Sounds like a valve stuck open - when the thermostat calls for heat it will open the valve and when the thermostat reaches temperature it will close the valve. Check the thermostat setting first and see if you can find the valve and check it is working. Some have a manual overide (a lever on the side). When you fix that you will probably find the water temp goes back to normal (it's losing heat through the rads at present).
Richard
December 2008
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