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Replacing a furnace door safety switch is a relatively simple task that can be done by following a few steps. Firstly, turn off the power to the furnace and remove the furnace door. Locate the safety switch, which is usually attached to the door or the frame. Disconnect the wires and remove the old switch. Install the new switch by connecting the wires and securing it in place. Finally, reattach the furnace door and restore power to the furnace. It is important to replace the safety switch if it is malfunctioning or damaged, as it ensures the safe operation of the furnace. Signs that the switch needs to be replaced include the furnace not turning on or off properly, or if there are visible signs of wear or damage.
If your furnace doesn’t turn on, the cause may be the furnace door safety switch. This is a simple safety feature created to prevent the furnace from turning on once the blower door is removed. The switch is also known as the blower door switch, and it operates like a refrigerator door switch that turns off the light after closing. Unlike the refrigerator switch, the furnace door switch cuts power to the blower when the door is open.
Since the switch is located on the external frame of the furnace or air handler, it’s easy to access and inexpensive to replace. Homeowners of any skill level can replace it, but before doing so, they should perform a continuity test to determine if replacement is necessary. Several components can cause the furnace to stop working, including the limit switch and flame sensor, so testing the door switch and discovering that it’s working properly can speed up the process of finding the real issue.
The Function of the Furnace Door Safety Switch
The furnace door safety switch prevents a gas furnace from turning on once the blower door is off. However, the reason isn’t to protect HVAC workers during routine maintenance or repair since they disable this switch by taping it down to conduct tests. The purpose of the switch is to prevent the blower from circulating combustion gases, like carbon monoxide, throughout the house. This can occur if the furnace starts while the cover is removed.
Many furnaces are installed in confined spaces, and if the blower turns on with the cover removed, it creates negative pressure that can pull combustion gases from the flue. This negative pressure also starves the furnace of combustible air, leading to incomplete combustion and the production of more toxic byproducts than normal. These byproducts, including sulfur dioxide, nitrogen dioxide, and carbon monoxide, are hazardous and potentially fatal.
Even a sealed-combustion furnace that pulls air from the outside through a PVC vent can malfunction if the blower cover is removed. The blower can pull air from the furnace room, which it isn’t designed to do, causing enough negative pressure to starve the furnace and produce incomplete combustion. A blower running when the air handler door is removed in an air conditioning or heat pump system can cause incomplete circulation or even freezing around the evaporator coils, significantly decreasing system efficiency.
Resolving Issues with a Furnace Door Safety Switch
A gas furnace door safety switch, much like a refrigerator door light switch, is made up of a spring-loaded plunger that is mounted to the appliance’s frame and is pushed in when the door is closed. If the switch fails, which is common due to it being a low-cost plastic part, the furnace will not turn on. However, since several other components can also cause this issue, it’s crucial to test the door switch early on in the diagnostic process. A DIY continuity test, which involves using a multimeter to measure resistance, is usually sufficient.
It’s important to conduct the test with the switch in both the open position, with the plunger extended, and the closed position, with the plunger depressed. Tape can be used to hold the plunger in place for the second test. The meter should be set to the most sensitive resistance ohm (О©) scale, and the furnace power switch or circuit breaker controlling the furnace should be turned off. The connectors from the switch terminals can then be removed, and the meter leads should be touched 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 the cause of the furnace shutting down, an “OL” reading will appear when the plunger is depressed. If a zero reading appears when the switch is extended, it means the switch is not providing the necessary protection. In either case, the switch needs to be replaced.
Where to Find a Replacement Switch
Regardless of the gas furnace or central air system model, a door safety switch operates in the same way, and different switches differ primarily in size and shape. It is possible that your furnace manufacturer produces a one-size-fits-all switch for all furnace models, and you may even come across a universal switch that fits your furnace. However, it is likely that you will need to purchase a specific switch that is compatible with your furnace.
Switches are available online at sites such as RepairClinic.com and eReplacementParts.com, and many other outlets. Your search will be simplified if you include the furnace make and model. If a third-party supplier does not stock the switch, you should be able to purchase it directly from the furnace manufacturer. You can also bring the old switch to a building supply center and look for one that is the same size and shape.
Replacing a Furnace Door Safety Switch
Step 1: Turn Off the Furnace Power
Turn off the furnace power switch, typically a light-type switch located in a box on or near the furnace, or switch off the breaker in the main panel (breaker box) that controls the furnace. It is not necessary to turn off the gas for this process.
Step 2: Take Off the Furnace Cover
It may be necessary to remove screws to detach the cover, although many covers have plastic knobs that can be turned manually, and some have no fasteners at all and are held in place by internal tabs. Remove the screws or turn the knobs, then take hold of the cover and lift it away from the furnace. If there are no visible fasteners, the cover should release from the internal tabs as it is lifted.
Step 3: Disconnect the Switch Electrical Connectors
Most switches have spade connectors that can be pulled off with a little force. It’s a good idea to take note of which connector goes where before removing them, but if this is forgotten, don’t panic. As a switch, the wires can be swapped around, and they will likely be positioned in the wire bundle in a way that makes it clear which one goes where.
Step 4: Remove the Switch
Inside the door, grasp the top and bottom of the switch body and depress the tabs by squeezing them. Then, pull the switch through the opening in the frame.
Step 5: Install the New Switch
Insert the body of the new switch through the hole in the frame and push it in. 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
Press in the switch plunger and secure it with tape. Turn on the power and increase the thermostat to activate the heat and ensure that the blower starts. Once it does, remove the tape to release the plunger and check that the blower turns off.
Step 8: Replace the Cover
To put the cover back on the furnace, reverse the steps you took to remove it.
FAQ
1. How does a furnace door safety switch work?
A furnace door safety switch is designed to ensure that the furnace door is securely closed before the furnace can operate. It is usually located near the furnace door and consists of a small switch that is triggered when the door is closed. When the switch is activated, it allows electrical current to flow to the furnace, enabling it to turn on. If the door is not closed properly, the switch remains open, preventing the furnace from starting.
2. What are the signs that a furnace door safety switch needs to be replaced?
There are a few indicators that a furnace door safety switch may need to be replaced. The most obvious sign is if the furnace does not turn on even when the door is closed correctly. Additionally, if the switch feels loose or does not click into place when the door is closed, it may be faulty. Another sign to look out for is if the furnace turns off shortly after starting, as this could indicate a malfunctioning safety switch.
3. How can I replace a furnace door safety switch?
Replacing a furnace door safety switch is relatively simple. First, turn off the power to the furnace to ensure safety. Locate the current switch near the door and disconnect any wires connected to it. Remove the old switch and replace it with a new one of the same type. Reconnect the wires to the new switch and secure it in place. Finally, restore power to the furnace and test the new switch by closing the door and ensuring the furnace turns on.
4. Can I replace a furnace door safety switch myself, or should I hire a professional?
While it is possible to replace a furnace door safety switch yourself, it is recommended to hire a professional if you are not experienced with electrical work. Working with electricity can be dangerous if proper precautions are not taken. A professional HVAC technician will have the knowledge and tools to safely replace the switch and ensure it is installed correctly. It is worth the investment to ensure the safety and functionality of your furnace.
5. Are there any precautions I should take when replacing a furnace door safety switch?
Yes, there are a few precautions to keep in mind when replacing a furnace door safety switch. Always turn off the power to the furnace before beginning any work. Use a voltage tester to confirm that the power is indeed off. Additionally, handle the wires and switches with care to avoid electric shock. If you are unsure or uncomfortable with any step of the process, do not hesitate to contact a professional for assistance. Safety should always be the top priority when working with electrical components.