Andy Burns explained on 25/03/2018 :
Harry Bloomfield wrote:

with no working boiler there is no easy way to work out which is the
flow, which is the return at the rads


Some TRVs (e.g. Myson 2-way) are marked with a double-headed arrow to
indicate they can be fitted either way round, maybe yours are too?

I have just dug them out and I must have bought 7 of them and they are
Drayton TRV4's. The instructions suggest they work in the flow or
return.


That said, I still try to fit them so it's flowing in via the TRV, just in
case it helps cut down on annoying whistling.

Can you tell by which end of the rads have the lock-shield valve being the
return, I suppose that's only a convention, no guarantee.

No, the valves at each end are identical, apart from the knob or
spindle cover and over the years, I have revised (swapped over) which
end the knob is on for easier access.