The problems could be: Sooted up boiler passageways, improper air supply, blocked fuel filter and fuel starvation at the pump, worn electrodes, dirt on the cad cell eye, smoky flame. The primary control on the burner is shutting down from any of these problems which can affect the flame.

If you have a power venter,(no chimney) the proving switch could be faulty. There is a 1/4" tube in the power venter that operates on sensing air movement to operate the power venter proving switch/diaphragm. This tube can be blocked with soot. All these things can shut down the burner.

If you don't understand any of these terms, you'll need to get someone in fix your unit.

Thanks for the response! This morning, Saturday, I pressed the reset button on the stack controller and the burner started up. I shut it off and then replaced the filter, replaced the strainer, brushed and vacuumed the inside - then I started the furnace back up and it ran. The flame, from my untrained eye, looks ok, not dark or smoky. I'm not convinced this will solve the problem as it has been less than a year since this was last done.

I don't have a power venter.

A technical at the hardware shop showed me where the cad cell eye is on a burner they had in the store. Still need to locate on mine and will then clean it off.

How would I know if the electrodes are worn?

Thanks for the help!

Generally, there is an air gap of 1/8" between two pointed electrodes. This gap is located about 5/16" above and about 1/16" in front of the center of the nozzle. Specs vary according to manufacturer, but that's close. Worn electrodes are rounded and spaced far apart. The air from the motor can blow out a spark if the ignitor is weak. Carbon under the two ceramic electrode clamps can short out a spark too.

If you have a stack control you won't have a cad cell. None of the afore mentioned issues will cause a motor reset. It's either the motor or the pump. Sometimes an older pump will stick and the motor doesn't have enough torque to break it free. In this case, since the motor will generally restart, I don't think that's the problem. You most likely have an older motor with a centrifigal start switch. With age they tend to hang up sometimes in the run position so on the next cycle it's trying to start on the run windings.

If your furnace is old enough to still have a stack control it's old enough to be carted out to the junk yard. A new one would cut your oil bill considerably.

Pushing both resets makes it impossible to determine which is the problem. Next time, push only the button on the motor. If the furnace starts, the motor or the pump is the problem and an amperage draw measurement is needed on the motor. If it does not start, push the reset on the primary control. The next time after than push only the reset on the primary control. See which is the problem and deal with just the causes involved.

If the motor reset trips then the primary reset will trip. If the primary reset trips first there will be no power to the motor, therefore it won't trip.

As dapoppa noted, I don't have a cad cell. The electrodes look ok - no cracks and the ends have a point. I also replaced the nozzle while I was in there.

The safety on the motor continues to pop, requiring a reset.

Improper air supply is still a possible root cause.

The motor being bad due to the centrifugal start switch is still a possible root cause. Where would I begin to research motors?

Can you think of anything else that could cause the safety on the motor to be tripped? Bad aquastat? Bad thermostat?

Thank you!

If the reset on the motor trips repeatedly and the pump/burner blower combo turns freely, replacing the burner motor is in order.

Look on the Grainger website. Compare the data on your motor's plate. You're probably looking for a 1/6HP, 1/7th or 1/8HP motor. You need to know the rotation and RPM (1750 or 3450) Rotation is viewed from the shaft side.

There are only a few motors for oil burners. Your style would be like the Grainger 4MA10, but without a dust cover and the proper RPM.

To address the possible root cause of improper air supply, today I removed the pipe from the furnace to the chimney to see if there was any blockage. There was a small pile of soot on the back of the furnace and on the entrance to the chimney. I don't think enough to cause the safety to trip. In the chimney however there was a lot of soot at the base of the chimney. I took out about a foot of soot in the bottom and there could be more but the hose on my little shop vac wouldn't reach further. I'm assuming that is not supposed to be there, is that correct? Could this building soot create an improper draft that would pop the safety reset?

As for the motor, you are right, there is only one that matches the specs of the one I currently have - Grainger Item # 4MA12. If this model is installed does that mean I no longer need a stack controller?

http://www.grainger.com/Grainger/oil-burner-motors/hvac-motors/motors/ecatalog/N-9y9Z1z0nw85

Just wanted to add that I have been in this house for five years and neither I or anyone I have hired to clean the furnace has ever opened the piping to the chimney to see what is in there. So the soot/debris has been accumulating at least for the past five years.

If your motor reset is kicking off, it has nothing to do with soot in the chimney. It is because the motor is overheating from a high amp draw. Possibly from a failing fuel pump.

You have a stack switch which I've only seen in training schools. The best thing you could do is to install a new Beckett AFG burner or equivalent and keep your old boiler. A new burner would have a primary control, burn much cleaner, and use less oil. The stack switch would be gone.

Figure about $500 plus installation. Or you could call an oil burner guy and maybe find a used one, possibly for free. Sometimes boilers get junked and the burner is still good. Just look for an AFG. Age doesn't matter all that much.

If your motor is shutting down because the stack switch is sensing an outfire, it wouldn't cause it to reset.

sigh... I hear what your saying.

tiffanys seems intent on confusing the issue. If the overload on the burner motor actuates, it is because the motor overheats. That would be because of excess mechanical load or electrical problems within the motor. Period!!!!
New stack-mounted primary controls are available, Grainger # 2E521.