bryant 90 plus model 398aaz...

[networkspr]

This is a 1984 model that runs ok most of the time. had a pilot light ignitor replaced 2 years ago. last year would not start so i cleaned the pilot light ignitor and then it started correctly most of the time. recently, started having same problems again. i checked the pilot light ignitor - looked clean but i removed and recleaned. also reseated the ignitor cable from the spark generator. would not start - tried everything, power resets, checked functioning of thermostat, removed intake vent to ensure no air flow blockage. left for a day and tried again - started ok. talk about frustrating!! symptoms when having problem - 1. Pre-ignition blower is running but no spark. 2. switch thermostat off/on - blower starts again, no ignition, and blower continues to run. 3. reset power to furnace using switch on side of furnace - no difference - no spark and blower keeps on running. Now... sometimes when I reset the furnace using the thermostat 2-3 times it will start and run ok for random period - 1 day to 2-3 weeks.. it looks like the pilot light gas is not being released because no spark has been sensed... has anyone experienced something similar and what did you do to correct the problem. I think it is something straightforward that I can repair, and do not have the cash for a new furnace. Also, what tools do you recommend for troubleshooting, and where can I get a manual? I have looked all over the web and cannot find a circuit diagram or manual for the 398aaz model ..thanks for your help forum!

[paul52446m]

This is a 1984 model that runs ok most of the time. had a pilot light ignitor replaced 2 years ago. last year would not start so i cleaned the pilot light ignitor and then it started correctly most of the time. recently, started having same problems again. i checked the pilot light ignitor - looked clean but i removed and recleaned. also reseated the ignitor cable from the spark generator. would not start - tried everything, power resets, checked functioning of thermostat, removed intake vent to ensure no air flow blockage. left for a day and tried again - started ok. talk about frustrating!! symptoms when having problem - 1. Pre-ignition blower is running but no spark. 2. switch thermostat off/on - blower starts again, no ignition, and blower continues to run. 3. reset power to furnace using switch on side of furnace - no difference - no spark and blower keeps on running. Now... sometimes when I reset the furnace using the thermostat 2-3 times it will start and run ok for random period - 1 day to 2-3 weeks.. it looks like the pilot light gas is not being released because no spark has been sensed... has anyone experienced something similar and what did you do to correct the problem. I think it is something straightforward that I can repair, and do not have the cash for a new furnace. Also, what tools do you recommend for troubleshooting, and where can I get a manual? I have looked all over the web and cannot find a circuit diagram or manual for the 398aaz model ..thanks for your help forum!

Well the next thing that is suppose to happen after inducer fan runs is the pressure switch has to close and the you get spark and pilot gas. So ether your electric board is not going beyond prepurge or you pressure switch is not closing. How many pressure switch's do you have and how many wires are on the pressure switch's? Paul

[networkspr]

Thanks for the prompt response! I do not have a manual, so I would appreciate if you would direct me where to look for the pressure switch and how to check if it is working correctly. Do you have a picture or diagram you can send? I cannot find anythiong online to refer to. Thks

[paul52446m]

Thanks for the prompt response! I do not have a manual, so I would appreciate if you would direct me where to look for the pressure switch and how to check if it is working correctly. Do you have a picture or diagram you can send? I cannot find anythiong online to refer to. Thks

The pressure switch's are about 3" round and have wires and small hoses going to them. All these hoses need to be taking off and make sure they are clear. and no cracks in them. Just a small crack will stop the furnace. When you take hose off , check the stub that the house was on and make sure that that is open.
What is the btu rating of your furnace? what size is your stacks , how far do they run and how many ell and 45's are in each stack?.
I do not have manuals on all the different furnace,s If you want me to see you parts then you would have to take pic. and start showing me. Some pressure switch's are 3 wires and some are 2 wire. Do you have a good wiring diagram on a door panel that you can take pic. of? Paul

[SeattlePioneer]

The thermostat turns on the inducer motor, which sucks combustion air through the furnace. The pressure switch needs to detect adequate negative pressure being generated by the inducer motor before it will turn on the spark and allow the rest of the ignition sequence to proceed. So very likely the pressure switch isn't closing reliably when the inducer motor turns on. You need to use a multimeter to check for 24 VAC being applied to the pressure switch. In the AAV furnace, I believe the pressure switch has C, NC and NO connections. The C terminal should have 24 VAC on it when the inducer motor is on. 24 VAC should be on the NC terminal when the inducer motor starts and switch to the NO terminal when the inducer motor comes up to speed. Check that and report what is happening.

[networkspr]

The pressure switch's are about 3" round and have wires and small hoses going to them. All these hoses need to be taking off and make sure they are clear. and no cracks in them. Just a small crack will stop the furnace. When you take hose off , check the stub that the house was on and make sure that that is open.
What is the btu rating of your furnace? what size is your stacks , how far do they run and how many ell and 45's are in each stack?.
I do not have manuals on all the different furnace,s If you want me to see you parts then you would have to take pic. and start showing me. Some pressure switch's are 3 wires and some are 2 wire. Do you have a good wiring diagram on a door panel that you can take pic. of? Paul

No diagram on the door... I took all the hoses off, trimmed about 1/4" off each and reconnected. there seems to be plenty of negetive pressure but it made no difference. I left the furnace for 1 day and switched it back on - it started and has run sort of ok for the past 10 days. It stopped once for no reason before it reached the set temperature - i switched it off and waited until the next day - it started ok. yesterday, it stopped again before it reached the correct thermostat temp and will not start. Once thing i noticed - when it did start the last time the ignitor stayed on for some time after the pilot light started.. is this normal? any other suggestions? Thks for your help.

[networkspr]

No diagram on the door... I took all the hoses off, trimmed about 1/4" off each and reconnected. there seems to be plenty of negetive pressure but it made no difference. I left the furnace for 1 day and switched it back on - it started and has run sort of ok for the past 10 days. It stopped once for no reason before it reached the set temperature - i switched it off and waited until the next day - it started ok. yesterday, it stopped again before it reached the correct thermostat temp and will not start. Once thing i noticed - when it did start the last time the ignitor stayed on for some time after the pilot light started.. is this normal? any other suggestions? Thks for your help.

[networkspr]

I will check the pressure switch as you recommended and see what happens. Thks

[paul52446m]

No diagram on the door... I took all the hoses off, trimmed about 1/4" off each and reconnected. there seems to be plenty of negetive pressure but it made no difference. I left the furnace for 1 day and switched it back on - it started and has run sort of ok for the past 10 days. It stopped once for no reason before it reached the set temperature - i switched it off and waited until the next day - it started ok. yesterday, it stopped again before it reached the correct thermostat temp and will not start. Once thing i noticed - when it did start the last time the ignitor stayed on for some time after the pilot light started.. is this normal? any other suggestions? Thks for your help.

If the ignitor stayed on after the pilot is on, that means it is not proving pilot flame, so the primary control will trip out on safety. When the pilot lite's, the pilot flame has to be a good blue flame. The flame has to be hitting the pilot burner and go across to its ground. All the pilot head and ground must be sanded with fine sand paper to take the coating off them so they will let the flame complete the circuit. They may look clean be they get a coating on them. Check the ground wire to make sure its clean an tight.
If the pilot flame is yellow it will not complete the ground. If the pilot flame lefts off the pilot head it will not work. So look at that pilot real close.
If every thing in the pilot is good, And your pressure switch is made. Or the pilot lite's and spark keeps going then you might have a problem in you primary control. Paul