...doesn't shift any water. The 3 position switch works and the rotor spins faster and slower as expected, but the boiler 'knocks' and overheats due to lack of water throughput. The pump is about 20 years old, can it be repaired?
If you have a gravity fed system, check that the ball valve in your CH header tank hasn't stuck closed and starved your system of water. If the water level falls lower than your pump it will have nothing to pump around. The header tank is normally in your loft. If the valve has stuck, release it and the system will refill by itself but you will have to bleed all the radiators.
plumber
June 2009
You don't repair these, only replace them....I guess the impeller has come off your pump. Anyway, it isn't such a bad job, so long as the shut off valves on either side of the pump are actually working!
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Try fixfault.com to fix home heating systems.