V3+ temperature overshoot

DM932187

First off, apologies if I’m asking a very often asked question here, but I couldn’t find an answer searching the forum.

I have a heat only, weather compensating viessmann boiler set up in S plan. It’s controlled by v3+ stats on relay.

I’m having a ‘fun’ ongoing conversation with apparently the sole member of Tado ‘support’. You probably know the drill. I raise a question. Wait a day. Get a question in response. Answer said question. Wait a day. And so this merry dance continues.

I have requested that my stats stop calling for heat when they just exceed set temperature. Currently they plough on for another 1C which rather defeats their purpose and with weather compensation on the boiler, usually means it almost never switches off. However, when it does, I have requested a minimum burn time (say 10 mins) because frankly, anything less with weather compensation doesn’t tend to get effective heat into the system. So relay ‘modulation’ is just a nonsense for my system.

Now these two calibration requests seem to be beyond the capabilities of my ‘friend’ at Tado, so I’m wondering if anyone here has managed to address either of these issues, or whether it’s time for me to ditch Tado entirely, and just replace it with a simple stat that actually just calls for heat when it’s cold and stops when it’s hot?

Thanks all.

Montage

I doubt you find much on here, as it’s not that sort of forum.

I’d suggest you don’t need a thermostat. Just run full WC. Once you have the curves sorted you are done.

DM932187

Thanks @Montage. Unfortunately, it's the older Viessmann basic weather comp with a fixed curve that you can move up and down, but not alter the slope, so it's not quite exact. It does work pretty well but I've found it's necessary to cap off with some internal control or it just keeps going (even with an array of (dumb) TRVs). House is also pretty old and leaky, so it's expensive to run 24/7.

I've used tado since beta and it's been ok but I recently had to ditch the v1 for the v3 and that's when the problems began as the v1 was far more on/off (and in my view, therefore better suited to relay).

Whilst I have a single heating zone (best for weather comp), I do have a few stats operating wirelessly around the house, just to keep an eye on any parts that colder. This has also improved comfort.

Nonetheless, relentlessly heating past target, and the occasional short and pointless burst from the boiler are both quite problematic for me in terms of system efficiency.

Maybe I just need to rethink how I set up the stats - i.e. think of them as indoor feedback sensors, albeit not affecting the boiler modulation from the WC.

Although I still think a few simple on/off relay stats operating as limiters might be preferable, if the tado guy(s) can't dumb down my kit to suit. Problem is, who makes those?

Montage

Ok, understood. I’m fairly sure Tado can turn off the unhelpful cleverness. In terms of others that avoid TPI, I think you can turn it off on some ESI stats.

Shame about the WC control. I presume it works ok, but leaves you cranking it up or down to match the time of year. Without building some automation, I can’t really see a way around it.

Does your boiler support an upper (outside temp) threshold to switch off? Mine’s set to got to sleep once it sees 20 outside. It’s circulating water at anything below that. Plus I have a setback for night temp.

DM932187

Thanks @Montage - No, no warm weather shutdown functionality on this one. Viessmann seem to last forever, and this one is 16 years old and still going very strong. I used to use IFTTT to link Weather Underground. It wasn't very reliable, but better than nothing. But then tado removed IFTTT. Since then, I've been looking for an indoor 'cold spot' location for a tado stat (i.e. if it's warm enough there, it's warm enough everywhere). We have an unheated eaves landing that's not far off, but unfortunately, that's not reliable enough, so I've needed to pick up a few extra tado stats on the cheap off eBay to create multi-point reference. That kept the place comfortable over the winter, but with tado's algorithms, everything often overshoots. Oh, and no boiler night set-back either, so have to turn down on the stats at night, so off they go again to attempt to reach (and overshoot) target temperature the next morning.

As for tado turning off the unhelpful cleverness, I seem to be unable to get through the unhelpful cleverness of the support desk, but I'll raise that question directly with them and see if I get anywhere. I suspect they'll just tell me that's not how tado works. I'll also look into ESI stats.

Thanks again for your thoughts.

Montage

Assuming you are starting with a clean system and balanced rads, I can see the steps to form a consensus on need for heat are interesting and then how to exert some control over the boiler.

Good to hear the boiler is working well after all these years. I do like the engineering involved. Too bad you can’t retrofit some of the features of the later boilers, as most of it is done in software.

DM932187

Thanks again @Montage. Tado support agent has apparently finally recognised the limits their knowledge and passed me on to someone who may know more. Could be a fob-off, but you never know, they might actually help eventually. I'll update if they do finally sort it, as I know overshoot is a common issue (albeit it very surprising, even with basic weather comp), and others may benefit.

DM932187

An update for users similarly suffering overshoot (presumably, on whichever tado version).

Eventually, I managed to reach a tado engineer, who offered the following solution after some to-ing and fro-ing:

We understand the importance of optimising your heating system’s performance with tado°. To help you achieve the best results, we offer three PWM (Pulse Width Modulation) options tailored to different needs:

1. PWM for heat pumps or pellet boilers: This setting is designed to improve the efficiency of pellet boilers and heat pumps by reducing the frequency of on/off cycles.
2. PWM immediate response and full heat request fulfilment: This option ensures an immediate response to heat requests for systems connected to underfloor heating valves and maintains activation until the request is fully met. It avoids energy waste by preventing the output from operating against a closed valve during low heat requests and helps reduce cycles for heat pumps.
3. PWM to minimise temperature overshoots: This setting helps to reduce temperature overshoots and mainly responds to moderate to high heat requests. It activates the relay only when the heat request exceeds 50%. 

tado° uses symbols to show heat request levels:

• 3 is a high heat request (66.6% - 100%)
• 2 is a medium heat request (33.3% - 66.6%)
• 1 is a low heat request (0% - 33.3%)
• 0 means there is no heat request

Please note:

• You can choose only one of these options for your heating system/device.
• You can request a change of setting anytime.

These configurations are made to the wireless receiver (or presumably, zone controller, where there's no receiver). Mine had been set to PWM2 after I requested adjustment to accommodate our weather compensating boiler last autumn. It seems this was resulting in one (but not both) of our sensors overshooting by around 1C.

Whilst I was given no explanation to why this might be (I did ask the question, in several different ways), PWM3 was suggested as a potential remedy, and actually, although it's only day one, and not exactly winter right now, it does seem to have worked. The offending sensor stopped calling for heat 0.1C below target temperature and only then fired again around 25 mins later, presumably to attempt to hold the target.

Unfortunately, that subsequent burn time was around 5 minutes, so only one of my problems seemingly solved, so I'll have to keep an eye on that one (after it stops being summer in April here in the UK).

However, it seems there is a solution to overshoot, if that's a problem you're suffering. Just ask them to check the PWM settings, and change as necessary.

Montage

Thanks for posting the information.

Should be cold again in a few more days. Let us know how it works.