The manual for a a Worcester Greenstar 24i System ErP says: "The appliance is equipped with an internal by-pass. The internal bypass is not intended to be a substitute for an external system bypass. An external automatic bypass should be used if the system flow can be significantly adjusted or stopped by zone valves and thermostatic radiator valves (TRV)."
Why does it need an external bypass valve as well as the internal bypass valve?
Why does it need an external bypass valve as well as the internal bypass valve?