Nest thermostat y plan with 2 port valve no hot water

[Denis k]

I have a y plan system with an additional two port valve because I had changed the hot water water tank to an unvented cylinder, The problem I have is when I installed a nest thermostat I get no hot water but with the two port valve disconnected the hot water works. How can I include the two port valve the system and still have hot water?

 

[CBW]

https://www.flameport.com/electric/central_heating/heating_wiring_Y_plan_unventedHW.cs4 Not sure if you need to be g3 registered.

 

[Taylortwocities]

Have you still got your old programmer in circuit?

 

[ericmark]

As the John Ward video shows the two port valve is basic an emergency cut off to stop the water in tank over heating, however as he says it is a simple over view, showing reason for having it, and the main reason for Y plan was so the water from the boiler could cool by heating the DHW, so you have one safety system over riding another safety system as shown by the video, so it is very possible that a plumber would wire slightly different depending on the boiler being used, with a solid fuel boiler the water can boil, and there needs to be some way the water can be cooled, not simply shut off, so I would suggest this is not a DIY job.

 

[AndyPRK]

They tend to have a bypass valve to route water back to boiler

 

[ericmark]

They tend to have a bypass valve to route water back to boiler

I would agree when boiler running, but once boiler stops, in many cases so does the pump, so boiler cools with thermo syphon it does depend on boiler, remember we have no idea if solid fuel, oil or gas, and in the main gas will shut off fast anyway so the two port valve not required, my worry is there is some thing we are unaware of, looking as John Wards diagram swapping a standard programmer for a Nest Gen 3 it would have run anyway, so there is more than meets the eye.