Selgas
Your problem is with the thermostat – to be absolutely accurate the differential bewteen the cut in and cut out settings are way too great.
If it is a gas water heater then the whole gas valve control has to be replaced with one that has less than a 12 degrees differential built in.
If it is an electric one then some of them can be adjusted but most cases a new thermostat usually fixes the problem. 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
