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Old 01-17-2016, 11:57 AM   #1
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Heil Furnace - 3 Flashing Lights Diag Code


I have a Heil gas furnace, model n9mp1. The furnace will run for a bit and then stop, without ever hitting the set limit. I look at the diagnostic codes and its flashing three flashes. I looked this up and its the pressure switch. I had an HVAC guy come out yesterday, he cleaned everything and was here for around 4 hours. When he left it was working but its now doing the same thing. He said everything looked normal but that's obviously not the case. He didn't think it was a bad pressure switch and himself was stumped. I'm not sure what I can do at this point? Call another HVAC guy?

My question is, if the pressure switch is stopping the furnace due to something obstructing it, why will the furnace start backup after a bit of time. I would think if something is obstructing it, it would never turn back on until the obstruction is removed? This is not the case with my issue.

Last edited by rj17; 01-17-2016 at 12:13 PM.
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Old 01-17-2016, 12:08 PM   #2
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Yes, he should check the suction on it with manometer.

Check your vent pipes outside for obstructions or snow sucked in if you have snow.
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Old 01-17-2016, 12:12 PM   #3
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Yes, he should check the suction on it with manometer.

Check your vent pipes outside for obstructions or snow sucked in if you have snow.
Im not sure if he used a manometer to check the suction or not, I wasn't with him the whole time. He did check the vent pipes outside and there was nothing there obstructing.

My question is, if the pressure switch is stopping the furnace due to something obstructing it, why will the furnace start backup after a bit of time. I would think if something is obstructing it, it would never turn back on until the obstruction is removed? This is not the case with my issue.
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Old 01-17-2016, 12:23 PM   #4
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You may have just enough draft for it to close but once the furnace fires up the draft changes from the density of the fumes/smoke it has to exhaust. Also if the condensate trap is dirty and not draining fast enough then water backs up in the secondary heat exchanger and trips the switch. When it stops it drains and restarts. There are a dozen reasons that switch can trip. Experienced Pros know them but the trick is to find one.
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Old 01-17-2016, 12:27 PM   #5
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You may have just enough draft for it to close but once the furnace fires up the draft changes from the density of the fumes/smoke it has to exhaust. Also if the condensate trap is dirty and not draining fast enough then water backs up in the secondary heat exchanger and trips the switch. When it stops it drains and restarts. There are a dozen reasons that switch can trip. Experienced Pros know them but the trick is to find one.
yes, experienced Pros is the problem I guess. The guy out yesterday didn't really offer a solution. He did say that the exhaust pipe outside the house was just coming straight out and should have some type of a 90 degree attachment on it. Not sure if thats the case or not. It's rather frustrating because I paid over $230 and the problem is still there. You sound like you know what your talking about.. Wanna come over? lol
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Old 01-17-2016, 12:32 PM   #6
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I understand your frustration. I train Apprentices and yeah some are as dumb as a bag of hammers and not too motivated or bright. Lots end up being parts changers.

If you have a large old well established HVAC company try them. Ask to talk to the service manager and see if he can send his senior tech. May have to kinda beg or schmooze but hey it it worth a try. Try the service manager at this tech's co and see if he can send a senior tech.

Usually the exhaust goes out straight and the intake has a elbow.
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Old 01-17-2016, 12:43 PM   #7
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Thats it, the intake is just straight out. He mentioned I should put the elbow in it. He didnt offer to do it himself. He could have charged more I imagine. I could try that, would it help?
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Old 01-17-2016, 01:04 PM   #8
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No. It should have a elbow on the intake so if the exhaust gets blown back towards it then it has less chance to be sucked in.

Technically every furnace has a specific setup as to how the manufacturer wants the outside pipes to be terminated/installed and that is in the install manual. I suspect he is guessing plus it has worked for a few years like that I assume.
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Old 01-17-2016, 01:11 PM   #9
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No. It should have a elbow on the intake so if the exhaust gets blown back towards it then it has less chance to be sucked in.

Technically every furnace has a specific setup as to how the manufacturer wants the outside pipes to be terminated/installed and that is in the install manual. I suspect he is guessing plus it has worked for a few years like that I assume.
Can you look at some videos I just uploaded to youtube on the diag codes of my furnace? The heat was off, I made a call from the thermostat to increase the temp, it didn't turn on. The following is that diag code. Its doing some weird read out, doesnt make sense. I have a Heil furnace:


What could that mean?

It then tried to kick on and then gave the following, three flashing, which is the pressure switch.


The furnace never turned on. I know at some point it will, but its freezing. lol
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Old 01-17-2016, 01:17 PM   #10
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Yeah it looks weird, kinda like a machine gun sputtering.

I don't have the list of your codes or what they mean.

All furnaces have a heartbeat ( normal power on ) code when they are not calling for heat. Usually one slow flash. Once it tries to run it is giving some weird flashes which look very odd. It could be your circuit board is faulty. They can give false codes when faulty. Does not look like any code I ever saw. Heil makes/made about a hundred different furnaces so that does not help.
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Old 01-17-2016, 01:30 PM   #11
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Its a Heil N9MP1. 90+ single stage. I even asked the guy if the board could be faulty and he said no. I will take tour advice and call the company back tomorrow and ask for the service manager.
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Old 01-17-2016, 01:43 PM   #12
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Yeah it looks like the board is shorting. ALL boards have a very distinctive pause between flashes to give you the error code.
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