On Tue, 12 Jul 2016 21:30:01 -0000 (UTC), Tim+
wrote:

snip

So, can someone confirm that it is where it is to be able to measure
the temperature of the main boiler tubes (and cut the boiler out if
above a preset threshold), if there is a 'typical' reason why they
might trip and if it could be a false positive (tripping prematurely)


Typically they trip because the boiler is overheating. ;-)

Ok ... ;-)

That's usually going to be down to poor flow.

Ok, well that not easy to measure is it?

Sludged pipes

This was an old gravity fed system and the pipes are comparatively
*massive* compared to a std system today. When the guy did the
bathroom a while back and I helped him put it back together, most of
the pipes were pretty clean (considering how long it had been running
etc). After re-commissioning we chemically cleaned the entire system
(left it running a week) and I even took the heat exchanger out and
found it to be very clean.

The system was then flushed several times (till it ran pretty clean)
and then was re-filled with suitable inhibitor.

Now, that's not to say it's not blocked somewhere *now* but judging by
how fast the pipework gets hot once the boiler comes on I'd say it's
not *that* blocked (if blocked at all).

or failing pump
probably.

It's yer classic Grunfoss 3 speed and sounds like it running ok (and
again, you have a good flow round the system).

If you fire the boiler up from cold you can see the flames running
nicely and it quite soon starts to modulate as the HW cct is quite
short (and always open, no valves etc).

From memory the are two paths though the heat exchanger and I guess if
one was blocked the boiler would appear to work but one 'path' could
'overheat'?

I'll see if I can get a temperature probe / IR thermometer on the top
of the two paths and see what they each read.

The other thing I was wondering was if the tube that carries the
overtemp stat probe had a hole in it somehow and the stat was being
'overheated'? I might be able to test that by blowing into it with a
suitable pipe.

Cheers, T i m