Hi all, looking for advice or ideas on what might have happened, what I might have missed and how to fix it.
I’ve got an Octopus installed, 8kw Altherma 3 operating in weather dependant mode. It’s given up adjusting flow temperature based on outside temperature, as if it’s operating in fixed mode. The outdoor temperature sensor works and gives seemingly accurate readings but the flow temperature does not change accordingly. Flow temperature is the same at 10 as it is at -2, for example. WDC is set at 28@10 / 40@-10. If I change the flow temperature manually using the offset, it responds as expected and changes the water temperature. If I reset the unit, the flow temperature on restart will be correct for that moment but will then stay right there, no matter what the outdoor temperature does, until it’s manually adjusted or restarted. It was working quite happily in WD mode since installation in February last year but after a cold spell in December, it just stopped following the curve one day. I noticed because it was overheating the house when the outdoor temperature rose up to 10 but the flow temperature stayed at 35. Everything else seems to work as expected. It’s been updated as and when updates come through the app but this change doesn’t correspond with a recent update.
I’ve spoken to Daikin who asked me to run it in fixed mode for a bit and then change it back to WD but this didn’t fix it. Octopus have been out for a look but didn’t think it was something they can fix so have referred to Daikin. Has anyone seen this behavior before? Any thoughts on what might have happened or what else I can try?
Hi Dave, Do you have OEM level3 monitoring? In my case, giving Octopus and Daikin sight of OEM graphs to illustrate the effect I was complaining about really helped.
Hi David, thanks. Yes I do and I’ll try to use it to show the effect I’m talking about. I suspect they’ll say it’s all down to my wdc settings…Daikin and Octopus already said that they struggle to understand how I can heat the house with low flow temperatures and said the temperature changes would be so slight with the curve set as it is that there wouldn’t necessarily be a noticable change in flow temperature. It used to be that at 10 degrees outside, flow would be 28 and at 0 degrees it would be 33 degrees - in practice those flow temps vary a couple of degrees because of the overshoot but that is what was displayed on the MMI screen as required flow temperature for those conditions. That no longer happens, neither the display or the actual flow temperature are adjusted. My impression is that they don’t like customers making their own settings and if something malfunctions, they will always assume user error. There’s definitely something wrong with it and I’m really hoping it doesn’t take a massive time commitment from me to prove it!
Your “idle” time could be months/years, but your “active” time should be small. You have your evidence : now it’s down to the suppliers to give a convincing explanation and close the case to your satisfaction.
Dave, I’ve the same model as you. As many people have said before Daikin’s don’t really like going below 30c flow temp, they get quirky. I was told by a senior Daikin engineer that regardless of settings, once working the heat pump always, always, uses the WDC curve. So I’m puzzled by the comment from the Daikin engineer. If you have an 8kw, can I assume your heatloss was between 6-8kw?and also you live somewhere really cold, Scotland maybe? If your temps have dropped as low as -10c, I’d be interested to know what your heat output was. I have mine set at 40c flow for -2 and 29c flow for 16. Depending on what your indoor temp is at 10c I wouldn’t have thought you’d get enough heat. However if you have big radiator capacity that would assist the especially low flow temp. At 10c outside temp I’d be at around 33c. I assume you’ve tried powering off and on again , doing a hard reset? Failing that I’m uncertain what support you can get. I’ve found daikin thoroughly unhelpful and ditto my supplier.
Yes, tried the old turning off and on again but no joy. Below 30 is less efficient but if set to be at 28 it will usually be more like 29 - 30 in reality. I assume this is because of the overshoot. It seems to be doing just about OK but with slighty reduced efficiency like this. 26 (28 - 29) is the lowest it will go without cycling but the efficiency loss is then very obvious. Daikin engineers visited previously because I was trying to get an official word on whether hot water heating is done by immersion only over a certain outdoor temp but I got different answers from different engineers and initially, they were both leaning toward settings being the problem, they seemed perplexed and questioned how I heated the house, even though the issue I was looking for help with was DHW, not space heating. Turned out that there is a maximum outdoor temperature for pump operation but that info maybe isn’t too well known at Daikin or Octopus but was understood by people on here. With this new issue, so far, there has been a similar line of thought and that the curve and / or other settings are at fault. I changed the WDC, after it was suggested this was likely the issue, to try to make the changes more pronounced. I’m in London and it’s only been down to -5 for a matter of hours in the last 3 years. 40@-10 is me messing about to see if settings make any difference but they don’t. A flow temperature of 33 - 35 has kept the house between 19.5 - 20 on the coldest days over the last year. It’s pretty well insulated which seems to be the root cause of confusion with most of the Daikin and Octopus people I’ve spoken with and had visit. A 9kw model was installed originally but this got swapped after 4 months because it performed poorly. A new survey was carried out before the 8kw was installed but it considerably overstates heat loss. The 8kw has worked much better and costs much less to run but once the outdoor temperature is around 8 or above, it overheats the house to uncomfortable levels unless I accept the reduced efficiency of a flow temperature under 30 or cycle it using the room thermostat, like a gas boiler.
Sounds as if your actual heatloss could be as low as 3.5kw. Going back to first principles how is your system configured? Does it have a buffer, do you have a madoka or a third party controller. If you do have a madoka have you tried using it combined with the WDC, I forget the exact settings at the moment, but what that does is make sure you don’t go over temperature, it may work for you. I didn’t like it because it does the hysteresis ( if that’s the correct spelling) thing, where you end up with a temperature variance, slight overshoot, then undershoot, it’s to stop cycling, as I’m sure you know. But it would avoid the overheating?
There is a volumiser fitted. The thermostat is the Madoka. I have tried using modulation but it doesn’t really work. I think because, when the WDC was working, the pump was usually trying to run near its minimum when it was 10 or above outside so had nowhere to go to reduce heat and turned off anyway. It wouldn’t then come back on until the internal temperature dropped by 1.5 degrees under the set point which was often noticable comfort wise. With modulation switched off, the Madoka turns the pump off when the indoor temp is 20.5 and then back on at 19.5 but more often than not at 19.7. That could take hours, if the sun’s out for example and hasn’t been an issue with comfort. It was working quite well in that respect, with very limited input, until it started ignoring the WDC and cooked the house after a cold spell.
Interested in how you managed to persuade them to change from the poor 9kW to the 8kW. I have been complaining since my HP was installed but they will not replace it. They just keep to the old mantra ‘ I must run it at the specified settings ‘ which is 50 @ -3 and if not then this is why the HP is not running efficiently. I have had an independent survey carried out that shows the heat loss at 5700w. Also had a visit from an area manager from Daikin who could not understand why a 9kW HP was fitted. With this new evidence I have now lodged a formal complaint with Octopus. Await the outcome.
Dave, when you say it is ignoring the WDC , what do you mean. You also mention offset, my suggestion would be to ensure the offset is set to zero. Also as I asked before how much radiator capacity do you have. I know @matt-drummer has a lot of radiator capacity and this can sometimes cause him to have over heating. He is pretty experienced, also now has an 8kw having as you had the 9kw replaced. He may be positioned to provide some suggestions/ observations.
Sorry to hear you’re struggling to get that resolved. I hadn’t settled up, maybe that made it relatively simple but Octopus were very good about dealing with it. I raised concerns about the survey before the installation based on gas usage and the EPC that Octopus arranged but they were adamant it was correct. After installation, it overheated the house, short cycled and had unreasonable costs. A series of people came to look at it, fiddle with settings etc and then a swap happened after a number of issues were found with the original survey. The updated survey result is also significantly higher than the real heat loss.
Best of luck, it sounds like you’ve got some good evidence to work with.
The unit has stopped responding to the WDC so it permanently sits at the same flow temperature, unless I manually change it using the offset feature in the Onecta app or on the MMI. It is not in fixed mode but external temperature changes currently have no effect on flow temperature, which is the same at -2 as it is at +12, for example. This has only been happening since sometime in December, it responded as expected before then. I’ve not seen this particular issue mentioned anywhere on this forum or elsewhere. I’ve read lots of the info that Matt has provided and it’s been very useful to get the unit running efficiently - thank you very much for that @matt-drummer.
Radiators are not particularly big I don’t think. I can’t remember the details but there was nothing extraordinary about the overall sizing. Two of the bedroom radiators (original ones) are probably too big. They are restricted by the lockshield valves to prevent the rooms getting too hot for sleeping in.
Dave, you may have done this already, but in case you haven’t, Mr AI says;
Based on common issues with the Daikin Altherma 3 8kW, especially those installed with smart controls, the issue of the WDC (Weather Dependent Control/Curve) stopping and holding a fixed flow temperature usually points to a logic hiccup in the firmware or a conflict between settings.
Here are the most likely reasons why your system has stopped adjusting the flow temperature based on the outside temperature:
1. The “Frozen Logic” Issue (Firmware/Controller Issue)
What is happening: Similar reports in the community (e.g., from Octopus Energy installs) suggest that after a cold spell, the Altherma 3 can get “stuck” in a certain flow temperature, acting as if it is in Fixed mode even though the settings screen shows it is in WDC.
Why it happens: A potential bug in the MMI (Man-Machine Interface) controller software where it stops querying the outdoor temperature sensor to adjust the heat curve.
Fix:A full power cycle is needed. Turn off the main heat pump isolator (rotary switch outside) and the indoor unit breaker for at least 10–15 minutes to reset the main control board.
2. Conflict with Third-Party Thermostat (Madoka or Other)
What is happening: If you have a room thermostat (like a Madoka) configured with “Modulation,” it can override the pure weather curve.
Why it happens: If the room temperature is slightly below the target, the thermostat tells the heat pump to “run hard” (increase flow temp). If the room gets close to the target, it might get stuck in a “maintenance” mode, holding the last known-good flow temperature to avoid cycling.
Fix: Check if your thermostat is set to “Modulation” and try temporarily setting it to “On/Off” or increasing the room temperature setpoint to see if the flow temp reacts.
3. “Eco” Mode or Minimum Flow Temperature Setting
What is happening: The WDC is working, but it has hit a maximum or minimum boundary that stops it from changing.
Why it happens: If the weather has been mild, the curve might be calculating a required flow temp of, say, 30°C. If your “Minimum Leaving Water Temperature” is set to 30°C or 35°C, it will not go any lower regardless of how warm it gets outside.
Fix: Check the “Minimum” and “Maximum” flow temperature limits within the installer menu under WDC settings.
4. Incorrect WDC Curve Parameter Set
What is happening: The slope of your curve might be too flat, or the points are set too close together.
Why it happens: Sometimes, after a firmware update or a power cut, settings can revert, or the unit might misinterpret a very gentle slope as a fixed value.
Fix: Re-enter your WDC settings (e.g., 28°C at 10°C outside / 40°C at -10°C outside).
5. Inaccurate Outdoor Sensor
What is happening: The outdoor sensor is faulty or being affected by sunlight.
Why it happens: If the unit thinks it is always 5°C outside, it will maintain a constant flow temp. If the sensor is in direct sunlight, it will report a false high temperature, causing a low, consistent flow rate.
Fix: Check the “Information” menu on the MMI to see what the outdoor temperature is actually reporting.
Verify setting: Go to Installer Settings → Space Operation → LWT setpoint mode to ensure it is definitely set to Weather Dependent and not Fixed.
Check Room Stat: If you have a Madoka, set it to act only as an indoor thermometer and let the Daikin handle WDC solely, or set it to a very high temp to ensure it’s not limiting the heat pump.
If the problem persists, it may require a firmware update from a Daikin technician.
Thanks for all your help, Terry. Much appreciated. I’ll give the reset another go. Octopus have now requested that Daikin arrange an engineer visit so hopefully someone will work out what has happened and get it fixed if the reset doesn’t work. Thanks all.
If you are using room temp control via the Madoka, one other thing to check is that there is no schedule programmed under leaving water temperature (LWT) control. If the unit has been previously set up as LWT control and then switched to RTC the LWT schedule still has an impact. The solution is to switch to LWT and then delete any schedules and weather dependent curves you find when in that mode. Then switch back to RTC.
Thanks for this, Colin. It’s definitely not to do with any ghost schedules, there are no schedules set up for heating in either mode.
Quick update - I did the ‘hard reset’ thing yesterday but that hasn’t fixed the problem. The unit restarted with a flow temperature (as displayed on the MMI) of 29 at 5 outdoors. This afternoon it was 12 outdoors but stuck at 29 degree flow. I restarted the unit and flow was then 27 degrees. Hopefully, the Daikin engineer will have a fix when they visit.