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

 

I wonder if air was introduced into the pilot gas line and needed to be purged.

 

No..... it's not air in the system.

A door that regulates air to the burner ? I have no idea what that is.
There should be motorized draft inducer blower.

I'm going to need a model number to help you without guessing what you have there.
The model number is in the ID tag near the burner on the inside wall.

 

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?

 

I need to look up your unit but if it's not sparking..... it's usually not the burner/igniter...... it's usually an open limit switch or the pressure switch is not proving closed.

 

I'm not familiar with that furnace but iIdo see the damper you mentioned.
There should also be a draft inducer blower.... is that running ok ?

 

Yes, the draft inducer blower is working fine, though of course it blows cold air.

UPDATE:

Just to make sure I understand correctly, my understanding is that's the big blower at the bottom of the furnace. I don't see that this model has any other blower:

https://www.questargas.com/ForEmploy...ox_G20_IOM.pdf

 

Here's the closest thing to a manual on this model I've seen:

https://www.questargas.com/ForEmploy...ox_G20_IOM.pdf

Where do I find those switches?

FWIW I've checked the interlock switch -- the one that opens up and turns the furnace off if the lower furnace door panel is removed -- and it seems to be working normally. If I take that door off, the switch opens, the damper door closes, and the furnace is off. If I then press that switch in, the damper opens and the furnace tries to start. If I release the switch to go get the door panel to put it back on, the damper closes. But when I put the door panel back on, the damper opens back up and the furnace tries to turn on.

 

Start by checking for voltage on the spark/ignition module. On a call for heat.... there should be 24vAC measured between the THS (stat) connection and GND. If the voltage is there but no spark..... the control module is defective. If there is no voltage there then it's a safety/limit switch.

There are three switches between W1 and the THS terminal.
1) Damper prove switch. This switch closes when the damper is all the way open.
2) Flame rollout switch which will be just above the burner. This switch should always measure as closed. If open..... it's tripped.
3) Blocked vent switch which should also measure as always closed.
These three items are shown in the illustrations in figure 1 and 2 of that manual.

 

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.

 

Just an udpate that during the last ~24 hours 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.

 

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 is NOT working with the new limit control. This is a new OEM part made by Lennox, supplied by a local Lennox installer.

PJmax I'm going to try your suggestions next.

FWIW 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.

 

You need to follow what I posted. You need to determine if the ignition module is at fault or is not getting powered properly.

 

I am getting about 24 volts AC across the THS and ground terminals, after turning the furnace on with the thermostat set to a high temperature.

I'm using an old pocket-size analog multimeter, and I'm not sure of the precise reading. But if I set it to 50 volts AC the needle goes about halfway across. Let me know if a precise reading is required and I'll go pick up a well-reviewed digital multimeter.

So it seems I need a new control module. It's a Johnson Controls G776RGD-11. The 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. What do you suggest?

 

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.

Anyway, thanks for the help figuring out what the problem was! :-)

 

At least you narrowed the problem down to the ignition module. That means all the safetys are good.

 

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.

Do you have 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?

 

It's probably a relay with arced contacts at fault. I don't really have a preference as
to replacement modules. I hadn't noticed one more reliable over the other.

 

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 be what made it work once today), but it hasn't worked again.

 

When you get the new one..... installed, up and running..... bust open the old one and check it out.

 

Okay, so here's what happened with the replacement control module.

It clicked but only 3 times. Then after 5 seconds it clicked 3 times again. It did that a total of ten times, at 5 second intervals. The LED was on solid the whole time. Then, after the tenth try, the LED went out; no blinking error code. And the damper closed partway. No ignition. I turned the furnace off for a full minute and tried again; same result.

I went to test the voltage across THS and ground when it called for heat, and got nothing.

I put the old control module back on, and checked the voltage there too, and was surprised to also get nothing there, even though I had gotten about 24 volts AC before. BUT IT ALSO IGNITED. It clicked many times as it does when working normally, and lit up and did a normal burn.

While it was doing that burn, I went to test the multimeter since the result on the old control module didn't seem to make sense. I got nothing in an electrical outlet that I had just verified works. Also if I set it to Ohms and touch the leads, the needle still doesn't move. I looked at the fuse but I'm really not sure if there is an extremely fine wire in there or nothing. I tried shorting the fuse contacts with a paperclip and testing the electrical outlet again; still nothing. My son had gotten to the multimeter last night and was playing with it; I wonder if he dropped it and broke something inside. It's a cheap old Sperry pocket unit; model SP-5A. I would go buy a replacement but the car is in the shop.

BTW the furnace completed it's burn, and now with the house cooled back down it's back to not igniting; same problem as before; it tries to start but no clicking.

What do you think of the replacement control module's behavior? Clicking 3 times at 5 second intervals, for 10 sets, then no blinking error code and the damper closes partway. BTW it's a used control unit, not new.

 

We keep coming coming back to the same issue. You need a reliable meter to make these voltage checks. If your meter has a problem.... it's not helping anything.

This should be a 1,2,3 diagnosis and repair.

 

I bought a much better multimeter tonight, a Southwire 16040T. I went to Lowe's to get the cheaper 10030S, but the auto-ranging 16040T was on sale for less.

Both control units read about 25.6 Volts AC across THS and ground when called for heat. The old one drops slightly to about 24.6 when it starts clicking.

After swapping the old one back in, it ignited again. I've noticed a pattern that all three times I've removed and reseated all connections on the old control module, I get one ignition and burn.

So I suppose this means both control modules are bad, though in different ways. Fortunately I can return the one I just bought. Tomorrow I'll call about a new OEM control module.

Any idea why I've been getting one ignition after removing and reseating all connections?

 

After last night's burn, it wasn't igniting again. I tried removing and reseating connections and retrying after each one until finally the last one I tried, the spark wire, worked. It's burning now. Any idea why doing that would allow a single ignition?

Still too early here to call about a new OEM module but I'll do that this morning.

 

Does that use push-on connectors or screw terminals ?
Push-on slides can be slightly corroded but are not usually that intermittent.

If it has screw terminals and tightening and loosening them helps.... that would indicate a problem where they are soldered on to the board.

 

The spark wire pushes on. The controller side of that connection looks like a nail sticking out. I've never seen anything else like it. The wire looks and feels more like a skinny hose than a wire.

Here's a good picture of the connector, though this is control module is a different model:

https://cdn3.volusion.com/hkvs7.73qo...jpg?1435641069

 

You are discussing the spark wire. That is a high voltage wire very similar to a spark plug wire. That wire just pushes on and doesn't need to be terribly tight.

The connections I'm talking about are the power connections to the board.

 

The other connectors are all push-on; none are screw terminals.

 

I just talked to the parts guy at the place here that sells OEM Lennox parts. He says new control modules for Lennox intermittent pilot furnaces are no longer made; instead they only sell conversion kits to -- here he said something I didn't catch, but I think he was talking about hot plate ignition.

He said parts alone for the conversion kit were about $800, and estimated installation about around $1,000.

Are you familiar with these conversions? Do you think it's a reasonable DYI project? Do you know if there are aftermarket conversion kits that would cost less? I tried googling but only got hits on what seemed to be various other kinds of conversions.

Anyway, for now, I'll be choosing between aftermarket control modules; I don't see another used Johnson Controls G776RGD-11 on offer now.

 

Try these guys.

https://www.parts4heating.com/Johnso...g776rgd-11.htm

 

Thanks, but I had already ordered the ICM 2902:

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I emailed ICM to ask about compatibility with the G776RGD-11, and they said:

"Yes, the ICM2902 will replace the JCI G776RGD-11. You will need to change the connectors on a couple of wires but it is a suitable replacement."

I understand from a customer review of the ICM2902 somewhere that this just means I'll need to remove some (all?) of the crimp terminals and put on wider ones.

Fingers crossed that this new unit will last longer than the used OEM unit that just went bad. It lasted 5 years. The original had lasted 17 years.

 

The ICM 2902 is installed. I only had to change two of the crimp connectors ( to wider ones), plus roll my own ground connection (The JCI has a metal back plate which grounds by being screwed to the furnace wall; the ICM has a plastic back).

It ignited and burned on the first try, and again on the 2nd try. Woo-hoo!

*HOWEVER*, I am concerned about two things:

(1) It clicks for a very, very long time, far longer than I've ever heard the other one do it.

(2) It doesn't close the damper door all the way.


Should I be concerned about those two behaviors? Any suggestions?