The heating system at our home is set to auto. I guess this means that our boiler will automatically operate as to maintain the temperature of the home. How much should we set the temperature of the hot water used for the radiator, to save energy? For example, if we set the home temperature to be 20 degree Celsius, how much should we set the temperature of the hot water?
I am assuming you have a condensing gas boiler.
If you have a hot water tank that is heated by the same system, then you need the boiler set to 70c, so that the hot water tank can get up to 60c, unless you have a very advance system that lets you let the boiler flow temperature to a different value for the radiators and the hot water tank.
If you have a combi, you don’t have the above limitations as the boiler heats the hot water direction on demand.
For the radiators, it is best to set the boiler to the lowest flow temperature that provides you with enough heat, as the boiler will then condense more. Ideally you want the water returning to the boiler to be below 30c, given that most radiators systems are designed from a temperature drop of 20c, then you need the flow temp below 50.
However systems are designed to cope with very cold days, on other days you can set the flow temperature lower, to get more condensing and turn it up on the few days in the year that are really code. A weather compensation system automates this process.