Everything You Need to Know About a Furnace Pressure Switch
Why Won't My Furnace Light?
I get this question a lot from readers and while there are various possibilities, they usually believe it's the igniter or the gas valve that is failing. Though these are valid suspicions of the failure cause, there is one that is often overlooked because it's not understood: the pressure switch. This
Let's take a closer look at this part: what it does, how it works, and what the purpose of it is so that if your furnace ever fails to light you might be able to figure out if the pressure switch is your problem and what you can do to fix it. Or perhaps understand why that blinking red light says "the pressure switch is open" and yet might not be the problem.
What does a pressure switch do?
A pressure switch is a pretty simple component. Like all switches it makes a connection so that power can pass through it. Think of all switches like a drawbridge but instead of cars it has electricity waiting to pass over it. When the bridge is open nothing goes across but once closed, traffic can pass.
In the case of a light switch, no lights will come on until you flip the switch by hand and allow power to the light but your pressure switch is activated by air pressure or in this case suction. The suction your furnace's pressure switch needs to work is created by the inducer fan or draft inducer. When that fan comes on it will pull a vacuum inside the pressure switch through that rubber tube you see connecting the two which then pulls the switch in an closes (connects) the circuit to allow power to pass to the next component which is the igniter.
The Pressure Switch Connects to the Inducer Fan Housing
The Purpose of Your Furnace's Pressure Switch
The pressure switch in your furnace is used as a safety mechanism. It is there to confirm the unit is drafting properly. If the unit is not drafting properly then it does not operate as efficiently and creates a safety risk by not removing unwanted gases that can make us sick or cause small explosions inside the heat exchanger. This is why if the pressure switch isn't satisfied with the right amount of suction it will not allow the unit to ignite.
What is a high pressure switch?
Different furnaces may have different types of pressure switches in them. Based on the size of the unit (meaning heat output capability, not the physical dimensions) the amount of suction the pressure switch requires to work will vary. But when talking about high and low pressure switches it typically has to do with the number of stages a unit can operate in.
While many of us still have a single stage furnace that runs at full capacity every time it turns on, there are two stage units that will run at a lower heat output and then when needed, kick into a 2nd stage for more heat. These types of furnaces require a dual pressure switch so it can monitor the level of suction in low fire and then another in high fire since they require different amounts of ventilation to function properly.
Some two stage furnaces will use what is commonly referred to as a pancake switch since the two pressure switches are "pancaked" together while others will have two separate pressure switches. Either way, one of these switches is called the low pressure switch and the other a high pressure switch.
Causes of Pressure Switch Failures
Inducer Fan Running Too Slow
Inducer Fan Motor Is Bad
Blockage In Inducer Fan
Blockage In Flue/Chimney
Crack/Hole In Suction Tube
Bad/Loose Wire Connection
Water Built Up In The Inducer Fan Housing (High Efficiency Furnaces)
Loose Suction Tube
A Pressure Switch Can Be Tricky...
This part is finicky so it can fail and then work again making it hard to pinpoint. A downdraft of air can cause temporary failure. A "tired" switch may work today and not tonight, then again tomorrow. Intermittent failures are harder to diagnose so watching and listening the unit run, or try to run, may be helpful in pinpointing your problem.
What To Do When Your Pressure Switch Fails
The table above gives you quite a few causes of pressure switch failure that are easily confirmed or eliminated as the problem either audibly or visually. You should start with checking those things since they are the easiest, don't require special tools, and cost nothing.
- First, as the unit begins to try and run (meaning after the inducer has ran for 10-30 seconds) do you hear the gas valve "click" open? The gas valve will not open if the pressure switch doesn't work so the "click" sound of the gas valve opening tells us that the pressure switch is fine and the igniter is likely our problem since we should also notice it didn't try and light just prior to the "click" we heard. If we don't hear the click...
- Listen to see if you can hear anything rattling around in the inducer fan. If so, see if you can remove it. (Be sure to turn the power off first) If you hear the motor whining, grinding, or running seemingly slow then you may need to replace it and the switch is likely doing what it's supposed to do.
- Look to see if the switch hose is tight and in tact. If it's loose try to tighten it up a bit and if it's cracked or has a hole, see if you can seal it or cut away the cracked material and reconnect. Be careful as the inducer housing may be hot if the unit has ran recently. Again, this would show the switch doing it's job and it should work now that the hose is fixed.
- Look for any blockage in the flue you can see. If one is found, remove it and your problem should be solved.
If you've taken a few minutes to rule these things out and still have a problem then we can confirm the switch is bad by testing the circuit with a voltmeter and maybe get the unit working temporarily for a little heat at least.
- If you have a voltmeter, shut off the power to the unit. Remove the wires from the pressure switch and test for voltage across the 2 terminals. If it measures "0" then the switch is closed when it shouldn't be (shorted or stuck). If it measures "infinite" (O.L.) the switch is not shorted and could just be sticking open. Either way we'll need to replace the switch but perhaps we can still get it to work temporarily and get the heat back on for a bit. To do this...
- Remove the hose from the inducer housing (again being careful since the inducer housing may be hot if the unit has ran recently) and gently suck and blow on the end of it. You should be able to hear the switch opening and closing. Do this a couple of times and then reconnect the tube to the inducer housing and see if it will work. Sometimes the switch will get stuck and just needs a little help loosening up. Even if this works, replace the switch soon. It is quite likely the switch will stick again and it's always better to fix it on your terms than the typically inconvenient timing it will choose to breakdown again.
Replacing A Pressure Switch
The actual work involved in replacing a pressure switch is quite simple and doesn't require any specialty tools that you don't likely have laying around the house.
- Turn off the power to the unit
- Disconnect the wires from the switch
- Disconnect the hose from the switch
- Remove the switch from the bracket (or bracket too if your new one has it mounted already)
- Fasten the new switch back in place
- Reconnect the hose to the switch
- Reconnect the wires to the switch
See how easy that is?
How Much Does a Pressure Switch Cost?
A single stage pressure switch whether from the factory or universal can cost anywhere from $20-$50 while pressure switches for a 2 stage unit can run in the $30-$75 range.
Finding and Buying A Replacement Pressure Switch
Like many furnace components, the pressure switch is not an item typically found in the "big box" stores like Lowe's and Home Depot. To find a replacement you will likely have to do one of two things: contact the contractor supply house that sells your brand of furnace or obtain a universal replacement from a contractor supply house or online.
Furnaces are much like cars and have parts that are specific to their brand. That is why if you want a direct factory replacement you have to find the supplier that carries your brand. It's a "don't call a Chevy dealer for Ford parts" sort of thing. Once you've found the supply house that carries your brand then be prepared to tell them what part you are looking for and have your unit's model and serial number handy so they can help you. That said, many of these suppliers are "contractor only" and will not sell to the general public so the more prepared you are, the more likely you are to get them to sell to you or at least give you a part number so you can look online for it.
A universal pressure switch however may be easier to find and purchase at any supply house or online. I keep an handy so that if my or a family member's unit breaks down at an inopportune time, I can make the repair without going hunting for one. These are nice because they can handle single stage and dual stage units as well as be set to various pressures. They also contain brackets that make mounting possible in more than one unit. The instructions walk you through the process of setting up the switch to suit your unit's needs. NS2 universal pressure switch
I hope the information I've given you regarding your furnace's pressure switch has helped you deduce whether or not the pressure switch is the problem you're having or if another part was a cause. If your still not sure what's going on then I recommend you contact a technician. We don't want to make matters worse or get in over our head.
~ We're all in this together ~
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