Mini Expansion Vessel

[Bollod]

First post since intro!
Worcester 28cdi in property when we bought. Worcester engineer when servicing says should have a mini expansion vessel in cold supply because a water meter is fitted. Boiler has worked with very few problems over the years so why is mini expansion vessel needed now? Always had a water meter and it is approx. 30 meters from boiler position.
Retired plumber friend says not needed because that length supply pipe gives plenty of expansion. (I understand the idea of hot water pushing back into cold supply)
However, since a liquid is not compressible how would it allow expansion.

Don't want to spend money unnecessarily so am asking for guidance

 

[Stanios]

To be honest is this was such an issue for combi boilers they would fit them internal as standard

 

[townfanjon]

To be honest is this was such an issue for combi boilers they would fit them internal as standard

One of the reasons I fit Viessmann

 

[Bollod]

To be honest is this was such an issue for combi boilers they would fit them internal as standard

There is no issue if no non return valve in cold supply. I've now read two installation instructions on line that do specify a MEV if that is the case.

 

[huddster]

As previously mentioned the mini expansion vessel will help reduce stress on the plastic manifold. If the boiler has pre heat on, massive amounts of pressure will build up (especially when not in use during the night) and weaken the manifold which will eventually pin hole. Ive only come across it once but apparently a very common problem on Worcester.