Now this brings me back to my original point.
What does "engineered to take advantage" mean - lower return
temperatures?
Hence my "silly" idea of putting the pool's heat exchanger in the return
flow.
mikej



I think that you are missing the point.

No I'm not if you read my earlier posts you will see that I've agreed that
the situation is complicated and I've more or less given up on the idea.

All I'm asking for is clarification of what "engineered to take advantage"
means in the previous post.

As you your reply - I follow what you are saying but I don't think the
outcome is conclusive because the pool isn't like a room heated by a
radiator - it doesn't have to come up to temperature quickly (and it has a
much bigger "thermal inertia").

Imagine a standard setup (forget the pool for a moment) and the return
temperature isn't low enough to get to the dew point i.e. boiler not working
at maximum efficiency.

Now put pool heat exchanger in return flow.
Pool water is very cold by comparison with return water flow and should drop
its temperature. It might even be enough to make the boiler work at its most
efficient.
As its in the return it can't alter the efficiency or temperature of the
house radiators unless the flow temperature drops.

The heat output from the radiators can't be changed by this because the
boiler will just work harder to lift the outflow temperature - this must be
possible because its
how the boiler is designed to work. I.e. if the house radiators were working
as required the return temperature would be exactly what we have with the
pool heat exchanger in the circuit.

The only down side is that that pool heat exchanger isn't as efficient
because its inflow water is cooler - but who cares?
The pools heat cycle is only 4 - 6 hours each day.
So just increase it to whatever it takes - 12 hours even.

As to what happens when the house CH isn't needed - just switch it back to a
flow and return connection as standard.

For a modulating boiler I can see that there is the additional problem of
the boiler always thinking that the house is cold but then there are room
stats for that.

mikej