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Old 11-12-2012, 10:40 AM   #1
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Heil Furnace problem - is is the undersized vent?


Hello -


I have a 100 mbtuh 90+% efficiency propane furnace manufactured by Heil, complete model number - N9MPD100J20A2. It is direct-vented outside via a concentric vent. Manufactured late 2007.

Ever since I moved in in 2009, I have been battling the same problem – the furnace would fire up just fine and start to heat the house, but one of two things invariably happened – at best, the furnace would periodically stop firing while trying to reach it’s target temperature, but would eventually get there. This was the story of the first winter. After that though, more and more what would happen was the thing would just stop at a certain point, returning the 3 flashes “pressure switch open when should be closed” code. I would then wait, assisted by a combination of power-cycling the furnace and/or resetting the thermostat, until the furnace sprang back to life. Sometimes I could get it up to target temperature, at which point its cycle was short enough that it would often work properly for the rest of that day. Then, allow it to cool down over night, and the same daily routing repeats for the rest of the heating season.


I have started learning as much as I could about my furnace, and have in the meantime had a couple of guys come out and look at it. One of them swapped out the pressure switch, and another suggested that the circuit board needed to be swapped out. I didn’t give this latter suggestion much credence – my knowledge of electronics leads me to believe that circuit problems are rarely this repeatable, and besides it turns out he was reading the error code list wrong
Swapping out the switch did nothing to help.

So this year I decided to get serious about tracking it down. I figured that if the furnace could be depended upon to start up fine and work for 10 minutes every day before it shut down, either it was overheating or water was getting in somewhere. There was rust on the tray underneath the tee-trap, and last year there had been some water in there, but last weekend I cleaned and flushed the relief tube and drain tubes, flushed and re-primed the condensate trap, and checked all their connections for tightness. Since the online suggestions for “pressure switch open when should be closed” often included clogged combustion vent/intake, I opened up an unglued section of the intake and shopvac’d it make sure it was clear. I couldn’t figure out a similar way to clean the vent (except prodding the final straight section to the outside) but a good solid stream of steam leads me to believe it’s clear.


The good news after this was 1) No more water leaks. 2) somewhat more of the “get up to target temperature in stages” and less “stop with 3 flashes” but the problem is still there. If I just let the thing run, after maybe 5-10 minutes of operation, the furnace would cut out with a 3-flash code. Mostly now though, I can at least “coax” it up to target temp by just increasing the thermostat temp by a degree at a time so that the furnace doesn’t have to burn for too long at once.


I was running out of ideas. Then a couple of days ago I started reading through the manual in a bit more detail, and something stood out to me. My model, the 100 mbtuh is the smallest model that requires a 3” vent kit. After measuring my concentric vent and checking online how to ascertain if you have a 3” or 2”, it turns out that the installer of this machine installed a 2” vent. Strikes me that the symptoms of “clogged combustion vent/intake” should be the same as those for an undersized vent/intake.


So I just wanted to ask here if you guys think I’m on the right track before I spend what looks like about $100 to replace the vents, or if you have any better ideas?


For the sake of completeness, a couple more things. When I started looking into this, I found that there were serious problems with the ducting. The panned return was only about a quarter of the size it needed to be, so I have fixed that with an additional floor return, but worse, seems like the installer had cut a wrong hole on the return air plenum. His solution? Turn the plenum upside down so that the hole faces down where it can’t be seen, stuff the furnace manual up against it in a vain effort to close it, and hope the previous owner didn’t find out – which he didn’t . . . I did. The crawlspace where this furnace lives is at present unfinished and very dusty. The furnace was circulating crawlspace dust through the entire house. Did I mention there was no filter in there when I first started looking at it?


Thanks for taking the time to read this


Malcolm


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Old 11-13-2012, 09:39 AM   #2
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Heil Furnace problem - is is the undersized vent?


(Shameless bump . . . )

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Old 11-13-2012, 10:38 AM   #3
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Heil Furnace problem - is is the undersized vent?


In your manual it should give possible vent lengths and sizing reccomendations. Sounds like the venting may be the candidate as most 100,000 units do require 3" venting. It is also possible that improper grade of the venting could also cause similar error codes. It can be as simple as the grade into the furnace casing itself sometimes. (depending on how your model ties the venting into the furnace) Coincentric vents can also become plugged on the intake with spider webs, leaves, etc. and can be difficult to clean.
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Old 11-13-2012, 12:15 PM   #4
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Heil Furnace problem - is is the undersized vent?


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Originally Posted by HVACDave View Post
In your manual it should give possible vent lengths and sizing reccomendations. Sounds like the venting may be the candidate as most 100,000 units do require 3" venting. It is also possible that improper grade of the venting could also cause similar error codes. It can be as simple as the grade into the furnace casing itself sometimes. (depending on how your model ties the venting into the furnace) Coincentric vents can also become plugged on the intake with spider webs, leaves, etc. and can be difficult to clean.
The vent does grade back down towards the furnace, quite noticeably actually - is it possible for the grade to be TOO steep?

I think I'll spring for changing out the venting - certainly can't hurt to do it right.

Thanks for your reply, Dave, much appreciated . .

Malc
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Old 11-13-2012, 07:23 PM   #5
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Heil Furnace problem - is is the undersized vent?


Make sure your condensate drains are free from obstruction and are draining properly
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Old 11-14-2012, 12:14 PM   #6
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Heil Furnace problem - is is the undersized vent?


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Make sure your condensate drains are free from obstruction and are draining properly
Thanks for the suggestion - as i mentioned in the initial post, I did take the time to clean and flush all the drains . .
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Old 11-14-2012, 12:20 PM   #7
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Heil Furnace problem - is is the undersized vent?


I've got to the point where i've located schedule 40 replacement pipes for everything in 3" size, as well as a concentric vent termination kit - Alpine has a reasonably priced one.

there is a mystery here though - at the point where the intake vent enters the closet - the "air intake coupling: - is sized for a 2" pipe. There is only one replacement part listed in the manual, covering all furnace ranges. If this is a 2" opening, this would negate the whole exercise, as a vent is only as wide as its narrowest point. Going to try to talk to someone at the manufacturers about thism though i don't feel too lucky . .
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Old 11-20-2012, 12:40 PM   #8
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Heil Furnace problem - is is the undersized vent?


So . . I completely replaced the vent system with 3" pipe. Problem still there, though i may be kidding myself, but it does seem to be burning for longer and stalling for less time with each improvement . . besides which, the spec called for 3" vent pipe, so I'm sure it'll burn more efficiently when all is said and done.

Noticed something though . . as an afterthought, I loaded up my iphone Level app (very useful app, BTW) and stuck it in various places inside the furnace. this furnace is 2 degrees away from level, from left to right, which is to say condensate can drain back into the blower and not towards the tee trap/exhaust. I assume this is bad. Could this be my problem? how does one re-level a completely connected furnace. i mean, the gas line is plenty flexible, but am I risking cracking the duct connections or, Heaven Forbid, my nice new vent work if I shim it a couple of degrees? And what to shim it with? Those plastic shims by the checkout counter at the hardware store?

thanks for your thoughts . .


Malc
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Old 11-20-2012, 12:47 PM   #9
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Heil Furnace problem - is is the undersized vent?


. . and i just noticed, Dave, that that was your very suggestion as to what might be the problem . .

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