No clicking sound, no ignition

[Greg Lovern]

I have a Lennox furnace that was installed probably in the early 1980s. It has an intermittent pilot light.

Normally when it's trying to ignite, if I'm standing next to the furnace I hear a repetitive clicking sound; several clicks per second. Then I hear a whoosh as the pilot light ignites, the clicking sounds stops, and soon the burners start up and the fan starts.

This year, when we first tried to get heat, cold air blew from the vents. I saw that the burners weren't lit, so I turned the furnace off, set the thermostat to a high temperature, and turned the furnace back on. A small motorized door that seems to regulate how much air gets to the burners closed when I turned the furnace off and reopened when I turned it back on.

But then, no clicking sound as I would normally hear at that point. After a while an indicator light on the electronics unit started flashing 1 second on, 5 seconds off, repeatedly.

I repeated several times, always getting the same result. I started unscrewing a panel to try to get access to the ignitor and pilot light to investigate. I had to leave for a while and when I came back to it the next day, I once again tried cycling it. This time I DID hear the clicking. And then the whoosh of the pilot light. and then the burners came on and it started blowing warm air.

So for now all is well, but since I did not actually do anything I assume it's probably going to happen again, perhaps soon. But of course it's difficult to troubleshoot a system that's working properly at the moment.

So my question is, what are your guesses as to what the problem might be? Is there anything I can check while the system is working properly?

Also, followup question -- can a furnace like this be upgraded to a hot surface ignitor at a reasonable cost?


Thanks,

Greg

 

[WorthFlorida]

A furnace that old may not be worth repairing. For one it sounds like a typical gas forced air furnace and most manufactures had basically the same design. A spark ignitor was really the first attempt to save energy by not having the pilot light burn 24/7 weather needed or not but it is old tech as compared to what is available today. I would say the ignition circuit is faulty (capacitor, coil, switching circuit) or the ignition probe where the spark jumps to ground may be worn out whereby the gap has opened too much for the spark to jump to ground. There is a safety circuit not to turn on the gas if spark is not detected, therefore, no clicks you should not smell gas. There is really no way to check it while it is running.

Most important is the condition of the plenum. A thirty five year old furnace there could be rusted through holes in the plenum that can allow carbon monoxide to enter the home. Furnaces of this era and today the plenums were made of stamped steel that can rust out and very old ones, pre WWII had cast iron that could last forever. If you never had the furnace checked out it maybe time to do so. The service tech may know if there are new ignition parts or upgrades are available for this unit. New furnaces are so much more efficient that a replacement probably would pay for it self in a few years.

 

[Sponsor]

 

[Dana]

What WorthFlorida said- a furnace that's over 35 years old that has run an average duty cycle is past it's anticipated lifecycle, and will in short years develop other, sometimes hazardous problems. At least consider replacement rather than repair.

In that consideration take the time to analyze the oversizing factor. Most gas furnaces out there are 3x oversized for the heating load- usually to save installers the time it takes to figure out the right size. Upsizing the furnace by 3x+ isn't a big expense nor does it have much impact on efficiency with hot air furnaces. The biggest impact is lower creature comfort.

Unless you're anticipating a cold snap reaching temperatures south of -50F you will be more comfortable with a furnace no more than 1.4x oversized (ASHRAE recommendation) for the heat load at the 99th percentile temperature bin, which is about +25F in Bellevue WA. With a heating history on the place it's fairly easy to put a bracket on the sizing with some wintertime gas bills and the nameplate efficiency of the existing furnace, using the furnace itself as the measuring instrument, as outlined here. This method reduces the guesswork around the magnitude of the distribution losses, etc. since there is no way to actually separate that out in the fuel use. There is some amount of error due to solar gains (even in cloud bedecked Puget Sound winters it's not zero), but those are pretty much canceled out by the error from other uses of gas, such as hot water.

Don't be surprised that even at 1.4x oversizing the max recommended furnace size is half that of the existing furnace. If that creates anxiety bumping it as high as 1.7X (the presumptive oversize factor in AFUE testing) is still fine, but less comfortable unless its a 2-stage furnace. The bigger the oversizing, the higher the air flows, which means more noise, more wind chill. A big oversize factor also results in short run times, a hot-blast followed by the lingering chill. A right sized furnace runs a very high duty cycle when it's actually cold out, and it's more comfortable to have a the warm summer breeze effect for the preponderance of the time than hot flash/chill cycles. Long run times don't mean it's "struggling to keep up", even if the cycles are longer than an hour.

 

[Greg Lovern]

UPDATE:

First, the problem is happening again.

Second, I was wrong about it being installed in the early 1980s. It was manufactured November 1996, so 22 years old. (Wasn't that kind of late for intermittent pilot? When did they stop doing that?)

Model Number:
G20Q3/4E

Serial Number:
5896K 2834

Third, I'm trying to get access to the ignitor but a panel still won't come off after removing all the screws that look like likely suspects. Still working on that, but if anyone can point to instructions that would be much appreciated.

Fourth -- now that the problem is happening again, can anyone offer any troubleshooting suggestions?

 

[WorthFlorida]

I search on your model number G20Q3/4E and a nice troubleshooting chart is on a last page. One thing that caught my eye was a safety cabinet switch or "interlock" switch, it's about the second or third item on the troubleshooting flowchart. Check any where the cabinet cover screws down to the unit.

https://www.questargas.com/ForEmployees/qgcOperationsTraining/Furnaces/Lennox_G20_IOM.pdf

 

[Dana]

That thing is a 100K furnace with 79K output- enough to keep my sub-code 2x4 framed antique 2400' house (+ 1600' of semi conditioned basement) at 70F at an outdoor temp of about -68F. Since my 99% outside design temp is +5F, that's a bit more than 2x oversized. Unless yours is a MUCH larger (and much leakier) house than mine, given your mid-20sF outside design temp it's probably at least a 3x oversize factor, which is all too common.

At 22 years of service, even if you can fix it on the cheap now to get it through another heating season or two, it's still worth thinking about replacing it with something newer (more appropriately sized for the load) before it's toast. Normal lifecycle estimates on hot air furnaces is 25 years, and you might get that (or more) out of it, at the expense of lower reliability and increasingly frequent repairs. A right sized condensing gas burner with half the output BTUs will likely still be oversized, but more comfortable, and knock 15% or more off the annual heating bill. A 40-60K single stage mid-90s efficiency gas furnace usually costs under a grand.

 

[Greg Lovern]

The interlock switch seems to be working normally. If I take the lower door off to expose it, which allows the switch to turn off, the damper closes. If I press the switch back in (lower door still off), the damper opens back up and the furnace tries to start (but no clicking).

If I put the lower door back on, the damper closes after I release the switch to go get the door, then the damper reopens when I put the door in place. So I think the interlock switch is doing its job.

 

[Greg Lovern]

Just an update that I suspect the limit switch. It sometimes works if I tap it, hold it at an odd angle, etc. But it's intermittent so I'm not sure.

Also I understand now that I've abused that switch by letting the filter go too long without replacing it. I do have a new filter in now. And I've posted a filter replacement log at eye level on the cold air return duct as a reminder. I walk by the furnace all the time for other purposes, so this should help me stay on top of it.

I've ordered a new OEM replacement switch, should be here in 2 days.

UPDATE:

It's been a day since I posted this and during that time it's been working normally with no intervention. It must have had several ignitions and burns during that time. Of course I still plan to install the new limit switch when it arrives.

 

[Greg Lovern]

UPDATE:

It worked for a couple days, then stopped working. After a day of not working, the new limit control arrived. I installed it a few hours ago and restarted the furnace. I tested raising the thermostat temperature. It it is NOT working with the new limit control. This is a new OEM part made by Lennox, supplied by a local Lennox installer.

Any suggestions on what to try next?

If I turn off the furnace for a full minute, then turn it back on, with the thermostat set to a high temperature, here's the sequence of events I see & hear:

1. When I turn the furnace off, the damper closes. Then I waited for full minute.
2. When I turn the furnace on, the damper opens.
3. Abut 25 seconds later, I hear a sort of hum or hiss. No clicking sound.
4. About 45 seconds later, the main fan starts up. No burn though.
5. About a minute later, the control module starts flashing an error code; 1 second on, 5 seconds off.

 

[Greg Lovern]

On the advice of someone on another forum, I checked for AC voltage across the THS and ground terminals, after turning the furnace on with the thermostat set to a high temperature. I found that it is getting 24 volts AC, and I'm told that this means that the problem is the control module.

The control module is a Johnson Controls G776RGD-11. The original control module died 5 years ago, and this was a used replacement I picked up then for $65.

I'd rather buy a new, high quality replacement this time than continue buying ever older used units getting a few years' life out of each one. I see that other brands including Honeywell offer replacements. Any suggestions on what brand would be best?

 

[Greg Lovern]

I reseated the THS terminal and the ground terminals several times, and tapped the side of the control unit a few times, and tried again. It worked.

So...now I don't know if it was just corrosion on the contacts that needed scraping away, or something going flaky inside the control unit that liked being tapped, for now. So I don't know if this problem is resolved or if the control unit is going bad. But for now it is working.

 

[Greg Lovern]

And...it's already not working again. My 9yo wanted to know what I did, so I walked him through the test, and it did not fire. Same problem as before.

I don't think corroded terminals are likely the problem; I think this more likely points to something going flaky inside the control module.

Any thoughts about whether it makes more sense to buy a new Johnsons Controls unit, or a replacement unit by another brand such as Honeywell?

 

[Greg Lovern]

I decided to go with the fastest way I could find to get a replacement control unit, which was an eBay seller willing to ship Saturday morning by overnight mail, so I should have it Monday (fingers crossed). It's a Johnson Controls, same model.

FWIW I've tried tapping the control unit several times since it stopped working (since that seemed to to be what made it work once today), but it hasn't worked again.