Boiler control versus water temp in radiators

Primrose

Which is the most fuel efficient:

1. Having the boiler control at at higher setting i.e. 4 out of 5 and having room thermostat set at say 18 degrees OR

2. Having the boiler control lower at 3 but setting the room thermostat at say 20 degrees?

Or isn't there much difference? Don't quite understand how the relativity works.

penrhyn

Setting your room stat to a lower temperature will reduce your gas consumption.
Leaving the room stat as above and reducing the boiler temperature will make it less efficient as it tries to meet the demand from the room stat and will probably use more gas.

anotherbaldrick

To get efficient heat transfer from your radiators to the room you need a good heat differential between the two so keeping the boiler temperature high is best. The boiler will then be operating at its optimum output . Turn the boiler stat down and the system will struggle and use more fuel.