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 the desired temp is reached. Sometimes this will occur 3-4 times depending on reaching the 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$(!). I ordered one myself and replaced it last weekend. Took me 5 minutes and the switch was 60$. 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 being blocked. The 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 a 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 by 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. We have a really good YouTube Video made by AC Service Tech, below that shows how to properly clean out a furnace’s condensate drain lines and trap. 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 furnaces tell how many feet of the vent pipe is 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 updraft 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 the furnace when vacuuming out the condensate line will ruin the diaphragm on the pressure switch. (7) A leaking heat exchanger. If your furnace has a leak in the heat exchanger combustion gas is allowed to mix with air in your home or in with the blower motor air. This hole in the heat exchanger will not allow the negative pressure that the pressure switch needs to close and operate properly. If you find that you have a hole in the heat exchanger then you need to replace the heat exchanger or get a new furnace because this creates a dangerous condition that can allow carbon monoxide into your home. 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 email@example.com. 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
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