In an earlier contribution to this discussion,
Kostas Kavoussanakis wrote:

Thank you John and Roger for the quick answer.


Thanks for your concern; what's uncertain about dropping the fuse? :-)
Sure, I will check for current at the terminals as well, isn't that
standard?

You *should* have an FCU (fused connection unit) which feeds the whole of
the heating system. Turning that off will isolate the heating system without
affecting anything else in the house. But it's always a good idea to check
for *voltage* on anything you're likely to touch, even when you think you've
isolated it. You'll need a voltmeter with a 300v AC range for this - don't
rely on neon screwdrivers!


The reason why I felt compelled to ask if it's DIY was the quote I got
from my gasman for this replacement: 70 pounds. Also, the standard set
of questions from Honeywell mentions a sub-base as a possibility; I am
not sure how that is defined, so I thought this could be an issue. In
general I thought I had underestimated its complexity.


The only 'complexity' is in understanding the function of each wire, and
being able to work out what to connect to what when the new unit is not
identical to the old one. A common problem is that some people feel
compelled to connect the old neutral wire to something - and end up
connecting it to the only spare terminal, which is 'Heating Satisfied' (C in
your case). The result is that when the house gets up to temperature and the
stat switches, they then have a dead short between live and neutral -
causing a loud, potentially expensive, bang!

Also, it's a moot point whether it's notifiable under the dreaded
Part P - since central heating controls seem to come within its
scope. I'm not sure how convincingly you could argue that it's a
'like for like' replacement.

That's important to me: who does one ask? My case would be that it's
"like for like" in the sense that the new thermostat performs the same
electrical task; it just does it in a internally different way.


Have a look at
http://www.planningportal.gov.uk/upl...F_ADP_2006.pdf If you can
wade through the treacle, you can probably find evidence to support the view
that changing a stat is *not* notifiable. [I wasn't sure when I posted
earlier, but it looks fairly safe from that document].
--
Cheers,
Roger
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