I have a dual-fuel system. The oil furnace is from the 1970's, and the heat pump was added by Sears in September of this year. I added that for the central air.
The thermostat is set to kick on the oil if the outside temperature goes below 35, and the heat pump works at all other times. The heat pump works fine- no problems. The oil furnace, though, is giving me headaches. When it's supposed to kick on, sometimes it does, and sometimes it doesn't. When it doesn't, I get a buzzing noise from inside the furnace. If I use the primary control reset button, sometimes it kicks the oil on and sometimes it doesn't. I took a rubber mallet and knocked gently on the underside of the oil burner and the furnace kicked on!
Now, this has gone on for about a month or more. I've had furnace maintenance performed (nozzle, filters, etc). I've had Sears come out and check it out, and they replaced two relays (or said they did- I didn't watch). I've replaced the transformer and the primary control box myself.
Nothing works. It's no better and no worse. It still buzzes when it doesn't kick over, and a rubber mallet strike on the motor or transformer kicks it over.
What can I do? I'm getting very frustrated.

I would check your low voltage transformer. Many times when other items are added the old transformer just does not have the VA to run them all. You need to be sure the transformer is rated at 24 volts with a 40 VA output. Capacity.

To low a low voltage transformer rate will allow things to buzz and not pull in as they should. Many primary controls have there own internal transformer and then you have to wire the thermostat by using the RH and RC separate.

What ever it is low voltage is playing a part of the problem. When you hit the reset that is usually a indication of a flame out situation with the oil burner.

This means that the burner is not firing in the allowed safety time limit. This could be many different things but you have replaced most of the parts.

Look at the low voltage wiring to make sure it is connected well and also check all grounds. The relay in the primary control is being kicked on by a mallet tap and that is all it takes to pull in the relay and make it run.

I would check your low voltage transformer. Many times when other items are added the old transformer just does not have the VA to run them all. You need to be sure the transformer is rated at 24 volts with a 40 VA output. Capacity

I checked this. The low voltage transformer is 24V and 40VA exactly - made in Mexico, if that makes any difference.


To low a low voltage transformer rate will allow things to buzz and not pull in as they should. Many primary controls have there own internal transformer and then you have to wire the thermostat by using the RH and RC separate.

The buzzing will occur as soon as the furnace is supposed to kick over. It doesn't start, then kick off, like it would with a no-oil situation. Also, I tested the primary control with a jumper test (works fine), and I cleaned the CAD cell inside and out. Could it be a bad CAD?


Look at the low voltage wiring to make sure it is connected well and also check all grounds. The relay in the primary control is being kicked on by a mallet tap and that is all it takes to pull in the relay and make it run

So could it be a bad wire? Or could the low-voltage transformer be bad? How can I check that?

One other thing: the blower cycles often and I don't know if this is normal just because I'm paranoid about the entire furnace. After the oil furnace gets going a few minutes, the blower switches on. After a few minutes, off. Then, on for a few minutes. Then, off again. I checked the limiter switch and I have it set at 100 degrees "off" 150 (or so) degrees "on" and 200 degrees at the top (factory setting)? Any advice on that also, and is that affecting the low-voltage performance any?

THANK YOU!

((If I use the primary control reset button, sometimes it kicks the oil on and sometimes it doesn't. ))

The only reason activating the primary control would work is if it tripped for some reason. Usually that is a no burner light off situation. Is the motor for the burner original? You said you could tap on the housing and sometimes that made it work and I am thinking armature/winding problem (dead spot). Lets face it you have changed every other part. LOL

Now it is possible the fan and limit control could be defective at least on the fan side. Under normal operating conditions tha burner lites and after a warm up time the blower comes on.The blower stays on until after the burner cycles off and stays on until the heat is blown off the heat exchanger. This is supposed to be a smooth operation without any on or offs during that time.
You need a fan and limit control because it is out of calibration so I would replace that first. Then if you have access to a amprobe to check current draw I would attach it to the burner motor and watch it as you activate the furnace to see what it does. If to high a draw it could be the motor failure. Next I would hook up a volt meter to the burner motor leeds. When you activate the furnace look to see if current is being supplied to the burner motor. If voltage is being supplied and the motor is not running then you have found the burner no start problem.

Is the motor for the burner original? You said you could tap on the housing and sometimes that made it work and I am thinking armature/winding problem (dead spot). Lets face it you have changed every other part. LOL

I believe the motor is original. I've been here 10 years and it came with the house. I'll have to get someone who knows what they're doing to test it for me.

As for the limit switch, I reset the "off" side to 80 degrees, and the "on" side to 140 degrees and the short cycling has stopped. Thank you for your advice there.

I was wondering about the low-voltage transformer. Is it possible that 40A is just not doing the trick? Could I bump it up to 60A and see what happens?

((Is it possible that 40A is just not doing the trick? Could I bump it up to 60A and see what happens?)0

40 VA should do the trick. You can try a 60 but I would be surprised if it made a difference.

Well, I've done the following: (1) checked the low-voltage wiring for tightness and connection, (2) cleaned the CAD cell thoroughly, (3) tested the *new* primary control box via jumper cable, (4) tested the thermostat with self-test for oil furnace, (5) had maintenance done on the furnace, (6) replaced the transformer.

The only things I haven't done are test the low-voltage transformer and the oil burner/motor,
Am I getting warmer (no pun intended)?

Get to the motor. There is nothing else left. When I had the shop I used to approach these type of problems as a challenge. Usually I lost $$$ because I always felt bad that the problem could not have been found sooner so I did not charge the customer. The good part is that I only had a very few of these type of problems in my career.

I spoke to a fellow at Wayne about my Wayne oil burner, and he agrees. The buzzing is the electrode igniting. Probably an open winding (as you said) is stopping the pump from moving. The only other options would be the coupling or the pump, but since the motor isn't even vibrating, I can't see that as it. Anyway, we'll see. I'll post when it works. Thanks for your help.

It was the motor. Thanks again.