EMC wrote:

I am just about to embark on upgrading my old oil fired gravity hot water /
pumped central heating system to a gas fired fully pumped system and am
unsure what arrangement of valves to use. What are the advantages /
disadvantages of using a 3 port valve instead of two 2 port valves to
control a fully pumped hot water and (single zone ) central heating system?

IMHO using 2 (or more) 2 port valves is better in every respect than a 3
port valve except that the 3 port (either mid-position or diverter
variants) is marginally lighter on material costs.

If you decide to go the 3 port way - be strongly advised that the
diveter valve variant (either but not both CH and HW) - has been found
wanting by many.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html