Blower won't run?

[scatterbrained]

Hi everyone!

Relative Noob here with another furnace question. I have a Standard furnace with an electronic ignition, only about 6 or 7 years old. About 6 months ago I replaced the blower motor and capacitor as they had frozen up. So I don't believe they are the source of the current problem. Filter is ok, too.

When my T-stat calls for heat, the furnace starts its "cycle" - draft fan starts, ignitor heats up, the gas valve opens and the furnace lights, but the blower does not come on. It runs this way for a minute or two, then shuts off. There's an LED that flashes its trouble code, which when translated says "OPEN HIGH LIMIT DEVICE". I found 2 limit switches in the start circuit according to the schematic on the door. I have jumpered across both in a effort to determine which has failed, with no luck. I cleaned the flame sensor and have continuity across the pressure switch and the 2 limit switches, and still no blower motor start. I jumpered in 120V to the blower motor, and it works fine.

Where else to look? Thanks in advance...

Scatterbrained

[HayZee518]

send me a copy of the wiring schematic - hayzee - srf12980@localnet.com.

[lonestarheir]

Originally Posted by scatterbrained

Hi everyone!

Relative Noob here with another furnace question. I have a Standard furnace with an electronic ignition, only about 6 or 7 years old. About 6 months ago I replaced the blower motor and capacitor as they had frozen up. So I don't believe they are the source of the current problem. Filter is ok, too.

When my T-stat calls for heat, the furnace starts its "cycle" - draft fan starts, ignitor heats up, the gas valve opens and the furnace lights, but the blower does not come on. It runs this way for a minute or two, then shuts off. There's an LED that flashes its trouble code, which when translated says "OPEN HIGH LIMIT DEVICE". I found 2 limit switches in the start circuit according to the schematic on the door. I have jumpered across both in a effort to determine which has failed, with no luck. I cleaned the flame sensor and have continuity across the pressure switch and the 2 limit switches, and still no blower motor start. I jumpered in 120V to the blower motor, and it works fine.

Where else to look? Thanks in advance...

Scatterbrained

Sounds like you have a bad fan relay, which would explain why it will run when you hook the motor up directly to 120V

[scatterbrained]

Schematic is posted on the back of the panel. Tough to email that...

[scatterbrained]

How do I test the fan relay? I didn't see it in the prints, or for that matter anywhere in the wiring. Is this on the circuit board? I read somewhere that the Standard (Trane?) circuit boards have a habit of failing. How much would a new one cost?

S

[HayZee518]

fan relay is a little gizmo about the size of a pack of cigs - brown or wine colored. if you can access the crossbar, the part that holds the contacts, use an insulated probe and push the crossbar in. this should start the fan. in its base you should see two terminals coming out of the coil. control voltage is probably 24 volts. if you have a meter connect the probes across the coil and when the unit is functioning regularly you should read 24 volts. you then found your culprit with a burned out coil. shut off the power, mark the wires and take it out and get a replacement.

[scatterbrained]

I don't see anything that looks like a fan relay. This doesn't show up on the schematic either. Help!

[Java_Bob]

does anything happen if you place the thermostat in fan on (instead of auto)??

[HayZee518]

I recall a previous post about a trane blower that I was able to help the user - only his problem was the fan wouldn't stop. Look for a circuit board about 2 inches by three inches. on this is a relay - part number WW24X0224. Get a replacement and your problems should be over. go to the trane site and find a dealer near you.
on the board it should say FDR

[scatterbrained]

Found an open limit switch on the circuit board. Ended up replacing the whole board for $75. On the plus side, the Father in law fixed the board, so I now have a spare, just in case!

S