On 18/08/2015 09:25, The Natural Philosopher wrote:

The *central heating* is on the primary side and is not pressurised
beyond what you allow it to be - usually 1 bar or so.

Only the DHW and cold water is at mains pressure.

Even that's not quite true as a PRV will be fitted, set to about 3.5
bar. There will be a safety valve after this set to open well before the
safe working pressure in the cylinder is reached (probably ~7 bar). This
may be combined with a temperature limit too depending on the heat
sources feeding the tank. Solar and solid fuel can get bloody hot.