I get my 25 yr old oil & hot air furnace serviced every other year.
Last fall there was a new kid at the place where I've had this done for 20 yrs & I watched him tune it up. He actually seemed to be doing a more thorough job than his more experienced predecessors. He was the first one in years to open the plenum & vacuum the top of the heat tubes.
But he didn't do the ?CO2? test-- where a probe is placed in the stack. I asked him about it & he passed it off with a 'we don't do that any more'. The old guy that was 'watching' him didn't disagree.
He left & for a couple weeks the furnace seemed fine--- but then it started this random 'stuttering'. [or I started noticing it] Maybe once every 10th [or 20th] cycle you can hear the furnace start-- then stop-- then start - sometimes as many as 5-6 times. The next cycle is always perfect. I've spent hours sitting beside this thing to try to figure out exactly what is happening, but I've never caught it actually acting up. The blower fan works steady through this stuttering so I've ruled out an open in the electrical feed. If I remember correctly the photocell will trip the reset button after the second try. So far it hasn't tripped the reset button at all. [ I 'tested' the reset button this fall when I changed the filter & forgot to bleed the line-- that tripped it.]
Another random and discomforting problem is some exhaust smell on rare occasions. I think I did catch this one once-- The burner fired & though it sounded good, there was a lot of smoke & it was coming out of an old probe hole in the stack-- and around the damper in the chimney.
This fall I finally decided to pull the nozzle & look at the ignitors. I was sure I'd found the problem as the ignitors were set to fire exactly even with the nozzle head. [.75, 80degree hollow cone] I re-set them to 1/8" ahead of the face of the nozzle, 1/8 apart & 1/4 above the hole. That seemed to do the trick-- but I've heard it 'stuttering' a couple times since.
I don't get any of the pre or post combustion 'pops' that would indicate unburned fuel being left in the plenum. I do smell burned fuel outside sometimes if the wind is just right. I don't know if this is just when the furnace is doing its 'stuttering' thing- but it is also random. I've never seen any smoke coming from the chimney.
This is a one line system & the *new* tank is outside and about 5 feet higher than the furnace. Filters were replaced this fall & there was no sign of gunk or water in either one. [I have 2 inline filters because I had an old tank under the eaves & I added a filter in the low spot of the line to catch water.]
Any thoughts on what might be causing the intermittent 'stuttering' of the burner?
Jim