If your furnace won't turn on, the culprit could be the furnace door safety switch, a simple safety feature designed to prevent the furnace from coming on when the blower door is removed. It's also known as the blower door switch, and it operates like the door switch in a refrigerator that turns the light off when the door is closed. The function of the furnace door safety switch is actually the reverse of a refrigerator light switch. Whereas the refrigerator switch cuts power to the light when the door is closed, the furnace door switch cuts power to the blower when the door is open.
This part is inexpensive and easy to install for a homeowner of any skill, and it's positioned on the outside frame of the furnace or air handler, so it's easy to access. If you suspect it may be faulty, you can go ahead and replace it without much trouble, but a simple continuity test can tell you whether or not that small effort is even necessary. A number of easy-to-replace components can cause the furnace to stop working, including the flame sensor and the limit switch, so conducting a test and finding the door switch to be working properly makes finding the real problem that much quicker.
Function of the Furnace Door Safety Switch
The furnace door safety switch is there to prevent your gas furnace from switching on when the blower door is off, but the reason isn't to protect HVAC workers when they're doing routine maintenance or furnace repair. In fact, they often disable this switch by holding it down with tape because they need the power on to conduct tests. The purpose of the switch is to prevent the blower from circulating combustion gases, which include carbon monoxide, throughout the house, which could happen if the furnace starts while the cover is off.
Many furnaces are installed in confined spaces, and if the blower comes on when the cover is removed, it pulls air from the room, creating negative pressure that can be strong enough to suck combustion gases from the flue. This negative pressure also starves the furnace of combustible air, and the reduced airflow leads to incomplete combustion and the production of more toxic byproducts than normal. These get sucked out of the flue and circulated throughout the vent system, and some of them, including carbon monoxide, sulfur dioxide and nitrogen dioxide, are dangerous and potentially lethal.
Even a sealed-combustion furnace that draws air from outside through a PVC vent can malfunction if it runs when the blower cover is removed. The blower can draw air from inside the furnace room, which it isn't designed to do, and enough negative pressure can be produced to starve the furnace and produce incomplete combustion. In an air conditioning or heat pump system, operation of the blower when the door to the air handler is removed can cause incomplete circulation or even freezing around the evaporator coils and greatly reduce the efficiency of the system.
Troubleshooting a Furnace Door Safety Switch
Like a refrigerator door light switch, a gas furnace door safety switch consists of a spring-loaded plunger, such as a rod or toggle, mounted to the frame of the appliance, and closing the door pushes it in. If a gas furnace door safety switch goes bad — which isn't unusual, since it's an inexpensive plastic part — the furnace won't turn on, but since several other components can also do that, it's important to test the door switch early in the diagnostic procedure. A DIY continuity test performed with a multimeter set to measure resistance is usually conclusive.
The test should be conducted with the switch in both the open position (plunger extended) and the closed position (plunger depressed). You can use tape to hold the plunger in while you conduct the second test. Set the meter to the most sensitive resistance ohm (Ω) scale, turn off the furnace power switch or the circuit breaker controlling the furnace and remove the connectors from the switch terminals. Touch the meter leads to the door switch terminals.
When the plunger is depressed, the meter should read zero resistance (or close to it), and when the plunger is extended, the meter should read "OL" (which stands for open line, or infinite resistance). If the switch is causing the furnace to shut down, you'll get an "OL" reading when the plunger is depressed. If you get a zero reading when the switch is extended, that's just as bad because it means the switch isn't giving the protection it's supposed to give. Either way, it needs to be replaced.
Where to Get a New Switch
No matter the model of gas furnace or central air system for which it's made, a door safety switch works in the same way, and different switches vary primarily in their size and shape. Your furnace manufacturer may make a one-size-fits-all switch for all of its furnace models, and you may even find a universal switch that fits your furnace, but chances are you'll need to get a specific switch that fits only your furnace.
Switches are readily available online at RepairClinic.com, eReplacementParts.com and multiple other outlets. Your search will be easier if you enter the furnace make and model. If a third-party supplier doesn't stock it, you should be able to get it directly from the furnace manufacturer. You can also take your old switch to a building supply center and look for one that is the same size and shape.
How to Replace a Furnace Door Safety Switch
Step 1: Turn Off the Power to the Furnace
Flip off the power switch for the furnace (usually a light-type switch located in a box on or near the furnace) or if there isn't one, turn off the breaker in the main panel (breaker box) that controls the furnace. You don't need to turn off the gas for this procedure.
Step 2: Remove the Furnace Cover
You may have to remove some screws to get the cover off, but most covers have plastic knobs that you can turn by hand, and some have no fasteners at all; they are held on by internal tabs. Remove the screws or turn the knobs and then grasp the cover and lift it up and away from the furnace. If the cover has no visible fasteners, it should unhook from the internal tabs when you lift it.
Step 3: Disconnect the Switch Electrical Connectors
Most switches have spade connectors, so give each connector a good pull, and it will come off. It's a good idea to note which connector goes on which terminal before you remove the connectors, but if you forget, don't worry. Because it's a switch, the wires are interchangeable, and they will probably be oriented in the wire bundle to make it obvious which one goes where.
Step 4: Remove the Switch
Grasp the top and bottom of the switch body inside the door and squeeze to depress the tabs. Then, pull the switch through the opening in the frame.
Step 5: Install the New Switch
Slip the body of the brand new switch through the hole in the frame and push. You'll feel a click as the tabs engage, and the switch is fully seated.
Step 6: Reconnect the Switch Wires
Push the spade connectors onto the wire terminals.
Step 7: Test the Switch
Push in the switch plunger and tape it to hold it. Turn on the power and turn up the thermostat to call for heat and verify that the blower starts. When it does, remove the tape to let the plunger out and verify that the blower switches off.
Step 8: Replace the Cover
Put the cover back on the furnace by reversing the procedure you used to take it off.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.