Hi All,
Goodman GMP100-4 80% eff. natural gas furnace circa 1993.
TLDR: 4th burner lighting inconsistently. All other burners (3) light immediately. Flame rod, igniter tested and working; burners, flame spreaders and general furnace has been cleaned and blown out with compressed air. Thoughts?
Full story:
Late last season I recall noticing 1st thing in morning it would take several trials for ignition to light. All burners would not light immediately, 2-4 trials required (it only locked out once or twice). Would sometimes smell natural gas. This season diagnosed as aging HSI during cleaning. New HSI lights burners on 1st trial and no more smell of natural gas. Win.
With the replaced HSI, it now appears we have a different issue with the 4th burner (last one associated with flame rod) having trouble igniting. Usually minimally delayed ignition from other burners and then it ignites and goes out and re-ignites few times. Burner flickers slightly for a few sec even after it is ignited and stays on. Depending on how quickly flame stabilizes the flame sensor may shut it down - it usually can stay on after 2nd or 3rd trial during recovery (after its been off over night or during day while we're at work), if system has had a run or two it will still flicker but will stay on with 1st trial. Same behavior of 4th burner regardless if trial for ignition occurs when only inducer motor is on or after main blower comes on for blowdown protocol from missed 2nd ignition attempt. Sometimes blowing on 4th burner helps maintain ignition, but that's qualitative at best.
Burners & flame spreaders have been cleaned (3x in varying intensity out of desperation), everything including gas manifold blown out. Manifold orifices have good flow. Issue acts like a flame spreader obstruction but order of burners has been changed and issue persists. I doubt a blockage could develop in heat exchanger and it wouldn't explain why once the 4th burner get's going it acts fine. We bumped the gas pressure >1/4 turn it seemed to help a little with time to ignition but issue persists... and realize this could be masking the real issue, potentially making it worse, etc. We have a manometer on order for arrival in a day or so. I hope it is the gas pressure but probably wishful thinking; a gas valve shouldn't unadjust and it is unlikely that gas pressure has always been too low given the furnace worked fine previously.
Initially I dismissed this could be a heat exchanger issue since there is absolutely no flame rollout, no change in flame pattern after main blower comes on, and no changes in flame pattern after system heats up. Did find a thread indicating could still be heat exchanger. I guess is could be an issue if air is being pulled in through a crack bypassing the 4th burner, but if there's a crack I'd expect to see impact from main blower turning on and changing the air flow pattern.
I'm resisting calling out a HVAC expert because I doubt they'll do any troubleshooting beyond what I've done, so it will just be $ without tangible results or explanations for behavior and they'll say we should replace the whole unit. Not opposed to replacing the unit, or even just the heat exchanger, but mainly I'd like to figure out what is causing the issue. Thoughts?
