John Rumm wrote in
o.uk:

On 03/11/2018 18:48, Tim Watts wrote:
On 03/11/18 17:50, DerbyBorn wrote:
After lots of checking a Heating Engineer has decided a mate's
heating system has a partly blocked boiler heat exchanger. Likely?
12 - 15 years old. Has been pressure cleaned a couple of years ago.


Quite possible in that timescale - the heat exchanger sees mains
water all the time, so hard water will scale it up.

You are assuming its a combi, and in a hard water area as well... I
was assuming he was talking about the main HX being (partly) blocked
on the primary side. Perhaps the OP could clarify?



??It is a conventional Boiler - not a Combi