Trane XL1200 Repair

mistercord

Hello,

I have a 2 ton Trane XL1200 a/c, with a 2&1/2 ton Trane a/h installed in my attic in 1991. Both have performed flawlessly until last summer when the a/h fan got a mind of its own. It was installed with a switch to keep it running for a couple of minutes after the a/c shuts off, but now the a/h sometimes keeps running or cycling on & off on no particular schedule. When it keeps running I can stop it only by tripping the circuit breaker or turning the unit's power off in the attic.

I tried to get it fixed but most repair outfits just want to replace both units. I am balking because the system was well-installed and cools as well as when it was new. I clean it and do routine maintenance myself. I have never had a service call, but did upgrade the thermostat last summer to no effect.

Any suggestions? Thanks.

EBEBRATT-Ed

Can you provide models & serial # for thermostat ,condensing unit and AHU. Then we can help you out

mistercord

I just tried to attach 3 images with the info. If it doesn't work I'll write into another comment.
Thanks.

HVACNUT

The "switch" sounds like a time delay relay was installed and is now faulty.
Like @EBEBRATT-Ed said, if you can post info and maybe a pic of the A/H blower compartment, we might be able to help.

JUGHNE

Did this start with the new T-stat?
If so you might try the old one again if you still have it.

The time off delay relay can be eliminated with little difference in performance. IMO

Hard to believe HVAC people can't figure this out without selling a complete system. If they can't troubleshoot this, what will they do with a new more complex set up when it goes south?

mistercord

Didn't work.

So, the thermostat is a Honeywell Pro 4000 (TH4110D1007). A-coil is TWX730 B140AO. Outside compressor unit is TTX724 E67499 (11.15 SEER). Delay fan relay is a BAY24X045.

I have no idea what this all means, but I am thinking it could be the delay fan?

Thanks again.

mistercord

The problem started before the new thermostat, which is why a new one was installed. The installer supposedly disconnected/bypassed the switch but the blower has not shut off when the a/c does -- always a slight lag.

HVACNUT

They removed the fan delay relay and the fan still runs after the stat satisfies? Do you know if it's an ECM motor?

mistercord

What's an EMI motor?

The guy from the firm (which I selected from a Trane site but had never heard of before -- none of the big names here are listed although all of them will install a Trane a/c if requested) supposedly disabled the fan delay relay as unnecessary, but then when that didn't work sold a new thermostat which magically had a delay feature. $300 later nothing has changed. Obviously the young guy is clueless.

JUGHNE

Your 1991 air handler most likely does not have an ECM motor as mentioned above. I would guess that your plain fan relay sometimes sticks shut, (much better than sticking open as your coil would freeze up).

This is sometimes solved by a hard slap to the side of the air handler which will jar the switch open, this is high tech trouble shooting but will tell you if the switch is simply mechanically stuck shut and the "wrack" unsticks it. Then you know it is now not an electrical problem.

They may have disabled the fan off delay relay but there is probably another simple relay that turns the fan on, I have seen those stick shut and cause run on of fan. Also they can fail and give you no fan run at all----freeze up of coil.

If you can take a picture of the wiring diagram which is probably on the inside of one of the doors then we would know what you have.

mistercord

I'll try tomorrow and post if successful.

G'nite.

newagedawn

the fan board has most likely failed

mistercord

Sorry I disappeared for a few days, but I had to go out of town.

I did open up the air handler, wiggled a lot of wires, cleaned parts, and applied a few thwacks. It has functioned perfectly since then but hasn't been stress-tested since it's been cool here.

I don't know if it has an ECM motor. But I have attached a wiring diagram.

Thanks for your help.

mistercord

This is the only circuit board I found.

JUGHNE

There might be another diagram that shows motor speed connections.
How about a picture of the motor that shows how many wires go to it?

HVACNUT

The "circuit board" is the blower relay.
If that's the problem, sticking contacts, giving it a thwack might open the circuit.
I'm not too familiar with Trane, so I don't know if they would stock an obsolete part like that. You might have to retrofit an actual blower control circuit board.
The first pic is just the low voltage (field) wiring diagram. There should be a full schematic showing line and low volt circuits.

unclejohn

Replace that relay and your problem goes away. If your any good at wiring you can just get a 90-340 relay. Trane will have the replacement part.