You don't say what make the boiler is? But in most cases on modern boiler the frost stat is worked of the main thermistor, it's set to operate the boiler when the internal water tempreture drops below 5 c it cuts off when the return tempreture is raised above 5 c. this should have the boiler on for no longer than a couple of minutes. If it is running for a lot longer you have other problem. or is it an external Honeywell type? if it is isolate the system at the mains, then slacken the single screw at the top and hinge the outer cover down until it can be removed. Then you will be able to see the ajustment knob as if a standard stat. turn down to a lower setting and replace the cover and turn back on.
Tony G, December 2010
