Am changing the radiator in our bathroom for a bigger radiator, because it has 3 outside walls and the current radiator is too small. As the room is much colder than the rest of the house (it sticks out at the back) the main thermostat turns the heating off before the bathroom reaches the set temperature, would fitting a TRV keep it on until that room is warm enough too (along with the rest of the heating) or do they only save energy in rooms that need to be cooler than the main thermostat?