Gravity H/W Pumped C/H

[secret squirrel]

Can someone clear up a puzzle that has developed in my very confused brain.

On a gravity hot water system with a pumped central heating the hot water has to be on to work the central heating.

There is no control on the cylinder so it is controlled by the boiler stat, if a 2 port valve is put on the primary flow to the cylinder and is now controlled by a cylinder stat. How does the central heating work, if it is only on when the hot water is on?

I'm missing something very small and I'm sure its how it wired and functions together but I can't get my head round it.

At the risk of rambling:

Central heating is on and H/w, H/w satisfied so valve shuts, central heating is calling for heat so is directly wired to the boiler and pump.......(but c/h only works when h/w is on)

So, can someone explain this to me please

 

[WHPES]

Can't do it that way - the gravity circuit must be unrestricted for a gravity DHW. You have to modify it to a fully pumped S plan system.

 

[Cds]

With a Gravity Hot Water and Pumped Central Heating you can either have HW only or HW & CH. As there is no control on the HW it will always be on with the CH.

If you add control i.e. 2 port valve then it should be possible to have CH only but the controls would have to be wired with 2 valves one for CH and one for HW.

As WHPES says it should be converted to a fully pumped system

 

[secret squirrel]

Thank you both, the fog has cleared.

There was a reason I was confused, it can't be done.

I'm much happier now.