The air handler fan on my HVAC system will not shut off. The system is about 12 years old. Split system. A/C compressor was replaced last year. Is my problem likely with the thermostat or is it somewhere else?

There are two items that turn on your blower.

In most newer, the fan is brought on by a timer. After a call for heat has brought on the burner, the fan comes on minuets later by a timer.

When you have a call for cooling the thermostat (T/S) brings on the compressor and the fan at the same time. After cooling is statisfied, the thermostat shuts of the compressor and blower. Some systems, the blower will run for about 3-5 minuets after to get all the cooling out of the coil.

Most thermostats have an "Fan" switch on the so you can turn on the fan to circulate air if you are haveing a big party but don't need A/C or Heat.

I would start at your thermostat. This is all low voltage (24 volts). If you know how to remove you T/S, remove it. If the fan stops, there is your problem. If your handy, you can just disconnect the low voltage Green thermostat wire for you furnace terminal block.

By the way, wire color coding is, or at least should be:

(R) Red Wire carries 24 volts to the T/S.
(W) White Wire calls for heat.
(G) Green Wire calls for fan.
(B) Blue wire calls for cooling.

Thanks WoW. What if the T/S doesn't fix it? Will I be looking for a relay somewhere?

Yes, there is a "fan relay" however it has been my experience that if it fails, it won't come on. There are a number of other items that could keep the fan on but like I said, I would start with the thermostat.

It is very hard to diagnose a system via the internet.