Photograph of a flue gas spill switch sensorFlue Gas Spillage Switch & Combustion Air / Backdrafting Test Procedures
Manufacturers' recommended test steps for flue spillage sensors & for inadequate combustion air

  • SPILL SWITCH & COMBUSTION AIR TESTS - CONTENTS: Flue Gas Spillage 6-Step Safety Interlock Test and 6-Step Combustion Air Test or Backdrafting Test for Oil or Gas Fired Heating Equipment
  • POST a QUESTION or READ FAQs about how to test flue gas spill sensors & how to test for backdrafting or inadequate combustion air in buildings

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Test procedures for flue gas spill switches and for combustion air adequacy or backdrafting at gas or oil fired heating equipment.

Adapting information from flue gas spillage sensor manufacturers we expand on that advice with practical warnings to give first a Flue Gas Spillage 6-Step Safety Interlock Test and next a 6-Step Combustion Air Test or Backdrafting Test for Oil or Gas Fired Heating Equipment.

Adjustments and opinions are offered about more or less aggressive test procedures and at the article's references section we include authoritative research on flue gas spillage devices, their designs and operations, and on NFPA, ANSI, ASHRAE and other standards for heating appliance combustion air requirements.

In this article series 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 also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Safety Tests for a Flue Gas Spillage Switch Detector & for Inadequate Combustion Air in Buildings

Flue gas spill sensor test procedure adapted from Tjernlund - citation in article (C) InspectApedia

[Click to enlarge any image]

Watch out: the tests described in this section are adapted from advice from equipment manufacturers, may not fit your equipment or building, and in any event are intended for performance by a trained heating or chimney service technician. These are not tests that we recommend for performance by a homeowner nor by a home inspector.

Watch out: messing with or testing this or any safety device may involve serious life safety hazards, and flue gas spill switch test procedures that we have reviewed (mostly the same procedure copied among websites) is dangerous as we will elaborate below. Check with the installation and testing specifications provided by the manufacturer of the particular flue gas spillage safety switch that you have installed.

Flue Gas Spillage 6-Step Safety Interlock Test

The following flue gas spill switch safety interlock test is adapted, illustrated, and expanded (with comments) from a procedure described by Tjernlund and is one we've seen replicated at various websites. This procedure is found in 24SP200 Safety Interlock System (P/N 950-2420) [Installation & Operating Instructions] retrieved 25 Jan 2015, original source given at REFERENCES

  1. Turn on the fuel supply (gas or oil) to the heating appliance
  2. Remove the flue vent connector from the vent breeching at the joint immediately above the flue gas spillage sensor devices. Note that this will normally be the flue vent connector joint immediately above the draft hood on gas fired appliances that use a flue-vent draft hood or on the small draft hood found on gas fired water heaters. This procedure does not apply to gas fired appliances such as some furnaces that incorporate a draft opening into the body of the furnace.
  3. Block the flue vent connector with sheet metal or other non-combustible material.
    We have used a piece of fiber cement shingle siding which had the advantage of a little weight to keep it in place. You do not have to block or seal 100% of the opening, the flat sheet metal or FC siding will close the opening adequately to cause plenty of flue gas spillage.
  4. Set the thermostat to cause the heating appliance burner to ignite. This may mean setting the room thermostat for a furnace or boiler to well above room temperature or for a gas fired water heater, either setting up the thermostat or simply running enough hot water to cause the water heater to fire. As Tjernlund points out,

    Flue gas spillage will emit around the draft hood, draft diverter, or on oil fired heating equipment, around the flapper opening of the draft regulator or barometric damper.

    Successful fuel gas spillage sensor switch test result: in less than two minutes (for most equipment) hot flue gases spilling at the draft hood/diverter will cause the flue gas spill sensor to open, shutting off the heating appliance. Different types of flue gas sensor switches may take different approaches to shutting down the heating appliance such as opening the thermostat circuit (stopping the call for heat) or closing a gas supply valve (stopping fuel supply to the burner).

    Failure of the flue gas spillage sensor test result: if the flue gas sensor switches do not "open" to turn off the heating appliance check for flue vent pipe leaks and seal them if necessary. It should be sufficient to trip the sensor switch if you feel hot flue gases spilling out at the draft hood or draft diverter or barometric damper for two minutes or longer. If after 2 minutes (or in our opinion 2-4 minutes depending on the sensor switch specifications) of continuous burner firing and gas spillage, if the sensor has not opened to shut down the heating appliance the switch or its wiring is defective and the heating system is unsafe.
  5. Re-set the flue gas spillage switch: Following a successful flue gas spillage sensor test, turn off the heating system by setting the thermostat to below room temperature, turning off hot water (for a water heater) etc. Then wait 2-3 minutes to allow the flue gas spill sensor switch to cool down, then pus the spill switch re-set button. The button should "click" in place and should remain depressed. If you press the button too soon and the sensor is still hot the switch may not reset.
  6. Re-connect the flue vent connector to the rest of the venting system and chimney. While you're at it be sure that the connections are properly secured (sheet metal screws may be needed) and are properly connected.
    Watch out: you can be burned touching hot flue venting system components

    When you have restored the system to proper operation don't forget to set your thermostat to its normal position or program.

6-Step Combustion Air Test or Backdrafting Test for Oil or Gas Fired Heating Equipment

Watch out: The following flue gas spill switch safety interlock test is intended to provide a chance of detecting inadequate combustion air supply in buildings that may be affected by ventilation systems, exhaust fans, clothes dryer and similar devices. This is not a comprehensive test for backdrafting hazards as performing the test at any given time cannot possibly represent all building conditions, weather conditions, or building occupancy conditions or activities that can significantly affect the building air pressure and combustion air supply.

This procedure is adapted and expanded (with comments) from a procedure described by Tjernlund and is one we've seen replicated at various websites.

This procedure is found in 24SP200 Safety Interlock System (P/N 950-2420) [Installation & Operating Instructions]

Warning from Tjernlund:

The 24SP200 Flue Gas Spillage Sensing Kit is designed to alert the user to a potentially hazardous condition. It is not designed to and cannot replace regular chimney inspection, appliance servicing, and combustion testing. Do not use the 24SP200 as a substitute for professional appliance maintenance. 

  1. Set up tight or closed building conditions by closing all doors and windows of the building. Close any fireplace dampers in the building. Tjernlund also advises that if the heating appliance is in a utility room or closet you should close the entrance door to this room.

    Watch out: OPINION: while closing the utility room door of a small utility room will often so drastically reduce combustion air as to cause improper and unsafe heating equipment operation, if the utility room is quite large, closing a door between this room and the rest of the building may actually reduce the impact of operation of building exhaust fans and vents which otherwise can also cause inadequate combustion air and backdrafting. In this situation you may want to conduct this combustion air test or backdrafting test in both conditions: with the utility room door open and with it shut.
  2. Turn on exhaust fans and appliances that exhaust indoor air to the outdoors. These include bathroom exhaust fans, kitchen exhaust fans and range hoods, whole house exhaust fans, and other one-way fans that blow building air outdoors. Place all of these exhaust fans at their maximum speed. See VENTILATION, EXHAUST ONLY

    Watch out: OPINION: Tjernlund advises that you should not operate an exhaust fan that is used strictly for summer exhausting, such as a whole house vent fan or attic exhaust fan used only in summer. I'm not sure we agree in all cases. For example if you are testing a gas or oil fired heating appliance that in fact does run during summer months, such as a domestic water heater, a tankless water heater, or a gas-fired cooling system, you may indeed want to see what happens when the whole house fan is running.

    Also note that the same backdrafting problems that these exhaust fans may cause for heating equipment can also subvert radon mitigation systems if one of those is installed in your building.
  3. Set the heating appliance thermostat to cause the appliance to operate continuously.

    Watch out: OPINION: even if you are only testing the flue gas spillage device on one heating appliance, you may want to turn on all of the fuel-fired heating appliances in the building. Having all of the appliances running at once maximizes the venting of indoor air caused by those devices and may detect interference among heating devices when they run simultaneously.
  4. Wait 5 minutes, allowing all of the exhaust fans and heating appliances to run continuously during this test interval.
  5. Failure of the Combustion Air & Backdrafting Test: if the spillage sensing device trips off, shutting down the appliance that it is intended to protect during this 5 minute interval not only is the spill detector switch working as it should, you have detected an unsafe building condition that can cause improper heating appliance operation and the production of potentially dangerous, even fatal carbon monoxide as well as other indoor air contaminants. In this case you should leave the unsafe heating appliance(s) off pending thorough diagnosis and repair by a trained professional.

    Successful (no problem found) Completion of the Combustion Air & Backdrafting Test: interestingly the Tjernlund instructions do not describe a "successful" backdrafting or combustion air test.

    OPINION: this omission may be because the company (and we) warn that this test and these safety devices cannot possibly detect all possible unsafe conditions. There are numerous possible chimney defects, for example, that are unsafe but that may not show up just by the test procedure described here. Additional chimney safety inspection procedures are described by articles given at MORE READING at the end of this article. One should argue the old saw that absence of evidence (of a safety hazard for example) is not evidence of absence (of the safety hazard).
  6. Restore normal operation if no trouble was detected. Return all windows, doors, fans and heating appliances to their previous conditions of use.

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

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.


Continue reading at CARBON MONOXIDE INSPECTION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




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