Nest Heat Link wiring - pump always on

[OliMortimer]

Hi all,

I'm sure this forum has been inundated with posts about Nest installs, so I apologise if this query has been answered before, but I couldn't see anything as I browsing...

We've recently moved into a house which is relatively new (4 years old), and I'm trying to install a Nest into the system.

To give you a quick summary of the system in place;
* Ideal Logic 15 boiler
* Siemens RWB29 programmer - CH and HW timer
* Siemens RAA20 thermostat

The boiler has 7 connections into it;
* Mains L
* Mains N
* Mains Earth
* sL
* Pump L
* Pump N
* Pump Earth

...this is where I think complicates things...

The pump connections on the boiler aren't wired to the pump (they aren't wire anywhere), even though the Ideal Logic 15 manual states that this should be, otherwise it voids the boiler warranty.

I successfully wired the Nest Heat Link to the boiler;
* Boiler Mains L to Heat Link L
* Boiler Mains N to Heat Link N
* Disconnected sL and replaced with Heat Link 'call-for-heat'
* Jumper between the Heat Link L to Heat Link common

...and replaced the RAA20 thermostat with the Nest thermostat.

I set the programmer to CH 'always on', and everything was great! The Nest successfully called for heat / cancelled heat, the radiators kicked in as before, and the HW sounded like everything was working as before. Excellent.

However, I noticed that the pump was always on, which I believe is because the boiler isn't controlling it, and is instead being turned on / off by the call to heat going through the system to the CH and HW valves.

Not to be outdone, I spent some time noting down all the wiring within the system, and trying to get my head around what goes where, so I can understand where I need to wire the Nest Heat Link so the pump only comes on when the HW calls for it (as before), and Nest calls for CH.

This is where I need assistance. I don't want to wire the Heat Link in the wrong place, and end up with a very expensive paperweight and now CH / HW. I'm guessing I need to wire the call for heat into the orange cable going to the CH valve.

If someone is able to advise on where I need to cable the Heat Link in, that would be fantastic. I've attached the heating diagram, which I hope is 100% correct, but would mean some cables magically change from white to grey in the wall (!?).

I believe it's an S plan system?

The black markers are marker pen 'dashes' on the cables, and the blue markers are blue sleeves on the end of the cables. The small dashed boxes are cables into / out of the area.

Original Wiring

• OliMortimer
• 4 Dec 2014
• 1

The UK Nest install guide is located here:
https://s3.amazonaws.com/support-as...ges/Nest-Thermostat-Installation-Guide-UK.pdf

 

[kbdiy]

Surely all you need to do is wire the pump from the boiler rather than where you have it at present?

The connection from the boiler is probably required to operate as an overrun to dissipate heat when there is no actual call for it (but would require some kind of bypass circuit for water circulation) and may be that doesn't exist in your system, hence why it wasn't installed that way?

It's worth noting that the Nest installation diagrams don't mention pump connections at all which is also why I suspect that is taken as read that it connects from the boiler?

I'm no expert in this but just trying to apply some logic to the situation.

 

[OliMortimer]

Surely all you need to do is wire the pump from the boiler rather than where you have it at present?

The connection from the boiler is probably required to operate as an overrun to dissipate heat when there is no actual call for it (but would require some kind of bypass circuit for water circulation) and may be that doesn't exist in your system, hence why it wasn't installed that way?

It's worth noting that the Nest installation diagrams don't mention pump connections at all which is also why I suspect that is taken as read that it connects from the boiler?

I'm no expert in this but just trying to apply some logic to the situation.

Forgot to mention that the boiler (kitchen) doesn't sit near the pump (upstairs airing cupboard), so wiring the pump into the boiler isn't an option at the moment.

 

[45yearsagasman]

Are you sure that only the pump alone is running and not the boiler also. If the pump connection is live then isn't the boiler switched live also active?
Are you sure that it's not one of the zone valve end switches sticking on.

 

[OliMortimer]

Are you sure that only the pump alone is running and not the boiler also. If the pump connection is live then isn't the boiler switched live also active?
Are you sure that it's not one of the zone valve end switches sticking on.

The boiler was sitting at 0 status (no call for heat), which is correct.

I believe it was only the pump that was still running when CH was set to 'always on' on the programmer - it was the only part that was 'humming / vibrating', but it may have been the CH valve as well.

 

[jackthom]

The heatlink should be wired so it powers the motor of the CH valve (switches the RS ON terminal live?) and the valve in turn should activate the boiler/pump when it has opened.

The heatlink certainly shouldn't be feeding direct into the boiler switched live.

 

[OliMortimer]

The heatlink should be wired so it powers the motor of the CH valve which in turn should activate the boiler/pump when the valve has opened.

The heatlink certainly shouldn't be feeding direct into the boiler switched live.

Would you mind suggesting how I would do this please?

I'm not sure if the Nest Heat Link 'satisfied' and 'common' would be needed to do this.

 

[jackthom]

Yes you would have Live linked across to Common inside the heatlink, then 'call for heat' would go to the wiring centre CH valve connection which looks to be the R/S ON terminal in yours. (Presumably where the original room thermostat was connected)

so in the heatlink, Live, Neutral, Linked live to Common, plus Call for heat going back to the wiring centre.

 

[OliMortimer]

Yes you would have Live linked across to Common inside the heatlink, then 'Satisfied' would go to the wiring centre CH valve connection which looks to be the R/S ON terminal in yours. (Presumably where the original room thermostat was connected)

so in the heatlink, Live, Neutral, Linked live to Common, plus Satisfied going back to the wiring centre.

Thanks Jack. The diagram is how the system stands now, without Nest wired in.

So;
* R/S ON to Heat Link satisfied
* Heat Link call-for-heat to where R/S ON currently connects to the CH valve?

 

[jackthom]

Sorry I meant to say the 'Call for heat' wire should be used and not 'satisfied' which isn't used.


So Heat link 'call for heat' wire will go to R/S ON in place of the old THERM wire which should be safely removed from the connector and covered up.

 

[OliMortimer]

Heat link'satisfied' wire will go to R/S ON in place of the old THERM wire which should be safely removed from the connector and covered up.

Sorry, I'm a bit confused now. So where does the call-for-heat wire from the Heat Link connect to?

If I disconnect the thermostat wire from R/S ON, then it would no longer provide power to the room thermostat for when I replace it with the Nest thermostat.

 

[jackthom]

What had you intended to connect to the old thermostat wiring that requires mains power?

I'm assuming the heatlink is mounted near to the wiring centre so will be getting it's Live and Neutral from that.

...and the actual nest thermostat itself can't handle mains except by using the USB power adapter plugged into a 13A socket.

 

[OliMortimer]

What had you intended to connect to the old thermostat wiring that requires mains power?

I'm assuming the heatlink is to be mounted near to the wiring centre so will be getting it's Live and Neutral from that.

...and the actual nest thermostat itself can't handle mains except by using the USB power adapter plugged into a 13A socket.

I was going to wire the Nest thermostat in place of the current room thermostat - this is where I had it originally wired and working, until I reversed the installation due to the pump issue.

 

[jackthom]

Was the old thermostat still connected up to it's wiring at that point?

How was the nest round thermostat getting it's power?

 

[OliMortimer]

Was the old thermostat still connected up to it's wiring at that point?

How was the nest round thermostat getting it's power?

No, the old thermostat was removed, and replaced directly with the Nest thermostat - brown from old therm to T1 on Nest, black from old therm to T2 on Nest.