Faulty ignition control?

Bosco1950

I have an old Lennox natural gas furnace (model #2003/4E-100-1) which is not igniting properly. When the furnace is cold, a call for heat results in the damper opening but no spark activity. Switching power off and back on results in 24 volts at the pilot valve, a short sparking cycle (maybe 3 sparks) but no ignition . Cycling the power again there is a longer spark cycle but still no ignition. Repeating 5 or 6 times finally ignites the pilot (sustained voltage at valve) and the normal start up sequence follows. If I then turn the thermostat down, and wait up to an hour or so before increasing the thermostat, the furnace will ignite normally. Waiting longer will result in the problem I described above.
My guess is that the ignition module is faulty (Robershaw SP735L) but I'm wondering if it could be the gas valve (Robertshaw 7100) instead.

EBEBRATT-Ed

First check all the wiring connections in all the 24 volt circuits. 24 volts isn't like 120 volts, it doesnt have much "push". The definition of voltage is "difference of potential". Corrosion, loose connections etc will stop 24 volts a lot easier than 120 volts.

Could be the damper proving switch, the ignition control, I don't think it's the gas valve if it isn't sparking. I would also remove and clean the igniter and check the gap. Also check the ignition cable for good connection on both ends and any deterioration.

These problems can be difficult to track down when they are an intermittant problem, best to not throw parts at it.

If you cant't find the problem prove the parts that work, that will narrow down the possibilities. Do you have a mulitmeter and know how to use it. If not you should probably call a pro after checking what you can check does not resolve this

EdTheHeaterMan

I read the sequence of operations for your system. There is a "damper open proving switch" that will allow 24V to energize the ignition control. If the "damper open proving switch" is not making good contact, the problem may be there and not in the actual ignition control. You may be able to test that with a jumper wire connecting W on the blower control board to TH on the ignition control at the proper time. (after the damper is fully open). If that test works every time, then the switch is bad, not the ignition control.





If you are not comfortable with performing his test, then call a Pro for help.

Yours Truly,
Mr.Ed

EdTheHeaterMan

If that switch is the problem, and you can't get the part right away, the repair I would use to get you heat is to bypass the switch and block the damper open, until you can get the replacement switch or until you purchase a new furnace. Don't bypass the other 2 safety switches S-47 and S-62 if you decide to do that. S-64 is the damper open proving switch

That damper is not a crucial part of the safe operation of the furnace, it is an energy-saving part that Lennox used to get a higher efficiency rating for that furnace. Bypassing its function won't make your gas bill noticeably higher.

Mr.Ed

Bosco1950

Thanks. I did pull and re-attach all the connectors and I'm showing a solid voltage (25.8 V) at the PV and MV when they cut in. I am fairly experienced in electrical matters having maintained some fairly complex woodworking machinery at work so I can check whatever's needed. The problem, of course, is knowing what to check. And yeah, I'm trying to avoid replacing perfectly good parts!
I did neglect to mention another symptom: after the pilot lights, the spark keeps firing until the main valve opens and the burners light.

Bosco1950

Thanks Ed. That looks easy enough to do, I think I have an alligator jumper that may be long enough. I'll give that a try tomorrow.

mattmia2

It will spark until it proves the pilot flame, then it will turn of the spark and open the main valve.

See the sequence of operations @EdTheHeaterMan posted above.

HomerJSmith

I have found on more than one occasion that the fault that you describe has been the result of the relay on the ignition board that has a intermittent contact with the foil pattern on the back of the board. This is, I believe, because of the vibration of the relay opening and closing which fatigues the solder joint. The main valve MV on the board never gets the power to open the main gas valve.

I don't know if this is your problem, but you can check it out. Look for a solder break on the MV relay or any other solder point.

I would also check the ignition cable where it might be resting on metal. Slip some cardboard between the cable and the metal chassis and try ignition again.

mattmia2

HomerJSmith said:

I have found on more than one occasion that the fault that you describe has been the result of the relay on the ignition board that has a intermittent contact with the foil pattern on the back of the board. This is, I believe, because of the vibration of the relay opening and closing which fatigues the solder joint.

It is a combination of fatigue and either improper design or control of the wave solder process, the joint does not have enough solder in it, either because the solder pad isn't big enough, the solder mask doesn't expose enough of the pad, or it didn't get the right conditions to fully fill the joint during the wave soldering.

Also possible it needed to be reengineered for lead free solder and wasn't.

EdTheHeaterMan

So what everyone is saying is, to determine if the problem is in the ignition control or the blower circuit board (BCB), you have to eliminate each item until you get consistent proof of failure. If the test I suggested earlier is not conclusive, then you need to place the jumper between the R on the blower circuit board (BCB) and the W on the blower circuit board to see if that resolves the problem.

My process would be to go from R on BCB to TH on the Ignition control (IGN) (with the damper open) three times. Don't let the burner run very long so as not to heat up the furnace compartment. If that operates the IGN normally every time, then drop back to AFTER the Prover switch S-46. If that works every time, then drop back to the BEFORE side of the Prover switch S-46. If that works every time then go back to the W that goes to the prover switch S-46 (not the W from the thermostat, it appears that there are two W terminals in the diagram) Each step of the journey you are leaving the other end of the alligator jumper on the R terminal

There appears to be a multi-wire plastic connector somewhere. Indicated by the ---<5<--- illustration on the diagram. This connector could also be the intermittent "Lose Connection" problem. That would make sense with the issue only happening when the system is idle for over an hour. As the temperature cools the connection becomes less reliable, as the system heats up and all the metals and plastics expands, the connection becomes more dependable.

When there are so many wires and connectors, it is difficult to pinpoint the cause. But a process of elimination will get you closer, each step of the journey.

Good luck with your troubleshooting.

Mr.Ed

Bosco1950

OK. This morning I turned the furnace on again. This time, though, there was no ignition action at all (no voltage at PV). I then attached the jumper as per Ed's instruction above, bypassing the damper proving switch. This returned the unit to the behavior I first described; first attempt was a very short burst of ignition activity (less than a second), second time a bit longer. On the fourth attempt the pilot briefly lit (1 second) and the PV closed. By the 7th try the pilot stayed lit and the sequence continued normally. I tried again without the jumper and there was no activity at all. I guess the next thing to try is Homer's suggestion to check the ignition board for a bad relay connection?

Bosco1950

OK. This morning I turned the furnace on again. This time, though, there was no ignition action at all (no voltage at PV). I then attached the jumper as per Ed's instruction above, bypassing the damper proving switch. This returned the unit to the behavior I first described; first attempt was a very short burst of ignition activity (less than a second), second time a bit longer. On the fourth attempt the pilot briefly lit (1 second) and the PV closed. By the 7th try the pilot stayed lit and the sequence continued normally. I tried again without the jumper and there was no activity at all. I guess the next thing to try is Homer's suggestion to check the ignition board for a bad relay connection?

EdTheHeaterMan

EDIT:
By jumping W to TH as above, you are not eliminating the BCB from the equation. Try a jumper from the R or the 24 of the BCB to the TH on the IGN. See if that eliminates the problem.
Look at the updated diagram.



You may have a bad ignition control and a bad BCB. Stranger thing s have happened. Can you take the plastic wire plug apart and look inside it? sometimes just taking the plug apart and putting it back together is enough to make the connection more dependable.

Is it the IGN the original White Rogers control? I would use a Honeywell S8610U3009 for the replacement. The Blower Control Board is a Lennox OEM part. There may be an aftermarket available, but I would need the part number for that control to check it out. At the age of the furnace, it might be time to consider a replacement. I would hate to put one or two new controls on there and find a cracked heat exchanger next season.

Best wishes with your diagnostics.

Mr.Ed

HomerJSmith

I always install a CO detector on any furnace that is over 15 yrs old. Costco has them on special pricing this week. CO is the silent killer.

Bosco1950

Thanks Homer. I'll check that out.

Bosco1950

Well, Ed, I traced the current problem back to the damper proving switch. Removing the cover I found a simple microswitch but no mechanical actuator lever! Apparently this has been absent all along and the circuit was being completed because the two terminals were shorted to one another. I had noticed that earlier and bent them slightly so they were no longer contacting. That is when the ignition cycle would only start with the jumper in place. Holding the switch closed the ignition cycle begins without the jumper. I will fabricate a new lever and that problem will be taken care of.
I also removed the pilot assembly and thoroughly cleaned it but that did nothing to solve the initial problem. In addition I inspected and reseated the plastic plug you spoke of but it still takes multiple on/off cycles before the pilot stays lit. After that the startup sequence proceeds as it should.
At this point I'm feeling more certain that the ignition board is the culprit here. The current board is a Robertshaw SP735L. Tomorrow I will pull the board and inspect it for bad solder joints as per Homer's advice before I order a replacement.

EdTheHeaterMan

Do you have a multi-meter? the ignition control will start its sequence of operation once the TH terminal gets 24 V. power with a return path from TR back thru pin --<7<-- that is connected to common on the control transformer. If you don't have 24 volts there, then the control will not operate. Turning the furnace power off and on may get the BCB to send power intermittently to the IGN. If you are getting Power to the IGN every time and the ign does not function properly, then you need to replace the IGN. If you are not getting power every time, the problem may be in the BCB, then you may need to replace the BCB. A meter will tell you if you are getting voltage every time or intermittently.

Once you know that, you can choose the proper part to replace.

Mr.ED

Bosco1950

Thanks. I'll check that tomorrow before I remove the ignition module.

EBEBRATT-Ed

@Bosco

Just be sure if you are not using the damper and have the proving switch bypassed that the damper is locked in the open position for safety

Bosco1950

EBEBRATT-Ed said:

@Bosco

Just be sure if you are not using the damper and have the proving switch bypassed that the damper is locked in the open position for safety

I do intend to use the damper so that won't be a concern. I'm curious, though, what function the damper serves. Is it to help establish a draft?

HomerJSmith

No, it traps the heat energy in the heat exchanger, preventing the hot air from moving up the flue.

EBEBRATT-Ed

@Bosco1950
The damper opens when the burner fires and closes when it is off to keep from losing heat up the chimney. Maybe they save a little fuel, not much

EdTheHeaterMan

The predecessor to that G20 Lennox furnace was 78% efficient. In order to meet with the new government minimum efficiency standards (back in the day), manufacturers scrambled to find some way to get the existing products to reach the minimum 80% efficiency standards of the time. That included the elimination of a standing pilot and shutting off the natural chimney draft thru the heat exchanger. Those 2 little changes with the development of a rating system that included the off-cycle losses as part of the savings (we call it AFUE) were enough to allow the equipment to be rated 80%.

You can still see some boilers that use a vent damper attached to the chimney connector pipe with many manufacturer's offerings like Weil McLain's EG boiler series and Burnham's Series 2, which have been around since the beginning of time.

If you close the door on the exhaust pipe, you will reduce the amount of heated air from inside the home from leaving up the chimney. This will reduce the amount of infiltration from outdoors, in turn allowing the off-cycle last a little longer.

Respectfully submitted,
Mr.ED

Bosco1950

Well, I installed a new ignition module and the furnace is operating properly. Examining the old board I found a burned pc trace where a small diode is located.  Many thanks to all of you who offered your advice and expertise!