replacing mechanical 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]

You've duplicated this post in the Electrics Forum and I've already answered it there.