Heil furnace pressure switch error code

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Old 12-25-15, 11:16 AM
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Heil furnace pressure switch error code

I have a Heil H9MPD100J14C1 forced air furnance operating on natural gas. It was installed about 7 years ago and has operated with no issues until recently. The unit shut down and gave me the error code "pressure switch not opening/closing"
I took all the rubber tubes off and made sure they were clean, made sure the filter is clean, the vent is clear and the drain is clear - still would not fire up so I changed the pressure switch, which did not correct the problem. Also checked the limit switches which are fine.

What happens is the t stat calls for heat and the inducer motor turns on and will usually run for 15-20 seconds (the first time) and then it shuts off. Occasionally it will get to the stage where the glow plug gets hot but shuts down before firing. Then the fan will cycle on and off sometimes running for only a few seconds before shutting itself off until the inducer motor stops, then it will start up again. I isolated the inducer fan and ran that independant of the furnace for some time so I know the fan is ok. It runs smooth and quiet.
I have 24 VAC at the R terminal and 24VAC at the W terminal when it calls for heat. When the inducer fan cuts out I still have 24 VAC on the W terminal but the fan is not running.
I have looked for obvious things like a loose wire or burnt connection but find nothing. The fact that the inducer fan only runs for a few seconds sometimes makes me think it is some sort of electrical breakdown and has nothing to do with the pressure switch even though the error code tells me it does.
I'm at a loss at what to try next and the only other thing I could think is the electrical board. Hate to put that kind of money into it when I'm only guessing?
 
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Old 12-25-15, 11:26 AM
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It doesn't sound like a board problem and very rarely does a pressure switch go bad. What is an extremely common problem is water in the condensate lines and a plugged fitting on the inducer.

The picture below is just a representative picture but the fitting on the blower is what needs to be cleaned. Small drills bits turned in by hand work best. The little port where the hose connects gets plugged with rust.

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Old 12-25-15, 11:46 AM
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This should be the manual for your furnace and has some detailed instructions in it for repair.
manual/download/lis_pdf/INSTL/L0502471.pdf
 
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Old 12-25-15, 12:23 PM
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Thank you for the quick reply and link. Do you charge extra for replying on a holiday?
the rubber tube from the pressure switch does not go to the induction fan but goes onto the heat chamber ( I guess that's what it goes to, it's on the main body of the furnace) and I did already make sure the tube was clear and the port it hooks to was clear (I ran a pipe cleaner in it)
 
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Old 12-25-15, 01:32 PM
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Look at the manual link I left. Is that your furnace ?

There are so many variations on a model that I need to see it to know which one we're working on.
 
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Old 12-25-15, 03:31 PM
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my furnace is set up just like the example on page 16 from that manual http://c.searspartsdirect.com/mmh/li...L/L0502471.pdf

is that the info you are looking for?
 
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Old 12-27-15, 02:48 PM
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PJ Max gets EXTRA CREDIT for all the illustrations he posts!
 
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Old 12-27-15, 03:54 PM
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I'm on board with that but I still don't have any heat
 
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Old 12-28-15, 11:22 AM
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the guy at the parts store suggested it could be a limit switch so I checked those and they are ok.
the part I don't understand is why do I have 24 VAC on the W terminal constantly even when the inducer fan is cycling on and off?
 
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Old 12-28-15, 12:46 PM
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<< When the inducer fan cuts out I still have 24 VAC on the W terminal but the fan is not running.>>



When you have 24 VAC on the W terminal, the inducer motor should turn on and stay on as long as that occurs.

Connect your multimeter to the 120VAC wires to the inducer motor. With the W terminal energized, you should have 120VAC turned on to the inducer motor.

If you have 120 VAC turned on an the inducer motor doesn't may have a bad inducer motor. If you have no 120 VAC being switched on to the inducer motor, you have a bad circuit board.
 
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Old 12-28-15, 04:27 PM
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I feel the inducer motor is fine as I isolated that and ran it separate of the furnace for a time without any issue. I put an amp meter on it and it would draw about 2 amp with no dips or spikes. (that was when I had isolated it from the furnace)

Is there a way to confirm that the control board is bad? If I have 24 VAC on the white terminal, and do not have 120 VAC at the inducer fan connection (which I would be quite sure is the case, although I haven't confirmed that yet) is my next step to replace the control board?

I understand that is what you are saying in your post but am just trying to be sure as I don't want to drop $200.00 on a part that can't be returned.

Just thought of one more thing.... if the control board is the problem then why am I getting a pressure switch error code?

UPDATE: I traced the inducer fan motor back to where it connects to the control board and tested it there. As expected, when the inducer fan dropped out the voltage at the control board connection did as well. Looks like I had better get a new board
 

Last edited by pushu; 12-28-15 at 05:05 PM.
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Old 01-05-16, 11:51 AM
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Replaced the control board today and at long last its working - thank you to those who offered thier advice
 
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