how does the heater know is day or night?

If storage heaters are fitted then you should have a duel electric meter, during the approx,hrs of 12-30 to 7-30 B.S.T. or 1-30 to 8-30 G.M.T. , you meter should change to what they call cheap rate electric, after these hrs the meter should revert back to normal rate.
the time clock inside the meter only allows the power to flow between the above hrs,this allows the heater(s) to charge up to max capacity providing the controls are set correctly
At any other time the supply should not be their.

wirecutterman
November 2009