Installed Bryant Plus 90 398AAZ furnace in 1991 as a backup to a heat pump so it was rarely used until 2006 when the heat pump died and it became the primary system. Furnace heats the 1st floor only. Since 2006 there have been 5 occasions (today being the 5th) when I wake up to a cold first floor - thermostat timer goes on at 6am, off at 10pm. On 2 of the previous occasions, I turned the thermostat off, waited a few minutes and turned it back on and there was heat, on one occasion it took several on and off attempts but it eventually came on.

Then there was the time last year when nothing worked so I called a serviceman. He came out opened the unit, checked wires, turned the switch on and off, turned the breaker on and off and nothing would work. He said that it was probably the Fan Blower Control Board or the Inducer Control Board, as he pointed here and there in the open unit. He said that he'd look into some prices for me and call the following day. He then put the bottom panel back on, pushing in the interlock switch and voilà, the unit came on. He offered to continue but I told him to leave it on at least until the floor was warm. He said to call him if it happened again but not to switch anything on or off so he could possibly see what the trouble was. That was over a year ago. The heater worked fine for the rest of last year's heating season and has worked fine for all of this, until this morning.

I've found issues with this furnace, on this forum and other forums but not this problem (except there was a suggestion to check for fuses on internal boards being blown, which I did). As an aside, I bought both of the boards that he mentioned but never installed them. Still have them. One more thing, I see from other posts that there is supposed to be an LED that should be flashing a status code. I'll look for this.

Today when I turn the thermostat on and off, I hear the relay clicking but no heat after waiting several minutes. And by the way, when I look at the relay, and press the interlock button, there is a bright greenish/white spark in the relay with the click. Could be nothing but wanted to mention it.

Does anyone have any suggestions? This time could it be one of the boards? Which one? Any troubleshooting tips? Any help appreciated.

If the system control was never switched from dualfuel to gas heat only(when you quit using the heatpump)-- That may be your problem. The control board on the furnace will give you a flashing code any time the furnace faults out.

You said when he pushed the door on the furnace came on?. Sounds like the door switch or the force of closing the door could have unstuck a safety switch like the pressure switch or limit switch. Something else you might want to check is the return air duct and filter. Make sure it's the right diameter for your unit.(3 ton you want at least a 16" diameter, 4 ton 18" and 5 ton 20".) Check that your filter is not clogged because that can also cause the unit to over heat and turn off. All of your supply air vents/grilles have to be open too. Let me know if you need more suggestions.

I don't think that's the cause of it not working just 5 times in 5 years but probably worth checking anyway.

It's back on and like the other times, I have no idea why. I'm leaning toward the relay with the spark as part of the problem. Three times, it has gone back on without touching the furnace, just switching the thermostat off and on; once it took a few attempts. When I called the serviceman last year, he had given up, was packing up, putting the unit back together. He put the lower panel on, pushing in the interlock switch, and before he got the upper panel on, we saw flame for the first time in an hour of fiddling with wires and boards. Putting on that panel, like turning the thermostat on and off causes that relay to engage. Yesterday, I was doing the same thing. I had given up, considering my next move, and noticed when I pushed in the interlock, a spark appeared in the left relay on the Fan Control Board, I pushed it 3 or 4 times to confirm that the spark always appeared, closed the unit up, and before I made it to the stairs, I heard the flame ignite.

You're probably right.

Wait a minute... so it doesn't spark all the time you turn the thermostat to heat?

I'm sure it does. When it's on, the lower panel blocks the view of the relay. Plus I'm just not there to see it. I hear the loud click of the relay from the 1st floor thermostat location directly above the unit. And it sparked 100% of the time when I could see it by pushing the interlock while the thermostat was calling, and it made that same loud click, so I'm sure it sparks all the time.

Try tapping on the gas valve when it starts clicking/sparking. Use something like a screw driver. If it turns on then you might have a bad gas valve.

The has valve is inside the furnace. Follow the gas line into the furnace and when it gets into a box shape component with wires coming out , that's the gas valve.