The pressure switch is a critical part of the modern furnace, without which the furnace cannot operate. It’s the only part that verifies the absence of dangerous exhaust gases, enabling the furnace to run without exposing you and your loved ones to gas poisoning.
Unfortunately, the pressure switch may sometimes force the furnace into a “lockout,” potentially keeping your home and the occupants in the cold for an extended period.
Below, we discuss common causes of pressure switch lockouts and what you can do.
What is a Pressure Switch?
The pressure switch is an electric relay switch. Relays are electrical switches that open and close depending on information from external sources. The switch receives an electrical signal from one source and closes to send it to another equipment or part of the appliance.
If the pressure switch, the relay closes if the draft inducer motor is working normally, completing a critical circuit necessary for the operation of the furnace. Otherwise, the furnace cannot run.
How the Pressure Switch Works
The furnace pressure switch works in tandem with the draft inducer motor. The draft inducer motor is a special fan located next to the heat exchanger.
Its sole purpose is to remove exhaust gases, including carbon dioxide, carbon monoxide, and silicon dioxide, from the furnace. The draft inducer motor is the first thing to come on when you switch on the furnace.
It comes on even before the burners to expel any exhaust gases inside the furnace, so these gases don’t vent into your home when heating begins.
Similarly, it’s one of the last parts to go off, typically staying on for up to half an hour after the burners go out to remove all exhaust gases from the furnace. Due to its unique and critical role, there has to be a way to tell whether the draft inducer motor is running or not.
That’s where the pressure inducer switch comes in handy. You see, the draft inducer motor is located in a closed “cage” so that it can easily pull and direct exhaust gases out of the furnace.
However, it also means that the motor creates a vacuum behind when it expels exhaust gases, which causes a negative pressure (conditions lower than atmospheric pressure).
The pressure switch tracks this negative pressure to determine whether the draft inducer motor works properly and uses the information to close the relay switch or keep it open.
Generally, pressure on the switch’s diaphragm compresses a calibrated spring. When the spring tension reaches or exceeds a set point, it moves the contacts from open to closed.
Once the pressure switch is closed, current flows through the rest of the furnace, and the appliance roars to life.
What’s a Pressure Switch Lockout
Furnace lockout refers to a non-responsive state in which the furnace becomes passive to most external controls. Typically, it means that the furnace will not restart no matter how many times you push the start button. A pressure switch lockout merely refers to a furnace lockout resulting from a pressure switch malfunction.
There are two broad categories of furnace lockouts – soft and hard lockouts. Soft lockouts usually come before hard lockouts and are essentially semi-shutdowns that lock the user out of critical furnace controls. However, you can still retry to ignite the furnace.
However, after trying for too long, typically three to five times, the furnace enters a hard lockout. You cannot restart the heater after it enters hard lockout until you manually restart it. Often, you may have to call an HVAC technician to perform the reset sequence.
4 Causes of Pressure Switch Cycle Lockout
There are several things to keep in mind here. For one, remember that lockout is the consequence of repeated ignition/restart attempts. So, it means that we’re dealing with a furnace that refused to start after multiple attempts, consequently entering lockout.
Thus, you need to ask why the furnace is not starting. However, since we’re solely concerned with the pressure switch, we must narrow down the reasons to pressure switch-related reasons.
In short, we need to find out the different ways in which the pressure switch can impact and even block the furnace from starting or restarting.
There are several reasons;
1. Pressure Switch is too Old
Yes, the pressure switch can become old and weak. Remember that that switch comprises several delicate parts, including a rubber diaphragm and spring wire. These parts can become worn and unresponsive over time.
For instance, the rubber diaphragm can lose its elasticity thus remain relatively static even when the pressure inducer switch is working normally. The spring wire can also lose its elasticity.
A common sign of an old or overwhelmed pressure switch is regular tripping even after you repair/fix it. Of course, you can also see the physical signs of age, such as worn parts.
Solution: The only solution if the pressure switch is too old is to replace it. A new replacement costs $20 to $50 while two-stage models cost $30 to $70. Professional replacement charges range from $60 to $150.
2. Internal Damage
Besides old age, the pressure switch can also refuse to close if it’s broken or damaged in a way. For instance, we’ve mentioned the wires connecting to the switch. Typically, two wires connect to the pressure switch to complete the circuit when the switch closes.
Now, imagine if they are pulled out or melted. For instance, mice occasionally enter the furnace and may gnaw on the wire, ultimately disconnecting them. The rubber diaphragm can also tear or develop if exposed to adverse conditions.
For instance, if the condensation process is compromised, sulfur dioxide from the exhaust gases can combine with the moisture to form sulfuric acid that can cause corrosion throughout the furnace and even damage the rubber pressure switch’s diaphragm.
Solution: Regular inspection and scheduled maintenance can help prevent and fix internal damages. More importantly, you must take excellent care of the furnace.
3. Dirt and Debris Buildup
You’re probably wondering how the pressure switch can become dirty. We’ve already mentioned that the pressure switch sits side by side with the draft inducer motor. This motor handles dirty exhaust gases, containing all gaseous wastes from the combustion process.
For instance, any trances of smoke pass through the same path out of the house. So, it can slowly trap smoke and carbon deposits (tar-like dust). Additionally, dust entering the house through the exhaust vent typically settle on the exhaust lining, way down to the pressure inducer motor.
The buildup can cause the relay wires to get “stuck.” If the switch is stuck open, the furnace cannot start and may eventually enter lockout.
Solution: Scheduled maintenance and regular inspections can help here too. Additionally, always keep an ear out for strange sounds when the furnace starts. If you pick up strange sounds, you may have a damaged or dirty inducer motor, which could mean the switch is also dirty.
4. The Draft Inducer Switch is Damaged/Defective
Finally, whenever you find that you’re forced to restart the furnace repeatedly because it keeps shutting down before heating even begins, you may have a damaged or defective draft inducer motor.
As we saw earlier, the draft inducer motor generates the vacuum necessary to create the negative pressure that closes the pressure switch. Now imagine the fan is not working correctly or not working at all. How would the pressure switch close?
The draft inducer motor can malfunction for many reasons. For one, an aging inducer motor can become too weak to generate the necessary negative pressure. Alternatively, some of the parts can wear and come off. For example, the motor and blower wheels can break down, or the wires become loose.
Solution: Regular maintenance can go a long way in preventing and fixing damages such as loose wires. However, you may need to replace the blower altogether if it’s dead or too weak to generate the required negative pressure.
Can You Bypass the Pressure Switch?
Yes, you can bypass the pressure switch. However, it’s extremely dangerous as the pressure switch is responsible for venting poisonous combustion gases out of the house.
Bypassing exposes you and your family to dangerous gases such as carbon monoxide that can cause asphyxiation or even death.
Nevertheless, if you need to bypass it momentarily to confirm that it’s dead, all you have to do is pull out the two wires connecting to the switch and connect the wires directly. If you do so and the fan works, you know that the pressure switch is defective, stuck open, or dead.
How Can You Reset a Pressure Switch?
Resetting is an easy way to restore the switch, especially if it was stuck open due to internal confusion. You can also reset it after cleaning. To reset your pressure switch, feel the back of the switch for a small button and push the button in, then release it. After that, reset the fan to see if it works.
There you have it – the four leading causes of pressure switch cycle lockout and how to solve each. As we always say, never hesitate to contact an HVAC professional when you’ve exhausted your DIY skills.