Furnace is about 14 years old.Comfortmaker GUI series.
Sequence of operations is normal until it is time for blower motor to start.
Upon the call for heat the induced draft blower runs for specified time.
Then the ignitor glows and the burners ignite. The burners run for several minutes and eventually shut down.
I then get a code of 4 flashes that says that there is an open high limit switch. I am assuming that I am getting this code because the blower has not started.
I can run the blower if I turn the fan switch to the on position on the thermostat. I usually have it in the auto position, But even in this mode the blower shuts down after a couple of minutes.
Is there any way of testing the blower motor without pulling it out?
Is the blower motor shot or maybe the capacitor?
Could it be the control module?
Please help as it is only 15 degrees and getting colder in Northern Ohio tonight.

 

You have a meter?

Otherwise, you could replace the capitor and give it a shoot.. But my guts are telling me that the motor is on it's last leg.. So you will have to pull the whole blower out to change out the motor.

 

I had a furnace in a rental that was actually limiting on four flashes, as claimed. To cure it, I had to physically push in a little red reset button on the thermal disc-type limit switch. This got me going again until it limited again. I ended up discovering an oddity: it was the programmable thermostat that was behind the whole fiasco. I changed over to a round Honeywell ( I had on hand) and the problem with MY furnace was over.

But I am not suggesting you go out and do this based on my story/situation. My story illustrates the need to thoroughly analyze everything.

Blower motors can be checked reasonably well by just disconnecting or cutting the blower motor wires [NOT RIGHT NEXT TO THE BLOWER!] (and wire nutting them back together later, after the test) and then do a continuity ohms test between the running wire and the neutral wire. (The running wire would be the wire that is not white (neutral), greeen (ground) or brown (capacitor wires), usually). If there is about full continuity, the motor is probably good unless you have a dead short to the body of the motor itself. Make sure this is not the case by continuity testing between the run wire and the metal body of the motor. If you show continuity, the motor is shorted and is toast. If not, the motor is probably good.

If you have more colored blower motor wires than black, white, green (ground) and two browns (starting capacitor), then you could TRY temporarily hooking up one of the other color (unused speed control) wires to the run, to see if the blower keeps going.

 

Fire the furnace with the bottom door off & listen. About 30 seconds after the burners fire off, you should hear a click from the control board. That's the blower relay closing. If you have a meter check the "heat" terminal on the board for 115 volts. If no voltage, the board is bad. The blower runs on high speed when you switch the fan to "ON", and a lower speed for the heat setting. 2 different relays on the board, 2 different fan speeds.
Try switching the wire thats on the "Heat" terminal with the one that's on the "Cool" terminal. This will use the high speed for heating. If this works, then the motor has a bad winding.

 

I thought of more on my way home last night..what I'd do if I ran into your problem:

I would turn off the furnace and spin the blower wheel by hand to see if it freely spins, to make sure bearings aren't going out. As the blower wheel spins, it gets warmer and the bearings can even seize up further. I would also then catch yor problem in the act...then quickly shut off the furnace and try spinning the blower wheel again to see if it got worse. Also note if you hear a hum coming from the motor, which coud indicate the motor is trying to turn the blower wheel, but if there is seizing up going on, it can't.

If you conducted motor wiring tests, or swapping out, as suggested, and you can also rule out bad bearings, then I would go back to the electrical end of things. Modern high efficiency furnaces use 24 volt safety switches in series, which is transformed down from the 120 volts at the transformer in the furnace. If the transformer is not delivering the 24 volts or if any of these fail: from the thermostat on through the pressure switch, or roll out switch, or high limit switch, this can shut off the furnace.

In my case that I told you about in my previous post, I started with the transformer and found I did have 24 volts coming out of IT. But then when I checked the 24 volt wire going IN to the thermostat, when it should have been calling for heat (yes, I had 24 volts going into that wire)...and then checked the 24 volt return wire out of the thermostat, back at the furnace, I got no power (no more 24 volts). So that is why the thermostat was the culprit in my case. My furnace acted just like yours. The blower would start to run, then quit. Then high limit. Then flash 4 times on the LED diagnostic indicator light, that the high limit was bad. The house could never warm up, and that is why I got the service call to the house.

What was happening is the short cycling of the furnace..the flame coming on, and then the blower shutting off prematurely, was causing heat to build up in the heat exchanger. That is why it would high limit out. I'd reset the high limit thermodisc, but it would do the same thing on the next cycle or two.

So...you will need a multitester (some people call them voltmeters) and start testing during the call for heat sequence if you are losing power ANYwhere in the 24 volt circuit from the transformer thru the thermostat thru roll out switch (if you have one or two), through the presure switch.

 

You can change the heat speed on the motor and see if thats it. The speed

I can run the blower if I turn the fan switch to the on position on the thermostat.

Then next Id say fan relay

 

I have a similar blower issue. The blower intermittenly will not start - always after being off for several hours at night. There's voltage on circuit board terminals and if I use the cooling winding, I can get the motor to start. After that, I put things back as they were and motor starts every time. Question: would a bad winding or bad capacitor cause a motor to intermittently start after being off/cold. This does not happen every day - unit will operate for a day or two without issue. Any advice on how to diagnose whether the motor or the capacitor should be replaced?

 

Question: would a bad winding or bad capacitor cause a motor to intermittently start after being off/

Yes To all of them. Try a capacitor first small cost

 

Thanks all for the thread on troubleshooting. I've got the intermittant thing going on also. Motor spins freely by hand and during long stays not active (at night when i set it to 63deg) it'll hum after heating the exchanger. If I coax it by spinning motor it seems to go. I've oiled both sides with provided access holes. So, like folks have said I'll try rewiring to cold high-speed if possible. Is that done at the thermostat?

 

blackhawks- just turn the fan switch at the thermostat to ON, that will give you the cooling speed. Monitor the motor and see what happens.