectors should be placed near the heating appliance and in other locations throughout the home.

  • Spill switches are no substitute for periodic chimney inspections, which are vital to the safety of a home.
In summary, spill switches are sensors designed to halt the flow of fuel to a heating appliance when CO gases flow in the wrong direction.  Inspectors can teach their clients about the necessity of these safety devices, especially as the seasons change to colder weather and heating appliances are in regular use.