Hi all. I have a Trane XL1200 rooftop combo unit, uses direct spark ignition, that recently was having intermittent issues, but now seems to consistently not fire.
What seems to happen is:
Call for heat
1) Fan noise inducer motor on the unit runs
2) Spark is heard (I have also seen it when observing with the cover off), the spark goes for ~5 seconds.
3) Nothing lights (assuming no gas to light), fan noise speeds up.
After some time (30-50 seconds estimated), repeats the process. It will do the process 3 times before timing out and going silent.
When the heater DOES (er, did, it hasn't worked for a few days) come on, the spark goes for maybe 2 seconds before the gas is lit (as opposed to ~5 when it fails to light). I was up on the roof observing it the other day, and after being lit for a couple minutes, suddenly it went out (assuming gas went away). It has also done this in the past, started blowing heat in the house then just shut off after a minute or two or three. It has a new flame sensor which I replaced last year hoping to fix it, but it did not fix the intermittent problem back then either.
My neighbor has the exact same unit, from the model # anyway, and when his furnace lights, I can't even hear the spark at all, it seems to light the gas instantly. Not 2 seconds like mine, when it lights, or double that when it tries to light but doesn't. I don't know if this is part of the puzzle or not?
A local tech has come out and replaced the air pressure switch, which made the heater work fine for about a day, then replaced the gas valve, which made it work fine for about a night. Now it's back to being how it was.
I would like to get your opinions on what it could be, and what I could test. I want to avoid throwing more parts and $$ at it, and would like to do some other tests before having the tech come out again and spending more $$. When I described the problem to him again he said it's probably the control board (but after he replaced those other parts, it was running fine for a few hours, so he has not "caught it in the act" yet).
make sure yu have a true 24V-26V off that transformer when things are clicking and sparking if you see a drop during the draws on it might be weak on the VA when stuff stars up the board spark relays and such