Nest App is not firing my boiler

[MordenMark]

My Nest App will not fire my boiler for Hot water only. The heating is now off. It was not a problem when the heating was running but now I have no hot water. The App goes orange, the heatlink gives me three green lights, I hear clicking, but the boiler will not fire up. Unhelpfully whenever I need to do something complicated with the App it just logs out. I have deleted the App several times and reinstalled it from the App Store. My iPhone is up to date. 

[Frustratedddd]

Does the system work as it should by using the display?

[MordenMark]

It does, thank you. The App on my phone talks t the thermostat. It just doesn’t fire the boiler unless I also put on the heating 

[Frustratedddd]

You state this in your first post, is this correct?

The App goes orange, the heatlink gives me three green lights, I hear clicking

I assume you get this when you call for Hot Water in the app icon, correct?

Do you have a combi boiler or system boiler?

[MordenMark]

Sorry, I have been away for a few days, so I have been unable to reply. The screen goes orange (mainly) on the app, and I hear clicking when I manually start the heat link. When I got home last night, I ran both the heat and hot water to ensure that I had water. This morning, the hot water fired up on its own accord when it was supposed to. It is the inconsistency that is driving me nuts. And having to use the heating when I don't need to

[Frustratedddd]

Just to confirm.

You can press the Heat Link button, 3 lights come on and the heating and hot water work correct?

When you call for hot water in the app, you get an orange screen BUT do you hear a click at the Heat Link?

Do you have any motorised valves in the system, if so have you checked they are working when you call for hot water?