When I turn my thermostat on, my oil-fired burner turns on, and the blower fan keeps cycling on and off. On for 5 minutes, off for about 30 seconds, on for another five minutes and so on. Meanwhile, because the building thermostat temperature still hasn't been reached, the furnace remains on, even though the blower has been turned off. And so on and so on. Occasionally the furnace will shut off when the blower does, but more frequently the blower shuts down and the furnace runs on for a time. This can't be good. I'm suspecting a faulty fan switch. Any suggestions?

When I turn my thermostat on, my oil-fired burner turns on, and the blower fan keeps cycling on and off. On for 5 minutes, off for about 30 seconds, on for another five minutes and so on. Meanwhile, because the building thermostat temperature still hasn't been reached, the furnace remains on, even though the blower has been turned off. And so on and so on. Occasionally the furnace will shut off when the blower does, but more frequently the blower shuts down and the furnace runs on for a time. This can't be good. I'm suspecting a faulty fan switch. Any suggestions?

The fan control/relay is out of range causing the blower to operate improperly. It might be best to replace it. Many times they look like the ones posted below. Make sure you get the right length bimetal element length since different furnaces require different lengths for proper operation. You might be able to adjust it but usually the bimetal gets tired from all the flexing and gives up.

This was the problem! Thanks so much for your help.

No problem