Bryant Plus 90 Burners kicking on then off

[HayZee518]

its hard to say what the problem is. its even more dificult to say because a home owner doesn't have replacement parts which to substitute in troubleshooting and to stock a spare gas valve, flame sense, thermocouple, gets expensive! we therefore have to rely on experience and a good meter with specifications on equipment. [allowable resistances, voltages at points in the electronic circuit.] what is passable, what isn't.
it's even more hair pulling when we try to troubleshoot a problem from 6,000 miles away is it the valve? the flame sense? the pilot valve?

[derringd]

I know I'm about six months after the fact in this thread but winter in Michigan came more quickly than usual this year. I have very similar problems as here and have scrubbed many sites but cannot seem to put it all together. My thermostat calls for heat, the fan kicks on, the pilot lights, I hear the clicking, the burners fire up and then blow right out. I did see on someone else's site that taking the cover off the box where all this happens can help troubleshoot so I took it off and every time I fire the furnace up, it works just fine. Put the cover back on, same problem. I've checked the air intake pipe and screen in the burner box without finding any obstructions. Based on this string it sounds like I probably need a pilot assembly. Could someone please help confirm or deny? Thanks.

[dw9663]

I am still having the same problem with my unit. I do remember that taking the cover off seemed to help. It seems to me that outside temperature does have an effect on the furnace staying lit (the problem seems worse in cold weather). My neighbor suggested cleaning the burners, he thought that dirt accumulation could cause some issues. I have not done that yet but plan to. The one thing that seemed to help was cleaning the contacts on the relay board located next to the exhaust fan motor.

Does your pilot seem to light? I changed mine and it did not seem to really make much of a difference. My conclusion is that the pilot has little effect once the burners are going.

The thing that surprises me is the abruptness of the burners when they shut off when you don't want them to.

[HayZee518]

the operating spool in the main valve is quick to operate because of the return spring inside. the pilot valve is held in place against a very small spring by means of the millivolt generator connected to a very small electromagnet. when the main burner is lit, the OPEN permissive has been met by the pilot flame sense. when the main goes out, there is still a very small air flow through the venturi that mixes air with gas to burn. there is also an air flow from the burner up the stack into the chimney. essentially hot gases create a vacuum. a heater with an inducer fan amplifies this upward travel of gases. that's why when the burner shuts down, the stack fan is also supposed to shut down and the stack damper closes. the pilot flame is ONLY gas - no air is mixed with it.

[dw9663]

My Bryant Plus 90 takes several attempts to start the main fan. The pilot lights, sometimes it extinguishes itself prior to the main burners lighting off, but mostly the mains start, run for 5-10 seconds and goes out. After several tires it will light but the new problem is the system will quit prior to heating all the way up to the thermostat setting. Any thoughts?

It is furnace dated 1982 and the house was built in '86 so it is 23 years old. The flame sense is new last year, and as mentioned did not really appear to solve the issue. I checked the exhaust stack and did not see any obstructions, along with cleaning the electronic inlet filters.

Thanks for all the advice on this forum, it is certainly very helpful.

[dw9663]

Can anybody describe the characteristics of a bad gas valve on the Bryant Plus 90 furnace? I have limped the furnace along, but the problem, same one as described above still persists. I have replaced the flame sense and it did not really make a difference.

In it's current state, it takes about 30-40 mins of cycling to actually get the furnace to turn on the main blower and then heat the house. About half way through a heat cycle, the furnace will basically quit like it has reached the set temp on the stat but is actually still a few degrees shy (the gas flame shuts off and the main blower runs for a couple of minutes to complete the cycle). Then it goes through the fire up cycle again numerous times before it heats again. I have the cover on the burner box off as it seems to run better with warmer air. I checked and have not found any restrictions in the system.

The furnace is a 1986 model, any idea on expected life of such a unit? I am debating whether to consider a new furnace or repairs to this one.

[paul52446m]

to have any shot of having these furnace to work right you have to do this Check the gas pressure both incoming and header side. there is a indent in the metal under the pilot ass.
drill a small hole in the bottom of this indent to let any moisture drain out so it wont short out the pilot ass. you have a separate pressure reg on the gas value for the pilot gas, turn this pressure up as high as it will go, this should help. Make sure your pilot is real clean. Taking the door off is not the answer. put the door on and get some washers or coins and start cutting down the air intake. Put the coins on the screen in the air intake, you might have to cut the air in half. The reason you are doing this is when the main burner comes on it pulls the flame away from the pilot switch and the pilot warp switch cools down and kills power to main gas. The inducer is pulling to much air through the burner area. make sure you turn that pilot pressure all the way up. later paul. Stay in contact and maybe we can get it fixed

[dw9663]

To close the loop and for anybody troubleshooting, turns out the vacuum line was plugged at the heat exchanger. This basically slowly deteriorated the vacuum signal to the point that the flame sense and burners would not activate. There is also a rebate on a Bryant Plus 90 burner due to a defect so you can get back about 1/2 of what it takes to replace it. The issue is the material used as a sealer, which ends up plugging the vacuum line. On the main fan motor, there is also multiple speed options as a setting, so if you need more / less fan speed, that is available. 4 is slow, 1 is fast.

[HayZee518]

THANK YOU for the feedback. I'm sure others will benefit from this. Maybe we should start a sticky thread on this type of burner to include symptoms and fixes. Eventually it will look like a flowchart troubleshooter and remedies.

[dw9663]

That is a great idea, the one thing I have trouble finding is a troubleshooting guide or manual for the unit. I have poked around a bit on the web but can't seem to come up with a manual. Any recommendations on good sources for a heat exchanger?