But there efficiency drops as the return temperature goes up.
And efficiency alters the time to repay - so with an old installation it
might make a difference.

All boilers drop in thermodynamic efficiency as the return temperature goes
up. The differences between a condensing one and a traditional one a

1. The condensing type is ALWAYS much more efficient than the traditional
type at ANY flow or return temperature.

2. The condensing type has even greater efficiency still at lower return
temperatures.

Don't get bogged down thinking about item 2, when item 1 alone is enough to
justify a condensing boiler. Remember also, that condensing boilers are
intelligent and will drop the return temperature if they detect they can get
away with it.

Thanks for the reply - but why wouldn't it improve the efficiency to put it
in the return.

It would increase the instantaneous thermodynamic efficiency. However, it
will cost much more to run, as the controls would not actually be keeping
everything at the correct temperature. There is more money to be saved
ensuring everything is only heated as much as is required, than by trying to
get an extra 5% on the thermodynamic efficiency.

Your scheme is likely to end up heating the swimming pool to higher
temperatures than you wish, which will cost you a fortune, even if that
heating was done efficiently.

Christian.