I have a 2 port buffer/volumiser, fitted pretty much as standard by Octopus on their installs, which I guess is just to make sure that there’s enough accessible volume to handle defrosting if someone shuts off half the rads! No secondary pump - the 9/11/16kW Daikin already has a monster. Since I have always had it, I can’t speak for the performance without it. Are you having drastic cycling problems? Mine cycles above 8C ambient, because I don’t have enough rad capacity to soak the minimum heat output at those temps, which is arguably a Good Thing on the basis that the house would be hotter than the sun. That’s what it’s supposed to do, after all, and these are 40mins on, 40 mins off roughly. If I want better, I’ll have to go smaller (see multiple posts from @KnightPhoenix !)
What size unit have you got and what’s your identity on the emon leader board interested in comparing. Issue is mainly on defrost tbh and a little cycling put I’m unsure having on return pipework will help defrost
Mine’s a 9kW EDLA09D3V3 - https://emoncms.org/app/view?name=DaikinEDLA09RevisedFlowT&readkey=798b7510494f9413958c7044e653480e
(It’s more helpful to link to the system view page on HPM site, so that people can read details of the property and installation → https://heatpumpmonitor.org/system/view?id=111)
Hi, I’m new here, and have been offered one of these 9kW Daikin’s by Octopus. Reading all this with interest/alarm. Octopus calculated heat loss as 7800.
Where in the Daiken specs do I find this minimum compressor power? Though I’m not sure if pushing back on Octopus is likely to get anywhere - they stand by their heat loss calc, and say the 8kW wouldn’t be enough. (Which may be true for the small number of days when it actually is -2 outside, but that’s not the problem, if I’m understanding this thread correctly.)
British Gas have speced an 8kW Daiken - I can’t actually find the total house heat loss in the docs - the sum of the losses for the rooms in the radiator schedule is 6154, but that (like the Octopus one) omits a couple of areas without radiators, which would add a bit to the total heat loss. (The corresponding total from Octopus was 73xx, suggesting the unheated areas add around 500W.) This is a bit closer to the rule-of-thumb value based on gas usage and degree-days - around 6kW.
Our 9kW EDLA Daikin runs at 950W min, of which around 180W is pumping and controls. It means if and when you can’t use ~4000W you’re accepting lower than Ideal efficiency. This aspect took me a while to appreciate, and doesn’t appear in any specs I’ve seen.
Does that manfest as increased cycling (hence wear and tear), uncomfortable temperatures (having to open windows), or is it possible to leave it running, but somehow just have it producing less heat, and therefore running at a lower COP. (I guess latter two options are much the same result - less heat in the house, whether by pulling less in from outside, or letting return the outside faster than it might normally have done so.)
[Octopus are charging £2k less for the install… that pays for a reasonable loss of efficiency, but is expensive if it actually reduces life.]
An oversized pump in my experience means you can get swings room temperature, as it’s ~4kW or nothing. If you reduce flow temperature then the actual flow temperature rises anyway, and then cuts out at a certain threshold. So cycling of sorts, but that might take 30 minutes. This results in a poor cop as well. Under a certain temperature your heat loss will be enough to use the power, and things will stabilise.
The main benefit you lose is the potential for low running costs and a low and slow running fashion. We can only use ours in bursts of a couple of hours when it’s above around 8C outside.
Please have a look at a similar post by @matt-drummer,
The 9kW can short cycle, overheating the house, not run correctly at lower flow temperatures (less efficient) and more.
If your heatloss is below 8kW I would call up and ask for the 8kW pump to be fitted.
Also if you have a current EPC pop on over to upgrades.heatgeek.com and see what size they recommend.
One other thing is to try using heatpunk and do your own calculations.
If you are 7.8kW then you would be ideally suited for the 8kW heatpump and remember, it has a nightly setback where hot water can be heated and you only ever need the full 7.8 kW for a few days a year (probably 10 at most) so all other times it will be oversized and have plenty of excess capacity.
One final note, if octopus won’t initially budge ask them if they will be willing to do it if you increase loft insulation to 400mm (cheap to DIY and can get it from b&q or other good retailers) or get a blower door test to confirm the number of air changes an hour as the defaults used are horrific (assume you live in an age related sieve) and you may find you end up closer to 6 or 7 with that taken into account.
Thanks… yes, I’ve been reading Matt’s posts with interest.
First thing I did was ask whether 8kW unit would be sufficient, but their reply was that “The 8KW unit has a max capacity at a design temp of 7050w”. (They’re designing for 50 degrees flow temp and -2 outside, and obviously aim to replace as few radiators as possible.)
I had been to the heat geek instant estimate (I’d forgotten about that), and they reckoned 5.8kW heat loss. (Which is in line with the rule-of-thumb based on gas usage and degree days.) British Gas have also done a heat-loss survey and are spec-ing the 8kW Daikin.
I did email Octopus to ask about air changes, and they are using 1.5 for most rooms, more for kitchen/bathroom. I reckon whole house is about 2kW per ACH, so using 0.5 rather than 1.5 ACH would indeed knock Octopus’ 7.8kW down to Heat Geek’s 5.8kW.
I already have 300mm loft insulation - isn’t it diminishing returns after about this point ?
Well at least they are right about the output as shown here for 50°c Daikin Technical Datahub
Slight improvement if you design lower but that is a non starter with octopus.
The returns on the extra 100 would be less than going from 100-300 but there is still some benefit.
I think you may have answered this yourself, get a blower door test and prove your air changes are lower (to allow them to be satisfied with you having the smaller pump).
Personally after having the bigger one and with the whole reason for installation being to go more green I would not be happy having a bigger less efficient unit cycling if there was an option but if you can get a good deal on electric or have solar and don’t mind using some extra /more wear and tear on the unit then the bigger one will work.
Is there a big price difference with BG?
£2000 difference in installation price buys a lot of electricity, as you say.
With your expected usage for the heat pumps, you should be able to work out the number of years that takes to pay back with a more efficient unit.
I’d say it’s going to be hard to get a SCOP much above 3.5 with a 9kW unit in the UK. I suspect SCOP well above 4.0 should be possible with 8kW.
My 9kW Daikin unit is now running okay, after an engineer disabled some always-on tape heaters 
. I’m running flow temps around 37-38C, as that’s what seems best for the unit and our radiators. Octopus also designed for 50C@-2C for us.
We like the house temperature between about 19-20C, which seems to be a bit cooler than other systems here. (But warmer than we typically had it with gas heating.)
We have the heat pump off from 10pm to about 5am at the moment; we can’t run it continuously as the minimum output is too high.
You can have a look at my system here and how long it is running for at a time:
https://heatpumpmonitor.org/system/view?id=120
It works pretty well once the temperature is above about 3C and there are no defrosts. We don’t have problems with short-cycles, we’ll often get several hours runtime. (The defrost cycles are annoying and hammer the efficiency, and I don’t have a good strategy to improve that yet - the unit likes to defrost every ~24 minutes when temperature is below 0C).
The Madoka thermostat is pretty poor, but it’s modulation controls can probably be made to work better with an 8kW unit that has more scope to reduce output to the levels the house typically needs.
If I could click my fingers and swap my 9kW unit for an 8kW, I would. (Apart from feeling bad about what happened to the old unit, hoping it wasn’t scrapped with all its embodied carbon cost…)
The 8kW and smaller units seem a lot easier to live with. Would I pay £2000 extra for an 8kW unit? Not sure, but maybe - for peace of mind and more stable temperatures it could be worth it.
If the 8kW unit fell short on a few days a year, you could either use supplementary heating or just run it flat out and hotter. (Or just turn the oven on etc. My office radiator is a little small, but once I’m in there and my PC and monitor are on, you can see it start to warm up quicker!)
But the 8kW unit would almost certainly be better on the majority of days.
So the best solution would be (obviously) to get the 8kW at a good price. As Zak has said, talk to Octopus and see how far off the 8kW threshold you are and if other measures can be taken to enable it.
To be fair to Octopus, if I’m reading the graph correctly, there is no flow temperature at which the 8kW can produce 8kW with an ambient temperature of -2 degrees. Seems like 4 degrees is some sort of crossover point where it hits 8kW at most flow temperatures - I wonder if that’s some property of the refrigerant ?
(Is that data available as a table rather than an interactive graph ?)
£2k
Another element is that Octopus offer what I’ve heard is a good price on annual maintenance - about £10/month IIRC. Not sure what BG charge.
BG are also replacing all the radiators, rather than just most. (And it would be nice to have a clean sweep.) The next set of sums I’ll be doing is seeing what flow temperature is actually required with those radiators (which are slightly overspecified at 50 degrees).
(Actually, is 50 degrees the flow temperature, or the average temperature of the radiator, which would be average of flow and return ?)
And when it’s below 3C ? Isn’t that where the bigger unit is less of an issue since you need more power anyway ? Or is it that the higher power draw means it frosts up more than desirable ?
Below 4°c you start to get into defrost territory so you will see a slight performance drop.
The lower flow temperature you can run at the less of an impact this will have (as the unit is not trying to heat the water as much so less frosting up.
You are right that the 9kW will perform better around and below 0, but how many days a year is that compared to the months a year spent between 4 and 16°c when you will be grossly oversized.
Some people may not agree but personally I would rather be on the limits of my heatpump at design temperature (so it is not grossly oversized) and then if I get 4/5 cold days a year put on a jumper or a 2kw electric heater for an hour to top up.
What manufacturers need to look into is a monoblock cascade system with a 8kW and a 2kW together (or 8kW monoblock and 2kW tank integrated that can also switch to space heating). While it may cost a little more to buy initially you have the benefit of spending much more time in the optimal size camp and you would also be able to run heating and DHW at the same time.
How long before we see the heat geek cylinder doing some dark magic 
We can look at how two 8kW systems performed on Jan 18th, which was the coldest day of the year:
- Farnborough - delivered 110 kWh over 24 hours at -2°C with COP 2.8 → chart
- Bournmouth - delivered 60 kWh over 24 hours at 0°C with COP 3.0* → chart
Interesting… the Farnborough one says it was installed by Octopus, but with a design temperature of 38 degrees… didn’t think they did that ?
Looks like a similar property to mine. The assessed space-heating requirement is similar to mine, Despite having a much lower heat loss at -2.
Dave,
Hi, Farnborough here 
It was installed by octopus back in April 23.
Due to some issues with the survey and vast differences in the upstairs and downstairs radiator sizing the ASHP was swapped January 10th from the 9kW (software derated 16kW / big unit) to the 8kW smaller unit.
The surveyor forget to add in the 400-600mm of loft insulation we have hence upstairs was all oversized and out heat loss was calculated to 7.3 instead of 5.4 (everyone makes mistakes)
Around February 10th 5 radiators were swapped to bring our flow temperature down to 38@-2 as we had upstairs sized for 28@-2 and downstairs 43@-2 (again due to the survey).
I ran all the heat loss calculations myself using a combination of heatpunk and a excel spreadsheet I designed to test flow temperatures and emitter sizing and 40°c was the sweet spot (subsequently lowered to 38 as it was too hot).
Octopus to their credit did all the changes FOC and even when I said we needed a k3 in the lounge they went ahead and made it happen. I cannot fault their aftercare so far but had I not paid up and requested the open energy monitor kit be installed up front I probably wouldn’t have noticed it being such an issue.
Just trying to clear some persistent air in the DHW coil now and then hopefully will be heading back up the charts again (DHW is currently killing me on SCOP)
Hello Matthew
I am also struggling with the 9kW Daikin Monobloc, out of interest who did you contact at Daikin to get support on an engineer visit to assess the HP suitability? Mine was installed by a company other than Octopus, where others have had success like yourself for a swap, so I am hoping to engage Daikin directly as they were the ones who referred me to the installer as they are in their installer network.
Many thanks for any advice you can send my way.