Central Heating Pump Over Run

swoopdave's Avatar (by Gravatar) by swoopdave in  Plumbing 

I have an old back boiler which works fine and have recently added a new programmable digital room stat. This has made the central heating more controllable but the water in the cylinder is now getting too hot, its taken me a while to work out why but I think its because when the room stat reaches temperature it shuts the boiler and pump off together and also takes the power away from the 3 way valve which in turn defaults the valve to hot water to tank position by the internal return spring, then the residual heat in the boiler thermals up to the cylinder. This was not a problem before but because the digital stat pulses on and of when it nears the set temp it is introducing multiple slugs of hotter than desired water from the boiler entering the hot water cylinder. So can anyone tell me how to correctly wire an over run timer to the pump while keeping the 3 way valve open to the radiators to take the heat away.

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