My thermometer doesn't start the furnace. I have to hit the reset button on the furnace to start it. I can tap the plastic housing around the reset switch and the furnace will fire as well. It's a 10 year old Burnham/Beckett combo and my thermostats are the cheap mercury type. I don't get the impression that it's the thermostats, because they'll send the signal, the furnace just won't fire. Any thoughts?

Oil or gas?

Sorry, it's oil.

The reset as you call it is a safety control.

Here are some possibilities

Loose wiring in and around the safety control causing intermitent problem.

The furnace needs a tune up and the safety control is tripping since the cad cell is not detecting flame all the time.

The safety control is defective causing the unit not to fire all the time.

Loose thermostat wiring at the safety control.

When dealing with oil burners there can always be more problems but I believe it should be one of the above. If you want to take the thermostat out of the system for test purposes just remove the two wires at the safety control and jump the two connections at the safery control with a piece of wire. This will cause the burner to run all the time. There should be a switch on the side of the furnace to turn off the unit completely.

It is always a smart idea to tune up a oil burner and change the oil filter at least once a year.

I appreciate your response, I'll let you know what I find.

This sounds like burned/pitted contacts on the primary control relay, if tapping on the control restores brief/temporary operation.

Will that be obvious when I pull the plastic cover? I had a similar problem with the contacts on my pressure switch on my water system's pressure tank.

If the contacts are burnt and pitted then it is time to replace the control. You might be able to tell if the relay is exposed and you know exactly what you are looking for.