OpenTherm & Weather Compensation

andyblac ✭✭✭
edited November 2022 in General Questions & Topics

Got a couple of questions, hoping some can advise.

1: Has anyone fitted weather comp to a combi boiler, when using using Tado with OpenTherm link?,

2: Is there any advantage over and above Tado's internet based weather comp?

3: more importantly wil thery work together or fight each other?


    1. No, but I have weather comp on my Viessmann and would not use Opentherm with it as it would override the weather comp.
    2. Boiler weather comp is hyper-local. Tado's is forecast-based. Boiler weather comp modulates your boiler. Tado's doesn't. Tado's load compensation (e.g. OpenTherm) modulates, but based on internal demand rather than external temperature. Boiler WC is proactive. Load comp is reactive. WC better suited to high thermal mass properties. LC to low thermal mass properties. Lots of informative content at
    3. See 1.
  • thanks, so basically you have answered my main question, can't use WC with OpenTherm, i'll stick with OT.

  • @DM932187, thanks for your interesting answer above. I've also been looking into WC on my baxi main eco (condensing system boiler). If Tado's WC doesn't modulate the boiler, that means it isn't maximising the condensing function and is burning more gas than necessary. I'll browse heatgeek some more, but what I've already seen there suggests that WC to control flow temperature is one of the most useful efficiency measures. It's a shame that both Tado and my boiler are compatible with OpenTherm, but they don't co-operate to control flow temperature. Am I missing something here?

  • Not sure if you're missing something, @DodgyRog but I'll try to clarify.

    WC on your boiler will modulate the boiler. You'd use that with tado in relay mode, with the tado temperature set as a high limit, rather than target, and aim to get your boiler flow temperature to hit your target. The idea is that tado doesn't stop your boiler during heating sessions. E.g. set tado to 21 and aim to get a steady room temperature of 20 from the boiler flow temperature. Depending on your boiler WC (advanced or basic) you may find you can't stop the room temperature eventually creeping up to the tado set point, and then tado will do its relay modulation, to hold the temp.

    'Weather compensation' on tado, is just reacting to the internet weather forecast. So the system may start/stop earlier or later, or switch on/off in relay, based on current reported weather conditions. It doesn't modulate boiler flow temperature. It relay modulates (on/off).

    OpenTherm is load compensation and works similarly to boiler weather comp in that it modulates the boiler flow temperature. The only real difference is that LC takes the reference from the room, and WC from the outside temperature. LC reacts to indoor changes, and WC preempts them, by adjusting flow temps before they impact the room (or so the theory goes). LC is usually best suited to low thermal mass, well-insulated properties because they are more susceptible to heat gains (solar, parties, cooking, etc.). WC is best suited to high thermal mass, less insulated properties, because they're most impacted by outdoor conditions, and less by additional gains. The best form of weather compensation (advanced) will allow full programming of the flow temperature curves, parallel shifting (flow temperature set back) and internal feedback, which fine tunes. Tends to come on the more expensive boilers though (like Viessmann v200).

    LC and WC can't work together because the boiler will attempt to set flow temp based on the weather, and the load controller (tado) will constantly change that to meet internal demand. For example, it's a bit chilly today, and tado fired up my Viessmann (v100, unfortunately) at around 5:30. It took about 90 minutes to address the 1C deficit in target room temperature. Why? <45C flow temperature from the boiler. Return temperature 25C. Lots of condensing. High efficiency. Now, if I'd been attempting to also use LC (eg. tado WC), it would have reacted to the slow warm up, fired the boiler up to maybe 70C, and then modulated back. It would have overridden the boiler WC resulting in faster heat up (which we didn't need), but very probably lower efficiency.

    Relay tado can work very well with boiler WC. The house is also almost rock steady around set temperature. The key is to not overdo set back (at any time) as the low flow temperature means the house will take ages to heat back up, particularly if it's high thermal mass (so first needs to reheat the walls, etc.). Kind of contrary to everything tado is about, but I was an early adopter and like the geolocation and remote access. Not sure it's saved me anything, but I'm sure WC has. We have a 2200 ft2 Edwardian house. Solid walls and some single glazed. It's not cheap to heat, of course, but I worked out if you compare average UK house size and gas consumption to ours, we're about 40% more efficient, which I'll take.

    Still doesn't really stop the 'WTF' moments when I see the monthly gas bill though!

  • @DM932187 Thanks for that detailed explanation, so which do you think I would save GAS from the following

    1: Max Flow 65, set back 17˚c, 8:00am till 17:00 18.5˚c, 17:00 to 21:30 20˚c

    2: Max Flow 45, set back 18.5˚c, 17:00 to 21:30 20˚c.

    atm i am doing option 1 and using about 48KWh per day. (I have a leaky Extension with full wall of glass doors, it triple glazed but still leaks heat)

  • DM932187
    DM932187 ✭✭✭
    edited December 2022

    Re your other post question on OpenTherm @DodgyRog , you need DHW priority to ensure the boiler fires at pretty much full rate to get your cylinder heated up, when scheduled. Otherwise, the modulated low flow temperatures (OT or WC) won't be enough.

    Same principle as a combi (which effectively has build in DHW priority), but using a 'heat tank' request instead of switching on the tap.

    What you gain from OT/WC on a system boiler, is the ability to run low flow temperatures when you don't need to heat the HW cylinder.

  • I'd love to be doing 48KWh right now!

    Based on my experience with WC, you'd probably find option 1 still works best. These are actually pretty similar to our weekday programme.

    You'd need to trial Early Start on tado to get it close to 18.5 in the morning. I then drop it back a little once everyone is out for the day.

    With WC, there isn't a max flow temperature unless you set it. On basic WC, you marry the curve up to the outdoor temperature (mine should be around 55C at -5C). On advanced WC, you set the low and high at outdoor temps and that creates the curve.

    In other words, the flow temperature bobs up and down constantly. As if you're turning the boiler up and down yourself, but you're not. Takes some trial and error to get it right, but it's worth it.

    You'll find the greatest savings come in the 'shoulder seasons' (Spring/Autumn), when the flow temperature cranks right down, but still just enough to heat the house.

  • thanks for that, someone is in all day, so it set to 18.5˚c all day.

    here is some results over past few days. with tests of setbacks and schedule changes.

    yesterdays test it not work well, lol as you can see 😂, 26/27th are warmer days 10˚c+.

  • @andyblac struggling to decipher the table, but I think you're showing me that set back works with a fixed flow temperature?

    Always someone here during the day too, but as it's me (rather than the Mrs) we can crank down a bit ;-)

  • andyblac
    andyblac ✭✭✭
    edited December 2022

    @DM932187 actually it really strange, I had Max Flow 55˚c yesterday, and it used more gas, trying to keep setback temps overnight, even though the boiler flow rate was around 26˚c (2˚c outside) at the time (3am).

    Something is really off with Tado˚ LC curves, last night after all schedules where done, I reset the Max Flow rate to 80˚c (mad I know), but it seems to have worked, overnight was way cheaper and it hit 0˚c outside over night and Flow rate was 32˚c when I checked it at 3am, as you can see it only used 17.157 KWh 22:00 to 10:00 next morning, compared to 21.231 KWh when it was 55˚c max flow and 2˚c outside.

  • Ok, so I'm speculating (because I don't use tado LC) but my guess would be that your 55C max flow temp was under heating, if it was giving you 26C flow temp at 2C outdoors. 26C is basically no heat at all, so unless your boiler has an incredible modulation ratio, it's gong to be switching on/off all night as it fires, overheats and so switches off again. By contrast, 32C is much more possible with modulation. It'll tick down to the lowest flame and hold it there, and give you plenty of condensation and thereby, high efficiency.

    The flow temperatures you need are dictated by three things. Emitters, property characteristics and indoor/outdoor temperature delta. If you have large emitters and very good insulation, and a small indoor/outdoor delta, you won't need a very high flow temperature. If you have smaller rads, poor insulation and a higher indoor/outdoor delta, well, you get it...

    The emitters and property characteristics are static, subject to alterations (new rads, windows, etc.), so the most important characteristic day to day is the temperature delta. In my case, that translates to a flow temp demand of around 55C at -5C outside, if I want around 19/20C indoors (a 24/25C delta). If the outdoor temp goes up, the flow temp goes down, and vice versa. It's not spot on (as the V100 has a fixed curve gradient that is slightly off for my house), but it's pretty close.

    In your case, because you're using LC, it'll have a shot at the right flow temperature (based on a curve/learning) and assess its impact. If the gap to target indoor temp isn't closing fast enough, it'll increase the flow temp, potentially impacting efficiency. When it gets nearer the target temp, it'll turn the flow temp right down. Possibly too far for your boiler to modulate, and that will also impact efficiency to some extent.