Boiler doesn't fire after simultaneous demand for heating and hot water

Let me explain the setup I have and what's happening.

We have a Vaillant EcoTec boiler and a hot water cylinder. There's a Honeywell controller with a valve that controls whether the hot water from the boiler is running through the rads, the cylinder or both.

So, all our hot water is pre-heated and stored in the cylinder.

When the boiler fires a radiator symbol on the display is shown (which is when heat is being called for I guess).

Our heating runs on a schedule which just varies the temperature a bit (lower overnight, higher during the day). The hot water is set on a fixed schedule in the early morning for an hour or two and again in the evening.

The problem we have and this has been an issue pretty much since the original Tado was installed is as follows.

If Tado is calling for heating and hot water at the same time, once the scheduled block for the hot water finishes, the boiler no longer fires for "just" the heating. It remains off. To get the heating to come back on again I have to hard power cycle the Tado boiler unit (which also resets the boiler and Honeywell controller as well). When I do this, after a couple of seconds I hear the relay in the boiler extension unit "click", the radiator symbol appears on the boiler and it fires up. Power cycling the boiler by itself makes no difference which suggests something in the Tado or the controller or perhaps the wiring between the two?

At this time of the year when the boiler is running most of the time this is now becoming a royal pain. Any suggestions on how to trouble shoot this and who could diagnose it for me?


  • i presume this is a Y plan setup. Could be the valve.

    Someone with a multimeter could troubleshoot it. Most of it will be mains level voltage.

  • anthonyodoherty
    edited February 13
    It is I believe a so called Y plan setup.

    The difficulty I have is recreating the issue "on demand".

    I will try scheduling a water heating block for during the day to see if the same thing happens.

    We have a HomeCare agreement in place so it will be interesting to see if British Gas could troubleshoot this. Anybody know how good Tado's installer helpline is these days?
  • Can’t you just boost the HW for an hour?

  • GrilledCheese2
    GrilledCheese2 ✭✭✭
    edited February 13

    I agree with Montage that it is most likely a faulty power head on the 3 port valve. When you power cycle the controls the valve will always return to its default mode which is hot water only. Very easy for BG to diagnose by using a multi meter to check the voltages on the internal switches.

  • Ahh okay. Right now I'm trying to see if I can reproduce the issue reliably during the day since I want to be sure I can demonstrate the issue to an engineer when they come out.
  • Thing is, the 3 port valve head was replaced not that long ago yet the issue persisted even after that which makes me think the issue could be elsewhere, but I'll let British Gas be the judge of that.
  • A question regarding the Tado boiler extension unit.

    When the Tado issues a demand for Heating or HW, I hear a relay click and then presumably after the valve head has motored to the correct position, the rad symbol appears and the boiler fires.

    Are there separate relays for Heating and HW? EG if I am only running heating and engage HW as well, would I expect to hear another relay click in the Tado unit?
  • GrilledCheese2
    GrilledCheese2 ✭✭✭
    edited February 16

    There are two relays in the extension kit. Normally they work independently and you can hear each one clicking.

    There is an old system in the UK referred to as gravity-fed HW and pumped heating, where the CH cannot be turned on without the HW being on. If Tado is configured for this system then the relays do not work independently.

    If a plumber is unaware that you have a problem with the CH operation they will simply replace the power head by substituting the new wires in the same terminal as the old wires. E.g. the old grey wire is just replaced by the new grey wire. Meaning a potential wiring problem is preserved.

  • Interestingly enough, I've only ever heard one relay click from the Tado. Our HW is gravity fed from the cylinder so I'm rather wondering if this means that the wiring for our Tado setup has been duff since day one. The original Tado setup had this issue and when it was replaced with a version 2 setup, the issue appears to remain.

    When BG come on site, is there an installer hotline that they can use to speak with Tado should the need arise?

  • Your secondary circuit (water supply to taps) may be gravity fed, but don’t confuse that with the primary circuit when describing your system to Tado.

    If the boiler has a gravity fed HW primary circuit it will have 4 pipes. Two for CH and two for HW. The CH primary circuit is pumped, but the HW primary circuit relies on gravity to circulate water through the hot water cylinder. No longer installed and rarely seen.

    When a boiler is fully pumped the primary circuit for HW and CH are both pumped and a motorised valve will allow the CH and HW circuits to be independently controlled. This setup is now the norm.

    A fully pumped heating system can still have a gravity fed secondary circuit for the HW. This is where the water coming out of the taps is pushed out by a high level water tank in the loft.

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