Air Conditioning - Carrier AHU running all the time...

I have a Carrier model# FK4CNF003 that was stuck on last month. The fan switch was in auto and the stat was off, not calling for AC or Heat. I turned the breaker off to the unit for a minute or two and that seemed to fix it. Well until this morning the units was running again with the same above conditions. I tried to reset the breakers again but the unit didn't reset as it did the last time. Went to work (9 hrs later) I turned the breaker back on and the fan came right on. I then went into the attic to write down the model numbers of the AHU and control board. In doing so I pulled the safety disconnect on the side of the handler got the numbers I needed, looked at the board, which there doesn't seem to be much to it. A few diodes, couple of resistors and connections to the blower. I fingered around with it for a few then put it back together, reinstalled the disconnect and the fan didn't start running. Have you seen, heard of this type of intermittent problem before??


Thanks for the help,
Dave

If you still have the same problem, put a meter on the fan motor, find out where is the power come from with T-stat off.

Could be a relay sticking on the board. How old is the air handler? When certain parts fail they also turn on the blower, I'm leaning towards the board or transformer.

The unit is about 10 years old. The board doesn't have a relay on it, I'm sure there's one somewhere but didn't see it when I had the cover off. The transformer is outputting 24 volts AC so I'm thinking it's okay too. The board doesn't look like much, 4-5 diodes which checked good, 2 resistors and one fuse that all checked good too. I would like to find a schematic for the AHU but haven't had much luck as of yet. Any idea where I could find one?

AHU:
Model # FK4CNF003
Serial # 0798A20419

Fan Control Board:
CESO 1300 35-00 (only numbers on the board)

Thanks for the help,
Dave

Wiring schematic usually is on the... blower it's self. Right on the housing should be a plastic bag with papers inside or on the inside of the door that you have to take off to access it. There really isn't much to an air handler. Turn off the power to the unit if you're going to open that side panel. It's usually has a model information tag on the door held in by roughly 4 hex head 5/16th screws.

While the fan motor is running, hopefully with the fan switch on Auto on the t-stat with it Off mode, try pulling the thermostat off the base and see if the fan stops. If it does, bad thermostat possibly.

Occasionally if the batteries get old in a thermostat and die, it'll just run the fan too. Provided it's battery powered to maintain the schedule. Doesn't happen too often but I do end up replacing batteries in thermostats about once a year.

Mad39er,

Okay round two... after the last time I removed power to do some checks inside the AHU and looked at the control board I turned the power back on the the system didn't run. So I waited unit 4:00 am this morning went it decided to come back on. So I read your last reply about changing out the batteries in the t-stat, they were the original ones I put in 11years ago. The new batteries didn't fix my problem, she came right back on!!!

The wiring schematic was on the blower housing but I couldn't read it. It had something on it that made it very hard to see. So if anyone had an electronic copy, that would be nice. There just isn't very much to the control board I was thinking there has to be relay in the blower housing somewhere..

Thanks for your help, My next move??

http://www.docs.hvacpartners.com/idc/groups/public/documents/techlit/im-fk4c-05.pdf

It appears that you have a variable speed motor.
I'll try to dig up a diagram.

I hope it is your thermostat.

http://www.docs.hvacpartners.com/idc/groups/public/documents/techlit/f-4sm.pdf


http://i151.photobucket.com/albums/s160/Houston204/FK4C.jpg

Houston204,

Thanks for the schematic, I'll print them out tomorrow.

I have removed the t-stat from the wall and the AHU still runs, changed the batteries, still runs....I'm going to work on it tomorrow when I get home from work. I'm hoping I'll find something burnt up.

Thanks again,
Dave

I would start by measuring for 24VAC between C (common) and G (fan).

If no voltage is present between C and G and the fan is running I would measure for 12 VDC between the 16 pin motor harness at terminals 1 to 6, 1 to 14.(motor end of harness)

Measure for 24VAC between terminals 1 to 15, 13 and 2. (motor end of harness)

This might help...

http://i151.photobucket.com/albums/s160/Houston204/varimotor-1.jpg

Houston204,

Thanks for the drawings. I haven't had a chance to work on my AHU until tonight. We haven't needed it, the weather has been nice here. The breaker has been off for a week or so.

Tonight I turned the unit back on and it started running again. Removed the side cover and started checking some of the voltages. I didn't know you had posted your last set of drawings so I was just looking around with the meter at J1 with the T-stat wires on it and while I was doing this the fan started to slow down and stopped. I was applying a small amount of pressure to the board with my meter leads. So I thought, maybe I could just have some bad solder joints, being this has been an intermittent problem. I continued to flex the board a little but couldn't get the unit to start running again. I notes all wire locations and removed the board to get a better look at it.... Found some pretty poor looking solder joints so I re soldered them and re installed and all is well so far. I have to wait to see if I have fixed it...

Thanks again,
Dave