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Old 01-30-2016, 12:06 PM   #1
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Heil furnace running cold, status light blinking 3 times


Hi there! I've got a 7 year old Heil furnace model H9MPD075F12C1, it started blowing cold air and the status LED is doing 3 blinks which means "pressure switch open when should be closed". I've since replaced the pressure switch and the draft fan, but this didn't resolve the issue.

I've done the usual advice I see on the net, pulling hoses off the drains and making sure there's nothing clogged. Any advice? If I jumper the pressure switch wires it runs just fine. So obviously the pressure switch isn't seeing the vaccum it needs for some reason. The question is - what's the reason? Got to be a clog somewhere, right? Thanks for your help.

Here's a pic of my furnace:


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Old 01-30-2016, 01:59 PM   #2
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I suspect the sensor you addressed is on the induction fan. Looks like a diaphragm switch. Does is make contact when you apply suction?

Our lennox furnace has several pressure switches. The one that went on our Lennox was one of two wired in series that verified the main supply fan operation and was located on the backside of the fan. This required enough dis-assembly to slide the fan out and access the switch. Since in series with the first, I replaced both. Local shops won't sell to DIY, but a parts search online and a quick conversation with the online retailer's customer service had the parts in my hands within a few days. Overnight was also an option.

Since your jumper enabled operation, I suggest you confirm you didn't jumper more than just that pressure switch and replace as required.

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Old 01-30-2016, 03:11 PM   #3
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Make sure the exhaust and intake are clear.
What draft is it pulling vs what the pressure switch is rated for?
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Old 01-30-2016, 07:18 PM   #4
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I think that unit has a trap in the bottom compartment comming off the back with the flexable pipe...check trap for clog..

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Old 01-30-2016, 08:58 PM   #5
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Quote:
Originally Posted by throrope View Post
I suspect the sensor you addressed is on the induction fan. Looks like a diaphragm switch. Does is make contact when you apply suction?
Yes - it does, and I've replaced it to make sure it wasn't the culprit. I also confirmed my system only has one pressure switch.[/QUOTE]


Quote:
Originally Posted by roughneck View Post
Make sure the exhaust and intake are clear.
What draft is it pulling vs what the pressure switch is rated for?
They seem to be - when I jumper the switch wires and get it running, it's blowing plenty of air and gets the home warm in short order. How would I go about measuring the precise amount the draft is pulling?


Quote:
Originally Posted by ben's plumbing View Post
I think that unit has a trap in the bottom compartment comming off the back with the flexable pipe...check trap for clog..
It does indeed, and it's clear of obstructions and the trap is full of clear water.
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Old 01-30-2016, 09:12 PM   #6
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You would need a manometer to measure draft.
Please don't jump anything out, that's not how to go about troubleshooting equipment.
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Old 02-12-2016, 09:56 AM   #7
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Good morning all.

So I picked up a digital manometer, and measured the draft inducer's vacuum at around 1.85 - 1.90 ". The pressure switch is rated at 1.80" so it should be activating but it's not for some reason. (I even bought a new switch to try but I guess they're maybe not super precisely calibrated) .

So it seems there isn't anything blocking the flow of air, it's a switch issue? Should I just get a switch that closes the circuit at 1.50" or 1.00" ?
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Old 02-12-2016, 10:01 AM   #8
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No, the only pressure switch that should be installed is the one approved from the manufacturer
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Old 02-12-2016, 10:10 AM   #9
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OK what would you suggest I check next?
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Old 02-12-2016, 10:36 AM   #10
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Blockages within the exhaust. Make sure all venting is installed as to instruction.
Not uncommon to find an install error many years after it was put in.
Also make sure that is the switch that's suppose to be in your furnace.

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