Fitting an expansion vessel

[MadAlicesDad]

We have an Alpha CD24S boiler that has over the last few months, required regular top ups and expansion vessel repressurisation. The vessel cannot be removed because of the fantastic design so the solution is a new expansion vessel.
Over the years since the boiler was installed we have added additional radiators and underfloor heating which has probably contributed to the failure of the standard 8l vessel, so I am going bigger (18l because it was cheaper than the 12l version)
There is space is the adjacent cupboard for it and the boiler return pipe is easily accessible. I estimate the pipe run will be about 1-1.5m but with several bends and elbows needed.
The boiler return is 22mm so I was going to fit a 22-15mm reducer to the return and run a pipe to the vessel in 15mm copper. But...
The standard vessel is connected by 10mm pipe, albeit a piece only 30cm long. As the purpose is to balance pressure and not take significant flow, could I use 10mm for the external vessel? It would make installing the pipe quicker and easier as no elbows would be needed. The only downside I can see is the potential for blockage with crud, but is there any other reason why 10mm should not be used?

 

[polesapart]

The only downside I can see is the potential for blockage with crud, but is there any other reason why 10mm should not be used?

No, but that's reason enough. I'd be using 22 myself, belt and braces.

 

[MadAlicesDad]

Fair enough and thanks, but I'll use 15mm because I already have the bits.
Thinking about it a bit more, I'm sure the choice of 10mm pipe by the manufacturer is primarily cost based, but also, if there is a short length of narrow 10mm pipe to the internal vessel, the volume entering and leaving the vessel during each pressure cycle would be enough to flush any crap through a short pipe (if only for the duration of the warranty).

 

[Madrab]

10mm within the boiler will probably be a space issue, some even use flexi pipes.

15mm is a good middle ground