Yes I do.
The other parameter you need to configure is Overshoot
4C overshoot mitigates cycling, and will inform your weather curve is tight/shallow enough if you keep going into the overshoot temp.
Minimum output of your unit compared to your emitters and heat loss will also play a part here.
I think as I may have said in another post I did hear that some Daikin heat pumps left the factory with an incorrect refrigerant charge whether this is my problem or not who knows.
The only way to check is for a qualified tech to come out drain the refrigerant and weigh it.
Thx for the info.
So if I understand the overshoot correctly if the WD curve is calling for 45c it will heat the leaving water to 49c and then only reheat when the leaving water temp drops to 41c?
I went and checked this setting on my HP and it’s already set to 4.
Rgds Rob.
The overshoot allows the heat pump to increase flow temperature within the set range to allow it to keep running. If it didn’t increase flow temperature it would have to shut down.
This mitigates short cycling as Stephen says when the heat produced can’t be delivered to the home, generally because the emitters are at their limit at the requested flow temperature.
What flow temperature are you running when it is something like 7c outside?
I am running straight LWT at the moment, no WD curve or Madoka influence. I am at 36c (in theory) as going any lower uses no less electricity. Plus these heat pumps don’t seem very happy at really low flow temperatures.
I find the heat pump takes so long to get into its stride that it is better to just leave it running.
I use no more electricity this way than turning it on and off.
I turn off the heat pump if we are out for a few hours, the house doesn’t lose much heat and it warms up quickly in any case.
I am controlling the house temperature with windows, sad, but it is what I am doing.
Based on my experience with this heat pump, the knowledge gained here and the size of your house I am certain your radiators are way too small to run efficiently at low flow temperatures.
It may well be that you can make it work in terms of overall energy consumption but the COP can never be good.
Your heat loss is probably the same or more than mine judging by what you have said, your house is bigger than mine and I doubt it is any better insulated than my house. Your downstairs is as big as my whole house.
I think Stephen’s house is about the same size as yours and he was getting a 9kW but ended up with an 11kW, not that it makes any difference and he’s actually better off with the 11kW, they should just give everybody the 16kW, you lose nothing.
It should work, there are just not enough/big enough radiators for this heat pump.
I would try and find out what they say your heat loss actually is and try and verify it, either yourself or with another heat loss survey from somebody other than Octopus, somebody reputable.
Then you will know how to proceed.
Hi,
So if my understanding of the MMI is correct then at 10c the set point for the flow is 33c.
I have all the TRV’s in the bedrooms set to 20c(18c at night) otherwise they get too hot all the common rooms bathroom, kitchen, dinning room, living room are set 24c and all sit at 21c quite happily.
The reason I set the TRV’s a couple of degrees above where I want the temp is the TRV’s seem to be optimised for GAS CH so start shutting off the valve as they approach the set temp but I have found this does not play well the lower flow from the heat pump.
Indeed, to avoid starving the heat pump of water flow (which also drives up the flow temp if you hit minimum output) you really want to try and set the downstairs TRV’s to the maximum setting (or remove the head entirely) - similar to what you’ve done.
Any differences in heat output/room temperature can be dialled in through balancing (assuming the radiators are generally able to meet the peak heat demand for the room)
It looks that way - assuming you have a heat curve defined? I’ve found going lower than sat 32C or so doesn’t really work that well from an efficiency perspective on my system. YMMV.
Switching to LWT mode and the 4C overshoot may result in some overheating, but you want to then reduce the weather curve (but not too far) - and it should allow balancing to be done if you can get to a steady state.
If rooms are not getting to temperature, ensure the valves (not the TRV as that should already be at max) are fully open.
Any rooms that get hot too quickly or overheat, start closing the valve little by little.
Unfortunately, most valves only start restricting flow rate at a certain point, and then they are very agressive, so you can turn and turn with nothing happening, and then all of a sudden the flow stops…its at that point you’ve found the “authority”, and then its’ trial and error:
room too cold, open slightly.
room too hot, close slightly.
There is a thread specifically on balancing here as well…:
OVO and heat Geek?
Exactly this.
They are chimpanzees.
You can’t trust them to do anything unsupervised but if you do hold their hand and point at exactly where the bits need to go to get their banana then they can join pipes.
They have money.
The well operated energy part of their business is rich enough to bankroll losses incurred by the heat pump division so you can get a discounted installation cost.
Unfortunately they’re dishonest and they have lawyers.
So they’re very happy to lie to your face unless you have overwhelming evidence of where they didn’t do things correctly and don’t get a banana.
Treat them the way that they treat you in their terms and conditions (as an adversary and with contempt) and can work.
Treat them the way they would like you to (trust in the brand) and they’ll run rings around you.
Here’s a fun idea:
Refuse to pay the final installment (release your tax rebate/BUS grant) until the job is demonstrably complete.
With “demonstrably complete” being it performing to design as measured by open energy monitor and MID measuring instruments.
MCS and DESNZ should be onboard with that. It’s the reason that you as the consumer must be happy with the installation before the government pays the installer on your behalf. 
If you have access to the upfront capital required - and there’s actually any availablity on the part of the installers - that does seem to be the peace of mind option for heat pumps today.
My 9kW is going, I have tried everything and I can’t live with it.
It will be replaced with an 8kW Daikin by Octopus.
No arguments now, it is obvious that my heat loss is a little over 4kW at the design temperatures, just as I have always said.
Keep us posted on how it goes Matt, it’s been an interesting, if somewhat trialling, journey for you. I’m sure you’ll put the new unit through its paces.
I’m am interested in the 8kw unit for myself. In particular what sort of saving it would give me per year to justify the change. My heat loss for yesterday with average outside temp -2.5C was 4.3kw and the worst day in November when it was more humid was still only 5kw. It’s a shame OVO wouldn’t give me access to the high res data from the meters to prove this in their 2 year heat pump trial, I tried but it didn’t work. It’s great that you and Zak have succeeded with Octopus.
Hi Matt,
I remember Octopus offered to replace your 9kW unit with 8kW model a while ago and you declined, at the time.
Can I ask what changed your mind? More data? Exhausting the available options, including a visit from a Daikin engineer?
I’m pleased you will get the 8kW unit. I think we know that will perform very well in your property and be easy to live with.
Hi Jonathan,
You are correct with everything you say.
Over the last couple of weeks I have been able to confirm beyond doubt what my heat loss actually is around the design temperatures.
Although it was obviously nothing like the heat loss surveys produced it is never easy to convince anybody without absolute proof, now it is beyond doubt.
I don’t think there is anything wrong with the 8kW but I feel that the 9kW is a more substantial and maybe a higher quality unit, I didn’t want to change really, I would have preferred to keep what I have.
But, the last couple of weeks have shown me how comfortable it is living with a heat pump that isn’t way too big.
I am dreading going back to the warmer weather, it’s just unpleasant.
I can’t continue like I have been and it has to go.
I now understand as fully as I can what is going on with the house, the heat pump and the Madoka, they will never work together above freezing.
I don’t like giving up and I think I have got the best I could out of this heat pump in our house. I think I have exhausted every way possible to make it work in our house when it’s warmer at a reasonable level of efficiency.
It isn’t just about efficiency though, I always had to make a choice between running the heat pump continuously and controlling the temperature with windows and doors, or running with the Madoka controlling the room temperature but living with no heat for 3 hours at a time. Neither worked very well, either it is too hot and uses too much electricity or parts of the house get cold for a while.
The Madoka is designed to load compensate by modulating the heat produced to maintain the set room temperature, it can’t modulate down when it is already at the bottom and this heat pump in our house is always at the bottom of its operating range.
I also had a problem with Octopus, although they offered a swap it didn’t come easily the first time, it wasn’t offered in good spirit and it came with a certain amount of intimidation, basically they didn’t believe I had a problem with the heat pump and they felt it was the correct unit for our house.
That seems to have changed now, they appear to accept that the heat loss is not what they calculated.
It makes a difference to me, it’s now amicable and not confrontational.
I will try and get the best out of the 8kW as you might expect but in all honesty I just want a heat pump that I can leave alone to heat the house.
I would expect my annual electricity consumption to be somewhere between half and two thirds of what it is with the 9kW.
Hi all, thx again for all the info no matter how long I have had this heat pump there seems to be more to learn.
I found a suggestion either in this thread or another to try a website called Heatpunk.
It seems very detailed you can enter the U values for your floors, walls, ceilings, roof etc.
Create a 2d map of your house with windows, doors and radiators.
Having a somewhat complex house with an extension and loft conversion I had to do a lot of digging around on various websites like Celotex for U values and cross-referencing that with my extension plans.
After lots of measuring and data entry, the site came back with a heat loss of 6672W as opposed to Octopus’s 7753W(I’m still not sure why they insisted on installing the 9kW heat pump).
The total area of the building is 129.30 m2, giving an average heat loss of 51 W/m2 at -3°C.
I’m still not 100% happy that these values are accurate as it states the radiators I have installed will not be powerful enough to keep the house at 21deg at a flow of 40 at -3deg but I have run the heat pump at that temp so it’s either overestimating the heat loss or underestimating the output from the rads.
It’s difficult to be sure though as I have the overshoot set to 4 degrees to try and reduce the cycling so my flow is oscillating by 8 degrees which is not great for thermal comfort at times.
I found once the temps increased outside my hallway would warm to 23 degrees and shut the heat pump off for hours and some rooms would drop 4-5 degrees before the heat pump would switch back on. So I have switched the control method to LWT with WD curve set to 40 at -3 and 20 at +18 all of the rooms are sat at a steady comfortable temperature despite the large overshoot setting.
I have put the two sets of heat loss calculations next to each other in the chart below and you start to see the problem.
Due to them overestimating the heat loss in many of these rooms, I paid to replace radiators that were perfectly adequate. The largest difference is a whopping 81.6% in the hallway(where the thermostat is).
Some of these I queried at the time I wish I had stuck to my guns now.
Anyway, with the temperature now back above 6 degrees we are back to constant cycling of the heat pump every 20-30 minutes I have no idea how long it is going to last like that.
I did try getting in touch with Daikin tech support to ask about the minimum power for the 4,6 and 8kW but all they seemed to be able to do was send me the tech specs in the brochure.
I will go back to Octopus with this data and see what they propose.
Hi Robert,
When was your heat pump installed?
Although the percentages on some rooms look big they are not actually large amounts.
Maybe the Madoka location is causing an issue as that area of the house heats up faster than the rest of the house and does not cool down so quickly. Maybe you need to move it or limit the heat delivered to the hallway.
The overshoot is not +/-4c, it is only +4c so you cannot have a flow temperature that fluctuates by 8c because of the overshoot setting.
Even at a heat loss of 6.7kW you are not that far away from the 9kW range.
My heat loss at design is more like 4kW and @KnightPhoenix is similar. The heat loss of my house is actually less than 3kW at design, it is just our conservatory that loses a little over 1kW that they now insist must be heated that caused all the trouble.
I have a heat loss that is much less than yours and I have no cycling at all. Apart from defrosting my 9kW Daikin will run for six hours at a time (the maximum) quite happily.
The minimum input for the smaller Daikin models is around 285w
Hi,
Yes, the thermostat location was part of the problem but I have removed that variable by setting the control method to leaving water temp. The heat pump just tries to deliver a constant water temp based on the WD curve.
It’s 9C outside so the LWT is currently set to 28C.
It’s constantly cycling as you can see from the power draw below measured with a CT.
I have smart TRV’s on most the radiators so I can show you the house temps as well. Most of the bedroom TRV’s shut off to stop the rooms from overheating but all the common rooms are set to 24 and are wide open.
Those rooms seem to sit at a constant 20-21 degrees without the TRV’s needing to interfere.
EDIT: The heat pump was installed Oct 22.
Hi Robert,
If your heat loss is really 6.7kW at -3c and you have to run at 40c flow then you will need the 9kW
Defrosting in those conditions mean your heat pump will barely put out enough to maintain the house temperature.
Do you know how much heat was required in those temperatures we have recently encountered?
My heat pump was installed in June 2023, the only heat pumps available at that time were the 9kW or the 7kW.
As my heat loss was calculated at over 7kW I got the 9kW heat pump. Maybe it is the same with you?
Nothing else was available.
As I can demonstrate that my heat loss is barely half of what they calculated then I can make a good case that my heat pump is too big.
Based on your own heat loss calculations I would say that yours is not too big, it is just barely big enough taking into consideration the flow temperature you need to run.
I have no trvs on my radiators, it is the last thing you want with this heat pump when it is warmer outside, you need somewhere for the heat to go.
I dare say that would be contributing to your short cycling. The bedroom trvs closing will cause your return temperature to rise and the heat pump to cycle.
As I said, my 9kW does not short cycle and my house is smaller than yours with a lower heat loss.
Unfortunately, your heat pump isn’t too big when you need it, it is just that it won’t modulate down to what you need when it is warmer outside, it is the nature of the 9kW Daikin.
I have sized all my radiators to match the heat loss of the rooms and the desired room temperatures, that way none of them ever turn off, they can’t anyway as I have no trvs.
The radiators need to be in proportion to the heat loss of the room to get the best out of this heat pump, trvs and zones are undesirable.
I do.
Here is data from the 19th pretty much the worst case for the heat pump.
-3 degrees outside
Between midnight and 04:00 it chugged along at 1kW
Between 04:00 and 05:00 it was heating hot water so the house cooled down a bit.
It does draw a good chunk of power to heat the house up between 05:00 and 06:00
Here is another sample 15th to 16th drops to -4
I guess the spikes about 6AM are defrost cycles.
Might not be relevant, as you may have tested this. Could the cycling be due to system volume - i.e. the TRVs getting shut off and thus only heating a single room.
We had this and removed all TRVs and actuators (UFH downstairs). Then balanced the relevant bits down to control temperatures in each room.
Also the LWT of 28 will cause the machine to cycle (I think you said you had the 9kw), the minimum temperature seems to be about 32…
Oops snap @matt-drummer
On days like today, I simply turn off the HP for a number of hours and then switch it back on at a given point. I’m nearly at the point of switching back to thermostat control but haven’t quite made the plunge - I’d love for LWT to just work…
Also we have a single Tado, which I don’t trust.
I’m awaiting Octopus to replace with Madoka which is proving harder than I expected…
This heat pump is great when it’s cold. It’s not the worst case for this heat pump, it is the best.
The problems come when it is warmer.
My radiators are massive, I don’t think I could possibly go bigger if I wanted to and I wouldn’t dream of fitting trvs.
If rooms are too hot then the other rooms that are ok need bigger radiators and the flow temperature reducing.
When I had to run at 40c at -2c outside the heat pump was defrosting every 20 minutes, it was all it could do to get to 40c in 20 minutes.
The heat output defrosting like that is a maximum of about 6.3kW.
Octopus insist on designing at 50c flow at -2.3c outside.
My experience is that this heat pump will never get to 50c as designed, it just isn’t possible.
This ends up with radiators that are too small usually, mine just about worked.
My radiators are now three times the output they were at installation.
It now works pretty well, so well that even though it is grossly oversized I am still a little nervous about changing it and I could live with it if I had to.
Cycling will be caused by either ice or the returning water temperature being too hot.
This heat pump isn’t really happy much below 35c flow temperature, most of the time this results in the minimum electrical input and reducing the flow further just results in less heat, a poorer COP and an unhappy heat pump.
