intermittent ignition, components replaced

Problems related to residential installations.

Moderator: juster

Post Reply
fredjr2004
New Contributor
Posts: 5
Joined: Wed Mar 25, 2015 12:31 am

intermittent ignition, components replaced

Post by fredjr2004 » Wed Mar 25, 2015 12:49 am

I have an older Heil forced air furnace. It's been working intermittently for a few months now. It started with no ignition coil glow. I cleaned the pressure switch and the ignition coil started working. Upon close inspection out also appeared damaged so I replaced that. It worked for another day or so but started to fail to ignite, the timing was off, glow with no pilot, pilot with no ignition ect... based on a recommendation a decided to replace the gas valve (Smart Valve 9401) after installing and purging the air out of the system it worked well for a few weeks. It started to fail to ignite again so I worked backwards from the pressure switch again, I tested the 24v transformer and the output voltage did not read properly. Next step wire in a new transformer. It worked properly for another week. It failed again today, no ignition just exhaust fan. After tinkering around with fan speed settings and fan delay settings it seems to be working again. Sounds illogical, I know. At this point the only thing left to replace is the fan control board (ST9120). I would really hate to replace this board and be in the same boat. Any insight would be appreciated.

Freon
Most Valued Contributor
Posts: 1019
Joined: Sat Jun 18, 2011 11:26 am

- intermittent ignition, components replaced

Post by Freon » Wed Mar 25, 2015 7:19 pm

You have two fans. One is the inducer and it is the first smaller fan to start immediately when there's a call for heat. The second fan is your main blower fan that circulates warm air in your house.

You need to focus on your inducer fan. There may be debris or condensation in the tube leading from the fan housing to the pressure switch. Or the inducer motor may be old and tired. To eliminate the inducer assembly as a possible problem, jumper the inducer pressure switch TEMPORARILY. See if you get consistent Hot Surface Ignitor glowing and main burner ignition. Unless the inducer pressure switch is closed, nothing else will happen. With the TEMPORARY jumper you can rule out or in the inducer assembly as a cause of your problem.

fredjr2004
New Contributor
Posts: 5
Joined: Wed Mar 25, 2015 12:31 am

- intermittent ignition, components replaced

Post by fredjr2004 » Wed Mar 25, 2015 7:39 pm

I jumped the pressure switch when the problem first started. There was some serious corrosion that I cleaned out. After that it worked ok for a week or so. Each time it's failed and before ordering another part I've gone through the pressure switch test again just to make sure that was not the cause, it hasn't been.

fredjr2004
New Contributor
Posts: 5
Joined: Wed Mar 25, 2015 12:31 am

- intermittent ignition, components replaced

Post by fredjr2004 » Wed Mar 25, 2015 9:59 pm

It seemed to be working when I got home this evening. Thanks for the reply freon. Hopefully it doesn't start with the failure to ignite again.

Freon
Most Valued Contributor
Posts: 1019
Joined: Sat Jun 18, 2011 11:26 am

- intermittent ignition, components replaced

Post by Freon » Thu Mar 26, 2015 3:38 pm

Check that the HSI grows white hot. When you replaced it did you get an identical replacement HSI and relocate it relative to the burner tube as before? Check for any debris in the burner tube and the orifice to be sure the gas flow is as designed. Lack of a hot HSI, improper HSI placement or improper gas flow might cause intermittent issues.

fredjr2004
New Contributor
Posts: 5
Joined: Wed Mar 25, 2015 12:31 am

- intermittent ignition, components replaced

Post by fredjr2004 » Thu Mar 26, 2015 10:08 pm

Freon, the HSI is an identical replacement. It wasn't working again when I good home today. I pulled the burner assembly out and checked the HSI position. Evening looked ok, then I pulled off the pilot tube and verified that it was clear. I put everything together again and checked my connections and it fired up. Left for dinner got back and now it's back to just running the exhaust fan. I checked voltages and jumped the pressure switch again. The pattern seems as if it will work if I take it out of service for a while. But always goes back to failure to ignite ands just running the exhaust fan. Sometimes it will last a day out sometimes a few weeks. Thanks again for the reply

Freon
Most Valued Contributor
Posts: 1019
Joined: Sat Jun 18, 2011 11:26 am

- intermittent ignition, components replaced

Post by Freon » Fri Mar 27, 2015 11:15 am

The fact the exhaust fan (draft inducer ) runs with no burner ignition suggests the main board itself may be bad. If there is no ignition the inducer fan should stop and there should be a flashing LED indicating the error code. Since the inducer continues to run after an ignition failure tells me something is not getting the right signal from the control board circuitry.

When you turn off the power you're also effectively resetting the control board so it will try and behave normally. Intermittent failures are the worst but I think you're down to the control board. If you can, give it a close inspection looking for cold solder joints on the PCB. Check all plug connections. It might be a physical (solder, plug) issue or an IC that's close to failing completely. If it's a failing IC you'll need a new board.

fredjr2004
New Contributor
Posts: 5
Joined: Wed Mar 25, 2015 12:31 am

- intermittent ignition, components replaced

Post by fredjr2004 » Wed Apr 01, 2015 11:16 pm

Freon, I ordered in a new board and installed it tonight. Everything seems to be working very well, thanks for your help!

Post Reply