No hot water but heating working

[JayBz]

Hi. We have a Nest system with two thermostats. One for the UF heating & hot water, and one for the Rads. All of a sudden the nest is no longer heating the hot water. When I activate the hot water boost, I hear the click and the electronic valve opens, letting hot through into the HW cylinder and I can feel it’s hot. But then about 2 minutes later the water starts running cold and then the electronic valve closes. Nest continues to say it’s boosting the hot water but it actually isn’t. The only way I am able to get hot water at the moment is to manually open the valve and click it to the side to stop it closing. Has anyone had similar issues? Plumber says he can’t see any issues with the other bits. I tried even replacing the stat that is on the hot water cyclinder but that made no difference. Help please! 

[Frustratedddd]

I assume you are in the UK?

Have you swapped the displays over, to see if they work that way?

One way to prove it's the Nest is overide by pressing the Heat Link button. That runs everything 24hr, run for about an hour, and if the Heat link lights stay on, I would assume it's a valve. Might be worth checking the valve circuit board as well.