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Posted: 1/17/2021 10:37:15 AM EST
Link Posted: 1/17/2021 11:13:03 AM EST
Link Posted: 1/17/2021 11:23:11 AM EST
[Last Edit: 1/17/2021 11:25:40 AM EST by fxntime]
You now need to check for 24V to the gas valve if the igniter has had power applied to it and OHM out the gas valve. You should get 24V to the gas valve from the board relay once the igniter has stayed on a bit, if you don't then it's a board issue, replacement is needed, if you get 24v before the gas valve and OL on the gas valve when tested [wires removed, no power] then the gas valve is bad and needs replacement.

Make sure all safeties have been tested and all are closed to include any in the blower area and at the burner assy.
Link Posted: 1/17/2021 11:26:53 AM EST
Originally Posted By TOTHEMAX:
Hey guys.

So my furnace will not ignite. The inducer motor turns on and the ignitor heats up but after 10 seconds or so it kicks off. In the past, the gas would kick on next and ignite.

What would cause the gas not to kick on? I checked all connections and they are tight. Some are a little rusty so I tried cleaning them up. The wires going to the ignitor look fine and I cleaned the flame sensor while I was in there. I checked the roll-out sensor and it is not tripped. I also checked the vacuumed controlled switch that is connected to the inducer and it seems to be working. You can hear the switch click when the inducer comes on so it seems to be working.

Model # CGU09516

Yes, it's old and needs to be replaced but it's not on the docket yet.
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Replace the flame sensor. Even if you cleaned it, it may still be bad and this is a cheap test.
Link Posted: 1/17/2021 11:31:57 AM EST
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Originally Posted By WVUSIG:


Replace the flame sensor. Even if you cleaned it, it may still be bad and this is a cheap test.
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Originally Posted By WVUSIG:
Originally Posted By TOTHEMAX:
Hey guys.

So my furnace will not ignite. The inducer motor turns on and the ignitor heats up but after 10 seconds or so it kicks off. In the past, the gas would kick on next and ignite.

What would cause the gas not to kick on? I checked all connections and they are tight. Some are a little rusty so I tried cleaning them up. The wires going to the ignitor look fine and I cleaned the flame sensor while I was in there. I checked the roll-out sensor and it is not tripped. I also checked the vacuumed controlled switch that is connected to the inducer and it seems to be working. You can hear the switch click when the inducer comes on so it seems to be working.

Model # CGU09516

Yes, it's old and needs to be replaced but it's not on the docket yet.


Replace the flame sensor. Even if you cleaned it, it may still be bad and this is a cheap test.


That proves that the burners have all fired, it has nothing to do with the gas valve opening and the firing of the burners initially, it simply uses the flame as a path back to the board to prove all the burners have lit. If it fires and then goes out after a few seconds then it's a flame sensing issue.
Link Posted: 1/17/2021 11:43:48 AM EST
I had a water heater that wouldn't fire because the sensor that detected the exhaust draft went bad.  The thing won't fire if it can't tell that the exhaust isn't going where it's supposed to.  I think it's called a negative pressure sensor or something like that.  Check that too.
Link Posted: 1/17/2021 11:45:44 AM EST
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Originally Posted By SWMP15AOP:
I had a water heater that wouldn't fire because the sensor that detected the exhaust draft went bad.  The thing won't fire if it can't tell that the exhaust isn't going where it's supposed to.  I think it's called a negative pressure sensor or something like that.  Check that too.
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If the igniter has power and is glowing, the inducer has closed the pressure switch and sent power to the board. OP is past that part of sequence of operation.
Link Posted: 1/17/2021 12:02:53 PM EST
Link Posted: 1/17/2021 12:14:58 PM EST
[Last Edit: 1/17/2021 12:16:40 PM EST by fxntime]
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Originally Posted By TOTHEMAX:


@fxntime

I got 24v for a split second when the ignitor came on then it dropped to 4ish. When it tried for the 2nd time it jumped to 6v then back down to 3v.

If I read your post correctly, I should replace the robertshaw control 100-00812?

I can hear it click then stop when I get the voltage jump
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Originally Posted By TOTHEMAX:
Originally Posted By fxntime:
You now need to check for 24V to the gas valve if the igniter has had power applied to it and OHM out the gas valve. You should get 24V to the gas valve from the board relay once the igniter has stayed on a bit, if you don't then it's a board issue, replacement is needed, if you get 24v before the gas valve and OL on the gas valve when tested [wires removed, no power] then the gas valve is bad and needs replacement.

Make sure all safeties have been tested and all are closed to include any in the blower area and at the burner assy.


@fxntime

I got 24v for a split second when the ignitor came on then it dropped to 4ish. When it tried for the 2nd time it jumped to 6v then back down to 3v.

If I read your post correctly, I should replace the robertshaw control 100-00812?

I can hear it click then stop when I get the voltage jump


Do you get a resistance read [OHM] when you take a read off the gas valve? [connections on the valve the leads from the board go to] gas valve needs to be set to on. If so, then yes, the board is bad, the relay on the board is defective and not closing properly. Time to replace the board.

Common and reasonably cheap.
Link Posted: 1/17/2021 12:18:18 PM EST
Link Posted: 1/17/2021 12:20:11 PM EST
Link Posted: 1/17/2021 12:23:04 PM EST
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Originally Posted By TOTHEMAX:
I was finishing up my post and did not see yours.

The furnace is running now and I did not check the ohm reading.

I will once it shuts off.
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How did you get it running?
Link Posted: 1/17/2021 12:40:06 PM EST
Link Posted: 1/17/2021 12:51:25 PM EST
Link Posted: 1/17/2021 12:51:43 PM EST
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Originally Posted By TOTHEMAX:


Not sure as the only thing is did was remove the control box and smack the 3 relays with a screwdriver. When I reinstalled that is when it decided to work.

Stuck relay?
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Originally Posted By TOTHEMAX:
Originally Posted By fxntime:
Originally Posted By TOTHEMAX:
I was finishing up my post and did not see yours.

The furnace is running now and I did not check the ohm reading.

I will once it shuts off.


How did you get it running?


Not sure as the only thing is did was remove the control box and smack the 3 relays with a screwdriver. When I reinstalled that is when it decided to work.

Stuck relay?


Probably, I'd get a new board on the way as it's not long for the world. You could also have a crack in the circuit board or a connection that isn't good. It won't last long if that is the problem.
Link Posted: 1/17/2021 12:52:20 PM EST
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Originally Posted By TOTHEMAX:
I removed the two wired going to the gas valve and yes they show resistance. Anywhere form 16-20ohms

The heater did run for approximately 20 minutes and then you could hear some intermittent clicking and when I came around the corner the flame had went out. Then the inducer motor kept running and the unit shut off.
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Valve is likely good.
Link Posted: 1/17/2021 12:55:29 PM EST
Link Posted: 1/17/2021 12:58:37 PM EST
[Last Edit: 1/17/2021 1:04:44 PM EST by fxntime]
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Originally Posted By TOTHEMAX:
Thank you for all the help. I will try and get a new board ordered for this thing.
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I did a quick search and you won't have any issue getting one.

I'm betting I have replacements [not always the same board number] right on the truck.

If you do get a generic replacement [which is pretty common with older furnaces] make sure you read the instructions WELL and adjust the appropriate dip switches or break off tab inserts correctly for the board number you replace. Don't just toss the instructions............
Link Posted: 1/17/2021 1:05:54 PM EST
Link Posted: 1/17/2021 1:24:25 PM EST
Link Posted: 1/17/2021 1:27:23 PM EST
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Originally Posted By TOTHEMAX:
Yep I can't get it to work properly now. I called my local hvac guy and he said he can get one tomorrow. I have one other call into another guy I know.
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I fix em for a living and I still have several electric heaters and 2 kerosene [12K and 30K] heaters simply because they always seem to take a shit on the weekend when the parts warehouses are closed.
Link Posted: 1/17/2021 1:53:46 PM EST
Link Posted: 1/17/2021 3:17:36 PM EST
Link Posted: 1/17/2021 3:20:37 PM EST
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Originally Posted By TOTHEMAX:
@fxntime

Is there any way to bypass the control pannel or trick the gas valve into thinking its working to get the heater to come on?
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There is but it's so unsafe that I wouldn't dream of telling someone how to do it. Needless to say, it's unsafe enough even I who have worked on them for decades wouldn't do it to mine.
Link Posted: 1/17/2021 3:30:37 PM EST
Link Posted: 1/18/2021 9:27:37 AM EST
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Originally Posted By fxntime:


Valve is likely good.
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Valves are very reliable.

Likely a board issue.  

Wanted to make sure that you are looking for ac volts not dc.
Link Posted: 1/18/2021 9:30:35 AM EST
They might not sell to your wife.  Supply houses are set up to sell to dealer.

Order on line and air ship in.

It’s going to be 300-500 for someone to come out and fix.
Link Posted: 1/18/2021 11:44:06 AM EST
Link Posted: 1/19/2021 12:13:53 PM EST
Link Posted: 1/21/2021 5:21:55 PM EST
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Originally Posted By TOTHEMAX:
Wife picked the part up yesterday and got it installed. Heater fired right up. It was still working when I got off shift this morning.

Thank you for helping.
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Excellent, glad you got it working.
Link Posted: 1/24/2021 11:47:28 AM EST
[Last Edit: 1/24/2021 11:51:09 AM EST by RIP-Yataski]
The FAU here started having issues with not lighting last year. The code being thrown by the board was for a limit switch. These aren’t really limit switches, but rather temp switches, but that’s not the point.

Anyways, after investigating, I determined that there are multiple (3-5) separate temp switches wired in series within an input control circuit. If any of them opens, then the unit goes into fault and shuts down/off.

Okay, fine. I start pulling the switches and continuity check them. Nothing conclusive, but I track down a shop that sells HVAC parts to the public & buy a couple of them. There are different styles. Anyways again, get it put back together and it eventually cycles fully & starts blowing hot air.

Fast forward to earlier this season: Same shit. Got to thinking that the fan motor hasn’t started when any of this has been occurring. Pulled the oil-filled capacitor and checked its capacitance. Yup, NFG. Replaced it and all had been good since.

The lack of air flow due to the fan not starting allowed to much heat to build which tripped the temp switch(s).

Maybe one issue is showing itself as another issue?...

ETA: Seems the issue was diagnosed & fixed. Good deal...
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