thermostat question

[I Podius]

Following the advice/discussion elsewhere recently I bought a Honeywell Room Thermostat (CM907 -

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

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

[Andy Burns]

Are you *sure* the yellow/green went to a terminal marked "Earth"? There didn't happen to be any sleeving or insulation tape on it?

What was the original stat?

When the thermostat calls for heat it makes a circuit from A to B,=20 thereby shorting Live to Neutral :-(

Don't replace with a higher fuse, indeed, don't replace any fuse until=20 you've understood and rewired properly.

Depending on the answer to the yellow/green wire question, it sounds=20 like the original installer has has used brown/blue as live/neutral feed =

to the thermostat (presumably it needed neutral as it had a compensator) =

and then been very stupid and borrowed the earth lead as the switched=20 live return to the boiler.

You may need to replace cable with 4 core fix this properly.

[Andy Burns]

Having read more of the PDF manual for the new stat, it is battery powered so doesn't need a neutral any more, so you will be able to use the brown as live supply, the blue (marked at both ends with brown sleeving) as switched live return. However this will require changing the wiring at the other end of the cable too, don't attempt this unless you're confident you know what you're doing, but will be easier than replacing 3core with 4core.

The PDF doesn't mention if the new stat is double insulated (does it have a square in a square logo on it) and the diagram doesn't show any earth terminal, did it actually have an earth terminal?

[Dave Plowman (News)]

Not really. Few thermostats need an earth, but many older types need a neutral for their accelerator coil. So ignoring the earth that would be line and neutral in, switched line out. Sounds like you've got typical 'plumber' wiring, where they've used the earth wire in flex as something else, which is very bad practice. The correct wiring for that type of thermostat is Triple&Earth cable which in old colours would be red, blue and yellow with a bare earth.

My *guess* is they've used the green/yellow as the switched output. But the easiest way is to check at the boiler or junction box as bodgers are never consistent.

Best would be to alter the wiring so that brown becomes the line and blue the switched line, and the green/yellow the earth. And sleeve the blue with brown sleeving at both ends. Your new thermostat is battery operated and doesn't need a neutral. The earth should be made off to the backing box though if you've used one and of course coupled to earth at the other end.

[I Podius]

Following the advice/discussion elsewhere recently I bought a Honeywell Room Thermostat (CM907 -

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

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

[John Stumbles]

^^^^^^^^^^^^^^^^^

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.

[Andy Burns]

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