Furnace Gas Valve Concerns

[someone.hasposted]

I have a Bryant gas furnace from the 80's. 376BAW0. It's a downflow. This type has a pilot that is spark-ignited with each call for heat. It has an inducer (exhaust) fan.

For those unfamiliar, the thermostat turns on a relay that turns on 120V to the inducer fan and passes along another 24V circuit (probably to limit the current draw through the thermostat. When function of the inducer is proved by a pressure diaphragm switch, power is sent to a thermostatic switch by the pilot light as well as the pilot gas valve. That switch, when cold, sends power to the spark ignitor. With the pilot burning that switch warms and the spark generator should turn off and, upon further warming, the main gas valve is turned on. It doesn't snap open; it takes about six or seven seconds for it to open. It's called a three wire pilot as the thermostatic switch is SPDT.

The actual gas valve is two valves in series for safety with the pilot gas tapped off in between. So what I call the pilot valve is the first of the two. Just to be clear we are talking about a single body with two valves inside. White Rodgers 36E93-301.

Over the years I have gone through (counting the original components), 3 inducer motors (the bearings fail), two gas valves, and three pilot heat switches. I recently noticed that the latter switch was not turning off the spark generator. I don't know how long it's been this way but so far as I can tell the sparker keeps sparking for the duration of the cycle. So I have ordered another pilot assembly which I will have in a few days.

The other night an odd thing happened. The furnace shut down due to the limit switch (a safety thermostat with a button one must press to reset) atop the blower inside the blower compartment. Over the years this has only tripped once, during a power failure. My thinking was the the power went out during a cycle and the very hot air convecting upwards (the blower being above the heat exchanger on a downflow furnace) tripped it. At the time I reset it and that was that.

So anyway it tripped the other night and I reset it. Next morning it tripped again. Not knowing if the limit switch had become hypersensitive for some reason I decided to do an experiment interchanging it with a similar one near the inducer. They might be for different temps; I am not sure, but this is just an experiment. I will revert eventually and replace if necessary. I figure it would be safer to do this than bypass it entirely when I don't know why it tripped.

(I think the purpose of the other one is in case the chimney stack is clogged; hot exhaust will come out of this sort of bypass thing next to the inducer and quickly trip the limit switch shutting down the furnace should that happen.)

So I did this and vacuumed the filters while I was in there and I closed things up.

Here is where it gets odd. When the furnace cycled, the main gas came on just a few seconds after the pilot. It is not waiting for the pilot heat switch to get hot. One might think okay, if the heat switch is keeping the sparker on maybe it's doing the same for the main burner. But it's not. I have an indicator (LED with suitable resistor) on the line from the pilot heat switch and that circuit turns on about 30 seconds later. So why could the main gas come on when that circuit is off?

(I acknowledge there are things I don't totally understand as regards the schematic refers to "pick" and "hold" for the gas valve. Maybe someone can explain that.)

It can't have anything to do with my test interchange of limit switches because those are just closed (on) switches like they are not even there.

But it may have something to do with why the limit on the blower was tripping.

I'm running through various theories. If that secondary valve was physically sticking open, then the main burner would come on instantly with the pilot. It is not. It's coming on the few seconds later similar to the delay when it is turned on electrically. But not knowing how the valve operates internally this delay could be some natural consequence of the 2nd valve not opening until it gets pressure from the first. So it could have been sticking but still take six seconds.

So cannot discount a gas valve issue.

It's not doing this all the time but sometimes. How this ties to the limit switch tripping I am not sure. Obviously, if the valves somehow kept the burner going even after the cycle completed -- the worst case scenario for a gas valve -- it would trip the limit but doing so would not shut off the gas if it's physically stuck. Fortunately that seems unlikely as that would require both valves sticking open. Which is why there are two valves within the valve body.

It's also possible they are unrelated. Maybe the delay that keeps the blower going after burner shutdown is too short (aging capacitor on main board). Maybe I should time how long the over-run is; it seems normal. (I would not mind a longer run to extract more heat from the heat exchanger.)

At this point I think I should replace the gas valve. What say you all?

[trader_4]

Sounds expensive. Does it really matter if the spark igniter sparks while the furnace runs?

Sounds logical. If you want to find out what's going on with either switch you could put them in an oven, rig up some wires to a VOM and measure them.

How does this sensor work? Is it a thermocouple or flame rod? Does it go directly to the valve or to a control board? Have you tried disconnecting it and seeing what happens? Is it possible the sensor detects the flame faster than you think? The flame rod type may be fast.

It is not waiting for the pilot heat switch to get hot. One might think okay, if the heat switch is keeping the sparker on maybe it's doing the same for the main burner. But it's not. I have an indicator (LED with suitable resistor) on the line from the pilot heat switch and that circuit turns on about 30 seconds later. So why could the main gas come on when that circuit is off?

I think at this point, another consideration is that it's probably time for a new furnace. Depends on what your fuel cost is, what the climate is. I replaced a 1984 gas furnace 9 years ago. Huge difference in gas usage, probably using 40% less. I assume it was an 80% furnace, but likely due to it's age it was running at less than that. New one is 94% two stage. If you're having so many issues and throwing money at a failing one, probably time for a new one. That would be true for sure if you couldn't fix it yourself.

[trader_4]

Another idea. The concern is that the gas valve opens just seconds after the pilot flame lights, you think it should take longer to detect that the pilot is lit. So, rig it up so you have your meter on the wires to the gas valve. Then turn off the gas and fire the furnace up. If the flame sensor is working, with no gas, no pilot, the gas valve should not get voltage to open.

[someone.hasposted]

Maybe not but if it burns itself out then yes. I presume they designed it to be turned off for a reason.

Sure, but I don't know at what temp they are supposed to pop. Given the other issue I'm leaning towards it not being hypersensitive and had a reason to trip.

The switch by the pilot flame is just a simple bi-metal thermostatic SPDT switch. Studying an old one it's possible that it's make before break. I hesitate to call it a sensor since that implies greater sophistication.

I know how long it takes to switch on. And even when it's doing the odd thing I am reporting on, it is still switching on later after it gets sufficiently hot. But the main gas is already on, coming on six or seven seconds after the pilot. That it does so despite the circuit from the pilot heat switch is the puzzle.

Yes, that is almost certain. A/C as well. But I wish to keep things going until I acquire full ownership of the house from my two siblings.

[someone.hasposted]

I know that main gas is coming on (when it's doing this) irrespective of the status of the thermostatic switch that gets heated by the pilot. I know this because of an LED I have on the hot side output of that switch AND another I placed on the main gas valve input wire on the gas valve. (I believe these are just opposite ends of same wire.)

With gas shut off as you suggest there would be no way to see that the main gas valve is opening. My indicator LEDs already tell me this is happening without the pilot heat switch being hot.

I just watched it and while I didn't run a stop-watch I'd say it normally takes about 45 seconds for the pilot heat switch to switch main gas on. Naturally this is going to vary with how long it's been since the burner last cycled and how much that part has cooled.

[trader_4]

What kind of "switch" is this that detects the pilot light? The only types I'm familiar with are thermocouple or flame rod, neither is a switch. But if it's a switch and I see you're saying that you've looked at the voltage and it behaves correctly, but the gas valve comes on before that, then the next question is how does that circuit work? If it goes to a control board, sounds like the control board is bad. If it's wired in series with the valve circuit, then IDK. But it comes down to tracing the circuit, where there is and isn't voltage.

[trader_4]

No markings on the switch that show the temp or where google might help? You could at least verify that they seem to behave properly. I'd think the plenum over temp is probably 150 or so, the one to detect flame coming out probably 250 or so.

K, then that likely goes to a control board, in which case the control board is suspect, because it sounds like it's turning on the gas valve without the switch being closed. Or the wiring has a short.

[someone.hasposted]

The “hot” out from the SPDT pilot heat switch goes to the main gas valve. It also goes to the main board where I believe it starts the time delay for the main blower. If something funky on the main board was sending 24V back to the main gas control line it would light my indicator LEDs. When it’s doing this weird thing they aren’t lit and don’t light until maybe 30 sec later when the pilot heat switch gets hot.

Maybe this is an open and shut case: “Anytime you see main gas on without 24V going to the main gas valve connection you have a bad valve. Period.” I’m putting that in quotes because I’m not certain of that. Hoping an expert can confirm.

I did realize a likely reason for the safety trip. If the main blower time delay works off the pilot heat switch, then the main gas coming on 30-45 seconds sooner than normal would cause hot air convecting up through the heat exchanger and occasionally it trips the safety atop the blower.m

[trader_4]

Old furnaces just used a plenum temp switch to turn on the air blower. My new one just uses a time delay. Seems unlikely anything would be based off the pilot light temp switch.

So, as I understand it the working theory is that the gas valve has two valves, one for the pilot one for the main and that they are in series, that both have to be open for the main gas to flow. And you think the main valve is stuck open, so the main gas flows as soon as the pilot valve part opens. The remaining question would be if your type of valve works that way. IDK. My experience I've had a standing pilot type and now a hot igniter that AFAIK just lights the main gas, not a pilot first. But if it's a valve like yours where it lights the pilot first, then it seems logical that they would design the valve so that for the main gas to flow, both the pilot valve and the main valve have to actuate.

[someone.hasposted]

Time delay blower start and overrun is common; perhaps more so than a stat on the heat exhanger. I do know that is the case here as the schematic says so. It does not detail the actual electronic circuitry of the time delay itself but a line coming from the hot side of the pilot heat switch (which also goes to the main gas valve) goes to the box on the schematic marked time delay.

I know about the dual sequential gas valve with pilot tapped off in between because I have the manual for the gas valve.

Yes, newer furnaces seem to dispense with the pilot entirely but they have microprocessors controlling the start. I don’t know much about hot surface igniters but I suppose it’s turned on first and then the gas. Some might use a sparker instead in which case it and gas can come on together. Either way, if fire is not quickly detected stop the gas and wait for the inducer to purge the unburned gas then try again. Try a few times and if it does not light lock out and throw an error code. But I digress.

[trader_4]

So then I guess it's time for a new gas valve.