hot water turns on the end of cycle
Hi, what could be a solution here?
Issue: in the hot water schedule the water is heated normally. When time reaches end of schedule, the hot water turns on again for a few seconds. Why? What could be the solution here?
Example: schedule is between 9:30-10:00 PM. Boiler starts up 9:30 and heats until like 9:42, when the requested temperature is reached. Boiler stops heating. Boiler starts heating at exactly 10:00 PM for ~5 seconds and stops again.
Hardware: V3+ (bridge, receiver, wireless thermostat, Baxi duo-tec compact E1.24)
Comments
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@soft_fins Hello, welcome. The fault description is a strange operation. The first question in my mind is whether the lights on the Tado wireless receiver show the HW calling for heat at 10:00pm or not? Second, is this a combi boiler or system boiler and S-plan controls or ? Is this a relay connection or ?
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This is presumably a system boiler as you have programmed DHW (combo being demand).
It might be pump overrun, in which case, it's normal. The boiler is dissipating the heat from the exchanger after running at full tilt to heat water. However, if it's always at exactly 10pm and always for exactly 5 seconds, that may not be the case, as I'd expect the boiler to switch on overrun depending on current conditions, which will vary from day to day - some days it takes longer to heat the cylinder, so the exchanger gets hotter and therefore overrun may be several times at random intervals. Ours kicks in whenever the exchanger exceeds 80C and runs the water around the primaries until it drops back below that level. If it creeps up again, it runs again.
Did this only start after installing tado?
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Hi, thanks for the replies.
@wateroakley: No, no change in the lights on the Tado receiver. It is an S-plan. Tado is connected with OT, not relay.
@DM932187: No, it is not a pump overrun, because the fire is also turning on for 5 sec and real water heating happens. I noticed after some months after installing Tado but I think it worked like this from the beginning. Before that I used Baxi thermostat and also a 3rd party relay stuff. None of those produced this behavior.
I think the following happens: at 10 PM Tado checks if the boiler is still running and if it is then it instructs it to stop the HW. But when it isn't - which is the most case - then this checking somehow triggers the HW production. Or the OT connection at this point stops for a few seconds as if I removed the OT cables and the boiler becomes temporarily factory 'dumb' and starts producing water according to its factory behavior. Tado guys said for my ticket that there is no issues found on their side.
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@soft_fins I'm not familiar with the OT wiring set up as my boiler has WC so I'm on S plan with relay. How does the DHW work in your set up - relay with cylinder stat or is tado controlling the water temp? If the former, I'd expect the call for DHW to simply be on (valve energised) for the half hour and then the cylinder stat to cut that call to the boiler when satisfied (so no tado 'check' at 10pm - just no more power to the valve). So from what you say, somewhere in your set up, you get a new call for DHW at 10pm and this was possibly pre-tado? What was your pre-tado set up?
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@DM932187 I'm not sure if my setup falls into your categories: I have a 130l external dhw tank with water temp sensor connected to the boiler. So the boiler can see the temp always. However Tado cannot - at least nowhere in the app is this data visible. Tado schedules only the 'open windows' and the target dhw value. The boiler pays attention on dhw temp if the hot water window is active. But I think Tado cannot read from the boiler if the temp is reached and even if the boiler is on or off for hot water production. Those functions are visible for heating.
The faulty behavior wasn't present before when I used a Baxi thermostat.
Yes, you're right, something triggers the hot water - or, the connection is lost for a short time, which also causes dhw production.
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@soft_fins - sorry, not checked in for a while. From your description, your boiler temp sensor sounds to be a more up to date version of the simple cylinder stat we have, but otherwise, the principle appears the same, in which case, you'd be correct that tado cannot see the DHW temp. Tado says 'hot water time', but the caveat for DHW production is whether the tank is already at temp or not. If it's not, the boiler fires for DHW, but if the target temp is already attained (and stays that way through the timed DHW window), the boiler won't fire for DHW (and should just keep running the CH instead, if the CH is on).
So that said, something else is triggering the DHW call at 10pm. As I said, I've got S plan running relay rather than OT because my Viessmann boiler has WC, and I'm wondering if the OT approach is indeed somehow the issue here.
As you have an S-plan, you're going to have the 2 separate DHW and CH 2 way valves, which need energising whenever there's a call for DHW and CH (per the Tado schedules), regardless of OT or relay. If you have OT, however, you have tado telling the boiler how to modulate the CH (and not the DHW). So (not being familiar with the OT wiring), I'm going to assume that logically, Tado is doing this during the CH window, and by so doing, should be holding the CH at the set temperature, rather than Turing the CH on/off/on/off (via relay).
So my first point of investigation, would be whether in any way your installation of Tado is interfering with the standard DHW S-plan set up (which, unless Tado is controlling the DHW temperature, which you've said it isn't), should logically be working as relay, shouldn't it? In your case, I'm guessing it's the boiler that energises the DHW valve?
As I have one lying around for just these things, I'd wire the DHW to a simple programmer and disconnect it from tado, to see what happens. If that did seem to stop this unusual behaviour, I'd then be looking closely at my tado DHW wiring in particular. My guess (and it is only that), is that your wiring might be slightly off on the DHW side. Specifically, I'm wondering if you have Tado DHW set up as if it Tado can read the DHW temperature, when in fact it can't, and that somehow, at the end of the DHW schedule, Tado can 'fire' up DHW, just for a last check.
All supposition, and probably sounds somewhat implausible, but hopefully that might get some cogs spinning and help you resolve it - or of course you may have done so already. Good luck!
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@DM932187, thanks for the reply. In the meanwhile Tado investigated the issue and discovered that there is a bug on their side: when the DHW window ends, the boiler is instructed for 10s to heat up to the default boiler value, which is 55 C for me. And because my DHW is always set around 40ish degrees, this makes the boiler to start again. They said that they will fix this, but it did not happen yet.
Another workaround would be to change the 55 default value to some lover one (like 30 C), but I could not do it as the OT disables changing such values. If I remove the Tado connections then the boiler starts making connection errors. Somehow the boiler should be set back to the pre-OT settings, but I don't know how to do that.
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