On 14/03/2019 08:17, Thomas Prufer wrote:
On Wed, 13 Mar 2019 13:07:50 GMT, lid (AnthonyL) wrote:

Analysis Typical Value
Hardness Level Moderately Hard
Hardness Clark 13.53

Maybe the heat exchanger between hot water and combi water is furred up. Low
flow, combi can't dump heat to the DHW fast enough. And local hot spots in the
heat exchanger cause more scale to precipitate, starting a vicious circle.

It's possible to descale them by pumping a suitable acid through the combi, heat
exchanger in situ...

Rothenberger, for instance, make units with an acid tank, reversible pump,
hoses, and bells and whistles. Nothing you can't jury-rig with a "drill pump"...

I've done one.
I took it out and did it in a washing up bowl.
It used a lot of descaler (Furnox DS-3).
There was also a lot of rust debris caught in the primary side, which
came out during the descaling.

--
Andrew Gabriel