On Sun, 17 Jul 2016 18:01:25 -0000 (UTC),
(Andrew Gabriel) wrote:

In article ,
T i m writes:
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

Presumably that was with a different boiler?

Nope, same one.

I never heard of a low capacity boiler being suitable for
a gravity system - it would overheat too fast.

With a pump?

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.

or failed pump run-on timer?

It doesn't have one and has been running like this for over 25 years?
However, the pipework was changed a while back and, coincidentally or
otherwise, the 'issue' did start about the same time. That said, it
has also been running for ~3 years after that (once it settles down).

So, for example, if the wind is in the wrong direction it can blow the
pilot light out. It generally re-lights ok but now and again it
doesn't. The last time it didn't (a whole back) it seemed that the
overtemp stat was introducing just a bit of resistance into the flame
failure detector cct as bypassing it allowed the pilot light to stay
on ok. Replacing it seemed to fix the problem.

Changing the FF detector seemed to make a difference but later the
same fault would return (and then be cleared after I tried a few
things).

This last time I noticed the upper limit stat had dripped and
resetting it allowed the boiler to run fine again (for a few days). I
went round there today and it seems like the pilot has gone out again
(hot water only warm) but I forgot to have a look at it (no issues as
Mums away for a couple of days).

The last time I looked into it I think the flow and return temps were
fine, everything seemed to be working fine (water flowing round the
system in general), it's just that the upper limit stat seemed too
'sensitive'?

Cheers, T i m