"keith" wrote
The water in the return line is probably close to room temperature.
He only
has to heat it from room temperature to his target temperature. If
he was pulling in cold water from underground, his heater would have a
lot more work to do. Yes, there are losses when he's done using hot
water, but probably not as great as the usage from heating up all of
that
40 F water.

But it's the *same* loss as that in the supply line, i.e. twice the
loss.

If it is heating season, there is no loss as you'd be adding heat to the air
in some form.