Vaillant GB VC112e fit smart thermostat

[rickcj007]

Got this boiler currently connected to an ACL lifestyle lp241 without a junction box and to an old Satchwell Thermostat.

I have purchased a Ketotek ktf0155c which I have wired into other boilers at our local community centre and working perfectly, but wanting to clarify the connections to this boiler. Would I connect it using the dry contacts?

 

[RandomGrinch]

Do you intend for the LP241 to stay in place, and simply swap out the Satchwell thermostat for the ktf0155c?

...and I'm afraid the way the earth wires are twisted together behind the LP241 needs sorting out immediately. These need to be connected together into a single terminal block - either a separate block, or clean up and find a screw for the earth terminal on the LP241 backplate.

 

[rickcj007]

The boiler only does heating, so probably no point in keeping the LP241. I'm actually thinking to replace it with a wiring junction box (https://www.toolstation.com/honeywell-home-10-way-junction-box/p51416) so the wires can be connected more safely.

Thanks.

 

[RandomGrinch]

The boiler only does heating, so probably no point in keeping the LP241. I'm actually thinking to replace it with a wiring junction box (https://www.toolstation.com/honeywell-home-10-way-junction-box/p51416) so the wires can be connected more safely.

Thanks.

That's a good idea.
I was trying to work out what was going on behind the LP241, but a lot of the wires just appear to be linking out the HW controls?
Do you have any zone valves in the system.

Would I connect it using the dry contacts?

If you are totally removing the existing controller, stat, wiring, and there are no other complications like zone valves; then yes, the dry contacts would connect to boiler terminals 3 and 4.

 

[rickcj007]

The white sheathed flex goes to the boiler and only uses:
Red - L
Blue - N
Earth - E
White - 3

The 3 grey sheathed flexes go to mains, thermostat and possibly a valve, but I haven't been able to check that. The boiler manual mentions 2 or 3 port motorised valves.

 

[RandomGrinch]

I wonder if you can tell me if this red wire goes to terminal 1, or terminal 3?

 

[rickcj007]

That red is going to terminal 3.

 

[RandomGrinch]

That red is going to terminal 3.

Thanks.
The wiring behind the LP241 seems truly bizarre!

The LP241 is a dual channel programmer with a standard layout...

There appear to be two red link wires that join L, with terminals 1 and 2.
This is odd, but not a big problem. HW off and CH off, aren't used in many systems.
In this case they will remain permanently live.

We can ignore the second cable from the left - this appears to be a red & black, twin and earth that is providing the incoming power to L and N.

The white sheathed flex goes to the boiler and only uses:
Red - L
Blue - N
Earth - E
White - 3

I believe the red is a brown and the white is really a grey, but that isn't important.
The grey wire connects to terminal 4 on the LP241 - this is the CH on terminal.
As soon as CH is timed on the LP241, the boiler will fire, the thermostat is bypassed.

If the 2nd cable from the left (with the red, yellow and blue wires), is the cable to the thermostat, then the red wire (and hence the stat) receives power when H/W is turned on.
When the stat operates, the yellow wire is a switched live, and that connects to the grey wire on terminal 4; completing the circuit and firing the boiler.

The 3rd cable from the left is another red & black, twin and earth.
The red wire is connected to a permanent live, the black wire appears to be connected to HW on.
There does not appear to be a neutral connected to this cable, so it can't power anything like a pump, or a valve; although it could be something like a tank thermostat, or another switch.

So from what I can gather. If CH is turned on by the LP241, the boiler fires and remains on without any stat control.
If HW is turned on, but CH is off, the stat becomes operational, and turns the boiler on, when the stat calls for heat.
If CH and HW are both off, if the 'switch' (or whatever it is), on the third wire closes, then the black wire on terminal 3 would become live. This brings the stat into operation and turns the boiler on when the stat calls for heat.

I may be completely wrong in my interpretation, but I haven't seen anything like this before; it would be interesting to know what is connected to the third wire from the left!

 

[rickcj007]

Thanks, I agree it certainly seems to be a strange wiring configuration.

Yes, that red is in fact brown, I typed it just referring to live!

I'm not totally sure how the boiler fires when there is only one wire connected to 3 in the boiler and don't think anything was connected to 4?

I'm going to try to trace the 3rd set of wires to see what they are connected to.

Thanks for your insight!

I'll come back once I've checked the other wiring again.

 

[RandomGrinch]

I'm not totally sure how the boiler fires when there is only one wire connected to 3 in the boiler and don't think anything was connected to 4?

That's not something to be unsure about.
The boiler has mains switching. Terminal 4 will be a live supply, just as terminal 5 is the neutral supply to an external stat etc.
Terminal 3 is the live return to the boiler - a single switched live wire to this terminal would fire the boiler.

I'm going to try to trace the 3rd set of wires to see what they are connected to.

Let us know how you get on, I'm intrigued!

 

[rickcj007]

I checked the connections in the boiler and it connects to Terminal 4.

The 3rd wires go to a froststat and is just twin and earth.

I picked up the junction box today ready to install.

 

[RandomGrinch]

I checked the connections in the boiler and it connects to Terminal 4.

Ok, live return is on terminal 4 then. Still no problem!

The 3rd wires go to a froststat and is just twin and earth.

Thanks, that confirms my thoughts that it was only a 'switch' and not a pump or valve.

So to update...

If CH is turned on by the LP241, the boiler fires and remains on without any stat control.
If HW is turned on, but CH is off, the stat becomes operational, and turns the boiler on, when the stat calls for heat.
If CH and HW are both off, if the frost stat closes, then the black wire on terminal 3 would become live. This brings the stat into operation and turns the boiler on when the stat calls for heat.

In this configuration, setting the timing on the LP241 for hot water would control the heating, in conjunction with the room stat.
Turning the heating on overrides the thermostat.
With both heating and hot water off, the frost sat would fire the boiler, if the room stat was also calling for heat.

....a very odd set up!

 

[rickcj007]

Yes, it seems it's been cobbled together to make it work in a way that the boiler isn't quite intended to!

So if I were to remove the existing thermostat, froststat and LP241 and then install the junction box, I guess I would use the dry contacts to terminals 3 and 4 in the boiler?

There is an unused black cable in addition to the grey (and brown, blue and earth) that goes between the boiler and LP241.

The other boilers I have used this thermostat with have had to use the N/O connection to the S/L in them.

 

[RandomGrinch]

So if I were to remove the existing thermostat, froststat and LP241 and then install the junction box, I guess I would use the dry contacts to terminals 3 and 4 in the boiler?

Yes, that should work perfectly.

The other boilers I have used this thermostat with have had to use the N/O connection to the S/L in them.

Alternatively, you could use the N/O connection to be the S/L to terminal 4 of the boiler.

If we didn't know how your system worked, I would advise to use dry contacts, as boilers with two control terminals may have (extra) low voltage controls. Your choice of stat can also cause confusion, as it's primarily designed for underfloor heating systems.
As we know exactly how your system operates, either way should be fine.

You could also wire the frost stat in parallel with the thermostat, should you be concerned that there was a risk of freezing.