Misbehaving shop furnace...


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Old 11-16-12, 04:52 PM
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Misbehaving shop furnace...

I've got a six year old ADP (Cayenne) furnace, model #FOAN-75-1-1-1.

I'm firmly convinced it's trying to drive me out of my mind.

Long story, short - the top tube in the heat exchanger got sooted up at the flue box end and resulted in the top burner tripping the flame roll out switch because the flue draft motor couldn't pull the flame down the heat exchanger pipe.

I removed the flue draft motor & flue box in order find out what the problem was - that's when I found it was partially blocked with soot. (unit had never been cleaned) I've got a new set of fiberglass gaskets coming, but I wanted to make sure that cured the issue with the exchanger. Well I get it all back together and it starts normally and I see it pulling the fire from the top burner down the tube, just like it's supposed to.

However, right before the main blower is due to come on, it shuts off the gas and starts blinking the diag LED at me - four slow blinks on this one indicates a limit or rollout switch. Well I check to make sure that the upper and lower roll out switches haven't tripped - they're good.

I _think_ the "limit switch" that it may be referring to (same brown wires as the rollout switches) is the culprit. However, I don't know if this is something I can test with a multimeter (does it operate like the rollouts? Can I just toss clip-leads on it and see if it is tripping?). The schematics for the furnace are pretty tragic. If this this limit switch is tripping legitimately, what would cause it to do so? What is it checking for?

I'd really like to avoid spending any more money with my local hvac guy - they wanted to replace the heat exchanger (they never inspected the end of the tubes on the flue box end), saying it was plugged with rust(?) and while the exchanger was under warranty, the labor ($1300!) was not.

Help!

Thanks folks!

Gene.
 
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Old 11-16-12, 08:30 PM
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Ok.....it's showing you a limit code. That would seem to suggest that maybe the heat exchanger is getting too hot before the fan comes on. Does that make sense ? There should be a heat sensor/fan thermostat that turns the fan on when the heat exchanger is up to temp. Try and locate that. You said that the flue draft fan pulls the flame down thru the tubes. Is that correct ? Usually the draft fan pulls the heat up thru the exchanger as heat rises anyway.
 
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Old 11-16-12, 09:17 PM
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Yes, you should check to verify that the limit switch is shutting off the burners.


Then you need to determine the reason why the heater sooted up. Very likely it is overfired, with too much gas going into the burners. That causes soot and carbon monoxide to be produced.

Also, you may not have done a complete job of cleaning the heat exchangers.

Is this a propane or natural gas heater? What other gas equipment do you have?

What is the rated BTU input for the heater, which is given on the rating plate mounted on the heater?
 
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Old 11-17-12, 05:04 AM
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Looks like a 75.000 by the model number he posted. They quote you $1300, you can buy a new unit for about $550 on sale? I assume you have a lot of dust in your shop?
 
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Old 11-17-12, 11:12 AM
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The N is for natural gas.
 
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Old 11-17-12, 12:22 PM
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The unit is a 75k BTU, powered by propane. It was updated for propane when it was installed.
The unit pulls fresh air in from the outside.

How does the limit switch work? What exactly is it limiting? Can it be tested with a multi-meter? The device can't be that complex as a replacement from comfortgurus.com is under $7.

I'll take pictures of the unit shortly so you can see exactly what I'm talking about.

tnx.

g.
 
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Old 11-17-12, 03:03 PM
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I may simply have a case of stupid.

Today I started fiddling with it again before I took pictures.

Here's the sequence I see.

1. Flue fan starts
2. 15-30 seconds after the flue fan begins running, the igniter(sp) triggers and we get fire.
3. 30 seconds or so after that, the main fan turns on.
4. 2-3 minutes later, the fire goes out and the diag LED indicates the limit switch has tripped.
The main fan & flue fan continue to operate.
5. 15-30 seconds later, I can hear a faint click and the diag LED changes to "call for heat" mode and the burners re-ignite.

Steps 4 & 5 repeat endlessly.

I've carefully checked the exterior surface of the enclosure as well as the inside wall of the furnace that divides the heat exchanger chamber from the burner & electronics chamber with a Fluke no-contact temperature probe. No point outside exceeds 79 degrees and no point inside exceeds about 85-90 after over an hour of operation.

Is this how these units normally operate and I've just become hypersensitive to how it works due to the issues I've had with it? I know that there's no rate adjuster on the propane valve, it's either open or closed, so this could be how it is designed to work.

Here's a pic of the inside, with a flimsy black arrow pointing to the device I suspect is tripping:



Working on this thing can be a pain because of where it's installed:





Thanks folks!

g.
 
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Old 11-17-12, 03:26 PM
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Nice shop too BTW, lots of room...I'm jealous
 
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Old 11-17-12, 03:31 PM
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It's very likely that the heater is overheating and shutting off on the high temperature limit switch.

The part you indicate is probably the high temperature limit. Check to see what is printed on the switch --- it's probably L with a temperature that opens the switch: L 110 or whatever.

Report back what is printed on the part and use your meter to verify that the switch is opening when the burners shut off.

The sooting problem helps confirm the overfiring problem.

You need a competent repairman to identify the problem and correct it. It might be too much gas pressure or burner orifices that are too large.

Because the heater has been overfired for years, the heat exchangers should be inspected for holes and cracking as well.
 
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Old 11-17-12, 06:12 PM
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Before I go chasing up those stairs again, I'd like my question answered - is the behavior I described today SOP for this type of furnace?

I would assume that the limit switch is the device turning off the gas as neither mechanically reset rollout switches are activating.

The furnace has been run no more than two days a week for the last six years, and even then only during the cold months.

Having the heat exchanger inspected ins't going to happen - especially with what they wanted to charge me to replace it.

Boilernut: Thanks! It's about 60x32 and I'm running out of space.

g.
 
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Old 11-17-12, 07:45 PM
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>


This is the correct sequence of operation, but of course the burners shouldn't shut off until the thermostat is satisfied.


The heat exchangers should be checked. If you haven't used the heater much, they may be OK.
 
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Old 11-17-12, 07:51 PM
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Having looked at your pictures, it's possible the venting on the heater is incorrect which might account for the sooting issue.


Most unit heaters, even those with inducer motors, are supposed to be vented with a rise in the vent pipe, while the picture seems to be of a vent pipe with an actual drop or flat run.

That should be checked against the manufacturer's installation manual to see if it is correctly vented.
 
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Old 11-17-12, 09:30 PM
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Looks like there are two vent pipes on the heater.
 
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Old 11-18-12, 04:59 AM
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Rule of thumb on the flue pipe is a 1/4 per foot rise...in case of a gas leak natural gas will vent out the flue, but not the case on LP since it's heavier than air.
 
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Old 11-18-12, 12:02 PM
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I'll try to get the back end torn open this coming week and see if there's any visible damage to the heat exchanger. What visible signs (beyond the blatently obvious) am I looking for?

If the HE does need to be replaced, is it a simple task of taking the cover off the right side (as seen in the pic I posted) and accessing attachment bolts from there or is disassembly(sp) from both sides required?

Thanks gents, I really appreciate the help!

g.
 
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Old 11-18-12, 12:28 PM
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Inspecting or replacing the heat exchanger are not good DIY jobs.
 
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Old 11-18-12, 12:34 PM
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f15sim...you need to take back the rolling ladder you "borrowed" from HD. Loss control will be all over you .....lol.
 
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Old 11-18-12, 12:48 PM
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It's a stack of steel tubes bent into an "M" shape. It's not exactly rocket science.

Unless you're willing to pony up the $1300 that the HVAC guys want to come dance with this thing, I'm it.

I guess I'll discover the answers when I start tearing it apart.

FYI, this is the kind of thing I do as a hobby: World's First DIY Collimated Display and F-15C Flight Simulator Project - so I'm not exactly without skills.

g.
 
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Old 11-18-12, 01:03 PM
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gunguy45: I bought that about six years ago when I had the shop built. There's a place local to me called "The Rusty Rack" guys. They buy racking and rolling stairs, refurbish and resell them. It was the only way I could install the lights in the shop - it's got 12' high walls. (should have been 14', but that's a whole nuther story)

g.
 
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Old 11-18-12, 01:19 PM
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f15...I figured as much. I worked at HD and couldn't believe what they threw away. Actually...they get a roll off dumpster delivered by a metal recycler who then re-sells whatever is repairable. OSHA or company rules say if a ladder got damaged, other than wheels...it was trashed. No welding, no bracing...junk it. Liability was too great.

Even worse is the metal racking. Every year or so, or after major remodeling all the spare beams and frames get junked as well. They offer them to other local stores, but rarely any takers. Too expensive to ship them out of the area so off to the scrap yard they go. People with shops like yours would love them.

We didn't have anything like "The Rusty Rack" in our area...not that I knew of anyway.
 
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Old 11-20-12, 03:41 PM
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As an update, I checked the heat exchanger and there's no indications of cracking that I can see. I mounted a carbon monoxide & gas detector near it and let it run for 48 hours and no alarm was tripped, so it's not out-gassing through a crack I didn't see.

The limit switch I'm suspicious of is labeled "L175-30F". The probe that goes into the heat exchanger compartment is in physical contact with the probe. If that's NOT supposed to be like that, it's no wonder the thing is tripping every 2-3 minutes. If it IS supposed to be in contact with the heat exchanger tube, could I have a failing sensor on my hands?

Thanks all!

g.
 
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Old 11-20-12, 03:44 PM
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The probe on the limit switch should be in the air surrounding the heat exchanger and not in contact with the heat exchanger. That might indeed be the problem.

Did you buy the heater new from a dealer authorized by the manufacturer? If so I'd call them, tell them what you found and see what they'll do for you. If the manufacturer installed the wrong part, I'd like to imagine they would replace it without charge.


But limits switches aren't especially expensive --- $30-40 probably. If you can get the OEM part you can compare it with what you have and see if the OEM part is shorter so it doesn't come in contact with the heat exchanger.

Good job in noticing that!


Still doesn't explain the sooting, which is a strong indication of a venting problem, overfiring or something else which needs to be identified and correcrted.
 
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Old 11-20-12, 04:22 PM
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Thanks for the quick reply SeattlePioneer.

The furnace was installed in late 2006, early 2007 by Advanced Filter when I had the shop built.

When the tech came out to look at the furnace (as a result of my work, I've got a serious bone to pick with them too!) he may have removed the limit switch to inspect it and re-installed it in such a way that the element now touched the heat exchanger coil. I'll get in there with a dowel from the side and see if I can't carefully adjust it a bit. This particular limit switch can be had for less than $7. I checked by part # (I've got a parts list from ADP) on comfortgurus.com and they have it if I need it.

There is a sticker certifying the installer properly installed the propane conversion kit and what kit # was used. What would cause over firing? If it was over firing, wouldn't ALL the heat exchanger tubes be covered in soot? The worst was the top one and it got rapidly cleaner as you go down (literally, top was worst, bottom was nearly pristine) the row of tube exits.

The picture that I posted that shows the furnace also shows the exhaust pipe - that's how they installed it. It is configured to pull combustion air from the outside.

Thanks!

g.
 
 

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