Sometimes Blower Does Not Turn on - Rheem Oil Forced Hot Air

jeffceo

Rheem Imperial Oil Furnace model ROBC-084QBEA. Sometimes the burner will run but the forced hot air blower/fan will not turn on to blow the hot air though the vents. Typically the burner fires and the fan runs within 30 seconds to a minute. When the fan doesn't run, you can switch the thermostat to manual Fan mode and it will work. This is intermittent and happens every so many times it runs.

The furnace was cleaned in the fall and the air filter is new. Here are a few pictures of the unit. Thank you

STEVEusaPA

Get your tech back out to troubleshoot. Could be a bad capacitor, bad blower motor, or something wrong with the board, for starters.

HVACNUT

Since the system is zoned, (How many zones?) Its possible the circuit is opening on limit if only one zone has a demand.
The furnace is from '96 so assuming it's not an ECM motor, amp it out with the blower door in place. Could be thermal overload on the blower motor. Are the fan blades clean? You'd be surprised what 22 years can accumulate.
Is there a barometric bypass for the zoning? Is the bypass damper operating properly?

DZoro

I believe that furnace has a fan delay timer inside the blower cabinet. Usually a small white board with 3-4 wires going to it.
It will definitely cause the issue.

jeffceo

Thanks guys. Don't think it is the motor or capacitor since it starts fine when you turn the Tstat manually to Fan. It has two zones. I'll test the amps and make see what they are and if it is even getting power. I'll check how clean the blades are. I don't see a barometric bypass anywhere. I'll check the fan delay timer. That makes sense that it could be on the way out causing intermittent issues. Thanks!

EBEBRATT-Ed

Just leave the fan switch on the thermostat in the "fan on" position until you get it fixed. You don't want the furnace running up to limit temp all the time.

You probably have a bad fan control board

DZoro

I believe that timer also controls a small relay contactor which is on the right side of your burner. Both can cause your issues.

Awesomestuff

I have exactly the same model (ROBC-084QBEA) with exactly the same problem. Blower motor does not operate on heat mode except very occasionally when the furnace is dead cold. I've verified clear exhaust/intake, replaced the fan control board and motor start capacitor. When the motor runs, it does so just fine...so thinking not the motor. Could really use a lifeline here!

SuperTech

Does your furnace have the timer mentioned above? Those are known to be problematic. 

Awesomestuff

Hello SuperTech. Yes it does have the timer and it is referred to as a "fan control" board. I have replaced this. As I mention, the blower runs when the furnace is left off and cold overnight. It will run until the thermostats cuts it off, but the next time it cycles on the burner runs but never gets the blower and just shuts down after 30 seconds or so. Puzzling. Schematic below.

jeffceo

Awesomestuff, I replaced the RHEEM/RUDD 47-102686-81 472289681 FAN TIMER CONTROL KIT. I am trying to remember if there was anything else affecting it... I want to say take a look at the temperature sensor/limit switch. I believe it is the blue wires above the yellow sticker in the picture. If this does not sense the temperature is adequate to run the fan, it will never signal to turn the blower fan on. Someone correct me if I am wrong.

jeffceo

Or it could possibly cause the burner to shut down prematurely, before the fan delay is over

Awesomestuff

OK all...the problem is solved. It I ended up having to get a furnace guy. He was baffled too as I'd already replaced the thermo switch, fan control and even the motor start capacitor. Ended up being the little relay on the 24VDC side that switches power from the thermostat command to the fan control board...an ordeal to locate! Pull-in winding would open when it warmed.