nest gravity

Hi everyone, So after reading the many other threads about problems with the nest, i cant find any issues which match mine. I'm pretty sure i have a Y plan system and that i have wired the Nest correctly, after wiring it in the heating and hot water all come on but the heating will not switch...

Just wondering if it is possible to have a nest on my heating system? The hot water runs on gravity only. The heating is on a pump. For the heating to work the water has to be live too. There is no valve, on cylinder stat. I know the system is old. I’d just like to have it working with the...