I have 8 radiators, all/each fitted with a TRV, all fitted horizontally as
recommended by the manufacturer, although the plumber who fitted them was
not aware of this(I also have a room thermostat, but the TRV in bathroom is
always set to maximum and there is also an automatic bypass internal to my
combi).

The TRVs are birectional so could have been fitted to the flow or return,
but the manufacturer recommends to fit to the return(coolest) end of the
radiators.

The plumber has fitted 6 TRVs to the returns(fortuitously, as he had not
been aware of the manufacturers recommendations), but the remaining 2 on the
flow.

The plumber has (or says he has) balanced the system.

My question is(if he has balanced the system), when the TRVs begin to
control at different temperatures and/orI fully close the 2 TRVs(in
bedrooms) which have been fitted to the flow, will the system still remain
balanced and function correctly?

Keith G. Powell