We are having an issue with our Rheem Classic Oil Furnace (model R0BD-1120BGA). We had the furnace repair man out 2x to try and resolve this and after about $750 in replaced parts it is doing the same exact thing still. I'm pretty handy, but need help troubleshooting this.

Here's what is happening. The furnace will fire up and run fine. it could run fine for several hours or a couple of days, but at some point I notice the house is cooler than it should be and I go down into the basement to see the red light illuminated on ththe Honeywell primary controller. I turn off the service power switch press the reset switch and everything fires back up immediately. I don't see a puff of smoke in the exhaust stack so I don't think the furnace has excess oil when I restart it.

Things I have done to troubleshoot so far:
*checked the light/fire sensor on the ignition coil to see if it has junk on it or is cracked. No issues there.
*checked that there is continuity on the F-F wires going from the Honeywell primary to the light sensor. No issue there.
*pulled out the over temp sensor in the exhaust stack and it looks pristine. No obvious corrosion.
*the nozzle was replace by the furnace repair man during the last visit. Still had the issue after the visit.
*checked the electrodes for corrosion between the spring connection of the transformer and the electrode. No obvious corrosion or discoloration.
*repairman replaced the combustion blower motor. It was making a little more noise than normal on startup and he thought it may be causing the furnace to kick off after a while. No change after spending good money to replace it. Although the furnace is running slightly quieter.

Could it be the Honeywell primary?? I'd rather do some troubleshooting to figure it out for sure before spending any more money if possible.

Furnace info:
Rheem Classic Model: R0BD-1120BGA
Beckett Oil Burner Model "AFG" (sticker is a little messed up so the F may be an R
Honeywell Primary Model R8184M 1051
Beckett Combustion Motor Model Y5L271C81
Danfoss Oil Pump Model BFPH 071N1151

Any advice you could offer would be appreciated!

 

Does the reset trip while the furnace is running or do you notice the room is below the thermostat setting? Any smoke or rumbling when you reset?

 

It randomly trips while the furnace is running. There is no smoke or additional rumbling other than the additional sound of the oil being burned in the combustion.

 

If it trips while running, I would suspect either a bad cad cell or high ohm reading. An old oil burner man's trick which will often tell if the relay is weak is to rap the side of the 8184M with the handle of a screwdriver. Don't wail on it just a good firm rap. If the burner stops, even momemtarily, replace the 8184M. Not very technical but...

 

I gave the primary controller a few stout blows with the handle of a screwdriver and the furnace stayed on. How would I test for a bad CAD cell or high ohm resistance?

 

You will need an ohmeter & something to serve as a jumper (even a paper clip will work).
Disconnect the wires from terminals F-F.
Connect one meter lead to each wire end.
Start the furnace & quickly install the jumper between the F-F terminals & snug down the screws.
Monitor the ohm reading for several minutes.
I like to see readings of around 600 ohms but even up to close to 1000 is ok.

 

I started the furnace and checked the resistance over several minutes. It started at 680-720 ohms and has dropped to 630-680 ohms.

The furnace repair man was out again yesterday and checked everything out. Did not find any obvious reasons for this intermittent problem. Replaced the nozzle and filter, but the furnace has kicked off twice again. We live in an old house and the furnace runs for long periods of time (1-1.5hrs) when the program kicks the temperature up from the 64F during the night to 70F during the day. Could the long run times be an issue? It has not been in the past.

 

Ohm readings are good. The relay in the 8184 seems good.
You bring up a good point about the long run time. Some things I've seen with long run times are:
The coupling between the motor & pump gets soft & starts to slip.
The motor becomes overheated.
If the primary gets hot due to radiated heat the coil for the relay can break down.
The fuel pump will get stiff & almost bind which will cause motor overheating.

This is going to be a tough one without being in front of the heater to actually watch & listen to what it's doing. You said in post #3 "...the additional sound of the oil being burned...". Can you elaborate?

 

Just thought of something else. Over a long run I wonder if the vacuum on the fuel pump goes up. It's easy & inexpensive to install a gauge but again, it would need monitoring unless you found a gauge with a needle which would stay at the max vacuum.