How to Install a Furnace Door Safety Switch
A critical safety device found on most modern furnaces is a furnace door safety switch. Even if you are not planning on replacing your furnace anytime soon, you may want to add a safety switch to your existing unit. You may also need to replace a safety switch on your furnace if it was already equipped with the device. A furnace door safety switch prevents the fan and burner from coming on while the access panel, or panels, are removed. This prevents injuries and from any loose materials or debris from being drawn into the blower or burner causing equipment damage or fire.
Some safety switches can be ordered directly from the manufacturer or through an HVAC supplier in your town. It is best to take the switch with you and also write down the manufacturer and model or serial number of the furnace so that they can cross reference the replacement part. You may not be able to purchase an OEM (original equipment manufacturer) part, but these switches are fairly common and off the shelf components should work well for this do-it-yourself repair.
Tools and Materials
Wire and armored cable, if required
Step 1 – Shut off the power
Turn the main power off to the furnace with the emergency shutoff switch or the main circuit breaker. Remove the blower door to access the control panel and with your voltmeter or circuit tester, verify that there is no voltage present.
If your unit is already equipped with a safety switch, turn the power back on to the unit and verify that it will not run with the access doors removed. If it does you either don’t have a safety switch or it is defective.
Review your owner’s manual to determine if a furnace door safety switch is present and if it is, find the location.
Step 2 – Prep the blower door
Some units have only one access panel, while others have two interlocking panels. The safety switch is usually located closer to the blower where the filter access is. If your unit does not have a safety switch, this is where you will want to install it.
Clean the door and position your new switch somewhere on the frame where it will come in contact with the door when it is in position.
Typical Safety Switch
Step 3 – Wiring the Safety Switch
Some safety switches will not come with installation instructions, usually just a wiring diagram. Some switches are inserted into a hole or slot while others can be screwed to the furnace housing.
Safety Switch wiring diagram (varies) Red oval is location of safety switch, small green circle is location of control board terminal, and green oval is location of relay switch.
Review the wiring diagram for the safety switch. The safety switch is an interlock device that cuts the power to the unit. When the door is removed the voltage will be 0, when the button is pressed in, the voltage should be around 120V. The switch typically is placed in series between one terminal (120V supply) on the control board to relay switch that is usually double pole single throw - DPST). Remove the wire from the terminal to the relay and run a new wire from the terminal on the control board to one terminal on the safety switch and another wire from the other terminal to the relay switch.
It may also be possible to break the circuit from the emergency switch to the safety switch, but you may need to consult with an electrician to verify the wire size and also run the wires in a shielded wire like armored cable.
Step 4 – Checking and Testing the Installation
Mount the safety switch in a hole or secure it with screws. Replace the furnace door and check that it makes contact with the button on the front of the switch.
Restore the power to the furnace and verify that the unit runs. Next open the furnace door verify that the safety switch shuts off the unit immediately.
Exisiting safety switch in unit