Had the same fault - all ok except no heat. Had checked thermostats and element continuity (30-40 ohms if good) and still wasn't heating.
Did what the doctor said - removed grey housing at bottom rear of unit (after cleaning condenser first) and found it to be a mix of fluff and water - there's a float switch in there and it was stuck. To remove the housing, lever a small screwdriver under the vertical grey tabs going into the back of the casing - the have grips on them, lever toward the sides to release them, and the unit comes off the casing. The bottom bit is the reservoir and the top bit is the pump and float switch assy. Clean this out thoroughly - I refilled with fresh hot water just as the Doc said, but don't know what this does...
All back together and all working OK. Try this next.
AndyPandy
June 2009