Following the advice/discussion elsewhere recently I bought a Honeywell
Room Thermostat (CM907 -
http://europe.hbc.honeywell.com/prod.../pg_cm900.html) to
replace my existing Honeywell room thermostat.

There are 3 wires coming to it, a blue, a brown and a green/yellow.
These were connected onto the old thermostat blue --- N, brown -- L,
and green/yellow -- E. (All well and good so far).

I have now fitted the new thermostat, and it all lights up and tells
me the day/temp etc, and I have connected the brown to terminal A, and
blue to terminal B (page 7, in the wiring diagram
http://europe.hbc.honeywell.com/prod...7uk07r0106.pdf)

BUT, when it demands heat - the flame symbol comes on, but upstairs
the 3A fuse in the plug supplying the power to the central heating
timer blows.

I have tried replacing with a 5A fuse but the same happens. I'm loathe
to try a 13A, as this is what was originally fitted and the heating
engineer was upset to see a 13A fuse in it when he replaced the
controller earlier this year. Also, the installation instructions say
that it should be less than 8A.

So, do I call out a heating engineer (another £75) or can anyone
suggest what stupid thing I'm doing wrong?

Thanks,
David

On Fri, 15 Dec 2006 21:07:33 +0000, I Podius wrote:

Following the advice/discussion elsewhere recently I bought a Honeywell
Room Thermostat (CM907 -
http://europe.hbc.honeywell.com/prod.../pg_cm900.html) to
replace my existing Honeywell room thermostat.

There are 3 wires coming to it, a blue, a brown and a green/yellow.
These were connected onto the old thermostat blue --- N, brown -- L,
and green/yellow -- E. (All well and good so far).

I have now fitted the new thermostat, and it all lights up and tells
me the day/temp etc, and I have connected the brown to terminal A, and
blue to terminal B (page 7, in the wiring diagram
http://europe.hbc.honeywell.com/prod...7uk07r0106.pdf)

BUT, when it demands heat - the flame symbol comes on, but upstairs
the 3A fuse in the plug supplying the power to the central heating
timer blows.

I have tried replacing with a 5A fuse but the same happens. I'm loathe
to try a 13A, as this is what was originally fitted and the heating
engineer was upset to see a 13A fuse in it when he replaced the
controller earlier this year. Also, the installation instructions say
that it should be less than 8A.

So, do I call out a heating engineer (another £75) or can anyone
suggest what stupid thing I'm doing wrong?
^^^^^^^^^^^^^^^^^

Replacing the fuse by a higher rated one when the blowing of the first one
should have told you there is something wrong with the way it's wired up,
which needs sorting out before re-applying power.

Someone who understands basic electrickery needs to check what's at the
other boiler/wiring centre end of the cable (and any junctions in
between) to make a circuit connecting the CM907's relay contacts to the
appropriate parts of the control system. Honeywell's diagram of a Y-plan
or S-plan (as appropriate for your system) wiring helps here. If your
wiring centre is very well laid out this is not too difficult, but if it's
a mass of wires choc-blocced together stuffed into the back of a pattress
box you'll probably find yourself out of your depth.


Tangentially: does anyone know of an online diagram of how to wire up an
S-plan using the sort of (Sunvic?) 2-port motorised valves which have a
yellow wire to open and white to close? (I know how to do it: I've just
spent an evening re-wiring a system I mistakenly wired up as standard
S-plan :-(. I just want something I could print out to put with the system.

I Podius wrote:

can anyone suggest what stupid thing I'm doing wrong?

Posting the same question again without reading replies that have been
provided ...