Furnace Question - Pressure Switch Stuck Closed

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Old 01-24-09, 10:42 AM
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Furnace Question - Pressure Switch Stuck Closed

My house has a Tappan G6RC 90+ Upflow Condensing Furnace.

Around mid decemeber it started failing to get the temperature up to the thermostat level, and short cycling. I opened the furnace up and found the condensation lines had become badly plugged. I cleaned out all the lines, and the furance ran fine for a month.

In the last week or so it started short cycling again. Both then and now, any time the furnace would fail to light, the blink code error indicates:

"3 blinks, pressure switch stuck closed"

With the furnace short cycling this time, i could get it to run again by manually draining the J tube trap in the condensation line of water. Though it was no longer clogged,it seemed just having water in the trap was causing the problem. So then I replaced the trap, and ran the condensation line straight (at a good downward angle) into the condensation pump with no trap.

The furnace ran for two days, then started short cycling again. Now, if i really fight with it, i can get it to ignite, but it cuts out after about a minute, flashes the 3 blink, pressure switch closed error code, and wont restart.

Please advise on possible causes of a pressure switch closed error. Looking for knowledgeable advice, not guesses, house is very cold, and having to use electric heaters to keep it from going to freezing....
 
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Old 01-24-09, 11:30 AM
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Furnace logic 101

Don't have a specific fix but perhaps I can explain the control logic and that may aid you in troubleshooting. It works like this:

When the control board senses a call for heat from the thermostat (24VAC on the "W" terminal) one of the first things it does is check to see that the pressure switch is in the expected state, that being open or non energized.

If it sees that it is open the board than sends power to the inducer and then checks to make sure that the pressure switch closes. If it closed it goes on with the ignition sequence (igniter, gas valve, etc.).

If it sees that the switch is already closed then it halts ignition sequence and either immediately flashes the error code or tries again two or more times and then "gives up" and flashes the error code, depending on your particular control board.

So in your case what is happening is that the control board sees a closed switch before it has turned on the inducer and figures something is wrong and for safety does not continue on with ignition. Look for something that is causing the switch to stay closed. Water in the hose or hoses going to the switch or water in the switch could be the problem. You also could have a defective switch. It is possible (but not likely) that you have a short in your wiring or (even less likely) that you have a bad control board.

You can do some troubleshooting by checking that the pressure switch opens and closes with an ohmmeter when you blow or suck on the pressure hose. Do this with power removed from the furnace.

You can also temporally replace the pressure switch with a manual switch. First make sure the switch is open and then give the furnace a call for heat. When the inducer starts running close the switch. The furnace should operate with no flash errors until you remove the call for heat. You may need to have someone else adjusting the thermostat because if the control board thinks that it takes the pressure switch too long to close after a call for heat it will give a "failed to close" fault.

BTW: I just went back and re-read your post. Are you sure of the flash code. In most cases, three blinks is a pressure switch "failed to close" fault and two blinks is the "stuck closed" fault. Well, regardless, I hope I helped a little.
 
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Old 01-24-09, 12:37 PM
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There is just a (large?) handful of things involved for you to look at, regarding the pressure in the furnace:

That fresh air can get into the burner. If you have closed combustion, make sure the PVC intake line is not obstructed. If you have an intake screen on an open combustion system, make sure the screen is not plugged with dust.

That burned gas can get out the exhaust vent pipe outside.

That water is not plugging the ability of condensation water to get out of the secondary exchanger and inducer -and that includes at every location that includes the exhaust port(s) of the secondary heat exchanger, the drain tube itself, and that the bottom of the condensate trap has no gook in it.

That the vacuum tube between inducer and pressure switch nipples is on tight.

That nowhere is that small tube cracked.

That where the small vacuum tube plugs into the nipple on the inducer motor - that down in the nipple/hole into inducer, that it is plugged in there. (You can ream that out with drill bit, wire coathanger, or whatever.)

That if you have a pressure switch where you can see the plunger between it and the switch, see if the plunger pulls in even further than how it is when it runs, if you suck in on the vacuum tube to it. It should not - ditto if you disconnect the 2 inch PVC vent from the inducer. [For safety, you may want to kill the gas supply or remove a wire on the control module or board that sends 24 volts to gas valve, so that only fresh air is coming out the open inducer hole, and not CO-laden exhaust gases.]

If you have the type of pressure switch where you see no plunger, then remove the pressure switch from the side? of the furnace, and with it still connected to the vacuum tubing, flip it around to see if you can see the diaphram through a hole in the middle of the rear. Then while running the furnace, see if that diaphram is going in and out some, as opposed to remaining steady. The pressrue switch is made so that the switch will open with the slightest change! VERY sensitive.

That the pressure switch itself could be bad.

If you are able to observe the plunger or diaphram, and see that some of the time, the plunger goes all the way in, but then at other times starts to bob around, then the issue is more likely you have a problem that is caused by the running of the furnace, which mainly would be the condensate issue, because of some blockage in the secondary exchanger, in the inducer, or the drain tubing or trap. The condensate issue is about all that can change anything while it runs.

Be sure when you blow out the drain tubing, that you also blow back into the secondary exchanger direction.

Do not leave water out of the trap. The trap is there to make sure water can get to the drain, and yet trap spent furnace gases, containing CO, from escaping through the condensate line or trap vent (tee fitting)!

......................................

The furnace where I had a pressure problem just days ago, that also was on the bubble - I ended up replacing the pressure switch. Was that the trouble? Even though days have gone by now without interuption, I am not positive, since my plunger would start to act up some, even on the new presure switch, after it ran a while. Yet, it did not kick off the furnace fire, the way it had been. Like I said, these things are designed so close to the edge of not working, even when everything is good. What I did discover is condensate water was flying around in the inducer fan housing, that was not drain/trap caused, and I found a ridge in the rubber coupler between the inducer exit and 2 inch pvc pipe and I carved a 1/16 inch ridge off to allow whatever that may have held back, to drain. Time will only tell, I guess. At least it faired through minus 15 degrees here last night!
 
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Old 01-24-09, 04:57 PM
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Originally Posted by xpogi View Post
Don't have a specific fix but perhaps I can explain the control logic and that may aid you in troubleshooting. It works like this:

When the control board senses a call for heat from the thermostat (24VAC on the "W" terminal) one of the first things it does is check to see that the pressure switch is in the expected state, that being open or non energized.

If it sees that it is open the board than sends power to the inducer and then checks to make sure that the pressure switch closes. If it closed it goes on with the ignition sequence (igniter, gas valve, etc.).

If it sees that the switch is already closed then it halts ignition sequence and either immediately flashes the error code or tries again two or more times and then "gives up" and flashes the error code, depending on your particular control board.

So in your case what is happening is that the control board sees a closed switch before it has turned on the inducer and figures something is wrong and for safety does not continue on with ignition. Look for something that is causing the switch to stay closed. Water in the hose or hoses going to the switch or water in the switch could be the problem. You also could have a defective switch. It is possible (but not likely) that you have a short in your wiring or (even less likely) that you have a bad control board.

You can do some troubleshooting by checking that the pressure switch opens and closes with an ohmmeter when you blow or suck on the pressure hose. Do this with power removed from the furnace.

You can also temporally replace the pressure switch with a manual switch. First make sure the switch is open and then give the furnace a call for heat. When the inducer starts running close the switch. The furnace should operate with no flash errors until you remove the call for heat. You may need to have someone else adjusting the thermostat because if the control board thinks that it takes the pressure switch too long to close after a call for heat it will give a "failed to close" fault.

BTW: I just went back and re-read your post. Are you sure of the flash code. In most cases, three blinks is a pressure switch "failed to close" fault and two blinks is the "stuck closed" fault. Well, regardless, I hope I helped a little.
I am 100% on the error codes, it has it listed on the panel, and in the manual, 2 blinks = stuck open, 3 blinks = stuck closed. Since the furnace will run fine, and go through the start up sequence, i am fairly certain that it is in fact working (no shorts, board is fine). If i disconnect the hose from the pressure valve to the condensate tank, it fails with the blink code "stuck open".

I dont have the kind of switch where you can see in it. It is sealed black plastic disk, with two wire leads, and two small tubes; one to condensate tank, and one that splits in a T joint to the burn chamber, and the gas valve. I have investigated the tubes to the condensate tank, but havent touched the two that go to gas oriented components. I can shut off the gas and check those two for blockages. I will also try taking off the pressure switch, so i can see inside it from the back, and make sure it works.
 
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Old 01-24-09, 06:34 PM
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Originally Posted by eisley79 View Post
...I can shut off the gas and check those two for blockages.....I will also try taking off the pressure switch, so i can see inside it from the back, and make sure it works.
No need to shut of the gas. The hose from the burner box and gas valve will have no gas in it. Kind of technical but it as a "vent" for the pressure switch and gas valve that goes to one side of the diaphragms in each so that the differential pressures caused by longer vent pipes (flue and combustion air) does not effect the rate (BTU/Hr) of the furnace or switching pressures of the pressure switch. Have you tried replacing the pressure switch with a manual switch?

Oh and you probably won't be able to see inside the switch from either side. You will either have to check it's operation with an ohmmeter or take it out of the circuit by substituting it with a toggle switch. If you don't feel comfortable doing this perhaps you better seek the advice of a professional. I am a furnace designer and tend to be a bit skittish about recommending "do-it-yourself" furnace repair anyway. Please proceed with caution and never ever leave a pressure switch, limit switch, or other component permanently bypassed in a furnace. They aren't just control devices... they are safety devices and are designed to shut your furnace down if there is a dangerous malfunction. Defeating them is like playing Russian roulette.
 

Last edited by xpogi; 01-24-09 at 07:09 PM.
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Old 06-22-09, 07:49 AM
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Hey there, im having a similar problem.

I have a coleman g8c07512mub12g HVAC system. I have recently had a control board replaced because of a lightning strike. The heat was working fine when the control board was replaced, but no one ever checked the A/C. I tried to turn on the A/C and the control board flashes twice (indicating pressure switch closed), and no fan or anything else turns on. The heat still works fine.

Would you think this is a bad pressure switch even though the same pressure switch works for the heat?

Please advice me on how to proceed.

Thanks
 
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