"CJT" wrote

I'm not convinced that replacing everything is necessarily the best
way to go. Of course you want it installed right. But the rest is
economics. It might be MUCH cheaper to simply replace the compressor
unit -- so much so that the additional electricity saving from going
all the way won't justify the additional cost.

In my experience, the greatest savings (assuming a working unit of
reasonable efficiency) come from making sure the ductwork is in good
condition so you don't leak half of your cooled air.

Again, it depends. If you use A/C a couple months out of the year, then you
could get by with just replacing the outdoor unit. If you can see what a 20
year old coil looks like, you can see some of the "physical" differences of
the improvements made to the new, high efficiency ones. One thing is the
size. More efficient ones have more surface area. An old 3 ton coil may be
12 inches high wher a newer one can be 20-some inches high. Another thing is
the metering device. Old ones used pistons where newer ones have expansion
valves. Not ALL new coils, but most. And, depending on the size, installer,
how much of a pain it is to change, sheet metal involved, adding a new
evaporator coil and new line set can be as low as an additional $500. Sure,
that's not pocket change, but if changing out the entire system can save you
50 bucks a month on your electric bill, it may pay to do it. Especially if
your cooling season is 5 or 6 months long. If you have a heat pump, forget
it. You'd be crazy just to change the outdoor unit......