Fitting TRV's on a communal heating system?

I've fitted TRV's before and someone wants me to fit some on their radiators when the communal heating is turned off & drained down in the next few weeks.
I imagine it'll be easy enough but am I right in thinking that I won't be able to test the valves & system until they switch it back on again later in the year & is there anything else I need to be aware of?

Personnaly I'd either do it now or when filled back up.heating can be isolated either in the ceiling before it enters or in the service cupboard in the flat.

Don't do it while it's empty as you'll not be able to test and also when they refill you won't be popular if they then have to chase your leaks.

That's true, I might isolate it & do it just before they switch it on again and pop back the day after it's live to refill & check it