Boiler wiring

[fattony]

Hi

I'm came home from work to find my boiler and the junction box in the airing cupboard with no power, the previous home owners moved the boiler from the kitchen to the loft and I suspect there is a junction box somewhere behind the kitchen tiles, the existing wiring is a mess, there is a 3 core (old colours) from the boiler to the junction box but harmonised colours in the boiler

Because of this I'm possibly looking at adding a spur in the loft which supplies the boiler and the junction box

The boiler is a worcester greenstar 28i, I had a look at the boiler wiring diagram which shows a switch live and neutral from the boiler to the junction box in the airing cupboard (connected to Ns & Ls) but looking at the diagram these are not required, currently I have a brown in Lr which feeds the mid position valve and it changes to yellow somewhere but nothing in Ns or Ls, is this normal?

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• fattony
• 28 Feb 2015
• 1

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[Adam_151]

The boiler expects the controls to be fed from Ls and Ns and to receive a live back on Lr to fire it up. While sometimes you do see a live fed back in that has not originated from Ls, this is not ideal as this will will not be running through the internal fusing of the boiler (which is likely to be faster acting than the 3A in the spur).

 

[fattony]

Thank Adam, it's been a while hope your well

 

[fattony]

Seems I have some wiring to do tomorrow

 

[Adam_151]

Depending on what the existing situation is. One get out of jail free card is to put Ls and Lr onto the normally open contacts of a 240v relay locally to the boiler. Leave Ns empty. Your controls can then be fed directly off the spur and switch the relay to call for heat.

 

[fattony]

To be fair it's pretty bad, cables purposely disconnected in the junction box still stripped back, the brown in Lr has the cpc just bare copper in the boiler not connected, I have a cpc sleeved brown in the junction box, list is never ending

 

[ericmark]

Now I am baffled. You have a combi boiler that means it heats the hot water inside the boiler and you don't have a cistern but then you show a S plan wiring diagram which does use stored water in a cistern.

The S Plan uses a room thermostat to control a motorised valve but does not monitor the return water temperature but any condensate boiler depends on the return water temperature and they are completely different systems as a result.

There has been a move away from the combi boiler as these don't lend themselves to alternative power. Having a water storage system allows solar, solid fuel and gas all to share in the heating.

The condensate combi boiler however has everything built into the boiler from pump to the brain which controls flame hight.

The system today is something often a thermostatic radiator valve switches radiators on and off as and when heat in that room is required. Often there is an external bypass valve which auto opens when all radiators are switch off and the boiler senses the return water temperature to adjust the flame hight. It even has anti-cycle software built in.

It does have connections for a time clock but it does not really require a electric room thermostat where they are used it's an add on which fits instead of the standard head on the TRV. Some times people do use clock/thermostat combination to act as a frost stat.

There is nothing to stop using zone valve instead of TRV with a room thermostat but there is no electrical connection between zone valves and boiler the latter is controlled by return water temperature.

The idea is the warmer the return water the lower flame hight until it reaches a point where it can go no lower at which point boiler switches off. A timer in the boiler will after a time switch it back on again. It then again measures return water temperature if high it increases time between tries and if low decreases time between tries so everything is controlled by return water temperature.

Trying to use a modern boiler on an old system i.e. no TRV's fitted is likely to fail as the warm return water will shut down boiler before all rooms are warm.

It is all down to energy saving the system uses the latent heat from the flue gases and to do that the return water must be cool.

So something seems wrong here.