Hot water getting stuck on

I've been using Tado for several years, and have recently noticed that my hot water is getting stuck on (for hours/days) despite it showing as "Off" in Tado.

I have a regular ("heat-only") boiler, unvented hot water cylinder, Tado Smart Thermostat, Tado Wireless Receiver (Extension Kit) and Tado TRVs.

It's an intermittent issue, and turning the heating system off and then back on at the mains seemed to temporarily resolve it, but then it would get stuck on again a few days later.

I initially thought it could be the relay in the Extension Kit getting stuck, so I got a replacement (Wireless Receiver) but this hasn't solved the issue.

I have temperature sensors in my hot water cylinder and on the flow and return pipes, so I can easily see when the cylinder is being heated when it shouldn't be.

I've asked Tado support, but they're not being very helpful, talking about other smart devices and bridge placement. But as the hot water light on the Wireless Receiver is off, when the cylinder is heating, it surely can't be a Tado wireless issue.

So is this almost certainly a fault with my boiler (5 years old)? Or could a fault with another Tado device be causing this?

Comments

  • davidlyall
    davidlyall ✭✭✭

    Is it an S plan system? If so, it could be the motorised valve for the HW tank that is sticking and keeping the system running

    If it's Y plan (I think that's right) then they 3-way valve may have a fault.

    Given that you've replaced the extension kit with a wireless received and the problem continues, I think it's safe to say it's not with the Tado side of the system

  • jcwacky
    jcwacky ✭✭✭
    Thanks @davidlyall

    It’s a Y-plan system.

    If the 3-way valve was faulty could it really keep the boiler running? I’d assumed it wouldn’t be able to.
  • GrilledCheese2
    GrilledCheese2 ✭✭✭

    @jcwacky When you say the hot water is stuck on, is the boiler actually running all the time? When the hot water is up to temperature the cylinder thermostat should switch off and cancel the call for heat. Under normal circumstances it shouldn't be possible to heat the cylinder beyond the set temperature - potentially dangerous.

    The way a Y-Plan system works is the extension kit relay provides power to the cylinder thermostat and the boiler is only switched on when the cylinder thermostat is closed (call for heat). When the extension kit relay is off the cylinder thermostat will be unpowered and unable to switch the boiler on.

    It is possible for a faulty 3-port valve to divert hot water to the cylinder when it shouldn't but this will only happen when the CH is switched on. A schedule for HW only will not energise the 3-port valve, because the valves default position is to open the HW circuit.

  • jcwacky
    jcwacky ✭✭✭

    @GrilledCheese2 Correct. It just heats to the cylinder thermostat set point, e.g. 60ºC but then keeps constantly kicking in every hour or so to keep it at this temperature, even when "off" in Tado. (Again, this only happens intermittently, every few days).

    Thanks for confirming how I understood it all works. So does that mean the fault is most likely to lie with the boiler itself? Otherwise, I don't understand how the boiler can be "on" when both Extension Kit relays are "off".

  • GrilledCheese2
    GrilledCheese2 ✭✭✭
    edited May 4

    Very unlikely to be the boiler - it's heat only and will be unaware if the CH or HW is calling for heat.

    I've just re-read your original post and realise you have an unvented cylinder, which means you have an additional motorised valve on the cylinder. It's a 2-port valve that gives additional protection against overheat. When you have the fault condition (boiler running, CH off, HW off) is this valve allowing hot water to pass through the coil in the HW cylinder? The micro switch in 2-port valves can stick in the closed position. Giving a condition where the valve is closed, but the micro switch erroneously switches on the boiler. Not easy to tell when a valve is closed because heat will be conducted to the adjacent pipework by the hot cylinder. But if that valve is closed when the fault condition occurs then it's very likely to be a faulty micro switch. Otherwise not sure what to suggest other than getting an electrician to check the wiring.

  • jcwacky
    jcwacky ✭✭✭

    Ah thanks, I'd assumed that the cylinder wouldn't be getting any power at all if extension kit HW was off, but if that's not the case then will look into the cylinder kit further.

    It's a Heatrae Sadia PremierPlus 210L that is about 5 years old. Boiler is the same age. The 3-port valve (Drayton BGMVSP23) is older.

    I'm happy enough using a multi-meter, just don't fully have my head around what should be live when with the system.

    It's so intermittent, i.e. occurs every few days, that I'm hesitant to get someone in to investigate if there's a chance the issue won't occur while they're here and so they fail to find anything. So I'm keen to narrow it down as much as I can myself first.

  • jcwacky
    jcwacky ✭✭✭

    Interestingly, it seems I don't have the additional 2-port value. (I'll raise this with my heating engineers who installed the unvented cylinder).

    However, after your suggestion, @GrilledCheese2, I just gave the 3-port valve a few taps, and I think I then heard a click come from it and the heating system then shut off a couple of mins later. So we may be on to something! I will try this again when the issue next reoccurs and if the same result will get the 3-port valve replaced.

  • davidlyall
    davidlyall ✭✭✭

    3 port valve will have microswitches to tell the pump and boiler to run when in position. Sounds like the HW microswitch is sticking so probably replacement of the 3 port valve or maybe just the motor head is the best option.

    You mention it's older than the 5 year old boiler and cylinder so could just be at the end of it's life.

    In the 14 years since my system was installed, I've had to change both motorised valves. First one would have been when the it was around 4 years old then the second at around 12 years

  • jcwacky
    jcwacky ✭✭✭

    Thanks, @davidlyall. That's very useful info.

    I'm now 100% sure that it is a faulty micro switch in the 3 port value. The HW got stuck on again, so I gave the 3 port valve a hard tap, and definitely then heard a little "click" come from inside, which was presumably the stuck micro switch becoming unstuck, and sure enough 3 minutes later (pump overrun), the pump and boiler switched off.

    I'll get the valve replaced. Thanks @davidlyall and @GrilledCheese2!