Danfross to Smart Stat

[bigrazza]

I have purchased a smart thermostat from Amazon, I currently have a danfross (tp5000 I believe) Im looking for advice on the wiring or if it can/can’t be done.

 

[stem]

The wires in the existing thermostat COM and N/O terminals are the switching wires, so they correspond with terminals 1 & 2 at the new thermostat.

However, the old thermostat is battery powered, and the new one is mains powered, so you will need to provide a 230V supply to the new thermostats N and L terminals to provide it with operating power. This supply should come via the same fused connection unit as the boiler.

As your boiler uses mains switching for its external control circuit, the aforementioned brown wire at the existing thermostat will be a live and can also be connected to the new thermostat L terminal, so that just leaves you requiring a Neutral. There is a grey wire at the thermostat that doesn't appear to be connected to anything. If when you locate the other end of it, you find that it is unused, and can be connected to a heating system neutral at its source, then that can be utilised to provide the required neutral.

 

[bigrazza]

Thanks for info, so I could bridge 1 & 4 on the new thermostat, as for the grey wire I will put a blue sleeve on it, as for the boiler end… do I connect the grey wire to the neutral on the 230 in or 230 out, thanks again

 

[Deleted member 267285]

I’m slightly confused (unless I’m being a bit thick), the live out is linked to the hot water live return, the heating live return is correct, but can’t see heating out

 

[stem]

Thanks for info, so I could bridge 1 & 4 on the new thermostat,

Yes.

as for the grey wire I will put a blue sleeve on it, as for the boiler end… do I connect the grey wire to the neutral on the 230 in or 230 out, thanks again

All Neutrals are connected together, so it will work with 'N in' or 'N out'. Convention has it that 'N in' is for the incoming supply and 'N out' is for the external controls. So I would use 'N out' But make sure you trace the other end of the grey cable from the thermostat to make sure it can be used, don't assume it is physically connected to the grey wire at the boiler. It may be, but also it may not. See my comments re wiring below.

I’m slightly confused (unless I’m being a bit thick), the live out is linked to the hot water live return, the heating live return is correct, but can’t see heating out

The cable at the thermostat is different to the one shown at the boiler, so there's a junction box somewhere. As is normal, it looks like heat resistant flex has been used for the final run to the boiler and then standard grey PVC sheathed for the thermostat. We can't see the point where they connect, however, in order to provide a live return (Lr) back to the boiler the existing thermostat will have a live supply connected from somewhere we can't see. I would guess probably the junction box; or maybe the fused connection unit.

This keeps the number of cores required in the boiler cable down (although there might be an unused grey that we can't see the other end of) and as you say the 'L out' has been used to link to the hot water Lr so it means that only one wire needs to go into the L out terminal.

It's not unusual to do this especially with installations that have motorised valves (eg S-Plan+) where the motorised valves pick up their live from the wiring centre and only the Lr goes back to the boiler. [@bigrazza don't worry if you don't have any motorised valves, if you only have one room thermostat on a combi boiler you won't have any, this is just to help explain to @CBW]

 

[bigrazza]

I’m unsure how to quote on here but yes there is a fused switch beside the boiler, with 2 grey cables inside (one from the thermostat and one from the boiler, neither are connected to anything) that could be joined together, I would take a pic but I’m not home at the moment,

So it’s:
brown in 1 on new thermostat
Black in 2 on new thermostat
Grey in 3 on new thermostat
And then 1 & 4 bridged

and the grey in to the N out on the boiler side and the 2 grey connected in the fused switch

 

[Deleted member 267285]

The cable at the thermostat is different to the one shown at the boiler, so there's a junction box somewhere. As is normal, it looks like heat resistant flex has been used for the final run to the boiler and then standard grey PVC sheathed for the thermostat. We can't see the point where they connect, however, in order to provide a live return (Lr) back to the boiler the existing thermostat will have a live supply connected from somewhere we can't see. I would guess probably the junction box; or maybe the fused connection unit.

Thanks @stem, yes I’ve seen this done with plenty of our boilers, think I was having a “moment” .

 

[bigrazza]

@stem thankyou for the advice, all wired up and working great

 

[DP]

I’m slightly confused (unless I’m being a bit thick), the live out is linked to the hot water live return, the heating live return is correct, but can’t see heating out

Perhaps Vaillant should stop fitting cable bushes as they are surplus to requirement

 

[stem]

@stem thankyou for the advice, all wired up and working great

Excellent and thanks for posting back how it went, it may helps others who may read this thread later.