Nest & Baxi system boiler

[smoothbadger]

Hello everyone

Thanks in advance for your help on this!

I have installed a nest 3rd Gen to the Baxi Megaflo 2 System 15 Compact GA boiler.

The boiler is not firing at all.....

However when I increase the temperature on the Nest thermostat, I hear a reley click (not sure if that’s from the heat link or the boiler itself)

This is my S Plan I have been working from.

I’ve probably done something fundamentally wrong, but need some help at this stage.

Many Thanks

 

[ianmcd]

you dont show the grey wires on your zone valves, there should be one in each valve, they are connected to a permanent live

 

[smoothbadger]

Must have missed those grey wires from the diagram.... my mistake. However I can confirm they are both connected to permanent live.

 

[ianmcd]

would be easiest if you posted a pic of your actual wiring, not the diagram,what controls did you have before fitting the nest ?

 

[smoothbadger]

Before the nest, there was a Honeywell ST9400c Controller with an analogue thermostat.

I’ll post some pics.

On the boiler, Terminal 1 is SL = Swithed Line, which is terminated at the S plan feed BOTH orange wires for the zone valves

HOWEVER
boiler Terminal 2 currently has nothing connected. What should that be connected to?

 

[smoothbadger]

A at the S plan is currently fed with SL Terminal 1 at boiler

 

[stem]

Installing the Nest Heat link in place of the ST9400c. Is a pretty straight forward swap one for the other, no need to alter any of the wiring at the wiring centre or boiler to do this part.

but, there's a crucial piece of information that you haven't told us. What did you do with the old analogue thermostat? If you have just disconnected it, the heating wiring will be permanently 'open circuit', even though the relay is clicking, it won't be switching anything. If on the other hand, the thermostat is still in circuit it should be set to 'maximum' otherwise it can interfere with the operation of the Nest thermostat.

In case you have just disconnected the old thermostat, here's how to decommission it properly.

1) Trace the thermostat cable from the thermostat back to find its origin. [Probably the wiring centre]

2) When you find its origin, make a note where the wires from the thermostat cable are connected, then disconnect all of the wires and remove the cable.

3) Add a wire to link the two terminals where the 'live' and 'switched live' wires going to the thermostat originated from so that they are now electrically connected. That closes the circuit. This single link is the only part of the rest of the wiring that needs to be changed. Nothing changes at the boiler. Provided of course that it was working correctly before.

Having said all of this, the hot water side of the installation should still fire the boiler up, unless of course the hot water is already heated.

 

[ianmcd]

You havent connected the orange wires from the zone valve to where they should be, they should be connected to here, age 26 fig 33 on the MIs

 

[ianmcd]

https://baxi-public.partsarena.com/Documents//DOC_002721/DOC_002721.htm

 

[CBW]

You havent connected the orange wires from the zone valve to where they should be, they should be connected to here, age 26 fig 33 on the MIs

View attachment 229773

Any ideas on what looks like the brown wire in the strip above it? Or is that supposed to be the orange?

 

[smoothbadger]

Any ideas on what looks like the brown wire in the strip above it? Or is that supposed to be the orange?

That’s where I’ve connected the orange wires from the valves to

 

[smoothbadger]

You havent connected the orange wires from the zone valve to where they should be, they should be connected to here, age 26 fig 33 on the MIs

View attachment 229773

Thanks I’ll give it a go

 

[CBW]

That’s where I’ve connected the orange wires from the valves to

Wire appears brown on photo, could be the lighting though?

 

[ianmcd]

Any ideas on what looks like the brown wire in the strip above it? Or is that supposed to be the orange?

yes mate that is the low voltage switched live, he may have damaged the board if that wire is 240v

 

[ianmcd]

Thanks I’ll give it a go

You really need to double check everything or you will damage the boiler, if that Brown wire in the other terminal block is the connection from the two oranges and the greys are connected to a permanent live you may have damaged the boiler, it is possible to use the grey and orange as low voltage switching, but the grey would have went into the terminal next to that brown wire and not a permanent live