Ok so the pre-heating radiators have infinate area, the glycol-water PHEs have infinite area.

Currently glycol can basically reach the hot water temps (oversized PHexchanger, excess of hot water).

Assuming we have an infinite size radiator (with excellent heat transfer coeff) and turbulent flow over the pipes&coils, air temp should be able to reach glycol temperature.

However the seasons come into this hypothetical: 0 degree winter air -> 50degC would save considerably more than 18degC summer air.

However: if we go with the average temp (10 C) and assume with an infinite sized radiator this can reach glycol temps of almost 50 C (air=25kg/s = 1MW).

Here glycol (13kg/s) would cool to ~29 C.

That is a lift of 40degC on the air. (from 10 to 50 C)

Currently, average lift observed is ~ 25 degC

Extra 15 degC approximately equates to £570,000.