Gas indoor furnace. Fan set to Auto.

When I set the thermostat to heat, the system turns on but the fan does start. Only way to get the fan to start is switching the fan to auto.

(Similar problem did not exist last winter, nor did it occur with cooling this summer)

Does this indicate a faulty thermostat? or a bad relay or similar on the furnace itself?

Thanks,
David

 

It's not a faulty thermostat cause the system responds, the same for the furnace. I would look at the control board.

 

When the thermostat is in AUTO mode.... the furnace tells the blower when to operate.
We would need a make and model number of your furnace to give you detailed information.
The ID plate can be found on the side wall, near the burner, behind the service door.

 

Thanks for the replies. Over the past few hours, it seems like on top of the original AUTO FAN problem, the system is blowing cool air.

The unit is a Trane about 12 years old.

The markings on the unit showed Model TUE100A960K3. Some other items shown in attached pictures.

I removed the cover plates and didn't see anything obvious to the untrained eye.

 

In the picture below I've circled the status LED. If the furnace has a problem with firing.... it will be represented in a series of flashes. The flash code should be on the inside of the furnace door. You'll need to look thru the sightglass on the front of the blower door as soon as the cover is removed the furnace shuts down and the codes are cleared.



The blower during furnace operation is timed to come on by the control board above. This may be an indication that the control board is defective.

 

Thanks, it's been an oddpast 24 hrs. I left the system totally off (no power) last night, and this morning turned it back on and the heat ignited just fine. Fan kicked in on auto for a few moments, then stopped, so I needed to set it to ON for it to circulate. The LED was flashing fast, which indicates normal operation while system is running.

So I'm back at my original problem...if AUTO is sensing properly is it likely a relay (which I can attempt to replace myself) or a new board (which means calling a tech)?

 

The LED is an aid to determining ignition problems. You said the system was blowing cool/cold air. The LED will help you to determine that problem.

As far as the fan coming on automatically.... that is a board issue. There is nothing to sense the heat.... the board turns the fan on xx seconds after the burner lights and off xxx seconds after the burner shuts down. I hate to say replace the board but I don't see any other problem.

 

Can't tell by the drawing,but is there a fan limit on that unit?something must be feeding info to that board.
Just a thought
Geo

 

I believe the flashing red light means it is calling for heat, not necessarily operating normally.

Based on your description, I am wondering if the limit switch is opening due to over-temperature. On some furnaces, that will shut the blower off and then through another circuit on the control board turn the blower back on and blow only cool air to cool off the furnace. Overheating can be caused by a dirty air filter, blower not moving enough air, blocked vent pipe, closed warm air registers, blocked return air duct, clogged a/c coil (if installed).

 

Do you have a multimeter?
I'd say the fault lies with the run cap, the control board or the motor.

I would turn off power to the furnace, remove the run cap and measure the capacitance of the run cap.


I have seen a weak cap cause a motor not to start in the lower heating speed before.







If the cap is under specs match the voltage and microfarad rating of the old cap when purchasing a replacement.

This is a 5uF by 370VAC example but I would expect the blower motor to require 10, 12.5 or 15 MFD.


2. Reinstall the cap if it measures within specs and restore power, give the system a heating demand and measure for 120 VAC between the heating speed fan terminal and neutral after burners ignite and run for over 30 seconds but before they shut off due to limit trip.



If the first 2 tests pass it points to the motor.
You can isolate and ohm the windings at this point but I would replace the motor and cap if it receives power and fails to run.

 

Make sure that the door switch is closed (jumped out,or taped closed)
Geo

 

Will the fan run with the fan set to the ON position? Seems you may have an error in the post,if it i fact does work fine in the ON position,and no problems in the cooling mode it would seem to point to bad fan limit control not a fan motor.
Unless I am missing something.
Geo

 

I don't see a fan limit in the diagram.
I have seen replacement boards that have a separate terminal for fan only. I have also seen boards that use the cooling speed tap for fan only on this site before but the heating speed is more commonly used for fan only.

 

David, if you'd like me to clean up the original post I will. You have conflicting statements, though I think everyone has figured out what you mean.

 

I was unable to read either Dia. Does that mean the fan starts regardless of plenum temp? I guess if it is that old of a unit it 's possible.
Geo

 

We're waiting for the OP to clarify some details.

He has said that the diagnostic light was blinking normally..... that would tend to tell us that a limit is not opening.

The blower turn-on and turn-off are strictly timed events from the control board. The blower isn't controlled by plenum temperature.