Problem:Hello, love the website and help that it offers. But I seem to be out of options with my furnace here. I have a Carrier HE furnace that is not even 3 years old yet. We have had repeated issues with the furnace running for 2-3 minutes, stopping, recycling and coming back on until desired temp is reach. Sometimes this will occur 3-4 times depending on reaching desired temp in the home. When I called a technician he came out and charged 170$ just to tell me my pressure switch was broken. And offered to replace it for another 150$(!). I ordered one myself and replaced it last weekend. Took me 5 minutes and switch was 60$. But, I’m still having the same symptoms. Sometimes the furnace will not even come on and we awake to a cold house. Other times it works just fine…??… it’s “buggy” as I describe it. I am getting a 3 to 1 LED code indicating pressure problems. But I have cleaned the water trap multiple times and there seems to be no issue. No water lines to or from are blocked. Exhaust is not blocked leading outside. Nor is intake. And again the pressure switch is brand new (supposedly). And I have also cleaned the flame sensor with newspaper (no steel wool). I don’t believe that is the problem. Seems clean. No residue on it. So, if you can offer any help please do! Cleveland is about to have sub zero Temps coming up this weekend! Chris
Answer: Hi! I am terribly sorry that you have encountered all these problems with your furnace. We have a furnace troubleshooting flow chart that I hope will help you out on the following page: Please click here if you would like to see our Gas Furnace Troubleshooting Flow Chart.I would start with making sure the thermostat is providing 24 volts between W and C terminals on the control board when the furnace cuts off. If you are getting a constant 24 to 28 volts AC to the furnace all the time between W and C when the furnace cuts off then this would eliminate the thermostat from being the problem. If the control board is correct in saying and showing that the pressure switch is the problem… then I would suggest looking at the things like you have already have, like the condensate trap and drain lines from the secondary heat exchanger. Please make sure the furnace is tilting forward slightly so the water drains out the front of the furnace. The installation instructions on these high efficiency furnace tells how many feet of vent pipe are allowed for venting. If the allowed vent feet are exceeded then you will have trouble with the pressure switch not staying closed. The 90 degree turns in the venting sometimes are calculated as 5 feet of piping. If you have exceeded the number of turns the furnace will not run right and the pressure switch will not work or stay closed. I remember one furnace I installed where I tried to use 2″ PVC for the venting and ended up having trouble with the pressure switch going off like your furnace is doing. I ended up having to tear the 2″ pipe out and install 3″ PVC vent. The venting is critical for these high efficiency furnaces to run right, to say the least! Some other things that cause furnace pressure switch problems are: (1) A restricted vent pipe (2) Too long of vent piping and too many turns which exceeds manufacturer’s recommendations (3) a stopped up condensate drain line if you have a high efficiency condensing furnace. (4) a dirty or stopped up draft inducer. The hole where the pressure switch attaches to the draft inducer could be stopped up with water, slug or dirty. (5) a slow draft inducer that might need a new capacitor or has slowed down. (6) someone has sucked on the pressure switch with their mouth or forgot to unhook the pressure switch from the furnace before using a vac on the furnace. Sucking on the pressure switch or leaving a pressure switch attached to furnace when vacuuming out the condensate line will ruin the diaphragm on the pressure switch. I hope you can find and fix the problem. If you need parts please send me your furnace’s product number. Please email us anytime if you have a question. Our email address is: firstname.lastname@example.org. If you think you have a bad pressure switch please click here to see the furnace pressure switches we sell. I hope you have a blessed day. Steve Arnold, https://arnoldservice.com
Problem: Can a draft inducer motor somehow get “weak” before it seizes up or shows some type of problem? My furnace code is telling me pressure switch/vent problem but I have checked both and they function fine. The problem is when the burners ignite the gas valve cuts off after only a second or two because the pressure switch much be opening for a split second (if I bypass the switch the burners stay ignited and everything works). With no vent problems and pressure switch working like it should I am down to thinking that either the inducer motor is not keeping up when the burner ignites and gives a little burst of pressure, or maybe the gas volume is set a little too high so that the burst of ignition is causing a pressure pulse that opens the switch. I’m not sure which one would be more common after running fine for 10 years. If I run the furnace for a few minutes with pressure switch bypassed, and then shut down, hook switch back up and restart, the system will ignite and run as it should, but later after being off for a while it will not stay lit and does the same thing and gives same code. This leads me to believe that when vent and system is hot it can draft better and switch will not open when burner first ignites so whatever the problem is, it is just barely giving enough pressure change to open the switch when ignited and system is cold. Inducer or gas volume or have you seen something in this situation that could be the problem? Thanks.
The second most common cause is the pressure switch tubing (line) being slightly obstructed with water or debris. If your Lennox furnace is a condensing furnace that produces water when it runs, then please make sure the water drain line on the furnace is draining well. Please make sure the pressure switch tubing where it attaches into the draft inducer is not obstructed with water, or the hole where the pressure switch tubing attaches to the draft inducer is not obstructed. If you are having a slight delayed ignition when the furnace first starts up then you might want to clean the burners with compressed air. Clean the pilot or furnace lighting mechanism to make sure the furnace lights smoothly. Too much gas pressure or too little gas pressure can cause a furnace to have delayed ignition. You can adjust the gas pressure if you think you might be getting too much or too little gas pressure. I show how you can do this on the following page: Please click here if you would like to see our Gas Furnace Troubleshooting Simplified Page. Please be careful when trying to adjust gas valve gas pressure. This can be dangerous when working with natural gas. A draft inducer’s bearings can get tight, or the inducer blower wheel can get dirty which could slow it down. Please make sure that the draft inducer wheel turns freely. A good test would be to watch to see how long the draft inducer wheel spins after the draft inducer goes off. The wheel should spin from 3 to 5 seconds after the power goes off to the draft inducer. If the draft inducer does not spin very long (only a second or two) then the bearings on the draft inducer might be tight and you will need to purchase a new draft inducer or a new inducer motor. If your draft inducer has a run capacitor you might want to check the capacitor to make sure it is reading in the right specs and not weak. A weak capacitor will cause the draft inducer to run slow. You said, the vent is not obstructed so this isn’t the problem. I would also suggest to look at the things on the furnace like the condensate trap and drain lines from the secondary heat exchanger. Please make sure the furnace is tilting forward slightly so the water drains out the front of the furnace. The installation instructions on these high efficiency furnace tells how many feet of vent pipe are allowed for venting. If the allowed vent feet are exceeded then you will have trouble with the pressure switch not staying closed. The 90 degree turns in the plastic PVC venting sometimes are calculated as 5 feet of piping. If you have exceeded the number of turns the furnace will not run right and the pressure switch will not work or stay closed. I remember one furnace I installed where I tried to use 2″ PVC for the venting and ended up having trouble with the pressure switch going off like your furnace is doing. I ended up having to tear the 2″ pipe out and install 3″ PVC vent. The venting is critical for these high efficiency furnaces to run right, to say the least! I hope that I given you some insight into what might be the problem. I am sorry we do not sell very many Lennox parts but I will be glad to try and help you out anyway I can. Steve Arnold, https://arnoldservice.com
Question: How can I test to see if a pressure switch is operating properly?
Set your meter to volts AC, turn your furnace on so it is calling for heat. The draft inducer should start.
There should be either two or three wires going into your pressure switch.
Probe one lead of the pressure with one lead of your meter while touching the other lead of your meter to ground. Ground would be any bare metal part of your furnace. This must be bare metal. I always try to touch the other lead of my meter to the furnace’s switch box.
If you have a two wire pressure switch you should be getting 24 or more volts between both leads to ground. By this I mean if you touch one terminal of the pressure switch with one lead of your meter, and touch the other lead of your meter to a ground, bare metal part of your furnace, you should get 24 or more volts (24 to 28 volts).
If you do not get 24 or more volts with the furnace running then you have a pressure switch problem.
A gas furnace pressure switch problem could be caused by the following:
Your vent could be stopped up with a bird’s nest, wasp nest or debris.
You could have a bad, dragging, dirty or slow running draft inducer.
The tube that runs from your pressure switch to the draft inducer could be plugged up. Many times the tube is plugged up with water. If the pressure switch tube has water in it then this is a sign that you condensate drain line could be stopped up or not draining properly. You can disconnect the pressure switch tube and run a wet vac to hopefully clear the stopped up furnace condensate drain line.
The draft inducer hole could be plugged up. I have seen the draft inducer holes get stopped up so bad that I had to use a small drill bit and drill to open the hole back up.
On high efficiency condensing furnaces the water drain line could be stopped up, causing a water back up and blockage in the pressure switch tube. *Make sure you disconnect the pressure switch tube before using a wet vac because the high vacumn of the wet vac can damage the pressure switch. I usually take the drain hose loose from the condensing furnace and use a wet vacuum to open the drain back up.
I wanted to post this because many people are telling me that they are sucking on the pressure switch tubing to test pressure switch operation. I have heard from other forums that you should not suck on the pressure switch to test it because this can damage the diaphragm inside the pressure switch. It would be best to use a manometer or test with a volt meter like we advise on this page. I wanted to pass this along so others will not damage pressure switches.
Please send us your furnace’s model number if you would like for us to recommend a pressure switch or part for your furnace. Our email address is: Support@arnoldservice.com
Below we have a real good Youtube video made by GrayFurnaceman that shows how to troubleshoot a pressure switch on an 80% furnace. Thanks to GrayFurnaceman for making this informative video! I hope this will help you troubleshoot your gas furnace pressure switch.
Below we have a video made by Buddyraccasac which shows how to unclog an AC drain using a wet vac. This is a similar to what I recommend with the furnace condensate drain. The condensate drain would be near the bottom sides of the furnace and you would need to make sure you disconnect your pressure switch tube so it will not damage the pressure switch when you use the wet vac.
Problem: Draft inducer will not start, ignitor will not glow or gas valve will not open.
FYI: *You would be surprised how many people go without heat or cooling for hours/days and all the problem they have is the switch on the side of the furnace is not turned on or the blower door is loose after they change a filter! One customer blamed his cat for turning it off! LOL! I try to go over this with my customers before I make a service call so I do not waste their money.
Solution: Any combination of the above problem or problems could be caused by a bad furnace control board or bad draft inducer. I always troubleshoot and test the least expensive parts first, such as the thermostat, pressure switch, limit switch and rollout switches to make sure they are all closed and operating correctly before I go to the control board. First, and most important things to remember is turn your electrical power off to the furnace. We have a real good Youtube video below that shows the 8 top problems when a furnace draft inducer will not start. This excellent video was made by acservicetech. Thanks to acservicetech for making this great video!
1. The first thing that is supposed to start when a furnace starts up is the draft inducer. If the draft inducer does not start then you probably either have a thermostat , thermostat wiring problem, electrical problem, control board problem or a bad draft inducer.
2. Tape the blower door safety switch shut (if equipped) and set your thermostat so it is calling for heat. Us a volt meter set to “Volts AC” and test between terminals W (usually white wire) and C (Com) and see if you are getting 24 to 28 volts between these two terminals. If you are getting 24 to 28 volts between the W and C terminals then your thermostat and thermostat wiring are in good shape and doing what they are supposed to be doing. If you aren’t getting any voltage then you might have a bad thermostat, bad thermostat wiring, not getting 110 volts to the furnace (make sure switch on furnace is flipped on and blower door safety switch is pushed in) or a blown fuse on the control board. We have a picture of a fuse on the control board below. Most of the time you can get these fuses at a local hardware store. You would be surprised how many people go without heat or cooling for hours and days and all the problem they have is the switch on the side of the furnace is not turned on or the blower door is loose after they change a filter! I try to go over this with my customers before I make a service call. If the fuse on the control board is blown this usually means that you have a short in the low voltage circuit of the furnace. Most of the time I find that the animals have chewed through the thermostat wires and shorted them out or the thermostat wires over time have been pinched around the body of the furnace or air handler which has caused them to short out. You will probably need to replace your thermostat wire if you find it has shorted the fuse out. If the fuse does not fix the problem with the low voltage then you may have burnt out the transformer. We sell some transformers on the following page: Please click here if you are interested in seeing the transformers we sell. You should have a constant 24 to 28 volts AC between the R (red) and C (com) terminals on the control board no matter how the thermostat is set. This is the power coming right off the low voltage transformer. If you do not have 24 to 28 volts between R and C then you probably need a new transformer.
3. Most control boards are located in the blower compartment. Many control boards have a fuse located on them to protect the board from getting burned up if you have a short to ground. The fuse is usually a 3 to 5 amp fuse similar to the fuses that you might find in a car’s fuse box. Please see picture below of the furnace control board: If you find that the control board or draft inducer is at fault we sell many different types of control boards and draft inducers. Please send us your furnace’s model number and we will be glad to look it up and see which control board or draft inducer your furnace uses. Our email address is: email@example.com
If your fuse is blown you will want to check and see if any of the low voltage thermostat wires are touching the metal frame of the furnace or grounding out. Most of the time I see fuses that are blown from animals chewing through the thermostat wires and from the wires grounding out to the body of the furnace where the wires enter or exit the furnace. The vibration of the furnace over time sometimes wears through the wires insulation and causes a short to ground. I would try a new fuse first, then if the fuse blows again you are going to need to find out where the wires are shorting out. This can be quiet time consuming. If you get tired of burning up fuses we have a tool called the LiL Popper that has a button you can push in when the fuse blows instead of having to throw away fuses. We sell the Lil Popper on the following page: Please click here if you are interested in seeing the Lil Popper tool that we sell.
If your fuse is OK, tape your door safety switch shut so you can view the status LED light on the control board with the furnace power on. Please remember to turn the power back off before touching anything inside the furnace. I do not want to hear of anyone getting hurt or shocked! Turn your thermostat up so it is calling for heat. Let the furnace cycle and see if the Status LED flashes a code. The directions for reading the code are usually located on the furnace door. The code will usually tell you what is wrong with the furnace. Technology is amazing! Best of luck on repairing your furnace. Please remember to turn that power back off before touching anything.