LARGER VIEW of a flue gas spill switchFlue Gas Spillage Switches on Gas or Oil Fired Equipment
Spill switch choices, installation, inspection, repair

  • SPILL SWITCH, FLUE GAS DETECTOR - CONTENTS: Flue Gas Spill Switches: Guide to Furnace or Boiler Flue Gas Spill Switches on gas fired boilers, furnaces, water heaters - Purpose, Inspection, Repair Troubleshooting Guide. Guide to Using Flue Gas Spill Switches for detecting carbon monoxide leaks at gas fired equipment. Troubleshooting heating system & hot water heater equipment safety controls
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Guide to flue gas spill switches on gas or oil fired heating equipment:

What is a flue gas spill switch or gas spillage detector? How do flue gas or draft hood spill switches work? Where should spill switches be installed? How are spill switches tested?

What causes nuisance or legitimate flue gas spill switches to trip? How do we correct un-wanted loss of heat due to flue gas spill detector switch operation or nuisance tripping?

Here we explain the installation, function, & troubleshooting 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.

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

Photograph of a flue gas spill switch sensor

[Click to enlarge any image]

Article Contents:

What is a furnace, boiler, or water heater flue gas spill switch?

This article discusses flue gas spillage detectors used on gas or oil fired heating equipment found in buildings.

Spillage detector switches monitor temperature or actual gas content of flue gases that might spill from the vent or draft hood or burner area of a gas fired heating appliance. When unsafe spillage occurs the switch operates to turn off the heating appliance using one of several methods. Most flue gas spillage or thermal safety switches must be re-set manually though a few models include an automatic reset feature.

Synonyms for flue gas spill switch include thermal sensor switch, thermal safety switch, and blocked vent safety switch.

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. Some flue gas spillage detectors operate to close the gas supply to the heating appliance.

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. Similar gas detection or safety device are also used on gas log fireplaces and on coal fired heating systems as well as in industrial applications.

Some flue gas spill detectors actively detect flue gases or carbon monoxide but the simplest models respond to temperature by detecting the hot gases associated with flue gases on gas fired heating equipment. This little flue gas spillage sensor 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.

What causes unsafe flue gas spillage:

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 building depressurization due to inadequate combustion air supply. We're not sure what data supports that view but more details are at REFERENCES where the effects on flue gas spillage due to building depressurization are discussed. One author warns that even where CO was not detected in some building depressurization studies other harmful flue gases such as NOX may be present.

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.

Where are combustion gas spillage switches installed?

Photograph of a flue gas spill switch sensor

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.

Watch out: as we explain in more detail just below at FLUE GAS SPILL DETECTOR MALFUNCTIONS, an improperly installed or improperly-located spillage switch detector can cause either nuisance tripping (spill switch shuts off heat when it should not) or unsafe operation (spill switch fails to detect dangerous flue gas spillage).

Watch out: also to be sure to select the proper spillage switch for the type of heater you are protecting. For example, Tjernlund's spillage switch instructions include additional details for spillage switch use on water heaters and on furnaces and boilers, from which we excerpt:

Gas fired water heaters: Millivolt water heaters require the Tjernlund Model JA-1 (P/N 950-0470), Thermocouple Junction Adapter (ordered separately). Connect ends of six foot cable to spade connections on Thermocouple Adapter.

Gas fired boilers & furnaces: Splice ends of six foot cable in a series circuit between the thermostat and gas valve or burner
control. Using cable routing tabs from wire routing packet, attach 6’ cable to appliance maintaining a
safe distance from hot surfaces, e.g. appliance vent pipe and hot water pipes.
- Tjernlund's, retrieved 12/10/1997, original source cited in detail at FLUE GAS SPILL SWITCH SOURCES & SELECTION below.

Oil fired boilers & furnaces: select the proper thermal switch for the heating equipment you are protecting, then see the instructions for the specific switch;

What goes wrong with flue gas spillage detection 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.

Improperly-installed flue gas spill switch: if the spillage switch sensing area is in contact with metal surfaces such as the flue or the draft hood, the heat of that metal may cause the switch to malfunction. The flue spillage switch sensor should be mounted so that it monitors the temperature of gases spilling out of the heating appliance such as at the draft hood, vent, or chimney or on some systems near the gas burners.

Misplaced flue spillage switches: similarly to the improperly installed spill switches whose temperature sensor should not touch metal surfaces of the heating system, 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 or OIL Fired Equipment Include a Reset Switch or Button

Field GSK Gas Spill Switch (C) Field ControlsFlue 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.

Many flue gas spill sensor switches, such as the Field Controls GSK-3, GSK-4, GSK-250M switches (which operate based on sensing temperatures of 180, 200, or 250 °F respectively) include a manual reset switch.

The manual reset switch is needed because a gas appliance pilot light can turn off for more reasons than a blocked flue or chimney problem that is resulting in dangerous flue gas spillage.

SAFETY WARNING: If your gas fired equipment has shut down in SAFETY OFF position it may be due to a re-settable flue gas spill sensor switch that has detected unsafe flue gas spillage. Flue gas spillage can indicate a dangerous blocked chimney or other unsafe conditions and may also risk potentially fatal carbon monoxide gas spillage as well as other potentially harmful flue gases in the building.

Check with your heating service company - you might think you can avoid a costly heating service call, BUT BEWARE: because flue gas spillage is very dangerous, including the production of potentially fatal carbon monoxide gas, don't simply reset the system without finding out what caused the problem in the first place.

Tjernlund gas spill switch reset button (C) Tjernlund

Here is a sketch of the Tjernlund Products Inc. Gas Spill Switch which also includes a manual reset button.

Tjernlund's sketch (left) shows the reset button right on the gas spill switch.

MORE SAFETY WARNINGS: in addition to our safety warning above, Tjernlund explains that flue gas safety switches are intended to alert the building occupants to a potentially dangerous condition.

But flue gas spillage safety switches are not a substitute for a regular chimney safety inspection nor do they replace regular heating appliance inspection and maintenance by a trained technician.

Those steps must be taken as well.

Readers should also see COMBUSTION AIR DEFECTS where we explain the causes and remedies for inadequate combustion air.

Flue Gas Spill Detector Switch Brands, Choices & Selection Guides

Field GSK-3 spillage detector switch installation locaiton - Field Controls &

Flue Gas Spillage Switch or Thermal Switches Available for Oil Fired Heating Equipment

Reader Question: Can and or should spill switches be installed on a oil fired furnace barometirc damper?

12 Sept 2015 said:
Can and or should spill switches be installed on a oil fired furnace barometirc damper?
This question was posted originally at OIL BURNER INSPECTION & REPAIR

Reply: yes thermal switches can be selected to protect various types of oil fired heating equipment including furnaces, boilers, steam boilers, and water heaters

Yes, there are flue gas spillage switches designed for the (usually higher) temperature range produced by oil fired heating equipment and that can be mounted at a barometric damper on an oil fired heating flue. Most thermal safety switches operate by being wired in series with the electrical power circuit supplying the burner (gas or oil). When the thermal safety switch has been exposed to a specific temperature for a specific time the switch "opens" to turn off the burner.

The spill switches discussed at the article series beginning at SPILL SWITCH, FLUE GAS DETECTOR discuss both oil and gas fired heating equipment flue gas spillage devices.

Watch out: Because most flue spillage sensors detect heat rather than the specific gases that might be emitted from the heating equipment the safety switch must be matched to

Field Controls, for example, offers such devices. The term usually used is "thermal switch" or "thermal switches for oil fired heating equipment".

These flue gas spillage devices sense heat, must be selected for the proper fuel and application (or else your system would be unsafe as it'd not be properly protected), and are sold in both manual reset and automatic reset spill switch sensor switch forms. Flue gas spillage or thermal switches also vary by their response time. You may have trouble with a too-fast-acting switch if in normal operation there is a brief burst of heat from a barometric damper - as happens on some oil fired heating equipment.

Field Controls Thermal switch models include FTS-4, FTS-6, GSK250-M (for steam boilers), GSJ0160-M for draft-induced furnaces, and about eight other models. You'll see that it is essential to choose the proper thermals sensor switch.

Contact Field Controls at Field Controls
2630 Airport Road
Kinston, NC 28504
Phone: 252.522.3031
Fax: 252.522.0214
Thermal safety switch specifications source: retrieved 12 Sept 2015


Test the Flue Gas Spillage Switch Detector or Die Inadequate Combustion Air

This topic has moved to a separate article at SPILL SWITCH & COMBUSTION AIR TESTS

More about backdrafting heating equipment, backdrafting causes, hazards, and remedies can be read at

Reader Question: spill switch tripped and we have no hot water

(Nov 14, 2012) B said:
I have recently had two GSK-3 Thermal Safety Devices attached to my Rheem Guardian Fury hot water heater. I have not had hot water since the installation. The HVAC company is unable to get to my home for some time. They are not from my gas provider. How do the devices work? The little tab is able to be pushed in. It sounds as if that should not occur. Any info is appreciated. I have found your other info very helpful. Thanks

This question was originally posted at COMBUSTION AIR REQUIREMENTS



The flue gas spillage switch you cite includes a manually-resettable feature that is probably the "little tab" that you describe. When the switch has cooled the button or "tab" is pressed in to re-set the switch.

Watch out: because flue gas spillage can kill building occupants due fatal carbon monoxide gas, if a heating appliance has shut down for any reason during normal operation (as opposed to during testing) the cause of the failure must be investigated and corrected before resetting the safety-switch and turning the heating equipment back on.

Here are more details:

The Field Controls 46086400 GSK3 Gas Spillage Switch is a detector mounted on the draft hood of a gas or oil fired heating appliance such as a boiler or furnace and used to detect flue gas spillage. The switch is wired in series with the burner so that in the event that it detects flue gas spillage it will turn off the burner.

This gas spillage switch is actually a thermal sensor - it is detecting heat, not flue gases. Field Controls notes:

Installation of a spillage SAFETY Switch is recommended for LP and Natural gas fired systems with draft hoods, draft diverter, or gas barometric draft controls.

This device is installed to detect flue gas spillage caused by a blocked flue system and/or inadequate draft. This device MUST be installed by a qualified installer in accordance with the manufacturer's installation instructions. Wiring MUST be in accordance with the National Electrical Code and applicable local codes.

Before and after adding this safety control on existing appliances, an installation inspection in accordance with the National Fuel Gas Code 54, Z223.1 Appendix H should be performed and a combustion analysis is recommended to determine the operating condition of the appliance.

CAUTION: Disconnect Electrical Power When Wiring Spillage Switch

This thermal switch can be re-set after it has tripped. The reset may be accomplished (on some switches) by pressing down a popped-up "button" component of the swtich. You have to wait 2-3 minutes for the switch to cool down, then re-set it.

Contact the manufacturer of the Field Controls GSK-3 / GSK-4, GSK-250M Flue Gas Spillage Sensing Kit at the address below, or click the link we provide to see the manual for this control.

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

This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs.

We describe how to inspect, troubleshoot and repair heating and air conditioning 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.


Continue reading at SPILL SWITCH & COMBUSTION AIR TESTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.





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