Central heating pump help needed

[D_Hailsham]

you missed my intent, I agree that heat out of the radiator will be the same, we cant beat physics but the return temperature to the boiler will be too high, which was the OP's concern. so the delta T is too low, and too much energy is being returned with the water to the boiler, and as such it may not operate at the most efficient level?

No more energy is being returned as the same amount of energy is extracted from the water for a 5C dT @ 10 litres/sec as for a 10C dt @ 5 litres/sec. The dT only has any effect if the return temp is below 55C as the water vapour in the flue gases does not condense above 55C (condensing increases as the return temp lowers). Look at any boiler spec and it will show output at 80/60 (non-condensing) and at 50/30 (condensing). For example, WB CDi system:

Input: 35kW
Output (80/60): 34kW
Output (50/30): 36.1kW

The 2.1kW extra output is due to condensing.

 

[John D v2.0]

The dT only has any effect if the return temp is below 55C as that is the water in the flue gases does not condense above 55C (condensing increases as the return temp lowers

Agreed, the ideal would be return was 10c ie outdoor temperature. Then the flue gas would be barely warmer then the intake so you'd actually lose no energy at all out the flue. Apart from maybe sound?
However the mechanics to do that would cost more than the gas saved.