Dan has been in the HVAC industry for 23 years with experience ranging from installation and service to sales and distribution.
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.
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 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
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 resistance (ohms) across the 2 terminals. If it measures "0" (or close to) 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?
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 this NS2 universal pressure switch 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.
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 ~
Feedback Is Important to Me
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Question: My vent pressure switch is working fine. I replaced the ignitor already. Is the furnace control module my next step?
Answer: That’s not really how it works. While replacing parts will eventually get you there and perhaps even end up still saving you money vs. a service call, you need to find where the breakdown is occurring in the cycle or by testing the circuits with a voltmeter.
© 2018 Dan Reed
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Denny on February 04, 2020:
I have 2 pressure switches. A .40 and a
.80, which hose connects to the inducer motor ?
Goodajj on January 24, 2020:
The thermostat is calling for heat and the control board says there's a problem with the presure switch. I discommect the tube from the nipple on the inducer motor and the inducer motor starts to run, i plug the tube back in and the system works as expected. Any idea what the problem is?
Kapilan on November 14, 2019:
Hey, so pancake dual pressure switch. When I have the high side hose off it works the furnace fires up but when I got the both hose off it gives me error 229 ignition on high fire.
Lennox furnace. EL26uh070
Michael on November 14, 2019:
Dan, thanks for this article. It's very helpful. The issue I have is that my unit blinks the code that says my pressure switch is closed. Once I disconnect the wiring to the pressure switch (and immediately reconnect it), that code stops blinking, the igniter starts, the natural gas comes through and we have heat. The heater runs until the temperature reaches the desired setting I've put on our thermostat. However, when the temperature in the room drops again, the same pressure switch code starts blinking, and the only way I can get the heat to kick on again is if I disconnect the wiring to the pressure switch. I bought and installed a new pressure switch today, but the same problem continues...Thoughts?
Daniel on March 16, 2019:
Does it matter which wire goes where when you're replacing a pressure switch on a Rudd silhouette heating unit?
Rick on March 08, 2019:
How do you determine the right Pressure switch to use in your furnace if the switch has no Vacuum rating on it? using the wrong one (ie universal) could be dangerous!
tyler watson on January 13, 2019:
I have a 100,000 btu Goodman furnace two stage. I have had people over several times to try and fix its issue but no one can seem to figure it out. It kicks on. Ignites. Then shuts off after 5-30ish seconds. Does it three times then waits a while before it tries again. The kicker is it doesn’t always do it. Last year and so far this year it only starts happening halfway through the winter. Anyways. It chooses its own days that it decides to not work. It will be working perfect for 2-15 days and then have a bad day. And sometimes it only does that for a couple hours and sometimes it attempts to start and continues to fail for twenty four hours. I was just wondering if you have encountered any problems like this. When I take the top half off it has a lot of water just sitting there. I don’t know if that has anything to do with it. The flame sensor is good. I replaced the low and high fire pressure switches. I’m about to buy the other pressure switch and replace it. But if that does not work do you have any advice on where to check next. The codes it’s throwing does not exist in the owners manual or online. So I am guesstimating. Any advice would be greatly appreciated. Thanks a lot.
Dan Reed (author) on October 16, 2018:
Patrick - I'm not sure other than perhaps the switch itself is bad. I believe you have a boiler here and I am unfortunately not as well versed in those as I am forced air systems. Sorry I'm not of much help.
PATRICK GILLESPIE on October 15, 2018:
I have been searching all over for someone that has had the same issue I'm having, but can't seem to find anything so I figured I'd reach out. I have a Weil-McLain GV-6 and I just tried firing it up for the first time this year. The inducer motor kicks on, then the pressure switch circuit closes, then I hear the burner ignite and the burner flame indicator light comes on. The burner is only on for a few seconds then the pressure switch circuit opens back up causing the system to shut down. I have checked all the lines and hose connections to the inducer housing and the pressure switch. I have pulled the vent pipe off at the exhaust and everything is clear. Do you have any thoughts as to why the system loses the vacuum once the flame is lit?