I've just moved to a new house and a few radiators don't seem to have any temperature adjustment. If i turn the TRV to off they turn off (cold) but as soon as the TRV is on (even if on frost protect) the radiators are on full and the rooms get too hot. How can i test the TRV's? Is the TRV head the clever bit or is the temperature sensor in the pipework (i.e. can i just replace the head and not have to drain the whole system). Any advice would be appreciated.