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Discuss Installing Nest 3rd Gen in the Central Heating Systems area at ElectriciansForums.net


EF Member
Hi all,

I just installed the Nest 3rd gen into my new house, which was a bit of a mess - everything was wired straight into the old Honeywell ST7100 without any sort of junction box. It’s a Y-plan system with a (very old) Glow Worm 35F boiler, a Honeywell V4073A1039 valve and a UnitFive pump.

First time I wired it - given that it was all a bit as-hoc, I just switched the wires around but must not have wired the pump properly, so the boiler started and went a little crazy! I’m now adamant it’s wired properly after buying and wiring my own junction box, but the thing seems to trip as soon as it’s turned on. Initially I changed the fuse at the spur and it turned on which worked but as soon as the pump kicked in (or could be the boiler) it shorted. Now even after changing the fuses the power doesn’t seem to kick in at all. The circuit breaker hasn’t tripped so could do with any advice on where to look for the fault and how to fix it!

Any help would be much appreciated!



Electrician's Arms
Call an electrician for goodness sake.

If you keep changing and blowing the fuse eventually you'll blow something else, be it the wiring, nest, pump or boiler. And that will be much more expensive to fix. It might actually be too late for that.


Forum Mentor
so you spend around £200 on a thermostat but don’t spend the £60ish to get it fitted by a pro and instead have a go yourself and probably knacker the boiler which will cost a few thousand to replace?

Yeah, that was a worthwhile cost saving!!


please let me back in to the prison cell.
Electrician's Arms


Respected Member
I will throw the OP a lifeline,and suggest that if it "tried",shorted,then does not function with all fuses functioning,there may be more than one fault,and possibly some damage.
The best course of action,would be to get someone competent,to test your wiring,then the components,and only then,connect the additional components.

Winter's comin' and duffle coats are scarce.

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