Advice on Y-Plan Heating System Issues

Discussion in 'Plumbers' Talk' started by hogman, Apr 7, 2021.

  1. hogman

    hogman New Member

    Hi All, looking for a bit of advice. I've just moved in to a new house a few weeks ago and have noticed that the central heating is coming when only the hot water is called. I only noticed this a few days ago after we had the hot weather, since I had the heating turned off but hot water still on - I have an older honeywell programmer that can only schedule heating and hot water to be called at the same time.

    I initially thought the issue could be with the motorised valve, as it seems to be stuck in an open position (there is no resistance at all on the manual lever, and I can feel the pipes heat up to both A and B feeds after hot water is called) - I replaced the actuator however there still seems to be issues.

    I've now noticed that the boiler doesn't even fire up when just central heating is called, and still have heating and hot water when just the hot water is called. My next thought is that this could be a wiring issue? Any advice would be appreciated.
  2. kiaora

    kiaora Guest

    With a 3 port valve, there are two types, 4 wires and 5 wires,

    if you have 4 wires there no mid position,
    If 5 wires, that’s normally how they work now,

    4 wire is hot water priority,
    5 wire, is c/h or h/w on demand either call.

    it may be wiring problem, or faulty programmer,
    A picture of the wiring center may help ?

    good luck
  3. The Happy Builder

    The Happy Builder Screwfix Select

    It sounds like a gravity hot water system and you can only have the central heating on with the hot water.

    Roughly how old is the heating system?
  4. Vin

    Vin Active Member

    Have you checked the valve itself isn’t seized?
    If it is then it could be stopping the actuator from travelling which in turn would stop the micro switch from operating and theeefore the boiler won’t light.
  5. BillyBobToo

    BillyBobToo Active Member

    If it was a gravity hw system it wouldn't have a 3 port valve.
  6. sam spade

    sam spade Active Member

    What is the model number of the Honeywell Programmer?
  7. hogman

    hogman New Member

    Hi Guys, sorry I missed these replies - new to the forum!

    The 3 way valve is a Honeywell V4073A1039/U - I replaced the actuator with exactly the same model when I though that was the problem, its 5 wires. I've tried to get a good picture of the wiring centre buts its an absolute rats nest and not easy to see what goes were on camera, is there any common wiring mishaps that I should check for?

    The valve body itself doesn't seem to be seized, as when I took the actuator off to replace it I was able to change the position freely with pliers - I was hoping it wasn't the valve body as really don't fancy draining the whole system!

    The programmer is a HONEYWELL HOME ST699 - it might be worth noting also that there is no room stat in this house, and it looks like the previous owners have just put TRVs on all the rads.

    Thanks all for your help so far on this
  8. kiaora

    kiaora Guest

    Check, port A to heating, port B hot water,

    hot water only, no valve action,

    the wiring must have a cylinder stat with a live when cylinder stat is satisfied, and the programmer must have a live when no hot water call, both wires connect to the white wire from the 3 port valve, check to see if that is the case,


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