Ask This Old House showed the installation of a system that captures the
heat from the AC unit to heat a swimming pool. The AC refrigerant line runs
through a coil inside a canister. The pool water runs through the canister
and the heat is extracted.

OK, that's all well and good. Efficient, free heat heat for a pool which is
shaded by trees and gets very little sun.

Let's think about this. When it's hot out, the AC is running and pool gets
heated. When it's cool out, the AC is not running, so the pool does not get
heated.

Sure, there's some advantage to heating the pool on a hot day, even into
the hot night. However, on a cool day or cool evening, when the owner would
really want the pool heated, he's right back where he was before the system
was installed. Early and late in the season, when it's not hot enough for
the AC to be running for any length of time, there is no heat available for
the pool - right at the times of the season when you would want it.

Considering that it takes a certified AC tech to capture the refrigerant,
adapt the AC piping and recharge the system, is it really worth installing
one of these systems? I don't have a pool, so I don't know how cold the
water in a shaded pool would be on a day hot enough for the AC to be
running.

After a few days of cool weather, how long would would the AC have to be
running for it to heat the pool to something that would be comfortable?
Yes, I know it depends on the size of the pool and how cool it was. The one
in the show was a decent sized in ground pool. Is it a matter of a few
hours or would it take all day or longer? I know there's a number of
variables involved...just looking for some idea.

It just seems kind of bass-akwards to only have heat available on hot days.