Quick update, I have downloaded OS V2.5.1, the 'Return Air Fraction box' is now available under 'Lights Definitions'.

I would have excepted that the sum of 'Lights Sensible Heat Addition [GJ]' in the 'Annual Building Sensible Heat Gain Components' report would have lowered, but it didn't.

Any idea why the heat gains due to lights did not reduce after having setting a 'Return Air Fraction' ?

Thanks !