Blower cycles before satisfying thermostat

[Willie]

I'm new here and in need of some help troubleshooting a problem with my Borg Warner gas heating system. I fired it up today for the 1st time in over 3 yrs. I had some problems getting it going but eventually succeeded. However, I'm still having one problem that has me stumped. The blower will only run for 45 seconds before turning off. It will come back on a minute later, but only runs another 45 seconds before turning off again. The blower will run without a problem if I switch to the Manual position on the thermostat. FWIW, the furnace continues to run while this is happening.
I'd sure appreciate it if someone would tell me where the problem might be and how to fix it.
Thanks in advance.
Willie

[how]

Blowers usually only turn off from a power interuption caused by the temp settings of a fan control/ fan relay faults on a board/ faulty dip switch time controls on the board / a failing motor run capaciter /or a winding fault on the heating speed (as opposed to your cooling speed) causing the motor to turn off for a while on an internal overheating switch.

How old is the * Do you have a board with a flashing diagnostic led light and a legend on the furnace that points out that *

[Freon]

Usually when you have the thermostat fan setting to FAN-ON (not AUTO) you are running the power directly to the blower via the G terminal, the same as when the AC is on. When you use the heat, the blower is run through the control board and fan relays. Usually the speed setting on the blower motor is different for AC than heat. Most motors have at least 3 speed settings available.

Since the problem behavior is with heating, it would point to to control board. However it could also be a faulty motor winding for the heating speed tap, as how pointed out previously. Assuming the blower motor power comes directly off the control board, a simple test would be to disconnect the motor wire for the AC from the control board (and tape it) and then move the motor wire for heating to the AC lug on the control board. Now run the FAN-ON from the thermostat and see if the blower runs continuously. If it does, the board is the most likely problem. If it does not run perfectly, then it could be a motor winding problem. Be sure to TURN OFF the power when making the motor speed changes.

[Willie]

Thanks for the responses. To answer the age question, My guess is that the unit is over 25 yrs old. I know you've heard this before but, it ran great the last time I used it (a couple of yrs ago). FWIW, the AC unit was replaced with a larger one (still not enough capacity) 12 yrs ago. Is the control board that Freon mentions on the thermostat or somewhere * I am interested in trying his idea.
THANKS!
Willie

[Freon]

The control board is usually on the front of the furnace. Best thing to do first is find the blower compartment door. Then be sure to turn off all power to the furnace. Open the blower door and find the wires exiting the motor. There may be 4 or more. See where they go. Where they connect SHOULD be the control board. But it depends on what type of fan control you have. Tell us what you find.

[Willie]

Thanks for the additional info. I opened the blower door only because I was curious about what I'd find. The 1st thing I noticed was the two wires for the door interlock had been wired together. Nice! Next thing I noticed was 2(*) wires connected to what appears to be a thermostatic switch on the blower housing. I'll take another look this weekend and report back.
Thanks again.

[Willie]

Okay, I just returned from taking another much closer look and have, what I think, is the cause of the problem. I want to start off by asking you to excuse the way I'm going to describe things. There is a Honeywell thermostatic control(*) for the blower. I popped the cover off and saw a wheel that has 3 movable set points. The low set point is at 90 and the middle one is set at 130. If I recall, the max is set at around 200. I watched the operation of this device when the furnace came on. As expected, the furnace heated up the air, the wheel moved and the blower came on when the wheel turned and tripped the peg at 130. After about 45 seconds, the cold incoming air (the house was at 45 degrees) caused the wheel to turn and trip the lower peg (90 degrees) which, in turn, caused the blower to turn off again. This continued until I manually turned the heat off.
My question now is....am I going to cause any other problems if I adjust the low set point down to about 50 * And, while I'm asking questions.....why is there a low set point to begin *
Thanks again.
Willie

[Freon]

You have found the problem... well done! Here is the only concern about lowering the Fan/Off setting on the Honeywell control. In a month or so your house may be 60 degrees at the coldest. Then you can see the blower fan will never turn off since you moved the fan-off setting is at 50. There is no reason not to set it to 50 as long as you know you may have to adjust it higher when things turn warmer in a month or so.

What should happen now it the cold air will move the dial to the Fan-Off set point and the blower will stop but the burner will remain on, heating the heat exchanger back up to the 130 Fan-Start set point. This cycle will continue until the house air is not cold enough to cool the heat exchanger to below the 90 degree set point.

FYI... The high set point is your high limit safety setting for shutting down the burner.

[Willie]

Thanks alot Freon! Its amazing how much help is available online if you're able to find good people that will respond to your questions. My belief in the power and benefits of "paying it forward" whenever possible continues to be substantiated. Not to preach or anything but I'm convinced the world would be a better place if more people practiced it.
Good things will continue to come our way, just keep doing what you're doing, spread the word and watch. Good feelings and smiles are right around the bend.
Thanks again,
Willie