Furnace won't stay lit

[Vector]

Ok, I have a new furnace, Coleman G9T 140K BTU Unit.

See the story at:

http://www.construction-resource.com/forum/showthread.php?p=30591

For background details.

The unit would start, and after a matter of seconds to a few minutes after the flame would light, the gas valve would shut off and the flame would go out. We verified that the unit was getting proper gas pressure (7" WC steady), verified that he problem happend with all sensors bypassed with the expection of the flame sensor which was checked and found to be dead-on spec at 5uA current when the flame was on.

The gas valve has been replaced. The circuit board was replaced. The grounding on the unit has been checked and rechecked and re-rechecked. The input voltage has been checked, the output of the internal transformer has been checked.

Damn thing still behaved the same.

The circuit board was swapped out for a third one. The connectors to the gas valve were reterminated. Somewhere in there the behavior changed slightly.

Now the board isn't cutting voltage to the gas valve, instead the flame is going out on it's own and then the flam sensor drops and the board cuts the gas vale. This is all normal, except the stupid flame going out inthe first place.

We've checked the pressure (as noted above), we've had the utility test on the high-side of the regulators, and they never drop below 56" WC (no-load they see 58", when the furnace lights they see 56" steady).

We've tried running the water heater at full-blast at the same time (175K btu) and it had zero effect, positive or negative on the behavior of the furnace.

We've stumped the factory people, the distributor's people, my installer, and lastly (and least, since I know jack about furnaces) myself.

Any ideas?

Sheetrock goes in in just under two weeks and we're going to need some heat by then for sure.

My leading opinions involve either my Husqvarna or a 12-gauge exorcism of the thing at this point.

[Sparks]

Interesting, what would stop a fire from burning other than lack of fuel, heat or oxygen? Did he install an ammeter in series with the 24v to the gas valve and confirm that the gas valve is really not being shut off? Confirming this with amperage other than voltage. Did he install a manometer and watch the pressure while the flame was on, not just static pressure but actual operating pressure? I'd be thinking still that something is amiss in the supply, being all that was already done. It's very possible that there is still a problem and that water heater is just not as sensitive as the furnace to a pressure problem. Checking pressure with the furnace running is a must. Typically 3.5 inches of WC manifold pressure while operating. I have a feeling you may not actually have this for some reason.

[rabadger]

Where are you reading 7" of pressure?
What size furnace did you take out?
Is the same gas line feeding the 175,000 BTU water heater?
How big and how old is the building?
When the gas vavle shuts does the fan and venter stay on, then it tries to light again? Does it do it over and over again?

[rabadger]

http://www.colemanac.com/PDFFiles/035-19592-001-C-0204.pdf

Your right Sparks, I have never run into a residential natural gas fired furnace that requires 7" WC pressure. I didn't see the gas pressure setting in the manual.

Vector,

On the rating plate of the furnace you should see the required manifold gas pressure. What is it? Get your hands on the blower specs for that model. Most units of the size and eff require lots of air flow.

[Sparks]

Taking the manometer pressure reading while the flame is on is important because this will reveal a problem with volume of gas being delivered. Pressure, like volts with electricity, is only part of the equation. Volume, like amperage, must also be known. The more I think about it the more it sounds like the gas volume is just dropping off. But, I'm way out here and that's just a guess.

[rabadger]

Chicago has some old large gas mains that work on low pressure. Pipe sizing can be tricky.

[Vector]

Wow, lots of responses...

Did he install an ammeter in series with the 24v to the gas valve and confirm that the gas valve is really not being shut off? Confirming this with amperage other than voltage. Did he install a manometer and watch the pressure while the flame was on, not just static pressure but actual operating pressure? I'd be thinking still that something is amiss in the supply, being all that was already done. It's very possible that there is still a problem and that water heater is just not as sensitive as the furnace to a pressure problem. Checking pressure with the furnace running is a must. Typically 3.5 inches of WC manifold pressure while operating. I have a feeling you may not actually have this for some reason.

No, we checked it with a voltmeter. But you can hear the valve closing, and it's remaining open until 2 seconds after the flame goes out.

Yes, we have watched the pressure with the flame on. There is 7" WC before the gas vale on the unit itself, and about 3.5" WC after the valve.

[Vector]

Where are you reading 7" of pressure?
What size furnace did you take out?
Is the same gas line feeding the 175,000 BTU water heater?
How big and how old is the building?
When the gas vavle shuts does the fan and venter stay on, then it tries to light again? Does it do it over and over again?

7" on the input side of the gas valve.
No idea. It was torn out my the asbestos abatement sub a long time ago.

No, the line was completely removed back to the meter and re-run. The meter goes out to a 1" which T's into two 3/4" runs. The Water heater is one one 3/4" run by itself, with a regulator just before the unit. The other 3/4" run goes to the furnace, it has it's own regulator that then T's into a 1/2" that feeds the furnace and another 1/2" that feeds the fireplace, garage unit heater, and cooktop (none of which are installed yet).

We have run a gastite line from the output of the regulator that connects to the water heater and connected that to the furnace. Same problem.

The building is about 3500 conditioned square feet. 2/3 of that is 100+ years old. The remainder is brand new.

Yes, it will relight (five times) and then goes into lockout for an hour (as it should).

[rabadger]

Does the venter motor keep running, does the blower stay on when the valve closes?

[Vector]

Does the venter motor keep running, does the blower stay on when the valve closes?

Yes and yes, until after the lockout, when everything shuts down.

[Sparks]

Sounds like your bases are covered but it still doesn't equate. A good burning flame doesn't just go out for no reason. Something must be missing, fuel, heat or oxygen. Maybe extreme mismatched burners from the factory? Orifices way too big for the BTUH capacity of the valve, who knows. What is your next step Vector? Sounds like maybe the factory should send out a rep to repair or authorize a replacement furnace. If you have 3.5 inches WC while operating, the flame shouldn't just go out by itself. What is the factory saying? Blaming it on the installer would be typical of a manufacturer.

[Vector]

The factory has no clue either.

The only thing I can think of is the burner too. But why would it sometimes burn for several minutes and other times only for 10 seconds?

I've spent way too much money and time on this thing already.

Who makes a good furnace that I can buy without having to get a whole new installer involved (i.e. i can buy myself)?

[Sparks]

You could get a Goodman from Johnstone Supply, just tell them you are a landlord and they'll let you buy from them. In fact, I've seen homeowners buy from them over the counter. The one I just put in my house was 90,000 btuh high efficiency, cost me $710 for the furnace. If it were me though, I would push the factory hard to resolve the problem or provide you with a replacement furnace free of charge but I'm sure they wouldn't pay labor, they never do.

[rabadger]

I think it is over heating. When a high temp safety switch opens the gas valve closes, venter motor and blower will continue to run. Furnaces are designed that way so the fans will cool them off.

Cycle of operation on a auto reset limit switch-

Thermostat calls for heat, venter motor starts, venter pressure switch contacts close. If you have a pilot assy the glow bar starts, pilot lites, flame is proven through flame rod, main valve opens, blower runs.
Temp keeps risings and limit switch opens. Gas valve shuts, venter motor and blower motor continue to run. When furnace cools down the limit resets, and the ignition sequence starts again.

If you have a direct ignition model the flame rod (sensor) is in the main burner fire and the glow bar will light the burners directly.

While all this is happening the therromstat never satifies and the equipment electronics could be counting the continued limit switch opening as a new trial for ignition. After several tries it is locking out on safety.

Has the contractor taken a temperature rise across the furnace? By the time the HVAC guys get to the job the furnace limit switch has cooled down and reset, so it checks out good. What is the temperature rise across the furnance, compared to the max on the rating plate? Are you sure you have a clean filter in it?

The next time it locks out look at the blinking light on the board. Count the blinks and read the code for reason of failure.

[rabadger]

Wait a minute.

You say you have only 7" WC before the valve and 3.5 after the valve? Is that enough to keep the valve open? What valve is it? WR, Honeywell?

If my memory serves me correctly the main valve plunger will open when 24 volts are applied to a closed valve. After that the 24 volts and gas pressure drop is what holds it open. Check the spec sheets on the gas valve. Check the min. lnlet gas pressure in the install manual.

Don't you need 5 to 7" pressure drop to keep it open?

Or am I getting to old for this?

[Sparks]

Like I originally said, I would be leaning towards a gas supply problem as well considering all that was done. The spec is always on the data plate, the min. wc input.

[Vector]

I think it is over heating. When a high temp safety switch opens the gas valve closes, venter motor and blower will continue to run. Furnaces are designed that way so the fans will cool them off.

Nope, we've bypassed the limit switch entirely and the problem is the same.

We've monitored the voltage or amperage (depending on the sensor) through every last sensor and they are not opening. We've jumped them out of the loop too. The flame goes out, the flame sensor drops from 5uA to 0uA, and two seconds later, as designed, the board cutrs voltage to the valve. The board is not shutting the valve.

[Vector]

Wait a minute.

You say you have only 7" WC before the valve and 3.5 after the valve? Is that enough to keep the valve open? What valve is it? WR, Honeywell?

If my memory serves me correctly the main valve plunger will open when 24 volts are applied to a closed valve. After that the 24 volts and gas pressure drop is what holds it open. Check the spec sheets on the gas valve. Check the min. lnlet gas pressure in the install manual.

Don't you need 5 to 7" pressure drop to keep it open?

Or am I getting to old for this?

11" WC at the input when the valve is closed. 7" at the input when open. I *think* I rcall it around 3.5" after the valve when operating, but to be honest, I'm not 100% sure.

WR sounds right for the valve, but again I'm not 100% sure of my memory. I'll check tomorrow if I get a chance to get over there.

[rabadger]

Check the rating name plate. for min gas pressure. I think Sparks is right. You have a gas delivery problem. If you are starting out with 11" wc and drop to 7" wc when the valve opens you are restricted. Time to get the dead mouse out of the gas line.

Way to go Sparks! ;)

[Vector]

Check the rating name plate. for min gas pressure. I think Sparks is right. You have a gas delivery problem. If you are starting out with 11" wc and drop to 7" wc when the valve opens you are restricted. Time to get the dead mouse out of the gas line.

Way to go Sparks! ;)
I will check the plate next chance I get to go over there, but...

As I said, the furnace does exactly the same thing when we run a flex line over to the other regulator across the basement. Which means the restriction would have to be in the ~4' of 1" pipe straight off the meter.

And of course, the 175k BTU water heater runs great off that regulator (regulators are 325K BTU units), and that's 35k more BTUs than the furnace.

And in front of the regulator it never drops below 56" WC.

Coleman's people seemed to think 7" was on the high side (but not out of range)... But I'll look for that plate.

I never wanted to know as much about furnaces as I do now. Ignorance was bliss.

[Sparks]

Ok, I hate saying stuff like this but here is what I would do. I would start the furnace, while it's running, I would install an ammeter in series between 24 volt transformer (or from board) and the main gas solenoid. This would keep the solenoid energized despite it's control. I would do this just to confirm what you are saying Vector, that the flame is indeed just going out by itself, and not being shut off by something else. Not to question you but, I've seen so many guys misdiagnose these types of problems because they think they are sure something is happening the way they think it is without an absolute confirmation. Then their diagnosis is based on erroneous conceptions or information. If the flame still goes out despite jumping the valve with the ammeter (the ammeter is used rather than a jumper wire for it's internal fuse and your ability to see the amperage draw of the solenoid, confirming it is indeed staying open) then I would continue with the belief of either a fuel delivery problem or something really weird with the burners (unlikely i.m.o). Obviously, this is dangerous to force a gas valve to stay open and I wouldn't recommend it to anyone not highly experienced with fuel gas appliances. We'd hate to see you pay for a new furnace, have it installed and have the exact same problem. What does the flame look like when it does fire? Is it a nice strong blue or a smoldering yellow or orange? Are you still reading 3.5 in wc when the flame goes out?

[Sparks]

On a side note, I was called in to a hotel kitchen. They were having a problem with one of their commercial ovens. It would light and then shut off by itself. Well the maintenance staff had replaced almost every replaceable component on this oven in an attemt to repair it. There next step was to order a new one but they figured maybe I should look at it first. These commercial ovens probably cost around 6 grand, I would think. After getting the rundown on what was checked by the staff and of course their opinion of what the problem was I asked them if the gas valve regulator vent was cleaned. HUH, that's got nothing to do with it! Was the reply. 10 minutes later the oven was working fine, all I did was clean the vent so the gas could flow correctly. This particular regulator required a vent, it was full of kitchen grease. They would've bought a brand new expensive oven and would've had the same problem.

[Vector]

90% cerrtain I just fixed it.

Initial problem was the control board. My HVAC guy fixed that.

Thatn, in the process of diagnosing the control board, a baffle on the combustion air intake wasn't correctly put back in place. I Just noticed it while taking the cover off the combustion chamber to look for any #s or marks to help verify the correct burners.

Son of a ...

Oh, the Max pressure for this model is 13.9" WC, and the Min is 4.5" WC, so that was all good. Manifold is 3.5".

[Sparks]

Oh man! Doesn't that get ya? Good thing you didn't buy a new one huh? Well at least we all exercized our grey matter talking about it.

[rabadger]

You going to send them a bill? If so add some in for us.:)

I would have never guessed the thing was put back together wrong. Air making the flame dance just enough to lose microamps.

[Vector]

Glad I didn't say 100%. I was wrong. Went to the house this morning and it was cold. Not as cold as outside, but colder than it should have been. Checked the furnace and it was back to it's old tricks.

I should explain some things here about this install.

My HVAC guy doesn't have a license to work in Minneapolis, which requires a city license, which is essentially impossible to get if you aren't union. My guy is an owner and sole employee, so being unionized doesn't make sense for him, and he gets plenty of work in the 'burbs. Not worth the trouble to him.

So, we have a friend in common who had a license in Minneapolis, and he was going to oversee the work. Unfortunately I took way too long in getting to the point where the install could happen that his license came up for renewal, and as he wasn't working in Minneapolis anymore, he didn't bother to renew.

We wanted to stay nice and legal, so the actual install was sub'd out to a firm that was licensed. They did a beautiful job, lots of custom sheet metal work, and everyone that's looked at it has been very impressed with the quality.

But the furnace problem (second problem, the first problem was that the control board was bad) was their fault 100%.

Turns out they installed the combustion air intake and exhaust too close together and the damn thing was sucking back in it's own exhaust and the flame was starving for oxygen. We put a sheet of plywood between the two and voila! it works perfectly. It's a matter of maybe an inch or two different and it would work just fine.

And yes, they're going to be backcharged for his time in locating the problem.

Too bad I can't charge for my time acting as an assistant. Grr.

[Vector]

But the good news is that during the course of this week I've passed electrical, plumbing and today the framing inspections (HVAC passed months ago). So insulation goes in next week and sheetrock starts the week after that.

[rabadger]

How far away is the vent. how many elbows, and is it vented in a north or northwest direction? Check the venting manual. Is your venter pressure switch cutting in and out causing the relighting and lockout?

Or did we cover that?

[Vector]

How far away is the vent. how many elbows, and is it vented in a north or northwest direction? Check the venting manual. Is your venter pressure switch cutting in and out causing the relighting and lockout?

Or did we cover that?

The venter switch isn't causing the problem. That has been covered.

The exhaust and intake are both about (guessing here) 20' and have 3x90° and 2x45° each. But the real issue appears to be proximity. They do exit to the north of the building.

The install was *not* done to spec. That's what caused us to try putting a barrier between the two to diagnose the problem.

It was toasty in the house when I left today :)

[Vector]

I'm going to try really hard to actually remember my digicam when i head over tomorrow to clean (getting everything out of the place for the insulation and sheetrock crews).

[rabadger]

Is this happening all the time or just every once in a while? How often is it happening?

[Vector]

All the time, until we separated the inlet and outlet. It *is* working great now.

[Sparks]

So the flame was missing oxygen. That's a good one, never seen that. It's always good to follow the manufacturers guidelines, I'm sure the backcharge will eat any profit they made on the job. The pvc must've been awful close for that to happen.

[rabadger]

When you started it up could you watch the flame change color and start to flutter before shut down? I've got to remember this one.

Every time I have run into a vent issue it has always been size and distance. If it where the intake in was from being blocked for some reason.

[Vector]

When you started it up could you watch the flame change color and start to flutter before shut down? I've got to remember this one.

Every time I have run into a vent issue it has always been size and distance. If it where the intake in was from being blocked for some reason.

Sometimes it would flutter a bit and then go out, but more often it would simply *poof* and go out all at once. That's why it was so hard to figure out because it seemed more like a gas valve or supply issue.

I didn't get a photo today, was way too busy. And my guy came by and installed a snorkle on the intake side, so now it's running great without the plywood divider.

[11chaos]

How old is your home? Have the gas company do a check on your system and determine if your lines are dirty. Good luck regardless. You could always call a priest to bless your home. :)