Weird Oil Furnace Issue

Joined
Sep 18, 2002
Messages
6,056
Location
VA
We have an old oil furnace at church. It randomly goes into safety mode. We push the button and furnace comes on and runs up to temp
just fine. May run fine for 3 hours or 3 days. But at some point within a week the safety light will come on. 3 different techs have looked at it.
2 of them are 60+ years old with 15-30 years experience with oil furnaces.

Today the oil "GURU" came by and said "I am stumped." It worked perfectly for him.

New air and oil filter, new electronic board, new nozzle, flu cleaned, fire box vacuumed out, perfect flame, limiter working fine, etc.

Any of you got any ideas? The only thing that hasn't been changed is the pump. Intermittent problems are the worst.
 
The test: Disconnect the two yellow flame detector wires from the electronic box "F" terminals, and connect to an ohm meter instead. The flame sensor is a light-activated variable resistor. Resistance should be very high in the dark with burner off. Then start burner, when it lights the resistance should go down to < 1k ohm. More than 1 k is a weak detection, it's prone to false trip off (at around 1.6 k, the timer starts running to trip the safety). Check that the detector cell window is clean and pointed directly into the firebox. If it's not that, replace detector cell.

Also not noted in the page but make sure the resistance from either wire to ground is open circuit. The F circuit should be isolated from ground.
 
Yes, definitely check the CAD-cell flame sensor as mk378 suggested. I just went through this with the 30+ year-old oil furnace in my barn. I tried everything to get it to run consistently. In the end, a new CAD-cell sensor from Amazon did the trick.
 
The old tech came by church just now to check it again. It is really bothering him.

I texted him what mk378 said. He called me and said they replaced the CAD cell. He said furnace was still working fine. I'll still put an ohm meter on it tomorrow morning before service.

He says the only thing that hasn't been replaced is the pump and the motor.
 
Weak ignition transformer will also cause problems. But you say everything but the pump has been changed so I would lean towards that. There is a serviceable screen in the suntec pumps that can be removed and cleaned.
 
years ago we had problems with the oil furnace , same issue as above furnace would cycle perfectly then it wouldn't ,turned out to be a bad spot in the pump motor , when the motor randomly stopped on the bad spot it would not restart.
 
Weak ignition transformer will also cause problems. But you say everything but the pump has been changed so I would lean towards that. There is a serviceable screen in the suntec pumps that can be removed and cleaned.
Most excellent idea!
 
years ago we had problems with the oil furnace , same issue as above furnace would cycle perfectly then it wouldn't ,turned out to be a bad spot in the pump motor , when the motor randomly stopped on the bad spot it would not restart.
Seems are leaning towards the pump. He estimated pump at about $100.
 
Check cad cell as mentioned, did you try jumping out the t-stat or replacing the thermostat? Did the techs check oil flow? Could have a bad pump or could be simple as a pump coupler.
 
Air in the line, a weak pump, and an old transformer. The line was not properly bled after a filter change, the pump's internal filter was never checked for 15 years (did not know one was in there) and it let crud in causing wear and needing a new pump. The transformer just could not always make a strong spark. It was a long cold week to get these problems sorted out. Works like it should now plus I learned a lot on how to keep it maintained. Sounds like you are on the right track. My repairman said these are not uncommon problem's he will be back in a few months for a check-up.
 
Shot gun method rarely works. Time for a new set of eyes. Call around to some oil companies ask them who they recommend. A older service person isn't always the best. Someone 35 - 40 been around long enough to see a lot not to old to for get. Some times when controls are starting to go bad has to get worst before the problem shows up. 40+ years of running service. Could be something like a bad ground. Anyone use a flue analyzer?
 
Depends on how old of a furnace I’ve seen units run just fine using the armor casing of bx cable for ground. Newer units with circuit board’s would be more dependent on a good solid ground.
 
Is the oil tank above or below ground? If above ground is it a straight run to boiler or excess long run thru rooms etc? Any oil line embedded in concrete on run to boiler?
 
Last edited:
Shot gun method rarely works. Time for a new set of eyes. Call around to some oil companies ask them who they recommend. A older service person isn't always the best. Someone 35 - 40 been around long enough to see a lot not to old to for get. Some times when controls are starting to go bad has to get worst before the problem shows up. 40+ years of running service. Could be something like a bad ground. Anyone use a flue analyzer?
We have had 3 different companies look at it. 5 different guys starting in winter of 2020. Yesterday after service, my hog man delivered me 85 lbs
of freshly ground sausage (and I split the hog with a friend, "big sow") and he knows everyone. I asked him who the best oil furnace man was and he told me the current guy I am using.

That's how all the new parts got on it. Each company installed something. LOL
 
Air in the line, a weak pump, and an old transformer. The line was not properly bled after a filter change, the pump's internal filter was never checked for 15 years (did not know one was in there) and it let crud in causing wear and needing a new pump. The transformer just could not always make a strong spark. It was a long cold week to get these problems sorted out. Works like it should now plus I learned a lot on how to keep it maintained. Sounds like you are on the right track. My repairman said these are not uncommon problem's he will be back in a few months for a check-up.
Transformer has been replaced. I'll pass on the other suggestions. Thanks!
 
Back
Top