External automatic bypass valve on bolier with internal bypass valve.

11 Sep 2014
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United Kingdom
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?
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The cynical would say its so that Worcester don't have to honour the warranty when they find an installation without an external bypass. I suspect though its because the internal bypass will have a limited flow capacity and the actual size/configuration of the heating system is an unknown to the manufacturer. Basically they are covering themselves. If its in the manufacturers instructions, it should be done. Not expensive and if the installation has been thought through, there should be space to squeeze one in somewhere.
apart from giving the appliance its min flow rate (which the internal one may do) fitting one on the system correctly reduces system pressures, reduces noise, increases efficiency and allows the appliance to circulate heat away from itself on over run.

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