Understanding wiring for wet underfloor heating?

[davesparks]

Thanks for that. I've only dealt with system boilers! Presumably Lsupply and Lreturn.

I think that is the logic, but it's not all manufacturers, the labelling of those terminals is arbitrary and could be anything.

 

[HappyHippyDad]

I now have a picture of the boiler wiring. The boiler is a vaillant vuw 306/6-3 (H-GB) ecoFIT pure 830.

The boiler has a L,N,E and RT terminal. The RT terminal stands for room thermostat.

Do I just need the LR in the wiring centre connected to RT in the boiler? This doesn't feel right as I'm completely ignoring the LS in the wiring centre. What exactly is the role of LS and LR? I realise this would also cause the S/L in the thermostat to become live. Both the boiler and wiring centre would be fed from the same FCU. It will also be a lot neater upon completion!

edit.... I've been thinking a bit more.
I think I need (as @Avo Mk8 said) a permanent live (PL) going to LS (this can just be a link in the wiring centre from the PL, or I can take it from the L in the boiler). Then a conductor between LR in the wiring centre and RT in the boiler?

 

[diditrain]

The RT terminal in the boiler is the switch return live.

 

[davesparks]

Do I just need the LR in the wiring centre connected to RT in the boiler?

Yes Lr will connect to RT at the boiler, you'll need to connect Ls to a permanent live.

. What exactly is the role of LS and LR?

Ls and Lr are just the normally open contacts of a relay which is energised to call for heat from the boiler or other source of heat.
In your case you need a simple 230V switched live at the boiler so you need only to give Ls a permanent live and connect Lr to the boiler.
If your boiler had a 24V ELV control connection you would connect 24V from the boiler to Ls and the Lr would become your 24V switched live to the boiler.

I realise this would also cause the S/L in the thermostat to become live.

The room stat needs to be disconnected from the boiler switched live terminal otherwise you'll have problems and the whole system will be uncontrollable.

There needs to be a motorised valve installed onto the flow pipe to the existing heating, which it looks like there is from your last picture.
From the wiring diagrams you posted it shows that you can connect the valve and existing thermostat into the heatmiser box.

 

[diditrain]

The RT terminal in the boiler is the switch return live.

You should be good linking permanent live to LS in wiring centre or take it from boiler or double up from your fcu. Your choice. If you’re unsure, just remove the plug from the boiler and do all your testing first, before reconnecting it to the boiler. At least if it goes bang, you’re not blowing the boiler pcb.

 

[HappyHippyDad]

Yes Lr will connect to RT at the boiler, you'll need to connect Ls to a permanent live.


Ls and Lr are just the normally open contacts of a relay which is energised to call for heat from the boiler or other source of heat.
In your case you need a simple 230V switched live at the boiler so you need only to give Ls a permanent live and connect Lr to the boiler.
If your boiler had a 24V ELV control connection you would connect 24V from the boiler to Ls and the Lr would become your 24V switched live to the boiler.





The room stat needs to be disconnected from the boiler switched live terminal otherwise you'll have problems and the whole system will be uncontrollable.

There needs to be a motorised valve installed onto the flow pipe to the existing heating, which it looks like there is from your last picture.
From the wiring diagrams you posted it shows that you can connect the valve and existing thermostat into the heatmiser box.

Many thanks Dave.

With regards your last point, I'd like to understand how it will cause problems. At present the thermostat is purely for the radiators which the plumber is planning to keep as it is, i.e. not being part of the heatmiser box.

If, on the one hand the thermostat calls for heat, this will cause LR to become 230V. I cant see how this is a problem?
If, on the other hand the heatmiser unit calls for heat, I.e. the switch closes between LR and LS, then the S/L terminal of the thermostat becomes live .....

Oh... I think I've just seen your point. Will this now turn the radiators on?

 

[Avo Mk8]

Oh... I think I've just seen your point. Will this now turn the radiators on?

Yes!

 

[davesparks]

Oh... I think I've just seen your point. Will this now turn the radiators on?

Yes the radiators will come on as long as the underfloor heating is calling for heat. And as underfloor heating takes a long time to get up to temperature it will be getting pretty toasty in the rooms with radiators!

 

[HappyHippyDad]

Yes!

But if simply livening up RT causes the radiators to come on, then it will do this anyway even if the conductor from the stat to RT is not there. This means there must be other changes that are required?

 

[davesparks]

But if simply livening up RT causes the radiators to come on, then it will do this anyway even if the conductor from the stat to RT is not there. This means there must be other changes that are required?

Yes, as I said above a motorised valve is required on the flow pipe to the radiators which there appears to be already installed in the picture.

If I have understood the photo correctly the pipes with the blue and red lever valves will be the flow and return to the UFH and the, electrically unconnected, motorised valve above the red valve will be in the flow to the radiators.