On Sun, 17 Jul 2016 23:37:30 -0700 (PDT), Cynic
wrote:

snip


Have you actually measured the control stat response?

No and I'm not sure how I would?

They do get lazy and settings drift.

In case it's relevant, Mum used to run the boiler at position 4 (out
of 10) and that gave her the right temperature hot water and
background central heating. After the most recent plumbing changes we
found that it seemed to 'hang in there' better when set on 1. So, if
you turn the boiler on from cold, the main burner flame comes up
pretty well (but not in any way 'violent') runs nicely till the water
temp starts to pick up and fairly quickly (minutes) modulates down
slightly till it reaches the correct temperature (thermocouple in the
return) and then shuts down. The pump runs all the time the timer is
on (7-8, 5-6 or summat).

The overheat stat could be entirely innocent in this saga

Understood, but without being there watching (or logging?) the process
the two hours a day it's on, I'm not sure what I can do?

Something Andrew said re a pump overrun timer got me thinking (as it
doesn't have one). A while back Mum had the system on 24/7 which would
mean the pump would also be on 24/7. This would mean there was never a
chance of any overtemp down to lack of water flow.

When I went round there yesterday (to do other things) I noticed the
water wasn't hot and therefore the chances are the boiler overtemp
stat may have tripped again. If manually resetting it works and the
boiler runs ok again, I could temporarily re-wire the pump to be on
24/7 and leave just the boiler on the timer? If that works, can you
get 'stand alone' overrun timers? [1]

Cheers, T i m

[1] I think you could years ago, or I made one with a delay timer
relay on my Uncles very basic CH system.