What Does a Furnace Pressure Switch Do?

furnace pressure switch

Photo credit: Felix Wong CC BY-SA 4.0

If you have a fuel-burning furnace, it will be equipped with a pressure switch. But you may have no idea what a pressure switch is or what it does. This article will inform you of what it is, how it works, and why you may need to know about it.

What is a pressure switch and what does it do?

A pressure switch is (not surprisingly) a switch that is activated by suction created by the inducer fan. It’s usually a small black disk located above your burner assembly near your inducer fan. Some units are equipped with two discs, but most will only have one. These will have two wires attached to them and a rubber hose that will travel back to your inducer fan assembly.

To know how a pressure switch works, you need to know about inducer fans. Before your furnace can turn on and start burning fuel, your inducer fan will kick on. This does a couple of things: First, it will create a vacuum at the bottom of the heat exchanger so that when the burners try and light, the inducer fan will pull the flames and gases through the heat exchanger. Second, it will create suction on the rubber hose that connects to the pressure switch.

The pressure created by the inducer fan is what engages or disengages the pressure switch. When the fan is on, the switch is engaged and essentially connects the two wires attached wires. This connection is what allows electricity to flow and the rest of the furnace cycle to continue.

The pressure switch is a safety mechanism. If your furnace cannot create the suction required to exhaust any fumes, it will not engage the switch and the furnace will not ignite. This can happen for various reasons, which we will discuss.

What are the signs that the pressure switch is not working or bad?

Any furnace has a cycle that it goes through when it tries to light. The inducer fan will turn on until the pressure switch closes, then your igniter will start, fuel is introduced, and finally, the blower fan will blow air across the heat exchanger and distribute the warm air through your home. Knowing this can help us to troubleshoot the issue.

If you attempt to turn your furnace on and it does not start, you may be able to listen to it and see which stage of the cycle it stops trying. If the inducer fan kicks on, and then nothing else happens until the inducer fan shuts off, then it is very possible that the problem is the pressure switch.

If you can get to your furnace, you can do a quick test. If you take the hose that goes to the pressure switch off at the inducer fan side, you can use your mouth to create suction while it tries to start. You should hear the pressure switch click when you do this. It is a very mild pressure, and you will have to hold it while the furnace is attempting to start. If it begins while you are doing this, the problem is not the pressure switch.

This could mean that the inducer fan wheel is rusted away or the motor is not turning. It could mean that the exhaust vent has something keeping the exhaust from escaping freely, therefore not creating enough suction. Check for other similar airflow issues that would keep it from creating enough suction. One example I saw was a wasp nest built inside an exhaust vent that caused it to not blow enough exhaust pressure. This caused the system to have a fault and not run.

How do they usually fail?

 The most common reason that a pressure switch might fail is that the points of contact inside them will no longer have continuity. You cannot take a pressure switch apart without ruining it, but from my personal experience, the most common issue is that they no longer make contact. You can suck on the tube and hear it close, but when you test it with a multimeter, the two wires no longer have the continuity between them that they need.

Sometimes the pressure switch can get stuck closed. You can do the same test by sucking on the tube, but if the switch is stuck closed, it will likely not make a clicking sound. This will mean that the leads inside of the pressure switch have corroded together to the point that they will not come apart anymore.

Another thing that I have rarely seen is water damage. If it is a high-efficiency furnace and the drain becomes plugged, it can back up into the inducer fan and get into the pressure switch. Or if you have a furnace under your house and it has flooded, then the dirty water and rust that forms can also cause your pressure switch to no longer work. If there is water in the pressure switch case at all, then it will not be able to operate correctly. Sometimes clearing the water out of the switch will work, but they usually will have to be replaced for proper operation.

If you have a newer or high-efficiency furnace, it will likely have a circuit board. If you can see the circuit board indication light from outside of the furnace once it has failed to start, then you can count how many times it blinks. Once you count the blinks you can pull the cover off and check to see what the code is by how many blinks you counted. The code will often be identified as “Pressure switch staying open” or “Pressure switch staying closed”.

How long does a furnace pressure switch last?

It’s difficult to determine a specific length of time that a pressure switch should last, but it should last approximately 50,000 cycles. This means that more than likely, they will last you several years at least, depending on how often your furnace is turning on and off.

Can you replace it yourself?

The short answer is yes. Pressure switches are not overly complex pieces of equipment, and they are usually installed in an obvious place. The problem would be purchasing the right one that you will need. Pressure switches have different levels of vacuum that will make them operate. So, when you go to purchase one you may have no idea how to answer when they ask you if you want a -0.5 or a -2-pressure switch. This should be indicated, however, on the switch that is currently installed.

Another thing to note is that some pressure switches will work off of positive pressure instead of negative pressure. This is rarer on furnaces, but not unheard of. So, you will have to be sure that it is a negative pressure pressure switch. They also make adjustable pressure switches that will allow you to set your own pressure, but that may be something you leave to a professional if you are unsure of what you are doing. If you have a pressure switch set to low, then it could be potentially dangerous. If the unit is not creating enough of a vacuum to engage the pressure switch, then there may be something wrong. If you put a lower pressured pressure switch on to try and solve the problem, you could potentially be putting yourself and your family in danger.

Once you are sure that you have the correct pressure switch, then it will be mounted to the inside of the furnace by one or two screws. You will remove those, unhook the two wires going to it, and the rubber hose. Then you will do the exact opposite with the new one. Connect the rubber hose, the two wires, and use the screws to remount it. Then you will be able to turn your furnace on and watch your repair get to work.

How much does it cost to replace?

A pressure switch is inexpensive. However, the majority of the cost lies in having to pay an HVAC repair company to do it for you and is where the biggest cost would be. If you were to go to an HVAC wholesale company, depending on which pressure switch you need, it would cost you between $20 and $50 and would be your only cost.

If you need to hire someone, then it will cost you the part plus markup and the service call. Depending on what HVAC repair companies charge in your area, it could be significantly higher. Pressure switches are commonly kept on HVAC service trucks. Once an experienced service tech has stepped on your property, they should be able to diagnose the problem and have it replaced in an hour or less. It is a common issue that HVAC service companies deal with and is straightforward with easy to evaluate symptoms.

Most components in gas furnaces are designed with safety in mind. This means that if you were to do something wrong or something didn’t work right, you may be unintentionally messing with the safety systems that are in place. This is why it is always recommended to hire an HVAC repair company to do any service on your HVAC equipment.

Do you have any experience replacing a furnace pressure switch? Tell us how it went below.

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