Lennox gas furnace lights intermittently but runs well once light

Hello all. New member and first post here, really needing some help figuring out a gas furnace issue.

I have a Lennox - G26Q3-75-5 natural gas furnace (14 yrs old) on a Honeywell programmable thermostat (5 years old). The symptom on the furnace is that the draft induction motor will run but the furnace will not light - some of the time. At other times through the day it’ll fire off the first cycle. It’s very intermittent. Once light, it’ll run as long as needed to reach temperature.

A few months ago the induction blower motor was screeching as it sounded as the bearings had dried out. I replaced the motor and fan housing as a unit with an “Armstrong Johnson Air ease Efficiency Ultra 90 Draft Inducer 115 Volts # FB-RFB547”. New one runs nice and quiet.

After replacing the motor, I noticed that the furnace doesn't always light. I do not know if this was happening before the motor was replaced, assuming it was which may have helped cause the first motor to fail.

The SureLight Status/Diagnostic code the furnace flags is “Pressure switch open or has opened 5 times during a single call for heat, or blocked inlet/exhaust vent, or condensation line blocked, (for condensing furnace’s only), or pressure switch closed prior to activation of combustion air blower.”

Things I have done so far:

• Checked the outside inlet and exhaust pipes on my house. Could not see any debris or blockage in either. When the furnace is running, the exhaust warm air steam can be seen 10 feet off the house which seems like it has good airflow.
  
• New furnace filter.
  
• Checked that the condensation line is not blocked. I poured a few pitchers of hot water through the line, disconnecting it from the bottom of the induction motor exhaust tube through the catch down to the drain. Flows freely. I did shorten the hose a few inches to improve the angle.
  
• Removed the thermal sensor. I cleaned the tip, which didn’t appear dirty, with steel wool. Ohm’d the sensor and all appeared in good shape so put it back in.
  
• Removed the igniter and ohm’d it also. Replaced it being careful not to touch the end for oil transfer. It does ignite since putting it back in place. Been told when the igniter fails, they fail, they don’t partially work.
  
• Checked the pressure switch voltages open and closed, saw 24 volts when open. From other online sources that appears to be a ‘good’ value. Can feel suction on one end and airflow on the other end of the switch.
  
• Checked all wire connections, including those on the control board, everything is tight.
  
• Gas supply line hasn’t changed, gas meter outside is working. I have a gas fireplace insert off the same supply line and it works fine. Also when the furnace is light, flames are bright blue and solid, no flicker or yellowing.

Outside temperature doesn't seem to make a difference, it has issues lighting be it in the 40’s or -10. We are in a bit of altitude here, roughly 6000 ft.. The inlet/exhaust vents are on the north side of my home, as is the gas meter. We have some strong winds at time but again, wind doesn't seem to impact this issue from what I have monitored.

At this point, I’m not sure what to do next with it besides potentially throwing parts at it or calling in a ‘pro’ to diagnose. Figured I’d reach out here first though and see if anyone has some other insights.

Thanks very much for your time!

 

You said you replaced your draft inducer with “Armstrong Johnson Air ease Efficiency Ultra 90 Draft Inducer 115 Volts # FB-RFB547.” Is that the original part or equivalent?

It sounds as though you are getting only a marginal negative pressure from the draft inducer and the pressure switch is making intermittent contact (perhaps). Do you have (or can you get) something to measure the negative pressure at the switch? See if the pressure is adequately over (that is, under) the switch set point.

Or maybe it is something totally different.

 

Hello bentlerf, yeah the Armstrong Johnson #FB-RFB547 crosses to the Lennox branded motor made by Fasco. Specs are the same.

To check the negative pressure I need a manometer correct? Don't have one of those... is there another way to test that. The switch voltage appears correct.

 

Hmmm. Sounds less likely that the vent blower is marginal then. But yes, you would need a manometer or some other pressure measuring device.

And you have checked for obstructions in the line. And the vent gases showing 10 feet from the vent sound like there is good exhaust velocity anyway.

You could hang your voltmeter on the pressure switch and try to make sure that it switches only after the vent blower starts and that it does not open/close intermittently afterwards. But that sounds pretty unlikely anyway if your vents are clear and the vent motor is brand new.

I guess it could be a leak in your heat exchanger (hope not) or some other air leak into the vent line (is anything rusting out from the condensation?). Or the control board.

Maybe one of the professionals on the board have some good ideas...