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javagrendel 03-15-2013 03:39 PM

Nordyne furnace, error code: Pressure switch stuck closed.
Hello all. I have an older Nordyne updraught gas furnace. Here is the official spec:

Model # FG6RA072C-12
Serial # FG6000908021

About 3 weeks ago, the furnace started acting up. It seemed to be continuously going through its start-up cycle without firing: Inducer motor starts up, ignition coil heats up; but instead of lighting the gas, the cycle would abort, and the inducer motor would shut down, and after a few seconds, it repeats this process. At this point, I turned off the main power to the furnace, and just let it sit for about 20 minutes. When I turned it back on again, it fired up just fine and continued to work.

Last week, it started doing the same thing. Once again, I turned it off for about half an hour. This time, however, it wouldn't light at all. I did some research, and figured out that the red blinking light on the main board was a code. I looked up the code, and it was "Pressure switch stuck closed" (there's a legend on the inside of the cover panel). We decided to let it sit overnight and try again the next day. No joy.

At this point, I called my gas utility co. for a technician to come out and have a look at it. They couldn't get one out until Monday. We survive on the space heaters over the weekend. On Monday, the tech shows up, takes a look around. At this point it's been two days since I turned it on. He cycles it on and off a couple of times, checks the pressure switch with a multi-meter and checks the pressure on the inducer motor. I can't remember his exact words, but he indicated that something was low about the inducer motor - either the voltage(?) or actual negative pressure - but that everything *appeared* to be okay. It's been about 10 minutes at this point, when he cycles the furnace off and on again. It lights up. And stays lit. I'm relieved, and I ask him what he did, so that I can try it myself if it happens again. He says he didn't do anything. It's very dusty in there, especially around the inducer motor, and he tells me to keep that area clean. He doesn't charge me for the visit, because he didn't actually do anything (which was pretty awesome of him - it's usually around $100 for a tech to just show up at your door). I'm confused, but relieved to have heat again. I thank him, and he leaves.

A couple of hours later - you guessed it - the furnace starts acting up again. Same old 3 flashes on the main board, indicating Pressure Switch stuck Closed. I tap here and there (lightly, of course), unplug the hose from the inducer motor, make sure it's clear, plug it back in. I've thoroughly cleaned the area. With the power off, I try flicking the blades of the inducer motor (gently, of course) and I can see that the blades move easily. I try leaving it turned off overnight, survive another night with space heaters; alas, next day, same story. It keeps going through its start-up cycle (inducer fan turns on, ignition element heats up, then the whole thing stops, main board blinks 3 flashes).

I also have a gas water heater, and it appears to share the same vent as the furnace - the two ducts join up near the rear external wall in the basement and go into what I assume is the flue for the chimney. We haven't had a problem with the gas heater.

At this point, my theory is that I have an airflow problem. Either the furnace cannot vent out properly, or the air intake is somehow obstructed.

1. Since the hot water heater hasn't stopped working, is it safe to assume that the actual flue into which the furnace vents is open? Or do the two appliances have different requirements for air flow? That is to say, the vent flue is just blocked enough that the furnace won't operate, but not so much that it prevents the water heater from working?

2. If the external vent flue *is* clear, it's possible there's a blockage in the duct work between the furnace and the spot where it joins up with the ductwork coming from the gas water heater. My next step is to try to disassemble that section of the duct to check it. This just seems unlikely to me - it's a very short section of ducting - about 3-4 feet long, and it's hard to imagine what could be obstructing it. Am I barking up the wrong tree here by trying to open up the ducting?

3. Where is the furnace's air intake? I can't find *anything* that looks like it's feeding air *into* the furnace from outside the actual furnace itself. Now, there's a big motor/fan sitting *under* the furnace - which appears to blow into the air filter (the one I replace 2 or 3 times a year) - is this the air intake? When the furnace is functioning normally, the big blower fan is the last thing to start up (happens almost simultaneously with the gas getting lit). Am I correct in assuming that this the air intake for the furnace? I kept expecting to find an external source of air for the furnace, but I guess it makes more sense to get the air from inside the house, where it's warmer, than to get it from the outside, where it is much colder. This motor/fan also appears to be fine. I've done my best to clean it out - the fan blades are also clean, and they move easily by hand (again, tested this with the power turned of, obviously).

Tomorrow being Saturday, I'm going to get up to the chimney, if I can do so safely, and see if the climbing ivy we have growing on the back wall of the house, all the way up to the top of the chimney, is in any way causing an obstruction.

Can there be another reason for why the pressure switch is stuck closed?

Your help is much appreciated. Even though it's spring-ish, it's still very cold, and we've been without a furnace for a week now.

beenthere 03-15-2013 04:09 PM

More like just a defective/worn out switch.

javagrendel 03-15-2013 04:11 PM

Thanks for your answer, beenthere. I had figured that because it was working intermittently, that the pressure switch was fine. I'll look into replacing it.

how 03-15-2013 04:29 PM

A pressure switch measures the vaccum created by a running inducer motor assy. When there is a call for heat, before the inducer motor turns on, the board verifies that the electrical circuit through the pressure switch is open. After the board verifies this, the inducer assy is powered and the pressure switch is pulled closed completing an electrical circuit that allows the furnace to continue onto the ignition cycle.
It sounds like you have a faulty electrical switch within the pressure switch. This whole PS will need to be changed.

javagrendel 03-15-2013 05:22 PM

I just tried by-passing the pressure switch by shorting the two wires connecting it to the main control board. The gas did ignite, but only for a couple of seconds. I tried it again, this time the gas did not ignite at all. Is it still worth trying to replace the pressure switch?

how 03-15-2013 05:50 PM

You've been describing an erratic electrical switch response in the pressure switch.
A code that says the PS is closed when it should be open will either be a faulty PS, a short across the wires that attach to it or a faulty board.

You can use an electrical meter to verify that electrical switch in the PS is stuck closed when it should be open.

javagrendel 03-15-2013 06:12 PM

A faulty board makes sense, I think.

I just unhooked one section of the duct so that it vents directly inside the basement. When the inducer motor starts up, I can feel air being forced out through the duct, but the gas still won't ignite. I took that to mean that it's not a problem of the exhaust vent being blocked. I also took the opportunity to inspect the duct itself, and it's not blocked with anything.

When I disconnect the two wires from the PS, the ignition sequence doesn't even start up. The MB led flashes twice, which, according to the legend on the inside of the cover panel, means "Pressure switch stuck open". Then I short the two leads using a small piece of wiring. The main board LED switches to ON (meaning it stops flashing), and the ignition sequence starts up again. (Note: if I disconnect the two leads while the ignition coil is heating up, the sequence aborts.) From this point on, the furnace behaves exactly the same as if the PS was still connected; the main board responds to the call for heat from the thermostat by starting the inducer motor. After a few seconds, I hear a click, and the ignition coil starts to heat up. When the coil is glowing a bright orange, it should ignite the gas, and then turn off. But it doesn't do that. Instead there's a loud click, and the sequence aborts. The inducer fan keeps running for a while longer, and the main board LED starts its error code flash (three flashes). After flashing the error code for a few seconds longer, the inducer motor turns off. The entire sequence begins again immediately, with the inducer motor quickly turning on again.

javagrendel 03-15-2013 06:56 PM

1 Attachment(s)
Ok I don't know what to make of this. I unplugged a couple of wires, blew some dust off the mainboard and connected everything back up and then restarted the furnace. It started up and now it appears to be working fine. In the picture you can see the vent flue switch that I disconnected and reconnected.

Attachment 67410

javagrendel 03-15-2013 07:15 PM

This actually happened when the gas utility tech was here. It worked for most of the day and then stopped working again. It has heated up the house to the point that the thermostat turned the furnace off again. I'll have to see how it does for the rest of the night.

The fact that it's so flaky tells me something is up with it. :-(

eboat 03-16-2013 12:04 AM

Unless you have two flu pipes going out the furnace your fresh air intake is coming from the room the furnace is in. Which means you have an 80% furnace. After the hot surface ignitor heats up you should hear the click of the gas valve opening and then it lighting the burners. I assume you have gas and your tanks not empty so your two choices would most likely be the gas valve or control board not functioning correctly both could work intermittently but sense you were getting a fault code adding pressure switch it almost sounds more like a board. You need a meter and a magnahelic to really determine the exact problem but with what we know I would replace board and pressure switch. The switch is very cheap and fail all the time so it'll be good to have that regardless.
Good luck lol

eboat 03-16-2013 12:09 AM

Also it's hard to see to much in that picture but what you unplugged was a roll out limit. Which doesn't have anything to do with your problem your having now. It's only there to keep your furnace from being damaged or your house from burning down if the flame starts rolling back out of the heat exchanger

eboat 03-16-2013 12:12 AM

On further looking that actually might be a vent switch in the picture. But if it was tripping you'd have to Manuel reset it with that middle button right on it. So either that's Ok

javagrendel 03-16-2013 01:07 AM

According to the spec that is a vent switch. I may have tried pushing the reset button before, but I don't remember for sure. The furnace has been functioning normally since I did that. We shall have to wait and see if it lasts through the night.

eboat 03-16-2013 01:22 AM

Ok lemme know if you have any new developments its a fairly simple furnace so im sure u wont have to much trouble getting to the bottom of this.
Gofd bless

javagrendel 03-25-2013 09:53 AM

So my furnace continues to act up, coming on sometimes and not at other times. I want to replace the pressure switch, but I'm having a little difficulty tracking down the part number. Does anyone have any suggestions as to where I might find out this information? In my original post, I linked the pdf documentation that I was able to track down, but it does not list a part number. I've tried searching in Google for key words "parts list FG6RA series furnace pressure switch" but so far I'm not having any luck.

Any help would be greatly appreciated!

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