It is in the settings menu when you set the nest up, can be changed if you upgrade the controls at a later date, gravity HW is exactly that, the boiler comes on and the HW circulates through a large coil in the HW cylinder through gravity, no pump is used , when the CH is asked for, the pump comes on and circulates water through the heating system, but the gravity HW will still be on, you can not get CH on its own with a gravity HW system, many people mistake what a grvity HW system is, as opposed to an open vent system, they are not the same thing,
@picasso , this is for others reading this , I know you will already know this but it is the open forum, and is advice to those that mistake open vented systems from Gravity HW systems
I am sure mine was set to system boiler, the problem is there are three C Plans all are gravity or thermo syphon, but some use a tank thermostat, and some use tank thermostat and motorised valve. I want to add a tank thermostat to mine, but need to get the wires from the tank in top floor to boiler at bottom floor, the gravity system I have is open vented, and yes not the same thing, but not as yet seen a gravity system which is not open vented, but no real reason which you shouldn't have a pressurised system which uses gravity, the old Morris 8 car used a radiator pressure cap but had no water pump still used thermo syphon. There is no relationship between open vented and gravity, it is like saying sports cars are never diesel, most sports cars are not diesel, but that does not stop you having a diesel sports car.
I know my dad had a motorised valve with his C Plan gas fired central heating, and he could select DHW and CH independently, it was upgraded to a combi boiler because the header tank started to leak, and he could get a government grant to fit a combi but not to repair the header tank. It however required re-piping as the thermo syphon used 32 mm pipes, and the combi only needed 15 mm pipes, and also the pressure was going to be a lot higher so every radiator changed, even the power shower had to be removed and swapped for a thermostatic shower, it was a big job to change it all, and would not have been done without a government grant.
It seems
@Tom Waller wants to in fullness of time to also upgrade the system, but wants old system to work for another year. So there is one big question, will fitting a motorised valve and a tank thermostat cause damage to the old Potterton Profile 80e boiler? I don't know the answer, I was told it may damage my Worcester Bosch boiler, not really sure that it would, but would assume the Plumber and Oil boiler service guys know what they are talking about so did not do it. I am not so sure on their skill level after a series of leaks after changing all the TRV bases, specially as can not see anything wrong with the old ones.
I did a google for 32 mm motorised valves and they seem rather expensive, 28 mm far cheaper, as to if 28 mm would be big enough I don't know? So to convert from early C Plan to late C Plan is likely even doing it DIY to cost around £150, if boiler only doing one more season I can't see it would be worth doing? Same goes converting to Y Plan or S Plan, if the idea was to keep the old Potterton Profile 80e boiler for 5 years then yes, but personally I would not bother changing from existing C Plan, I found being able to set the on time for DHW down to ½ hour every other day stopped the problem of using too much oil, the old system would only run 1½ per day minimum which was longer than required. And I am using Nest Gen 3 with a old C Plan and oil boiler.
