Gas and Oil Home Heating Furnaces - Gas furnace intermittant lighting

I have a Rheem model RGDA-15EE-JR, about 18 years old. In December of 2007 I had problems where the furnace would not light and traced the problem (or so I thought) to the ignition module. I replaced the Ignition control (Robertshaw 100-00812-12, model HS780) with a Honeywell S8910U1000 and all appeared fine.

Now, first time starting the system this year, the furnace lit, but intermittantly it won't. What happens is it cycles thru igniting the glow plug, but the pilot bar won't light. This seems somewhat similar to what was happening last year. This morning I checked and the thermostat was calling for heat, but nothing was running. Checked the module and it was blinking 3 times indicating internal error. I shut power to the furnace to reset everything and sometimes this will do the trick and it will light.

Do I have a flaky ignition module, or is this something else?

My reasoning for replacing the module last year was (and I cannot remember exact details now) but the voltage output for turning on the gas switch appeared to be too low. I can't remember the exact numbers, but if I needed 24 volts, I was only getting 2 or 3. When I replaced the unit I was seeing proper voltage. I have not checked the voltage yet on this newer unit.

Thanks

Make sure the spark/pilot asemebly is clean near the burners.

When the spark lights the pilot, make sure the pilot is strong and wrapped around the sensor.

I'm suspecting a gas valve problem unless the gas valve is not getting 24 volts. We know the module is putting out 120 volts for the hot surface ignitor but how about the 24 volts for the gas valve? Can you at any time during the ignition sequence hear the gas valve click?

Shoot, I thought that model of furnace had spark, not HSI (Hot surface ignitor)..

Is the ignitor glowing when it is trying to light on call of heat?

On that model I believe the igniter is also the flame rod. Make sure the pilot lighter bar/tube is not blocked by a spider nest. (happens all the time) It can be removed by using a pair of channel locks and pulling it from the orifice. It is a wedge fit if memory serves.

Thanks for the replies.

This model uses an HSI to ignite. The HSI appears to be working properly as it does glow. When it does not light (ignite the pilot bar), I do not hear a click of the gas valve opening. Unfortunately I did not have a voltmeter around at that time to check voltages. Watching it I see the HSI glow, waits for a number of seconds, then it shuts off - no audible click signaling the gas valve opening. Eventually the HSI is turned off and the process repeats itself. The problem now is that it DOES work most of the time, so I don't always have a good opportunity to diagnose it. I just checked this evening and I am getting 24V to the gas valve, and it is working (clicking, opening and allowing gas thru).

At this point it is really intermittent. It’s getting up in the 70’s outside so there isn’t much call for heat, and when I have turned the thermostat up, it works just fine.

Any suggestions on what I could check out/on at this point?

Besides possibly a faulty control board, I think the problem might be that the furnace has determined, at those times, that the pressure inside is not right. It's for reasons like you are having that I love those pressure switches where you can actually see the plunger connected to the diaphram, moving in and out. Sometimes you can actually witness it asort of quiver in and out, while it is running. And then if you observe where the plunger is when the furnace is off, and where the plunger is, when the furnace is working, you can guess if at those times it does not come on, if the plunger is right in that gray area.

Still having periodic issues with this furnace. Tonight it would not light. I checked this time and I am NOT getting 24V to the gas valve. HSI is operating correctly, however no "click" for the gas valve, instead the HSI just turns off.

Sometimes just turning off the heat @ the thermostat or killing power to the furnace and then turning back on seems to correct the problem.

Are there any other switches/sensors that would prevent the voltage from going to the gas valve? Or am I dealing with a a buggy ignition module? The latter would really upset me as it was just replaced 12/07.

I checked this time and I am NOT getting 24V to the gas valve. HSI is operating correctly, however no "click" for the gas valve, instead the HSI just turns off.
Did you check at the igniter/board to see if you got power from there for the gas valve?

I am getting 24V supplied to the board, verified at the board terminal. I have also checked the terminal supplying 24V to the gas valve, and when it does not work I am not seeing any voltage there. When it does work, I am seeing voltage there (24V).

After monitoring this thing for a while now I can tell when the gas valve is about to open (after the HSI has been on for a bit). When it fails to light, it's right at the time the gas valve should open, instead the HSI is turned off - and no gas valve open. It then repeats itself until eventually it gives up and I get the status light blinking 3 times - indicating "internal fault", whatever that is...

Reading your most recent post, it sounds like you've found the source of your intermittent ignition. If you are getting power to the board, all safety switches are closed, the ignitor glows, & nothing out to the gas valve, it would certainly seem the problem lies in the board.

And now another question...

While checking out everything, I'm questioning the HSI currently installed. The original ignition control was a Robert Shaw (100-00812-12, model HS780) which I've replaced last year with the S8910U1000. Looking over the doco for the S8910U1000, it says I should be using a Norton HSI 201 or 271 or equivalent. I've got a White Rodgers F767A-372 HSI installed, but how can I verify that this is the right/equivalent HSI? Does it matter?

I wouldn't worry about the HSI especially since it is not used to prove flame.

Thanks Grady (and others) for the help. One final question...

As I have just replaced this board less than a year ago, is this possibly indicating I have a problem elsewhere that might be the root cause, or, should I chalk this up to some bad luck?

I have seen (somewhere) a cross-reference for the ignitor numbers. Check the W-R website. Someone had actually installed the 'wrong' model number on mine, but since the ignitor still got hot, it still got the job done.

blue3

Appears it was the ignition module... And it pays to ask questions - questions like "Is this part by chance covered under a warranty?". Turns out that Grainger has a 1 year warranty on their parts, so my replacing it 12/07 got me a new one in exchange!

Thanks all again for the assistance!

Glad you got it fixed & even better the control was under warranty. :thumbup: