Shower Expansion Vessel

2 Nov 2006
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United Kingdom
Combination Boilers and Thermostatic Showers. My Mira Excel Thermostatic Shower instructions state that an expansion vessel must be fitted to the domestic water supply to protect the shower from undue rises in pressure (water hammer, etc.). Mira do not state a size for this vessel but other thermostatic shower manufacturers (eg Triton) state that it should typically be 0.16 litres in capacity. Another figure that I have seen banded around is that the expansion vessel should be at least 4% of the capacity of the domestic water system. My system (small detached) is around 10 litres which equates to an expansion vessel of 0.4 litres. The smallest expansion vessel that I can find available in the UK is 2 litres. Does anybody know where smaller vessels can be sourced?
I am aware that below a certain size vessels containing pressure do not need to be certified as pressure vessels and therefore are not subject to p.v. regs. Does anybody know what the limit is before they become classed as p.v's.?
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