Honeywell ST9100A to Nest 3 - CH Only

12 Feb 2018
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United Kingdom

I am trying to fit a Nest 3 from the existing wiring in the Honeywell ST9100A unit.

I replaced the wiring like for like and taped the earth. The Nest powered and the Heat Link clicked but the boiler did not fire up.

There is a wall thermostat fitted which I will leave there and use the Nest wirelessly.

Can anyone help please?

Thanks for your time,

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So has the Honeywell in the photo has gone and the Nest is in its place?

When you say "like for like" I assume that you are referring to the function of the terminals, and not that you moved the wire from terminal 1 at the Honeywell to terminal 1 at the Nest and from terminal 4 at the Honeywell to terminal 4 at the Nest.

Why did you tape up the earth wire? the Nest Heatlink has an earth terminal it should be connected to.

Also, if by "There is a wall thermostat fitted" you mean an existing room thermostat, it should be turned up to 'maximum' so that it is permanently on, otherwise it will override the Nest. Ideally it should be removed and the wiring modified to take its removal into account.
Thankyou for your reply.

For reference it's a 230V combi. I can't remember if the room thermostat was on full, any advice on the below as to whether I have wired correctly will lead me to rewire and check the room thermostat.

I did subsequently fit the earth.

I've taken it back off but I wired into the heatlink in this way :

Honeywell - Heatlink

N - N

L - L


4-4 (although I'm not sure this has any relevance?)

I then put a loop in from Live - 2 following a YouTube video - I also tried live - 3 (in with the call for cable)

On each attempt I had power, and the unit clicked on prompt from Nest unit but the boiler did not receive signal and did not fire.

Thankyou again for your time and any further suggestions are welcome!
Oh dear, I hope you haven't damaged the boiler! Guessing with wiring is a dangerous activity physically and financially.

As you are replacing a single channel programmer, you will not be using any of the hot water terminals (4, 5 or 6) It should be:

Honeywell N blue wire = Nest Heatlink N
Honeywell L brown wire = Nest Heatlink L
Honeywell 1 grey wire = Nest Heatlink 2 'heating common'
Honeywell 4 black wire = Nest Heatlink 3 'heating call for heat'

Don't forget to connect the earth.

On no account add any links. The way your boiler is wired doesn't need them. You risk serious damage if you do.
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The boiler seems fine - would damage be obvious?

The Honeywell is working fine also, so I'm assuming my foolishness has not cost me...?

Ill follow your guide and put the thermostat up full, thankyou for your help.
If the boiler is still working, then you have been lucky, and possibly for you, two wrongs have made a right. Connecting one of the heating wires to the hot water section of the Heatlink would mean that is wasn't in circuit, so you may have got away with it because of that.

Some boilers use 24v for their control circuits and so the switching wires are kept totally separate from the mains wiring. This is how yours is wired. Add a link to the L and you can put 230V directly on a boiler circuit board designed for 24v and destroy it. When that happens, the boiler doesn't work and it usually costs about £300 to £400 to get it fixed.

Having said all of that, your boiler may actually be wired to use 230V for switching and could still be connected to the Honeywell in the same manner with the power and switching wires kept separate. In which case the same risk isn't there.

However, regardless of the voltage used, you still wire it up as per my previous post. Without any links. If you needed links they would be present at the Honeywell now and they're not.

The Heatlink will probably be OK, unless you have blown any fuses at any time during your experimenting.
Thankyou again for your responses.

The heatlink is still powering on and was "clicking" so I hope that means it'll be ok.

I'll give it another go shortly and report back, again, your help is much appreciated.

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