Interesting @chrisg
I stand corrected then. I have a 4-8kW version which does what I described, I assumed the 9-16kW one followed the same control logic your data shows not.
Agree that defrosts are a different story, there is some logic to throwing as much heat at it as possible following the defrost to get back up to temperature. It can create its own issues with frosting up again immediately but at least there is some logic behind it.
Very interesting conversations. We are currently looking at an ASHP through octopus. Need a hotwater cylinder, 9 radiators and theyâre saying we need a 15.5kw Grant system, which I think is rather larger. We live in a 1930s semi with 4 large doubles and a single box room. Itâs all open plan downstairs with energy rating of B on the house. They have calculated heat loss of 10kw and putting the radiators up to 10.3. So why a heat pump of 15.5kw and is this exactly what has happened to alot of you good people?
Welcome, @Glenn_Rodrigo to the OEM forum.
If you type up your post outside this site, it is detected as spam 
, and held until one of us approves it. So if you can, please type directly into this page. Donât worry about losing what youâve done, it will still be here if you go away for a while.
As for your question, first a caution: I donât own a heat pump, I am simply reporting what has been written here by others.
You only have to do a bit of reading here and youâll see it is rare for a heat pump NOT to be over-sized for the property, and there are quite a few owners here who have convinced their supplier to swap the heat pump for a smaller version. Also, beware the rating of the heat pump - often a lower rated one is identical to the next size up (or two sizes up) and is simply limited in the software. The unfortunate result of this is it cannot turn down low enough as the weather warm up and cycles, something which is best avoided.
Thank you. Will post the relevnat outcome of our conversations with octopus and other suppliers.
I cannot recommend Octopus Energy Services for heat pumps. They just donât know what they are doing. I was left with a partially working system from day 1. They left off a whole room in the manifest and threw in a rad and hoped it would work (it didnât), left another rad in a non-working state and despite many requests for help, noone came so I had to fix it myself, by trial and error. They sent a junior to resolve one issue who didnât have a clue. They did eventually send a competent engineer for a minor ESBE value adjustment but overall, utterly hopeless.
My advice is walk away, donât use them unless they are super cheap and youâre happy to fix things yourself.
Make sure if you get a quote from Octopus they provide you with a heat loss survey that meets the MCS standards. I am currently trying to have a smaller unit fitted and after a FOI request I have discovered that they did not provide one.
Thank you. Tbf the engineer that came out did do a thorough survey. We now have an EPC for the property on the gov website with a high B grade and they provided us with a heat loss calculations with a schedule for radiator replacements that included current outputs, heat loss per room and new radiator outputs to match the calculated loss. There were some odd values though, with our lounge which has only 1 external wall loosing more heat than from our loft conversion, which is the whole loft area, so much larger. Got another company coming tomorrow, so will see how they stack up against octopus.
Thanks for the input. Will keep you updated. Incidentally one company suggested that Grant ASHP hey used to use 4 years ago werenât great at low temperatures, so they stopped using them and now use Bosch as preference.
bw Glenn
Just out of interest. Is there anyone who has had the issue of overheating and large cycles in heating sorted out by reducing the size of the heat pump?
Glenn
I went from an 11kW Daikin to an 8kW Daikin. Iâm now running longer cycles at lower electrical input power. 1920s, solid stone semi with lift insulation and double glazing. 110 square meters total floor space.
The 8kW will modulate down to ~250W and run without cycling for days in the right conditions. Whereas the 11kW wouldnât go much lower than 800-900W and cycle frequently.
My home sola/battery is now lasting longer on cold ,dark winter days. Iâve done one winter with each.
Itâs now much easier to get the house to sit at a comfortable temperature.
Fyi, my heat demand number from heat pump monitor is sat at 4.2kW which is roughly the same heat demand number I was getting when I had the 11kW.
Originally Octopus wanted to fit the 16kW Daikin⌠
Thanks for that info. Although I do feel a bit out of my depth with all the jargon 
. Today had a smaller company come in. They always put in a water buffer, which is like a store of hot water that the HP uses during defrost cycles. Octopus never put these in and the HP draws on the radiator water, which can cause a rapid drop in temperature which triggers the aggressive heat cycle. They have also suggested a 12kw rather than the 15.5 kw Grant. He didnât think much of Grant either, and though Octopus put larger HP in as the COP deops of signicantly at low temperatures. I had heard tgis feom another company who use Bosch HPs. The guy today said vaillant are good but expensive compared to Ideal. They also prefer to use Ideal, which is British with a large Nationwide coverage of engineers. Of course they are sales people 
, which h is why these groups are good.
Glenn
How many kwh batteries do you have. We have 6.5kwh and are thinking of increasing this to 13kwh. We charge with Octopus from 00.30 to 5.30âŚ..currently 6.5p/kwh
Also, what was the calculated heat loss and radiator output, including any that were changed by octopus.
I believe that Octopus always fit a volumiser, for exactly the reasons you suggest, to ensure there is always sufficient accessible water volume to make defrosts happen in the event that the owner uses the mandatory TRVs to restrict flow. This is because of the constraint of following MCS regs to comply with the BUS grant conditions, and ensure the installation nominally supports various owner scenarios.
There are many examples of Octopus-installed volumisers on this forum, including my installation.
We have about 17.4kWh nominal battery which works out to be somewhere between 14-15kWh usable. Combined with 6kWp east west split solar. This covers us for about 90% of the year, with only the coldest, darkest winter days where we deplete the battery completely, usually in January. Note that they specified an additional 2 kW electric plinth heater for the kitchen to make up for the radiator output shortfall, as we didnât want to replace the column radiators, however we never fitted it and the kitchen has never been the slightest bit cold.
And yes we have a volumiser fitted as standard by Octopus.
We are running flow temperatures way lower than Octopus designed for.
In hindsight, the algorithm Octopus used for the heat loss really didnât do a good job with our 1920s solid stone house. We suspect that the house is no where near as draughty as Octopus estimated.
I was able to provide enough data (spreadsheets with tens of thousands of data points) over a winter to prove to Octopus that an 11kW was massively oversized, they agreed and swapped for the 8kW.
Overall its working very well and we are happy with it but we did get it when Octopus heat pumps were crazy cheap (ÂŁ2700) a couple of years back.
Link to my data
My current weather curve, vastly different to what Octopus left me with.
Radiator schedule, note the kitchen shortfall, which in reality has not been an issue.
Heat Demand from Heat Pump Monitor
Data from the 25/26 heating season
Did you receive a heat loss report as laid down by MSC or just this? This is a similar document to what I received. I am now in dispute with Octopus about the HP they fitted. This document does not meet the requirements laid down by MSC for a heat loss report. I have had an independent survey done and received their report, properly laid out with all required information, which shows my heat pump is far too large for the property.
Hi Derek,
who did you use for your heat loss survey and what did it cost?
Bw Glenn
Hi. I used a local firm, Radiant Renewables from Lincoln. It cost ÂŁ300. You get an in-depth report.
