On 09Jul2020 0058, Timothy Collinson - timothy.collinson at port.ac.uk
(via tml list) wrote:

> (Also, I'm not entirely sure what the purpose of the 'expansion tank'  > is. I pinched it from an RV plan online but have been unable (fairly
 > rapidly admittedly) to find anything online that gives a *clear* >
explanation of why you'd want one. At first I thought it was a kind > of
overflow safety thing, now I think it might be to do with > maintaining
water pressure. Certainly in the latter case and perhaps > in the
former, I'm inclined think scrap it - "Trav Tech" (TM) can > handle not
having it!
It's a sealed tank that has both air and water in it (separated in a
modern tank) that's there to smooth out pressure spikes from water
hammer and to stop thermal expansion doing terrible things to the
plumbing. Wikipedia has a more complete description, FWIW.

--
Rupert Boleyn <xxxxxx@gmail.com>