I have a Rheem Criterion II furnace with a White Rodgers 50M56-843 control board. Blower runs in manual ON or COOL mode, but will not start when needed for heat. Exhaust blower starts, burners ignite and burn until shut down due to over temp condition.

The control board was replaced in the summer when the original board failed. Not certain the blower wires were installed correctly or if the heat mode was tested then. I have Black to Cool, Red to Lo Heat, Blue and Yellow are parked. Hi Heat has nothing connected. I am unable to find a color code that describes the functions of the blower leads. This is a single stage unit with a constant speed fan. Instructions for the control board say that one of the parked wires should be connected to Lo Heat for constant speed. That's confusing given the way it is currently wired. Looking to verify the blower connections, and if they are correct, what I should check next. Thanks.

 

Welcome to the forums.

Yes..... the instructions are not terribly clear.

Activating "fan on" at the thermostat turns on the Lo Heat terminal in your application.
Black(high) for A/C is good.
The Hi Heat terminal is your normal heating terminal. The speed used for this terminal is usually determined by the heat rise of the furnace. It would probably be blue or yellow. Try yellow first. You could use any remaining wire for the low heat terminal.

color --> speed
Black = high
Yellow = medium
Blue = medium low
Red = low

 

Fixed it. As I suspected, the blower was not properly connected by the tech who installed the controller. Using a multi-meter, I measured 115vac on the Hi Heat terminal when the blower was called, but the red blower wire was connected to Lo Heat instead. Moving the red lead fixed the problem. Still confused by the control board instructions that said one of the parked wires should be connected to Lo Heat. This serves no apparent purpose, and I'm inclined to leave it as it is.

 

PJmax,

Thank you for the clarification. Lo Heat does appear to be controlled by the "fan on" setting, so I will rewire the controller per your instructions. Thanks again!

 

Using the Red wire as your normal blower speed maybe too slow. Make sure your furnace runs without cycling on high heat.

The "parked" wires are wires that are connected to dummy terminals. This just holds the motor connections from hitting each other. You can remove those wires and use the speed you need.

 

I have the same problem with a gas furnace blower not kicking on with heat (it will run if I switch fan to ON), but works properly for cool. However, I hadn't had any recent work done before this started, so I'm thinking incorrect wiring isn't the problem. Any idea what this might point to? I've searched about this subject, but this was the only place where I found someone else mention that the fan kicks on fine with cool, but not heat. I have a Pulse 21 by Lennox.