How to heat water only without central heating

ADSLUSER

Trying to determine if my Central Heating system supports heating hot water only without central heating via the boiler. 
Current systems consists of the following:
- Standard Gas Boiler setup with Water tank in attic and Cylinder in hot press
- Two zone (I think) i.e. Central Heating and hot water as I have two motorized valves in hot press with Thermostat for upstairs and downstairs
- Nest 2nd gen as main controller connected to boiler
Note: I was told by a plumber previously that I cannot just switch the heat on upstairs only, the down stairs must also be turned on as together they are treated as one zone.

Question:
I recently replaced my old boiler and considering buying a Nest 3rd gen which allows to control hotwater and central heating independently. My old boiler didn't support this even though there was a setting for hot water only. I was wondering if this is a piping issue or a boiler issue and whether my newly installed boiler supports this? Asked the plumber at the time but he said it's best to use the existing setup based on the existing piping. So confused. Hopefully someone can shed some light on my central heating setup and whether I can just set the boiler to heat hot water only without central heating. Right now, the plumber said I can do this by turnoff the two analog thermostats on the wall upstairs and downstairs. However, I wondering how Nest knows when to turn off the boiler if radiators don't turn on unless I manually set it to 1hr in the morning and 1hr in the evening?

blackbox

It depends on the physical layout.

We have our cylinder upstairs and the central heating boiler can be used to heat the water only if the circulation pump is turned off.

This will not work with a cylinder on the ground floor as there wouldn't be circulation from convection.

ADSLUSER

Our cylinder is upstairs. I’m hoping for a solution where my existing system allows for the Nest 3rd Gen to use the hotwater only function. If not, it’s not a big deal to turn off both thermostat and just set Nest to run an hour or two daily during the summer months. It just would be nearer to control hot water and heating independently using the Nest.

theflash

I have the nest 2nd gen. I can't use the hot water only function due to a plumbing issue. The plumber explained it but can't remember why. But it made sense at the time.

ADSLUSER

The 2nd Gen Nest doesn’t support controlling hot water only but I have a sneaky suspicion our setup and many others alike cannot do it anyway without changing some piping work.