The temperature of the radiators will be determined by the boiler and how hot it makes the water - there is usually a user adjustment for this.
Some discussions on the web suggest that a high temperature is more efficient but there is little evidence to support the theory (some).
A key factor is how hot do you want the radiators to get? Not too hot to burn any visiting children or old people. And the shower/bath - not too hot to scald or burn. My boiler has a numbered dial for this from OFF to MAX in 7 steps - I run our boiler on the 2 setting - my radiators are comfortable to the touch and the tap water just too hot for comfort. I don't mess with it and if my rads suddenly became very hot I would suspect the boiler control had been set too high.
Apart from the temperature of the water coming from the boiler there will be the Room Thermostat that sets the air temperature of the house. The radiators will continue to push out heat until the room stat says stop.
The room stat will not change the temperature of the heated water - just the air temperature.
Peccavi, December 2012
