Octo-Confused. Ensuring right-size Heatpump, long term effeciency

Hardware Heatpump
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So, done lots of reading on the forum of other experience with various heat pumps, Octopus supplied or otherwise. Asking after research as I am still a bit bewildered.

Had a survey done by an independent late 2024 when I was certain we would go ahead spring 2025 with a heat pump, but the cost came back very high - £8-11K (on top of the £7.5K grant), didn’t go ahead.

Sat on the idea for a while, but paid the Octopus £200 “refundable” deposit earlier this year for them to come and do the detailed survey, with intention of proceeding with them if acceptable costs and broadly inline with expectation on HP size/rad changes to earlier detailed survey.

Survey went ok, they said mostly what I expected, need to change at least 3 radiators (we know two are undersized from when the house was new). Cylinder appears to have a 0.87m^2 coil so needs to be replaced, but I was expecting that as well.

Octopus estimated heat loss at 5.5kW (estimate house at 156m^2), 6kW heat pump, designed to 50c flow and -2.2. This was first point of contention as the website offers you options at initial price quote stage to select lower flow temp for high efficiency - why have that if you then flat refuse to design at anything but 50c? I repeatedly reminded the surveyor I was looking for lower flow temp for best SCOP.

The quote is very basic, its based on a Cosy 6, new HW cylinder (of unspecified type/make) SCOP 3.45 on heating, 2.7 on HW, no details other than radiator table - not clear what ACH they used. I challenged their usage calculations at 9MWh Pa for the heating and 4.7MWh for the HW - we only consumed 9MWh of gas total last year! (will exclude about 500kWh of immersion heating the water with excess solar). So I know their calcs are out but with no “proper” survey provided I am not sure where the delta is to real life. I suspect ACH but also they measure insulation in loft at 250mm and its double layer I always assumed 150mm*2 =300 mm.

Previous survey suggested 4.7kW heat loss, approx same footprint for house, 45c flow temp, -1.7 external 5kW HP giving SCOP 3.83. Just feels like the Octopus model is a rough guess, let the install team work out the details and hope for the best?

They are pushing their Cosy 6, but I read the threads here about noise from the Cosy pumps, and have asked for a Daiken quote as well (and learned a 6kW Daikin seems to be an 8kW Daikin software limited - so the 8Kw makes more sense for more headroom as it drops down to same lowest output?)

Now my head is spinning and I am down the rabbit hole of do I continue to ask sensible questions, but get answers read from a script (inexperienced person is my contact, they keep leaning on someone else) or go to an independent, or another large supplier for another quote?

I don’t mind investing up front for long term efficiency but it’s hard work when their “proposal” is a price and MCP minimum info only. Coming out at just under £4k, heat geek estimates suggest one of their independents will charge £1.5K more for the same thing (assuming hot water cylinder is swapped).

Anyone got any tips, or suggestions, persevere with them or look elsewhere?

Conduct my own heat survey? Pay for an ACH blow test? seems the two survey data points are close already and some “educating guessing” around 5kW heat loss is all that’s needed. My “worst” day last winter was 98kWh of gas usage, which as another data point suggests that the 18 hours the heating was running that day I was not losing more than 5kW per hour (probably less when you take out hot water usage).

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Take anything Octopus survey people tell you with a big pince of salt. After years of fighting to get my HP replaced my advice is to get an honest independent survey completed and stay away from Octopus if they will not accept the results.

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I wouldn’t pay for more tests. Your worst gas day shows that you’re going to want a 5-6 kW heat pump. 8kW would probably be fine too.

Don’t get too hung up over design flow temp. My heatgeek design was 50C and I have never seen over 45C for heating on the coldest days. Unless you add a lot more radiators or insulation you can’t do much about flow temp. More important that radiators are roughly balanced so you can run them all fully open (-ish).

Do consider smart-control ability. I find that using home assistant to adapt my Vaillant heating to reflect Agile pricing saves me more money than chasing COP. E.g I heat water when it’s cheapest rather than most efficient.

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Thanks Derek and Andy.

I had another chat with them yesterday, asking questions about the old gas controller being removed showed up some holes in the survey so more answers pending.

6kW or even an 8 seems fine, I have pushed for a valiant quote as while they are pushing the cosy the AlthermaESP option would give me more monitoring and control.

Octopus flat refuse to install the OEM flow and heat meter which is a shame.

I did get one other estimate yesterday, local heat geek installer, they are coming out twice price of Octopus at 8k. They do seem to be proposing the Grant HP that Trystan installed and did a video for on YouTube and a better cylinder though.

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Hello @whitecitadel

Sounds like the heat loss 5.5 kW & 50C flow temp will more likely run at 46C flow temp if heat loss is actually lower (4.7 kW) as you suspect. Your 9000 kWh gas reference would put the heat loss closer to 3 kW, which could take your running coldest day flow temperature down closer to 40C.

I think Cosyies auto optimise their flow temperatures to actual requirements don’t they? so it should do a fairly decent job of running at the right flow temperature for your actual heat loss? Perhaps the Octopus design will be ok?

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It was @glyn.hudson who installed and did a video about the Grant. I did also install one but for a family member and have found the controls a bit fiddly - but then I haven’t had much time to spend on it recently. I keep meaning to start a thread on here to ask for advice on getting the right Grant control settings :sweat_smile:

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Thanks Trystan, and sorry for confusing you and Glyn!

It feels like its all in the right ballpark, and the earlier independent quote being a 5kW (against their 4.7kW loss) makes me confident the 6kW will be fine and lower temp flows over the 50c design.

The cosy I was a bit wary that I could not adjust the weather compensation, but actually I found the Cosy manual on the octopus site yesterday and it has weather compensation on page 8 onwards, and lets you adjust the max flow temperatures for warm and cold weather in the app. Not sure it’s quite as advanced as other heat pumps as you just set the end stops of the curve, but it seems adequate to tune it post install based on real world data.

I am leaning towards its probably just fine, as would be the Daikin of the same output - although that has R32 not R290.

Cosy also seems to get FW updates OTA from reading more here on the forum, and the August update last year I think it was seems to have boosted hot water performance and economy so it seems Octopus continue to develop and invest in the pump.

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Oh and the other thing that surprised/concerned me about Octopus was that when I asked about G98/99 for the Heatpump impacting my current import limit from the DNO imposed when I fitted batteries (12.5kW, hoping to negotiate increase, have 100A fuse) the person I was speaking to seemed to think that a heat pump was not a notifiable item. The MCS Website did not agree with them!

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Mm that’s strange, we’ve had a number of customers successfully get Octopus to install a heat and electric meters for OpenEnergyMonitor monitoring. There’s currently 3 independently monitored Cosy systems on HPM

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I saw that, I think either octopus changed their policy, or those that managed to get them installed were persuasive with the install team. The posts I found mentioning octopus agreeing were older dates.

I still plan to order the flow meter and try my luck on install day if I go with octopus.

I saw where you mounted yours inside the wall which gave me some ideas where to mount mine.

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Your battery/PV export limitations G98/G99 is not an import limit. Your import limit is the incoming fuse, your consumers (ASHP, kettle etc) are nothing to do with export limits. So heat pump has zero impact to do with the export limitations.

A 100A import limit is huge in reality.

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My Octopus office handlers repeatedly refused to include installing an OEM hp monitor. I can sort of understand them not wanting to expand the scope of the installation to include configuring the OEM kit.

What I did was to wire up the hardware and get the comms all working before Octopus arrived. At that point it was a simple ask of the install team - “please cut-in this heat meter tube to the flow and return pipes”. They installed the fittings without question.

ie. It was no more involved or complicated for them than making two more plumbing joints.

Looked at like this, it would be churlish of any tradesman to refuse: the relationship is customer and supplier. The installation belongs to the customer, and it must be possible for the customer to adapt the design without the installer taking responsibility for the adaptation.

Besides, these guys take pride in their work (witness the beautifully parallel multiple pipe runs you see in photos) and it must be preferable for them to work with a customer who equally wants to make the best of the job.

My OEM hp monitor has always worked fine and imo is an indispensable tool presenting its data in clear and understandable ways.

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Sorry your making an incorrect assumption here, I have an import limit as I wrote which was imposed by SSE when the G99 for the batteries was done, this is separate to the export limit your talking about.

The Tesla powerwall gateway confirms as an import current limiting device, so limits grid import to 12.5kW (and export your talking about to 7.5kW) which is about 53A limit despite the 100A cut out I have.

I have no idea why SSE did this, no one else including my direct neighbour with PV/Battery I know has this imposed. I want to get it bumped ideally when he heat pump goes in. In winter when car and battery are charging my ability to consume off peak is constrained by the limit.

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Thanks, that’s my hope too. They are already re-using the 28mm piping in floor fro the boiler to the airing cupboard, I hope they will just see it as “another bit of the customers’s existing pipework”, even if they have to stick it on the end :wink:

I ordered my EmonPi in 2015 when I had solar installed so been a long term user, I do of course need the Heatpump data in the system or my life would not be complete. My original EmonPi (Pi2) is a bit long in the tooth now, backups take a very long time with the size of the database and slow CPU so it might be time to upgrade this year.

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That’s great to hear. Is there any chance you could contribute on this thread a screenshot from your Octopus app to compare with the OEM MID heat meter data? It looks like Octopus is over-reporting COP

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Hi Glyn, Sorry, I can’t do that. My HP is an 8kW Daikin. Apologies if I was unclear about it.

But I can give comparison of the Daikin vs MID consumption SCoP since new (21 months).

Daikin MMI 19776/4646 =4.257

EmonHP MID 21643/5199=4.163

Is that in line with expectations?

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Just a point to pick up on from your OP, you have in fact got a lower flow temperature design from Octopus - if you’d gone the ‘Turbo’ route then they would have designed around ~65c flow temp rather than 50c.

FWIW I have a Daikin 8kW at 50c design flow temperature from Octopus and my actual peak flow temperature in operation was 44c-46c, and my ESPAltherma/Shelly data suggested a 4.9 heating COP September - April so I honestly wouldn’t worry about flow temps that much.

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Ah right, I didn’t realise that. That’s a useful data point. Maybe you could start a new thread with Daikin vs MID monitoring accuracy. That’s a good result! One of the closest match in terms of scop reporting. Of course this is only one data point, it doesn’t meant that all Daikins will have onboard monitoring this accurate.

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If the cosy is cheap then go for it. If you have batteries; scrabbling for scop doesn’t mean much in the face of cost.

£5k in your pocket goes a long way…

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I’m just trying to get a few things in writing, like they take feed from the power wall gateway and re-use boiler pipes that were agreed with the surveyor. The surveyor had put the volumiser in the loft not the AC as well which he had not told me, now corrected, so more indicators I would need to keep a close eye stuff doesn’t get lost between all the handovers.

Can cosy cool? Current heatwave has the wife asking, the problem with cosy is it’s completely closed, no install manual or spec sheet to reference just marketing. Suspect with only rads not underfloor cooling capacity may be limited, if only a few c on a 33c day like today would be ideal.

The new Valliant looks very nice, not out yet taking orders June onwards it looks like: https://www.myvaillantpro.co.uk/products/product-overview/heat-pumps/arothermplus