Gas and Oil Home Heating Furnaces - Gas Furnace Fires Up Intermittently

I've watched my Bryant/Resco furnace during the boot process many times now. About 15% of the time, the furnace will fire up. Here's what happens when it doesn't. The inducer motor starts and the pressure switch activates correctly. I've measured 25 VAC out of the switch and also checked continuity back to the PC board. The surface ignitor doesn't light up and everything shuts off. I get an error code of 31, "Bad pressure switch or auxiliary switch". I don't think I have an auxiliary switch, but not 100% on that. I've ohmed both sides of the pressure switch back to the board and it shows continuity (That's why I don't think there are any other switches in the circuit). It may be coincidence but it seems to fail more often during storms.

Thanks for any help,
Steve

Check the hose on the pressure switch.. You should read 0 volts when the switch closes. if the hose is fine, then you may want to check your vent pipe to make sure it's clear.

Hi Jay,

What I observed was that, when idle, there is no voltage on either side of the pressure switch. When the thermostat calls for heat, 25v is put on one side (yellow). After the inducer motor starts, the switch activates and the 25v appears on the other side (orange). I can see the switch being released during this process. Am I right in concluding that since I can see the switch activating that the internal pressures, hoses, vents, etc. must be OK?

Thanks,
Steve

When a switch cloes you should read 0 volts on both sides.. if you are reading 24 volts, the switch is not closing.

yes, you will read 0 if the system is not running.. But when it runs, the currect will send power to the switches to make sure everything is closed.. if it's not fine, the path of power is NOT copleted, the system will not fire.

An auxilary switch would be any number of switches, I bet, that are in the pressure valve loop which could be any number of those safety switches. Test all switches for volts while running the furnace to see if you are completing all the loops. Do you have only ONE pressure switch and is it the one that has two wires or three wires on it? (With markings by the terminals that say "NO", or "NC", for normally open or normaly closed.)

If I am not mistaken, you can remove the one tha says "NO" as this, I believe is a safety device activated by a bad pressure situation, where the NC one needs the suction going on to make the furnace operate, You can jumper the NC one to then see if indeed you have a pressure switch problem.

Jay,

> When a switch cloes you should read 0 volts on both sides.. if you are
> reading 24 volts, the switch is not closing.

When a switch closes, you should read the same voltage on both sides.
It doesn't have to be zero, though.

Ecman,

Both sides of the pressure switch are shorts back to the board (yellow and orange wires). Besides, I can see the wires don't go anywhere else; i.e. no other switches in the loop. I am beginning to suspect a bad solder joint since I've read they're common with these boards, my problem is intermittent, and it's in a high vibration environment. Does that sound likely to you?

Thanks,
Steve

What do you mean by a 'short'? The fact that current is going to the board? THAT wouldn't be a short. That would be what it is supposed to do.

And if NO current were going throught the switch, that wouldn't be a short, either.

Clarify.

Ecman,

By a short, I mean the ohmmeter measures ~0 ohms from the pressure switch to the PC board, with power off, of course. I'm just verifying continuity. If there was another switch in the loop, you would measure open on that side. This assumes we're talking about Normally Open switches, which my pressure switch is.

Steve

A switch is NOT a load.. So when you take a reading on a swtich, if it's reading 24 volts, you are reading the "otherside".(Open circut) if you read 0 volts, you are on the same side..

If you take a reading from one side of the switch to ground you will read 24 volts.. now on ther other side of switch, if you read 24 volts to ground, then the switch is closed.. If you read 0 volts to ground the switch is open.

Let's look at this another way as to not get confused over open and closed and what side wires are being tested, and how this works, as we are at an important juncture in the test of your furnace.

You need current to continue flow to the board, plain and simple, in a nutshell. With the furnace running, if you put your voltmeter to volts and touched the terminal of the pressure switch that outflows to the board, and the other end of the voltmeter to ground, it should yield full voltage. In fact BOTH terminals on the pressure switch, tested to ground, should give this voltage. If NOT, you do have an issue with the pressure switch itself or why the suction TO it is not being generated.

Edit: Oh - and if you don't even have juice to the supply side of the pressure switch then you have other issues UPstream (the board would be DOWN stream). Continuity tests are fine and dandy, but you have to find out what's going on with the supplying power and where it's being terminated.

And the furnace has to be running while you do the voltage tests. (As long as the draft inducer is running, that is all that matters while you are doing this test.)

> When a switch cloes you should read 0 volts on both sides..

Jay,

After reading one of the other threads, I realized we were talking about two different situations. All my voltage measurements were referenced to ground.
I wanted to see which side was driving the switch and which side was sensing. I believe you were talking about measuring across the switch.

Sorry for any confusion,
Steve

Ecman,

See my last post to Jay. My readings were all ground-referenced.
I guess I should check if the voltage is making it all the way to the micro-controller. That will tell me if there's a bad solder joint along the way.

Steve

Yes.

And read my post again as I kept editing it like 6 times! :)

Just an update. I found that by pressing against the connector on the PC board, which contains the two wires to the pressure switch, the furnace would start up and run through one heating cycle. I took the board out and resoldered that connector and found another bad solder joint. Unfortunately, the board didn't work any better. I found a couple of sites on the net that sell the board for $200. Then I checked EBay and found that several boards a week are sold there for $75-$150. I needed one right away so I did a BIN for $150. Should arrive by the weekend.

Steve