Furnace pressure switch conundrum


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Old 10-15-08, 11:39 PM
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Unhappy Furnace pressure switch conundrum

I have a Goodman GMP 100-3 furnace that failed to fire-up on the first attempt this fall. After testing the voltage on both sides of the pressure switch and decoding the blinking light on the circuit board I've determined that the switch isn't closing. I removed the tube, sucked on it and the furnace fired right up.

In an attempt to determine whether the diaphragm in the switch is perforated, I sucked on it, pinched it off and held it for 30 seconds. The furnace continued to run while I held the hose pinched. (leading me to believe the switch is fine)

The tube leading from the pressure switch to the draft inducer appears to be in good shape and fits rather well on both nipples.

I stuck a wire about 1/2" down the hole on the draft inducer nipple to insure it was clean, and it seems to be.

The high-temperature sealant where the draft inducer meets the furnace appeared a bit cracked, so I applied some duct sealing foil tape (knowing this wasn't a safe long-term fix) to see if maybe it was sucking-in fresh air at the seal and therefore wasn't creating enough suction at the nipple to close the pressure switch. No dice.

Finally, I disconnected the vent pipe from the chimney to see if it was obstructed. I could only see to the 90-degree elbow at the base of the chimney, but that at least was not obstructed. To see whether an obstruction in the chimney was the culprit, I attempted to start the furnace while it was disconnected from the chimney. Again, nothing.

What am I missing?
 
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Old 10-16-08, 07:23 AM
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You are doing all the right things.

You never jumper tested. But even if you did, that would not prove the switch over a venting issue, as they are tied together. Yes, you would make the furnace work, but it would not prove which one.

Your suction test may have some issues with that test you may not realize. For example, maybe when you suck in hard, you are doing something to the diaghram that normal slow, minor and steady suction does not do, like a pinhole on the one side of the diaghram up against the metal housing, or something. I don't know. Just saying. Vacuum draw through the tube leading to the pressure switch is so minute one can hardly feel it on their finger. Wetting your fingertip helps. The suction is that miniscule.

As tempting as it is to be so careful as to not replace a good part, you might have to bite it in this case, and buy another one and see, even though it might not be it. Because if it isn't, you may have something really major, like perhaps a bad secondary heat exchanger or primary heat exchanger with a major crack or hole in it (I've actaully seen that and diagnosed it first hand. And secondaries can go bad also. We've had them replaced, professionally). I can't recall for sure if the probele is ususally visible or not, on a secondary. It is possible that they may wind up with debris inside that you can't see externally. You can view the bottomside of a secondary (on an updraft furnace) by removing the blower motor and looking up, but that might not prove anything, as stated.

Oh. I just thought of soemthing else: Be sure to blow out the condensate lines both down the line and up the line! This can effect the draw!
 
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Old 10-16-08, 10:13 AM
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Thanks for your response!

My plan is to grab a new pressure switch on my way home tonight and give it a go.

Regarding your point about the condensate lines, where are these? The pressure switch I have only has one nipple (no condensate line here). Are there lines elsewhere on the furnace I should be checking?

Sounds like if replacing the switch deosn't do the trick, I'm going to have to call-in the pros.
 
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Old 10-16-08, 03:49 PM
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Those other more like 1/2 inch sized rubber tubes that wind up going to a the combination drainpipe/exhaust gases PVC pipe, that should be vertically mounted alongside the furnace. The condensate from the condensing secondary heat exchanger collects in that tubing. A high efficiency furnace is also known as a 'condensing furnace'.

Is this an open combustion furnace where it only has the PVC exhaust pipe?
 

Last edited by ecman51; 10-16-08 at 06:12 PM.
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Old 10-16-08, 10:03 PM
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My house is 109 years old, so the forced-air system was installed in a somewhat unconventional manner. The only candidate for a condensate line is a green garden hose running to the floor drain. It exits the furnace immediately below the insulated line that goes to the a/c condenser outside. I thought this was only for the a/c...but maybe it's also for the furnace?
 
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Old 10-17-08, 06:56 AM
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Originally Posted by ecman51`
Oh. I just thought of soemthing else: Be sure to blow out the condensate lines both down the line and up the line! This can effect the draw!
E,

This furnace is an 80% furnace, so there is no condensate line on this.

MN,

Have you gone up to the roof to be sure the vent is clear up there?

Also, when you took the pipe off at the furnace, did you look into the inducer assembly to see if the blower cage is not rusted, missing blades?
 
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Old 10-17-08, 08:26 AM
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I was so close!

I replaced the pressure switch last night and nothing changed so I kept my appointment with the furnace repairman.

The guy walked in this morning, I told him what I'd checked and replaced, he said "ok, so you've determined everything is clear on both sides of the draft inducer. Sometimes the little hole where the hose to the pressure switch attaches gets clogged up." He disconnected the hose from the pressure switch and blew through the hose. Sure enough it was clogged. He took a little Allen wrench and jammed it in the hole, reconnected the hose and it fired right up.

The frustrating part is I stuck the tip of a coat hanger in there myself, but didn't want to get overzealous and ruin the insides of the draft inducer.

Oh well. At least my trouble shooting eliminated his need to do it and charge me for the time. And I learned a lot about how a furnace works.

Thanks for your help e and j.
 
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Old 10-17-08, 04:06 PM
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That is a shame. Here I thought you had that hole cleaned out. Oh well. These things happen sometimes. At least you can feel good in the fact that you pretty much knew what to try to do, even before I ever started suggesting stuff.
 
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Old 10-24-08, 12:04 PM
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Unhappy this has been more helpful then any thign else ive seen

Hello,

I have the GMP100-3 as well and my issue is this if any one can help me out, I get absolutely no activity except for the LED blinking 3 times (pressure switch failure to close) I have tried to use suction to close said switch by gently sucking on the hose the only result I get is that the light will stay constant for a moment then continue blinking again I have taken off the exhaust pipe and cleaned out all the soot and grit in there and inspected it from below and above it is clear.

I have cleaned other components as well such as the flame sensor and the ignition element (not the right term) but I get nothing if you can suggest any thing else it would be oh so helpful as I donít have the money to call in a pro I will just have to deal with the cold and its starting to get very cold.
 
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Old 10-24-08, 01:22 PM
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Does the small inducer fan run?

Don't suck on the hose till the fan runs, then suck on it to see what happens.. You'll have to the switch closed (sucking all times )

If you get the error code again while sucking the whole time, then I'd check make sure the wire is in place and clean all the way back down to the control.
 
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Old 10-24-08, 03:36 PM
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Unhappy

no the fan does not start up, I get nothing but the blinking red light, I have maintained constant suction on the hose and checked the wires all the way back.

I jumped the pressure switch wires and it then gave me 2 blinks (pressure switch stuck closed) beyond that I get nothing at all.
 
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Old 10-24-08, 05:48 PM
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i got it going

so it seems the power connector on the venter motor was loose I used a zip tie to tighten it up and were back in business I appreciate the help thank you
 
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Old 10-11-10, 09:02 AM
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vacuum test

Thanks for the tips on testing the pressure switch. One other trick I found helpful: use a wisp of cotton pulled from a cotton ball to check for the presence of vacuum -- it will show even the minute vacuum at the pressure switch.
 
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Old 12-15-10, 07:46 PM
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Similar situation......

I have a Carrier 58PAV... and here's what's happening. It sequences fine, Inducer fan comes on, pressure switch closes, ignitor glows, gas valve opens, flame ignites, flame sensor senses flam to keep it lit, then the circulation blower comes on. After about 5 minutes the inducer fan stops, flames go out, and I get a flashing red light code of 31- " Pressure Draft Safeguard, Auxiliary limit or Vent Clogged or Switch did not close or reopen"

Things I have done-
1. cleaned the flame sensor
2. Last year replaced the inducer motor
3. removed pressure switch hose- it's not plugged
4. looked up the exhaust vent- not clogged or blocked.

Could use some help, it's 58 degrees in here and not warming up outside anytime soon!

thanks-
 
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Old 12-15-10, 08:09 PM
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It's confusing when people piggyback problems on top of each other.

My suggestion is to re post on a new thread.
 
 

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