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    Vaillant Ecotec Plus 637 won't heat water in Unistor unvented cylinder

    I have a Vaillant Ecotec 637plus system boiler feeding a Unistor unvented cylinder for HW and radiators in just one heating zone controlled by a VRC470F, weather compensation sensor, VR61 control box and VR10 cylinder thermostat. All installed about seven years ago. Two or three months ago it...
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