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Rheem Criterion upflow gas furnace always tripped negative pressure switch?

Rheem Criterion upflow gas furnace always tripped negative pressure switch?

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Old 11-17-14, 08:42 AM
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Rheem Criterion upflow gas furnace always tripped negative pressure switch?

Hi, first time home owner here, please bear with my silly questions. So I have this 20-year old Rheem Criterion upflow gas furnace when I closed this house last year. Last winter, it gave us some trouble when blower is running but no gas out, so no heat. An HVAC tech came over, did a few measurements and found that the pressure switch right above the burner is not getting voltage, so he opened up the circuit board, unplug/plug back the two wires connecting the switch and the board, and then the furnace is back on until a couple of days ago, same problem occurred.

But this time, the old trick did not work wonder for long, the gas would stopped after a few cycles of on and off, and most likely in the middle of the night ( it starts and stops more frequently at night). I found out that there seems to be some contact problem on the board, for example, if I reset the switch by unplug/plug the wires, and the furnace started to burn gas, the moment I tap somewhere on the wire bundles or the board, it shuts off. And furthermore, I noticed that if it shuts off, you don't even have to unplug/plug the wire, tap the wire or even hold the board and drag it towards me a little would get the thing restarted (And yes, the board was fixed on the place in a non-level way, the gap at top and bottom between board and the metal casing is different. The bottom gap is much smaller, almost touching the metal casing).

Have you guys had similar experience before? Last time the tech told me if similar problem happen, I will be expecting an expensive board replacement,7-800 at least. I don't know if it's worth it for this old furnace. Please advise. Thanks a lot.Name:  mmexport1416238109245.jpg
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I attached the diagram of the circuitry and the pix of the wire bundles FYI.
 
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Old 11-17-14, 09:36 AM
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Welcome to the forum!! I suggest you turn power off to the furnace. Remove the 9-wire connector that attaches to the control board. Inspect the plug for corrosion and make sure none of the pins are sticking further in or out than the other pins. Inspect the control board where that plug goes into and check it for corrosion. There should be a ground wire (usually green) that attaches to the furnace cabinet. I believe on your furnace the ground wire goes from the 9-pin connector green wire to the safety door switch area. Follow that wire and see where it attaches and make sure the screw is tight and there is no corrosion. You may also have a ground wire coming from your gas valve. Grounding issues can cause a problem such as you are experiencing. Your wiring diagram photo is not clear enough to make out. To better assist, please post the model number of your furnace.
 

Last edited by firedawgsatx; 11-17-14 at 10:22 AM.
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Old 11-17-14, 10:43 AM
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Thanks for the reply. I actually never thought of the grounding problem. Not sure about the grounding inside the electronic case (bottom box of the furnace), but I know that the green ground wire in the top case where burner sits (ground wire from gas valve you referred to) actually just loosely attach on the metal case) it was never really screwed on stably, but we never had such frequently shut-off as well.

BTW, where shall I check for the model number and SN? I took apart both the top and bottom metal box covers, but never really see any specific model number information.

Another thing is, I never really clean the vent of drain for the system. Where shall I start? I saw the vent of this furnace goes up with water heater in AL tubing, since the system is in the basement, I suppose the vent is connect to the chimney, how would I clean that up without climbing up to the roof?

Also I do not see any drain pipe at the bottom of the furnace, where would it normally sit? And will the clogged vent and drain cause the same symptoms that I'm having?
 
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Old 11-17-14, 11:02 AM
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BTW, where shall I check for the model number and SN? I took apart both the top and bottom metal box covers, but never really see any specific model number information.
The data plate with the model number should be located on the cabinet wall inside where te gas valve is located.

All ground wires need to be tight and clean. I highly suggest you start there. As far as cleaning the vent pipe you will need to make sure there is nothing inside of it such as a bird's nest, wasp nest, dead bird or other animal. You will probably need to get on the roof to properly inspect the vent pipe unless you can take the pipe off of the furnace and push it up and look inside.

Your furnace is not a condensing furnace so the drain pipe is not an issue.
 
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Old 11-17-14, 11:25 AM
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Size:  32.7 KBThanks. Can you elaborate why grounding issue would cause the gas shut off? And what are other possibilities for that symptom?

I also found two old pictures of the gas valve?
 
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Old 11-17-14, 11:34 AM
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Check the grounds to see if they are good. Good grounds are required if you want your furnace to properly operate. Another possibility is if the vent pipe is blocked/restricted the pressure switch will not close or stay closed and the gas valve will shut off.
 
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Old 11-17-14, 11:54 AM
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I don't think I have ground on the gas valve. The loose wire touching metal case I was talking about is the green/yellow wire attached to the fan if you look at the first picture. Don't know if that's a possible cause, but I'll tie it down.

Another thing is about the vent, I'm at a 3-story townhouse, so without a really long ladder, I don't think I can access the top of my chimney. I do have a fireplace in the basement directly vent to the chimney though. Is there anything I can do to kinda determine vent is not blocked (say, maybe access from the attic?) Otherwise, I may need to hire someone to do the duct cleaning and chimney sweep to make sure it's not clogged.
 
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Old 11-17-14, 12:00 PM
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You should have a ground that looks something like this (two green wires). As far as a wire on the gas valve the wiring schematic should show if one is required. Some schematics list a ground on the gas valve as optional.
 
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Old 11-17-14, 12:10 PM
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Here is a better image of the wiring diagram. And btw, the fan is making big noise when slowing down or starting up, and if I pressed the fan case or some attached wire, the vibration and noise diminished. But I do not see any place that I could tighten a screw. Is that a sealed part?
 
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Old 11-17-14, 12:28 PM
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And btw, the fan is making big noise when slowing down or starting up, and if I pressed the fan case or some attached wire, the vibration and noise diminished.
Are you talking about the inducer motor? Did the noise just start?
 
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Old 11-17-14, 01:05 PM
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Sound are front the "fan" area in my 1st gas valve image, so I guess that's the inducer motor? It's a clicking sound frequency goes with the fan speed. It's obvious when starting up or slowing down and gets buried in the other sound such as gas burning hissing and the air blow sound. Been like that since end of last winter.
 
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Old 11-17-14, 01:24 PM
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Yes, the "fan" in your photo is the inducer motor. The noise you hear is possibly the bearings going bad in the motor. There is also a possibility something could have entered it through the vent pipe causing the noise. The inducer motor creates a vacuum to close the contacts in the pressure switch. The fact that the hvac technician said you weren't getting voltage at the pressure switch may be related to the inducer. You would get 24V on one side of the pressure switch from the control board but if the inducer vacuum does not close the switch you would not show 24v on the other side. When the inducer motor is running and the pressure switch is closed you should read 24V to ground on each terminal. Testing across the two terminals the reading would be 0V if the pressure switch is closed. You can remove the inducer motor and see if you can spin the wheel by hand and evaluate any noises.
 
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Old 11-21-14, 07:34 AM
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OK, I want to report back and express my gratitude to @firedawgsatx. After cleaning and securing the grounding and stuff, the furnace was up and running well for the past couple of days, no overnight stoppage.

Had an HVAC tech come over for tune-up, but he did not do anything after seeing the furnace was running OK and the chambers looked clean. I was disappointed since I was expecting something magic from the "pro", "checking the sensors, cleaning heat exchanger etc per some check list. But the dude got his point, furnace this old, a lot of techs wouldn't touch it, since you might just mess something up and create some problems instead of solving them. I wasn't expecting these from a "pro" though. Anyway, what he actually helped is to secure the induce motor with some steel wire (to pull it inward a little, because the three pins that the fan was fixed on the induce motor case were loose, so the fan was unbalanced and hitting the case, making the sound I described), he also secured the three bolts that put the induce motor on the chamber too.

Another thing he noticed is that my freon was leaking inside. Outside unit seems dry and fine. But there were two copper pipes in the AC part right above the furnace, he touched the pipe and found some oily stuff on his finger on one pipe (thin one), and told me that "1000% the freon was leaking". Well from what I can tell, it was just a little bit of oily, no dripping, no puddle, no ice built on this passing summer. I did notice that my top floor was 2-3 degrees higher than the main floor this passing summer. He said to properly repair the leak, he would have to get all the freon sucked up and refilled, and the cost of material would be 4-500 hundred, plus the "ghetto way" fix for the induce motor probably would not last too long as well, so it might be the time to replace the whole AC+furnace.
 
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Old 11-21-14, 07:38 AM
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Glad to hear your furnace is doing well and thanks for the update!
 
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