Bryant troubleshooting help

[whacker]

I'm getting near the end of my rope, but luckily I found this forum (and intend to explore others as time allows).

I've got an early '90s Bryant Plus 90, Model398AAZ06060AAKA. It worked flawlessly up until two years ago, when it started to intermittantly not start up, but let me explain. In the sequence of events, no one (myself or any furnace repair guy) has ever been able to witness for the sake of troubleshooting what exactly takes place (the nature of intermittant), but here is the outline:

The inducer motor/fan will start up in response to the thermostat, but it quits before the burner ever comes on. Whether or not the pilot light makes it on is anybody's guess. This apparently puts the electronics into a lockout mode. If you switch off the furnace, either with the main power switch (furnace mounted) or at the thermostat, then switch back on, it will come back and start over, this time successfully. The furnace will work normally for anywhere from 24 hours to two weeks, then will fail again, whereupon you can reset it with the power off/on routine and go from there, with the results being another undetermined length of normal service.

The service people who've work on this thing have kind of come to an impass, and I've educated myself on the sequence of operations, based on the manuals that came with the furnace, so I could do my own troubleshooting, as I have a background in electronics. Not that I have anything against service people, but it has simply become a matter of parts swapping, as they don't seem to have any broad history of this particular problem to go by (plus it's expensive!), so I've turned to the 'Net to see who might be able to help.

Let me also say that, along the same time this problem started to develop, a leak in the secondary heat exchanger was detected (water in the blower compartment and then on the floor -- never to any level that would immerse the electronics), and that was replaced recently.

The service techs started out checking the inducer intake for obstructions -- no problem there. Then they swapped out the ignitor/lockout module (ILO -- a logical choice as I've come to learn). But because the furnace never fails in the presence of anyone (could take weeks), they pack up and consider it solved, until we call again. Generally, if someone is home, we just switch off/on the furnace and keep the house warm. The ILO has been changed twice!

Then the following parts have all been replaced: The inducer motor relay board; the pressure switch; the inducer motor itself (which, when it didn't solve the problem, they swapped back out for the old one, fortunately for me); the thermostat itself; the pilot assembly; the blower control PC board. I also, on a hunch, decided recently to bypass (put a jumper on) the high-temp limit switch, thinking that this might have become degraded due to the water possibly being blown around in the blower compartment from the leaky exchanger. Nope. It still fails and must be reset with the off/on routine.

The only thing left I can think of is that there's a problem in the redundant gas valve, perhaps with the opening of the pilot gas flow, but that's why I'm here today, folks. Keep in mind, the inducer motor seems to shut off early in the sequence, before the burners are ever on (that's what I keep trying to assess, but I'm never there or paying close enough attention when it shuts off).

Does anyone have a clue what might be going on with this thing? I'm not inclined to have the gas valve replaced unless the evidence is strong (mucho dinero, you see). I know the problem must be electrical/electronic in nature, but practically every part has been replaced. Has anyone ever had a problem with the 115VAC/24VAC transformer becoming intermittant? Haven't replaced that part yet. Or any electrical connectors becoming corroded, perhaps just a ground connection? Things in both compartments actually look pretty clean.

Aren't there any diagnostic devices devised that can be connected up to this to more closely pinpoint the area of trouble? That would be a good electronics class project!

I need help, or I may have to get a new furnace, which the two local outfits that sell/service Bryant would love to provide! But really, this has got to be something so simple. Everything else works fine on it, and it's only 15 yrs. old or so. Just a baby, really. But the services techs can't apply themselves in this sort of a problem for any length (understandably), so I'm trying to find out the fix myself. HELP!

[HayZee518]

how about the flame detector? once the pilot is on, the flame sensor detects the flame and opens the valve. if it is a millivolt thermopile generator, perhaps it isn't providing the 500 millvolt output to the main valve. on the millivolt line inside the valve is an insulator and a copper button. if the insulator is shot or near shot sometimes you'll get the low voltage and the valve operates normal. other times the output shorts out and the pilot never lights.

[whacker]

As I understand it, flame detection is accomplished on the modular pilot assembly; it's part of the unit that was replaced when the service tech changed that part. I could be wrong, but that's the way it looks, and also as it appears on the schematic I have of the furnace. If you know different to a certainty, please elaborate. I'm getting desparate.

And thanks for your reply! You're the only one so far.

[HayZee518]

send me a copy of the schematic - maybe I can see something you are bypassing.

[whacker]

That's going to be difficult, as I don't have a scanner available. Maybe I can borrow one, but it'll be a few days...

[cfwolnia]

90 plus

Hi, I have a similar furnace (Bryant 90 Plus) from early to mid 90's. Not sure about the model #, but on mine if I pop off the cover, there are two round plugs about the size of a half dollar. Behind one of those is an LED that flashes with an error code. Look in your manual to see what the cause is.

I had a similar problem recently, where she acted like she was going to fire up, but then shut off. Turned out it has a sensor that checks the airflow. In my case (and I would guess in yours too) I had a dirty filter which restricted proper airflow. I didn't have a new one on hand, so I simply vacuumed the old one and put it back in. Ran it that way for about a month before finally replacing the filter.

Don't know if this will help you, but good luck!

[whacker]

cfw-

The only filters on mine are the main furnace airflow filters, and they're fine. They sit outside the unit where the return airflow comes in. Mine is an upflow style unit.

However, I did discover that the high temp limit switch (sensor) might have been worth checking out. It is in the internal airflow (near the heat exchangers). I made a little jumper to "go around" it, and for a few days I thought it was doing the trick, but no, eventually (within two days), it stopped coming on again. But I turned her off, and back on, and she fired right up and ran for eight days before failing again.

CFW, if your unit has a diagnostic LED, maybe it's a Plus 90i model. That's a generation newer than mine. Mine has a one speed AC motor for the main blower, and the control electronics are run from a 24 volt AC transformer tap. No microprocessors. How large was this "filter" you're referring to, and whereabouts was it located?

I still am leaning to the idea that it's the redundant gas valve (it's the only part that hasn't been replaced - unless the sevice tech or I missed something). There are two servos or relay operated valves on it, one for the pilot and one for the main gas burner. It's either that or some really obscure electrical connection problem, corroded or loose connection.

I was trying to get a scanner to send images from my owner's manual over to HayZee518, but that turned out to be a no-go, so I'll probably have to go to Kinko's and scan the schematic so I can send it. We'll see. It's been mild out here in Michigan up 'til now, but that can't last forever!

Thanks for your input, though. I'll keep you posted!

[Joe_Shmo]

Simular Bryant Problem

Whacker,

I have a mid 90's Bryant +90 natural gas furnace. I recently experienced a similar issue to which you have described. When the thermostat would call for heat the furnace vacuum motor would come on followed by the thermo igniter. Once the igniter would get hot "orange" I could hear the gas valve click and turn on and what I think is the sound of gas flowing. However, after about 8 seconds no combustion would occur and the gas valve would shut off. The furnace would retry 2 more times and then shut down in a lockout mode for 3 hours.

The problem with my unit was in the gas valve. Even though the valve would turn on there was insufficient gas flow sometimes. I ended up removing the brass plug on the gas valve and turning "in" the flame adjustment to let more gas into the combustion chamber. If you try this you need to be careful to not turn it up too high or else the thermo switch will trip stop the unit from turning on until it is manually rest.

I hope this helps,

[leone184]

furnace problem

This sounds like a problem my furnace was having. It turned out to be the flame-senser. For some reason the senser did not register that the gas jets were ignited (they were). This made the furnace think the jets were releasing gas that was not being burned. This would shut down the cycle after about 60 seconds. The senser looks like a thin gray rod of metal with a porcelain cap, and a wire sticking out of the top. The rod itself sticks down thru a hole in the frame, and right in front of one of the jets.
Every year before the winter season, I have to remove the rod and run it through the wire brush on my bench grinder. It does not look dirty or have any noticible carbon build-up on it, but the cleaning seems to last the entire season with no other problems.

I hope this helps.

[wlc0216]

Bryant Plus 90i

My furnace is not running properly. When it goes through the start up process, the gas valve opens and flames for a short period then I hear a clicking and notice puffs of air from the combustion chamber and the gas valve shuts down. This process repeats continuously. I checked for a flame sensor as described by the previous posting but there is no such rod with a wire attached positioned infront of the flame area.

I checked on the error code displayed and it was 3 amber flashes followed by 1 green flash. I am hoping to repair it myself and would appreciate some advice.

[wlc0216]

Problem identified!

Here is an update. Error code 31 indicated "high pressure switch fault". The switch is located on the gas valve with a six wire plug on top. The switch is fastened to the valve by one screw and at least two rivets making removal impossible. The switch has an adjustment knob on it that has movement limited to 180 degrees and it was positioned in the center of this adjustment range. I checked the wires for good connection and dialed the adjustment all the way to the right and retried the furnace (of course I shut off power to the furnace while working on it). The furnace still had the same problem. I then dialed the adjustment all the way to the left and again retried the furnace. It then began to operate normally and has been ok overnight until nearly noon today.

I dont know how the emergency heat function works. I see it appears to be activated by a switch below the circuit panel. Could someone explain?

PS The links in the message automatically happened when posted, not my doing.

PSS OK now they're gone, whats up?

[HayZee518]

the underlined links in your script are a function of the forum.

[wlc0216]

The problem returns

I am back to square one. The furnace has the same symptom as before; the burner ignites and burns for about 20 seconds, the valve makes a clicking sound and puffs of air are heard escaping from around the cover to the burner area and the valve shuts down. This cycle repeats continuously.

I planned to replace the gas valve today but the replacement valve specified by Carrier was not a match. The old valve has a six wire connection collected on a single plug, while the new replacement has a five wire connection with separated 1/4" terminal tabs. I had the parts supplier research to see if Carrier provides an adapter to match up the wiring, but they do not. The old valve is an Essex SX345NSRX-14 which I'm told is obsolete.

In my checking around, I was told that the error code 31 may not indicate "high pressure switch fault" as stated on the unit, but it could also indicate an inducer problem or a problem with the heat exchanger.

Would the emergency heat function provide a short term solution? If so, how is in activated?

Any help would be greatly appreciated!

Bill

[Narroc]

Furnace

The problem as I see it is no one has been able to see exactually what happens when it faults.
Sequence is: Room Thermostat calls for heat>Induced fan starts>Vacuum (sucky) switch makes >electronic board gets signal that fan is going (there enough suck from the fan)and turns on spark and open gas valve>flame sensor sends signal back to board that flame is established and locks gas valve open>Main fan starts either by fan stat or timer. Heat continues until the room thermostat yells out enough enough .

There is of course a time gap between the induced fan and the flame approx 15 seconds.

So some were along the way it must be faulting. Has water got into the main board? is there a hair crack on the circuit board? Is the flame probe sensor CLEAN. Take it out and clean with fine sand paper. Is the tube from the induced fan to the sucky switch clear? Hasn't got a leak in it?
I had one similar to this last year finally cleaning the flame probe fixed the problem.

[HayZee518]

there is one post on ebay for the same valve and a robert-shaw equivelant