wrote

The heat pump will no longer shut off. The fan control at the
thermostat works, but no matter what, the outside unit continues to
run.

She switched the circuit breaker off upon realizing the problem.

The unit is 20 years old. If it matters at all, the unit is branded
"Payne high efficience heat pump", and is white/off-white with a green
grill. Again, if it matters, her house is a 20-yr. old double-wide
mobile home.

It was last serviced in June of '05. The service was to top off the
coolant, and "check it over", at least in theory.

She lives about 30 minutes from town, so I assume a service call would
be somewhat expensive for her. Should she call for a repair or would it
make more sense to just go for a replacement at this point? Thanks,

If it's 20 years old (and a Payne!) she probably should just change the
system out. When the fan continues to run, is it putting out heat? The
reason I ask, if it's just the fan that continues to run, it SHOULDN'T but
it won't hurt anything. If the fan is running AND putting out heat (when it
should be off) then it sound like a sequensor is stuck or maybe a wire
crossed (or several other possibilities) and that is not good. It will never
shut off and it may get kind of hot in the house.

She needs to get at least 3 REPUTABLE companies to come out and give her a
quote (not an estimate) on replacing the system. If it's just the fan
running (and not the heat) she doesn't need to rush into it. A lot of people
run the fan 24 hours a day for filtration purposes. Sure it uses a little
bit more electricity, but not that much. I only say this because you don't
want to jump into this without "shopping" around. Don't worry so much about
the brand as much as the guy installing it.

Now to really confuse things: You may want to just have someone come out and
repair it. It might be something as simple as a stuck sequencer or fan
relay. But if they came out and June and "topped it off", there may be a
leak and future problems.

Good luck....