primatic cylinder

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I do not think that you can connect a feed & exp tank to the cylinder untill you change it as u would therefore be feeding the heating via 2 source's which could cause problem's !!If you google in primatic cylinder there is a site with various dia's that might give u a better understanding ??

Many thanks for your help.

Just one further question. If I was to turn it into a fully pumped system I understand that the expansion pipe, the supply pipe and the pump have/should be close together. Therefore, would the easiest way to achieve this would be by adding an additional pump on either the flow or return pipe next to the cylinder as this is where the vent pipe is connected?

For a fully pumped system the cold feed & vent pipe should be behind the pump , or if u like on the negative side of the pump another alternative is to connect the cold feed & vent at the boiler ? the cold feed & vent pipe could be close coupled eg cold feed tee .d into vent providing the boiler has a high limit or overheat stat if you are changing the cylinder converting to a fully pumped system with the relevant temp control would be sensible in-fact as I understand it this requirement when changing clinder;s on gravity system's may be written into the building reg's ?
You will of course not do any of this with a primatic !!