On the Trent the FOHE is always on. There are no controls at all on the fuel flow side, and there is only a bypass on the oil flow side to enable to oil to bypass at very low temp (when the oil is below zero when starting)
The oil temp control system controls the AOHE which is normally closed, and opens at high oil temps to pass fan air through a heat exchanger to cool the oil.
Afaik the bypass on the oil side is thermostatic, and diverts oil flow through or around the FOHE to maintain oil temperature. It is designed to cope with a wide range of fuel temperatures from freshly loaded ambient to altitude-soaked cold. With very cold fuel, most oil will bypass the cooler with only a bare minimum passing through to maintain oil temperature. The heating effect on the fuel will be minimal.
Sooty