replacing mechanical room stat with digital

[Speedy471]

I have just bought a new digital room thermostat to replace a Honeywell mechanical type. I tried what I thought would be the correct wire up but ended up blowing a PCB fuse on the boiler. Luckily there was a spare on the inside of the case!

I had a Honeywell stat with this wiring

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• Speedy471
• 3 Dec 2012
• 2

The set up consists of a honeywell programmer and Y plan valve and bosch Worcester 18i system boiler.

I am replacing with a home expert/honeywell digital room stat (THR840DUK)

Here are the wiring options

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• Speedy471
• 3 Dec 2012
• 2

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• Speedy471
• 3 Dec 2012
• 2

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• Speedy471
• 3 Dec 2012
• 2

When I blew the fuse i connected live to A, neutral to B switch C

 

[stem]

I'm not surprised. You connected a live and a neutral to the same switch, so that when the switch operated they were joined together. If you are lucky, the fuse is the only thing that has blown. If not, your new thermostat may be damaged, depending upon how much current passed through it before the fuse blew.

You do not need the neutral connection. None of the diagrams you have posted show a neutral connection to the thermostat. Why did you connect it?

The Red wire should go to 'A' and the Yellow to 'B'

The blue wire should be isolated in a small screw connector and tucked safely out of the way.

No connection to be made to 'C'

 

[Speedy471]

who knows why i did it. thanks

 

[Taylortwocities]

who knows why i did it. thanks

You did it because you did not, first of all, look in the wonderful WIKI which has many solutions already mapped out for the DIYer.

The WIKI is the very first thing you see when you enter this site.

The bit YOU want is here //www.diynot.com/wiki/electrics:how_to_replace_a_room_stat

Meerkat

• Taylortwocities
• 11 Dec 2009
• 53

Simples!!