I have an oil fired warm air furnace with a single speed blower. The blower is not coming on and the high limit is turning off the burner. There are four wires going to the blower. Black, brown, green and white. I was wondering if I could safely jump out the blower to see if it is working or dead. What else could be stopping the blower from coming on?
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The furnace should have a manual switch for the blower at the front with the temperature switch box. Most are white, about the size of a pencil and simply push in and pull out. Others have a more regular switch in the controls area. Try this switch first.
If no go, check the blower motor for an integral overload breaker. If one, reset it and try manual switch again.
If no go. Got to check leads to motor for power (120 volts) when switch on. If power and no go, motor has problem- repair/replace.
I have no experience with oil furnaces but gas furnaces also have a low limit switch to bring the fan on. The burner fires and runs to bring the plenum temperature up to a preset temp. and the low temp switch then brings on the blower fan. This avoids getting a really big blast of unheated or cold air from the ducts durring start up.
Do oil fired units have the same set up?
I have also seen dual switches in gas units. The low (start) and high limit switch in one unit. The low side will also keep the blower running after the burner shuts off, to cool down the heat exchanger and get as much heat as possible out of the unit.
Many units still have a basic wiring schematic inside the door, or single line drawing that will help trouble shoot such problem.
As I said I don't know much about oil fired units, so I may be old in the cold on this :)
Dave
When I was in grad school I had a mobile home with an oil furnace.Dont' remember the details but that had a dual thermostat control on it for the fan and burner.IIRC the one thermostat was on the lower part of the heat exchanger and it was SPDT and when it got hot it started the blower and shut off the burner.The as the blower started moving air it cooled off the lower thermostat and the upper one close. The upper one just controlled the blower. So if the house thermostat was still calling for heat it the burner would refire. Otherwise you just got one slug of hot air and the blower ran until the top thermostat cooled off enough.But that was almost 40 years ago and it was 10 yo MH at the time.
I took a vocational school course 8 or 9 years ago that covered oil fire furnaces, but that is to long ago to even take a SWAG at this one. The only thing I remember for sure is that the annual maintenance requirements were to clean and inspect the injectors, filter and pump assemblies. Most of the class material was not in the cheesey book we had, and my notes are a long time gone.
Dave
Thee four wires suggests that this is a dual speed motor, with a higher speed being used for AC. It could be that a relay controlling this has gone bad. And it's a hair tricky to jump-start the motor -- you can burn it up if you do the wrong thing, due to the dual speeds.
But first check if there isn't a fan switch in the same assembly as the limit switch. You should be able to tell just from the extra wires. This kind of fan switch does fail occasionally.
No electrons were harmed in the making of this post.
At the T stat,jump the red to the green. If the blower does not srart, its a faulty fan relay.