furnace main gas valve cycles 2 to 3 times per second

furnaceNewby

We have a Bryant model 394A forced air furnace (circa 1988). When the thermostat calls for heat, the pilot flame lights, and the small blower that forces air up the flue comes on. The next thing that is supposed to happen is the main gas valve opens and the big flames come on solid. This works fine for one heating cycle in three.

However, sometimes the main valve cycles on and off at this point. Rapidly. Like two or three times per second. The main flames go on-off-on-off like this until either they stay on - and the heating cycle continues and completes normally - or the pilot goes out and a 15 or 30 seconds later the cycle repeats. The pilot relights (the flue blower has remained on this whole time), and soon after - the main flame either lights successfully, or does the rapid cycle (2-3 times per second) until the mains can stay on.

This is hard to explain. Below is a video.
In the first case, the main flame tries once then the pilot goes out.
In the second case, the main flames cycle repeatedly, 2-3 times per second, and then the pilot goes out.
In the final case, the mains stay lit and the heater runs as designed.

All three cases in the video are from the same "call for heat" period from the thermostat. I edited it for brevity, taking out all the dead time waiting for things to happen.

If you have any suggestions, or if I can provide more information, please let me know!

https://www.youtube.com/watch?v=ViRZM2x40uk

JStar

Looks like the pressure switch or some other safety needs to be tested or have its electrical connections cleaned. I would highly recommend having a pro look at this.

icesailor

You need a Professional to service it.
Look carefully. Every time the gas valve clicks, the pilot has gone out. You can always see the pilot lit when tit tries to start, but when the gas valve goes out, the pilot has been blown out. One time, during the "dark" view, it goes out and the pilot flame doesn't re-appear. In the final "bright" view, the pilot never goes out and the flame never goes out.

You need a professional. Only a trained professional can decide what the problem is.

furnaceNewby

Thanks for the quick replies!

I'll contact a professional, but not today (New Years Day). The unit works: it always starts. Sometimes it starts on the first try, sometimes it takes a couple minutes of trying. But it always starts, and the heating cycle always completes normally. The house is warm.

I will try cleaning the pilot unit to perhaps keep the pilot from going out when the mains come on. Once the main flames are stable and the blower comes on, the main flames look great throughout the remaining portion of the heating cycle (10 mins?).

I looked at the pilot. Nothing much to see or clean.

Ray_Landry

Where are you located? Do you have CO detectors? Could be a cracked heat exchanger

icesailor

If you don't have installed CO detectors, go out and get them ASAP. They are cheap at 10X the price. Far cheaper than a session in a hyperbaric chamber or the mortuary.

Seriously.

Super Wally's is open today. Get a battery one and fire it off. Put it behind the exhaust of your car. make sure it gets a reading. Let it zero out. Put it inside. If it at ALL starts showing even a "1", whomever you call to look at your pilot, tell them about the reading.

If you called me, you could follow me back to my house by the tire turning marks in the road from my house to yours. For those of us with an aversion to scorched air, we don't always think about cracked HX'ers.

furnaceNewby

I am in Colorado. The furnace is in my basement. I will get a CO detector. I watched the flames through the whole blower cycle, and they look great. No floating, no yellow, nothing strange.

The effects of a cracked heat exchanger show up once the blower has come on. The blower doesn't turn on while this furnace is trying to light the main flames. It comes on after the main flames are stable.

What I mean to say is, I suspect that the issue of the furnace not lighting the mains is not caused by a cracked HX. But, I am not a professional, so who knows.

icesailor

Maybe. Maybe not. The exhaust fan comes on long before the gas valve opens after
"proving" draft.

IMO, the reason the burner cycles is because the pilot keeps going out and re-lighting.

I'm not there, I won't be the one fixing it. You still need a professional. There's a reason the pilot goes out and causes the recycling. The reasons are too numerous for an untrained person to figure out.

IMO

furnaceNewby

The professional came to visit today. He listened to the description of the problem, turned the furnace on (initiated a call for heat) and immediately (before the problem showed itself) began to bang around on things with a screwdriver handle. When he hit the pressure switch, the furnace shut off. Well, the gas to the pilot shut off. Then he called up and ordered a pressure switch (the one he had on his truck wasn't compatible with my furnace). The switch will be here Monday.

I'm skeptical of the fact that he did not re-create the symptoms shown in the video I posted.

How do people normally test the pressure switch? and if the pressure switch is making and breaking, is that an indication of a problem elsewhere?

unclejohn

You have a Bryant furnace not a Rheem. Also you may have a bad gas valve or three wire pilot. They need to be checked with something other then the end of a screw driver. It sounds like the pick coil of the valve keeps dropping out. Tim can explain it better then me.

furnaceNewby

Thanks for your reply. Yes, I do have a Bryant, thanks for correcting me. I have a volt meter from my electronics days if that helps. I agree the gas valve is switching from on to off and back again, causing the main flame to pulse off and on. What I don't know is whats causing this to happen.

I've read other discussions about wire brushing and cleaning the pilot assy. Is that a good place to start?

Tim McElwain

That model of Bryant furnace is a little tricky and probably beyond your ability to diagnose. Was the tech you had there from a Carrier, Bryant, Day Night Payne dealership? If not I suggest you get hold of one with those credentials. There is still the possibility that it could be the pressure switch so let them put it in and see how it goes from there. If you need me to go over the operation of your furnace I will do that. Or even better than that I can mail you a Troubleshooting Guide for the Bryant 398A furnace. Just let me know what you want to do?

That pressure switch should be checked with a manometer and the normal function is that the normally open pressure switch contacts make at a pressure of at least 0.82" negative Water Column. If it does not make at this pressure or is erratic replace the switch. Banging with a screwdriver is not a real way to test and is guessing at the best.

Do you have this 90 plus furnace serviced every year?

furnaceNewby

I am guilty of not servicing the furnace every year. My bad.

I did replace the motor this past November. It was having trouble starting in October so we oiled the old motor, but it was not to be. Failed to start again a month later, so we replaced the motor.

Yesterday, we vacuumed a bunch of debris out of the exhaust vent and around the area where the proving blower (?) connects to the exhaust piping (at the top of the furnace unit). It seemed like sand, or cement/brick mortar that was in the flue. I can't figure out how it might have got there. My water heater also uses this flue.

I called this particular pro at the recommendation of my neighbor. He uses this guy all the time. Not sure of his creds, but he does work for a local HVAC business.

Today (and yesterday during the day) the furnace is running fine. Mains are firing on the first try, every time. Outside air temp has been above freezing. This morning, outside air was well below freezing, the unit had the most trouble firing the mains.

As for troubleshooting, I am interested in fixing my unit, but I am now wondering if 26 years isn't more than enough to expect from it. I suspect that whatever is happening, it will eventually fail hard, and not heat my house. That is starting to bother me, to the point where I am trying to decide to replace the whole unit.

I have to make it through the weekend regardless of what I do, and we are more than willing to try anything to keep it firing the mains on the first try.

Thanks for any input.

unclejohn

You need a new furnace. The fact of the matter is you could spend hundreds of dollars with someone who knows what he's doing . Were not supposed to discuss pricing here but I will tell you I repaired a similar furnace last year, customer said if a new furnace was a dollar more he would fix it rather then replace. He but over a thousand in a 28 year old furnace. Possible causes for your issue, pressure switch, three wire pilot, gas valve, pressure switch circuit board. I probably left something out, plus you already but a new fan motor in. Cut your losses and replace. The least of your worries is the sand stuff, that's in all those furnaces.

furnaceNewby

I tend to agree. We are looking at our options in that area now.

One other thing the pro did while he was here was to disconnect the tube connected to the pressure switch, and blow into the tube toward the firebox.

So, in summary, the things that have been done are: disconnect/reconnect all wires on the gas valve, the pressure switch, the pressure switch circuit board. Blow air into the tube (towards the fire chamber, not towards the pressure switch). And some banging with a screwdriver handle on the various parts: gas valve, pressure switch, and 3-wire pilot/igniter.

Since the pro left our house (27 hours ago), the furnace has started the mains first time, every time. Our furnace runs three (sometimes two, sometimes four) cycles per hour, so that is over 75 cycles without the multiple tries on starting the mains. Prior to his visit, two cycles in three would have multiple tries to start the mains (as seen in the video above). I hope I didn't just jinx it!

The other thing is the outside temperature hasn't gone below 20F since he was here. I only mention that because the furnace had the hardest time starting the mains when it was coldest yesterday morning (prior to his visit), and I think it may have been below 20F outside.

I am not calling this a final fix, in fact I expect it to go back to using multiple tries to start the main flames again soon. So, we are actively pursuing the replacement options.

furnaceNewby

It has now been 5 weeks since the repairman was here. The furnace has started first-time every-time since he was here.

Still looking at options for a new furnace.
Thanks!

furnaceNewby

It has now been 15 weeks since the repairman was here. The furnace has started first-time every-time since he was here. We've had a bit cooler spring and so our furnace is still heating the house although not all day and not more than once or twice per hour in the cooler times of the day.

Still looking at options for a new furnace.
Thanks!

rick in Alaska

Thanks for the updates!
Rick