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Guide to Flue Gas Spillage Switches on Gas Fired Equipment
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Here we discuss Flue Gas Spill Switches and we provide a Guide to inspecting Furnace or Boiler Flue Gas Spill Switches on gas fired equipment such as heating boilers, warm air furnaces, water heaters. We describe the Purpose, Inspection, Repair Troubleshooting Guide for flue gas spill switches which are installed at dampers or burners on gas fired equipment. This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs.

We describe how to inspect residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects. We include product safety recall and other heating system hazards. Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks. We continue to add to and update this text as new details are provided.

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Guide to Furnace or Boiler Flue Gas Spill Switches on gas fired equipment - Purpose, Inspection, Repair

What is a furnace, boiler, or water heater flue gas spill switch? A spill switch may be found at the draft hood on any modern gas fired appliance, such as a heating furnace (hot air heat), a heating boiler (hot water heat or steam heat), or a water heater. This little sensor, or two or more of them, form an important safety device that feels the heat of escaping combustion gases that ought to be going up the flue or chimney. Since escaping combustion gases in a building are dangerous (forming a potentially fatal carbon monoxide hazard), if the sensor gets hot from flue gases flowing past its surface, it is designed to turn off the fuel supply to the gas burner.

Where are combustion gas spillage switches installed?

Combustion gas or flue gas spill switches are usually installed at the edge of the gas fired appliance draft hood. Some appliances may also have a spill switch installed at the gas burner opening itself. This photo shows a spill switch at a gas fired water heater draft hood. In the somewhat blurry photo of dog hair blocking a heater draft hood (above) you can also make out the spill switch and its wire at the right edge of the draft hood in that photo. In the photo at left, a spill switch was not installed but had been simply left loose, disconnected, atop the water heater.

What goes wrong with flue gas spill switches

Missing flue gas spill switches: we've seen these switches removed from modern water heaters, gas boilers, or gas furnaces when they were originally installed. If you see holes drilled into the edge of a draft hood or other marks indicating that a device has been removed, or if you see the devices themselves lying loose, perhaps on or near the equipment, an expert service technician should examine the heater promptly as it may be unsafe.

Misplaced flue spillage switches: the spill switch needs to be installed in the proper location so that if a chimney blockage or some other operating problem causes combustion gases to spill out of the appliance into the building, the flow of flue gases, while still warm, will pass over the switch sensor. (There may be other flue spillage switches which sense carbon monoxide (CO) or other gases directly and without depending on the gas temperature.

Older heaters with no flue spill switch: On older heating systems these safety switches may not be installed at all. A spill switch or a set of them can be added to almost any gas fired appliance, but it is likely that the gas control valve/regulator will need to be replaced too, since the old regulator may not have a point to which the spill switch's sensor wire can be connected to tell the valve to close.

Flue gas spill switch operating failure: While a spill switch could simply fail to sense passing hot gases and thus not perform its safety function of turning off the heater, or while such as switch might simply fail internally, forcing the heater to turn off when it should not, in our experience these are rare events. We do not have at hand industry failure rates for this device but we suspect that installation errors or omissions are far more common.

Some Spill Switches on Gas Equipment Include a Reset Switch or Button

Flue gas spill switches normally connect to the gas valve on gas fired appliances and the switch will shut the valve after sensing flue gas spillage such as that which could occur if the flue becomes blocked.

Where are all the heating system reset buttons? If you are looking for the main reset button on heating equipment you'll want to see: Aquastat Functions and Cad Cell Relay Switch Flame Sensors (hot water boilers and some water heaters), Stack Relay Switch on older oil fired boilers and furnaces, SPILL SWITCHES (gas fired equipment), and also Low Water Cutoff Controls on steam heating systems. At ELECTRIC MOTOR OVERLOAD RESET SWITCH we discuss the thermal overload switch and reset button that is found on many electric motors including those operating air conditioning fans, heating system oil burners, and furnace blowers and motors.

What causes unsafe flue gas spillage:

• Initial combustion of a gas fired appliance can cause brief flue gas spillage at the draft hood because the appliance may need time to warm up and start a good draft a cold chimney into which it vents. This is normal and the spillage should stop in less than three minutes as the flue and chimney are warmed. Flue gas spill sensor switches are designed to avoid false-tripping due to this condition.
• Improper chimney installation: such as venting a small BTUH appliance (a water heater) into a large masonry flue. Especially in cold weather the appliance may never develop adequate draft. We see this occurring when a building converts a heating boiler or funace to a direct-vent system, no longer venting into the chimney, but where the water heater is left trying to vent into the old chimney flue.
• Inadequate combustion air supply to the heating appliance - located in a room too small with no outside air supply; located in a too-small utility room with a solid door that when closed, blocks air;
• Building depressurization - turning on fans in the building depressurizes the utility room or area around the heating appliance, overcoming the natural draft in the appliance's chimney. This condition might occur also with power-vented equipment in some conditions.
• Other reasons: Contact Us with suggestions. Other reasons that a gas flame may be lost or a gas fired appliance shut off on safety may have nothing to do with bad flame or bad combustion air. For example a common part failure on gas fired heating equipment is the thermocouple that senses the pilot flame. If we can't keep a flame lit we suspect the thermocouple first.

Bachrach Corporation, a manufacturer of heating system test equipment opines that gas fired equipment is more likely to have flue gas spillage from a blocked chimney than from buildnig depressurization due to inadequate combustion air supply. We're not sure what data supports that view.

Certainly home inspectors find many heating appliances installed in tiny closets with no outside combustion air and a door that, when shut, blocks off air to the appliance. We have also observed that gas fired heating equipment operated just fine in a building until a new owner installed a whole house ventilation fan system.

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• Field Controls provides instructions for the installation of LP and Natural Gas spill sensor switches, for example for their Gas Spillage Sensing Kit Model GSK-3, GSK-4, GSK-250M switches. Contact your heating service technician directly, or contact Field controls at fieldcontrols.com for more information. These switch models include a manual reset switch. Field Controls, Kingston NC 28504 - Tel 252-522-3031.
• Tjernlund Products provides instructions for the installation and use of their controls, including the WHKE Millivolt Interlock Kit for use with their UC1 Universal Control, MAC1E or MAC4E auxiliary controls for gas fired equipment. Contact Tjernlund Products at tjernlund.com or at 800-255-4208.

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• Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
• Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
• Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
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• Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
• Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
• Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
• National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
• The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
• Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
• The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
• Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
• "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
• Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
• Installation Guide for Residential Hydronic Heating Systems
• Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
• The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)

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