Central heating problem

Discussion in 'Electricians' Talk' started by ejenner, Sep 26, 2021.

  1. ejenner

    ejenner Member

    Hi everybody.

    Got a strange problem with our central heating that I'd like to get sorted before the winter. I'm posting in 'Electrical' because I believe it's a wiring problem rather than component failure or anything to do with the heating pipework.

    The issue is the hot water cannot be turned on without the heating. It can work the other way around though. i.e. you can turn on the heating by itself without the hot water.

    The obvious issue here is that you might want only hot water when the house doesn't have to be heated for whatever reason.

    Something which makes me suspicious about the wiring is that the H/W and C/H sides on the programmer have been reversed. You can see somebody has written on the cover to show the controls have changed sides! That's no good!


    I've taken off the junction box cover and traced all the wires. I've made a wiring diagram to try and help me understand the setup.

    heating wiring diagram.jpg

    My suspicion is the little jumper wire connected along the top in the junction box linking terminals 5, 6 and 8 is a potential problem. It has the effect of connecting the C/H thermostat to pin 3 and pin 4 on the 3-way valve and probably joining 3 & 4 on the valve so they effectively do the same thing.

    Anyone at a sufficient level of understanding of these systems to see any problems?

    I've got a diagram from the manufacturers website which looks as close as I can see to what I should have. We have the two-wire room thermostat and no tank thermostat. manufacturers diagram.jpg
  2. Mike83

    Mike83 Screwfix Select

    Can’t see it working correctly without a cylinder stat on a y plan.
    Would work on a s plan without a cylinder stat.
    Programmer also been wired incorrectly.
    Probably best stripping it out, adding a cylinder stat and rewiring it.
    Should be easy enough with all wires identified.
    If it’s already neat and tidy then it would just be swapping some wires over and adding in the stat.
  3. ejenner

    ejenner Member

    Thanks for that. I can't see any 'S' plan diagrams which include a mid-point valve like I have in my setup.

    So have we worked out what the issue is? i.e. if there's no tank thermostat it's not going to work properly? They seem to have wired it up as 'Y' plan.

    My tank is a bit difficult to get to (aren't they always!) but as a novice is the thermostat easy to see on the tank? I'm assuming it'll have one but they've not hooked it up.
  4. ejenner

    ejenner Member

    Thermostat actually has more than 2 terminals on it... but only 2 are connected.

  5. Lectrician

    Lectrician Screwfix Select

    You need a cylinder stat.
    Your room thermostat won't be accurate as the neutral is not connected for the accelerator resistor.
    Programmer is incorrectly wired.
    I would be ripping out and starting again.
    nigel willson likes this.
  6. sparko69

    sparko69 Screwfix Select

    The valve is not wired correctly and you need someone to install another valve so you have a separate valve for the heating and water. A Y plan needs a 3 port valve and an S plan needs a 2 port.
  7. Lectrician

    Lectrician Screwfix Select

    He has a 3 port valve. Looks correctly wired to me.
  8. ejenner

    ejenner Member

    Don't think I'll be doing any 'ripping out' as it's a reasonably neat job... just a bit wrong apparently!

    So I should be looking on my tank for a thermostat and the room thermostat should be rewired so all three terminals are connected as per Y-plan shown above?

    It really puzzles me that whoever installed this in the first place appears to have made it up as they went along and it appears they managed to get to the end of the job and walk out of here. (under the stewardship of the previous owners of my house I must add).
  9. The Happy Builder

    The Happy Builder Screwfix Select

    The best way forward is to disconnect everything, then reassemble it having identified all the cables and components installing any that are missing.

    Otherwise you will waste a hell of a lot of time.
  10. ejenner

    ejenner Member

    The more I find out about this installation the closer I am to this view of things. As I said above initially I consider it neatly done and don't want to unnecessarily rip... however, the more I dig into it the more I find wrong and the closer I'm getting to this view.

    Honeywell do an alternative junction box which they call a 'wiring centre'. Having looked at that I think it might be the way to go. It labels all the terminals so you know exactly which wire to connect where.
  11. ejenner

    ejenner Member

    I mentioned at the beginning that the red wires jumping the terminals along the top of the junction box seemed wrong to me.

    If you look at this chart it says for hot water only the orange wire (pink on my diagram) should be live but not the others.

    Due to terminals 5 & 8 being linked it would be impossible for the orange (pink) wire to be live by itself as it is jumped to the white wire!

    Sundial Y Plan.jpg
  12. ejenner

    ejenner Member

    I think they might have jumped the terminals to compensate for the lack of a hot water cylinder thermostat!
  13. ejenner

    ejenner Member

    Little lunchtime excursion to Wolsleys and picked up these two. So something for me to do this evening after work!

    Thanks all for your help, can't really see how I can go wrong from here.

  14. MGW

    MGW Screwfix Select

    The idea is the TRV controls room temperature, the wall thermostat is only there to switch off heating in the summer, and the large hysteresis is actually good when using a modulating boiler.

    I would guess one of the micro switches in the three port valve has stuck, and you will need to swap head or whole valve.

    The TRV control system has a problem, it can't turn the boiler fully off, it can cause the boiler to first modulate and then cycle off/on, but not fully off, so the wall thermostat is required to turn off the boiler on warm days.

    I would agree that the wax thermostat head is not very easy to set, having *123456 is not very helpful, personally I have eQ-3 and Energenie TRV heads in my house, but the wax heads do work if you take the time to set them.
  15. The Happy Builder

    The Happy Builder Screwfix Select

    The room thermostat is required to prevent the boiler cycling and wasting fuel thus reducing your bill.
  16. ejenner

    ejenner Member

    yeah, the room thermostat has been rewired with all the required connections so should work as intended.

    The boiler was originally connected with 4 wires but the wiring centre has space for 5 wires. At the moment I have L, N, E connected and then 'PL' for Pump Live. There is a space for 'SL' Switched Live but no wire to connect to it. Bearing in mind how many compromises were previously baked into this setup I'm going to read the details on the boiler and find out if it has the SL function.

    I doubt there is anything wrong with the 3-way valve but if the system doesn't work when I'm finished installing everything I'll double-check the valve is operational.
    Last edited: Sep 28, 2021
  17. ejenner

    ejenner Member

    This is how things currently stand. I'm yet to install the tank thermostat so you can see the space for it on the top left is currently unoccupied. Also I've wired the 3-way-valve to the wrong terminal so I'll have to move that to the other side.

    When those two connections have been finalised I can cut some of those white bars to set the box up for the Y-Plan. I'll have to read up on that as it's not immediately obvious which ones have to be cut.

    new heating wiring.jpg
  18. The Happy Builder

    The Happy Builder Screwfix Select

    Do you mean check if the boiler supplies and controls the pump?
  19. ejenner

    ejenner Member

    The problem with the original installation was the whole thing was wrong or half-measures. Accordingly with there being two types of boiler supported in Y-Plan typically our existing setup was somewhere between the two.

    The options are either:

    1. Earth, Neutral, Live


    2. Earth, Neutral, Live, Pump Live (PL) and Switched Live (SL)

    Number one configuration has 3 wires. Number 2 configuration has 5 wires. Our boiler was originally connected with 4 wires. So that makes it neither of the two supported configurations but closer to a 5 wire setup.

    I am expecting when I dig deeper into boiler type that it will be a 5 wire boiler.

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