Octopus quote for Daikin 11Kw - oversized?

Hardware Heatpump
1

I saw a similar thread on this topic, but I’ve just signed with Octopus and have 11 days to reconsider.

My concern is that from our bills I can see our estimated annual gas consumption has been 17500kWh (rounded up), using a 20+ year old gas combi boiler, but actual has been 16500kWh (from manual review of bills from 2024).

2024 kWh
Jan 2748
Feb 2134
Mar 487
Apr 1006
May 500
Jun 490
Jul 328
Aug 405
Sep 1023
Oct 1639
Nov 2880
Dec 2890
Total 16530

This reduced may be because half way through last year we go rid of our gas hob and changed to induction.

On the 4th Jan with weather -7c to 0c our gas consumption was 197kWh. This is post all building work and best example I have of a really cold day.

Yesterday on 7th Jan I put the UFH and house thermostat up higher than normal and we used 148kWh gas from -4c to +2c.

This using an ancient Worcester Bosch 35cdi ii which I believe is 78.5% SAP efficient.

We have a 1930s 4 bed semi-detached house (basically a 3 bed semi with attic conversion), with mixed wall insulation. A bit messy, but partial new extension ground floor from last year with UFH, rest of ground floor cavity filled, 1st floor solid wall (no cavity), and attic conversion from mid-90s spec (pretty good).

Octopus survey stated hot water (2000) + heating (10000) = 12000kWh energy requirement.

My own Heat Punk calculation came to around 10490W (not quite sure why it’s W in that calculation), with similar recommendations, so I can’t fault the Octopus survey outcome, but it doesn’t tally with our real-world gas use.

With a more efficient system, upgraded radiators, etc. I’m concerned the mixed nature of our house if throwing things out a little.

Octopus have designed the system with 50c flow temperature, which is probably to keep radiator changes to a minimum.

Any thoughts appreciated.

2

Hello James,

They design the system to work at a minimum expected outside temperature and constant room temperatures indoors. The room temperatures are set by the MCS and are a condition of the grant.

So, was your house at around 20c inside for the whole 24 hours on the day you used 197kWh of gas?

3

One rule-of-thumb is to take annual consumption and divide by 2,900, so 5.7 kW.

Divide by 24 to get average power for the day, and multiply by the boiler efficiency comes out at 6.4 kW of actual heat required.

What were the average indoor and outdoor temperatures for that day?

148 kWh ÷ 24 × 78.5% = 4.8 kW

What were the average indoor and outdoor temperatures for that day?

That is a high temperature for a heatpump, and you’ll get better performance if you can upgrade your radiators to run at a low temperature (e.g. 40), especially if you also have UFH running at 30°. (Octopus favour the high temperature design in order to minimise disruption)

Octopus (and other installers) do regularly overestimate the heat loss of a property (mostly the air change rate), with several homeowners then having to battle to get them swapped out for a smaller unit.

Based on the information above, 11 kW seems too big, and I would guess that 8 kW would be a safer bet. (Note that the 9 and 11 kW Daikins are the same inside, whereas the 8 kW is different and can modulate lower in mild weather)

1 Like
4

As a point of reference, our house is similar to yours: semi-detached Victorian solid brick, ~145sqm, new insulated ground floor extension with wet UFH, new insulated loft conversion. 4 bedrooms. First floor rooms and entire front and side elevations are uninsulated solid brick.

Calculated heat loss: ~10kW @ -2C outdoor, 20C indoor.

Actual, measured heat loss at -1degC outdoor and 21C whole-house indoor: ~5.5kW.

I managed to get our Vaillant Arotherm bumped down from the 10kW model to a 7kW, by persuading the installer to factor in the near-future installation of EWI on the side elevation…

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5

If you have daily consumption and temperature data, I recommend working out the heat loss coeffiecient over a number of cold and mild days. This can give you a much more accurate idea of how much heat your property actually needs in cold weather:

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6

Your Heat Punk calculation of 10490W is the calculated heat loss at the design inside and outside temps you entered, which is effectively a requirement for an 11kW heat pump. kW is the instantaneous power, kWh is the total energy required over a period of time. Hence your 197kWh for a day, which is an average power of 8.16kW for 24hrs. Sounds like your Heat Punk calculation needs some input changes - what air change rates did you specify?

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7

No, it was at 19c/19.5c depending on room and a setback of 17c at night. Let me bump it up for the next week.

8

I didn’t mess with the air rate changes. I just put in all the real-world insulation we have and left as defaults.

9

So all, what’s the best way to approach this scientifically? Set the master thermostat to 19.5c and put the UFH in the kitchen to 20c? All 20c (not sure we can sleep with it that hot!)?

10

No.

It is just that your heat loss at the design temperatures is a bit more than you gas usage would suggest.

Using the gas consumption during a whole day doesn’t tell the full story unless the house was at the design temperatures for the whole day.

They will design at 20c in living areas, 18c in bedrooms and 21c in bathrooms.

If you are too hot in the bedrooms when it is 21c in the living areas then you can resize emitters to correct it.

11

Interesting. They left the bathroom radiators alone.

Screenshot 2025-01-09 at 11.07.53
Screenshot 2025-01-09 at 11.07.532088×958 231 KB

12

Wow.

Your radiator output is about 60% of what mine was when my 9kW Daikin was installed

It was nothing like enough.

I have about 25kW now at dT 30c, almost 5 times what you have.

That 11kW in your home will be a nightmare.

What on earth are they thinking?

How is an 11kW heat pump going to work with radiator output of only 5,417W at a flow temperature of 50c?

13

For some reason the UFH in the kitchen/diner isn’t showing. I think that’s why it look ridiculously low output.

Screenshot 2025-01-09 at 20.44.03
Screenshot 2025-01-09 at 20.44.03739×619 68.8 KB

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14

OK, that explains it

15

The weather looks perfect over the weekend to test out at a constant temperature. I’ll report back on Monday.

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16

Have you got space for a large (say 200l) volumiser if you need to add one later on the central heating return pipe? If not I would be fearfull to have that heatpump.

The mix of radators and UFH always makes systems more complex or harder to operate. If it was 100% high thermal mass UFH I would be less concerned with oversized heatpump other then Daikin not likeing low UFH flow temperatures.

17

Good morning all and thanks for the engagement here.

There was almost perfect testing conditions over the weekend. Results below…

House changed to constant 20c living, 19c bedrooms

Saturday 11/01/25
-6 to briefly 2
Average temperature -3c
202kWh gas used

Sunday 12/01/25
-3 to 3
Average temperature 0c
180kWh

78.5% efficiency boiler (35cdi ii = 27.5 max output)

18

Saturday @ -3&°C = 202 kWh ÷ 24 hours × 78.5% = 6.6 kW.
Heat loss coefficient = ~330 W/K.

At 10°C the property will only need 2.3 kW of heat to maintain temperature. If that is below the minimum output of the heatpump, it will be forced to cycle. The 11kW Daikin can modulate down to maybe 4kW? Flow temperature will need to keep high to get sufficient output from the radiators.

So yeah, this spec feels oversized to me.

19

My 11kw unit will go at its lowest 700w input (exc circ pump) for ~3kw output.

20

So what would be the correct heat pump?

High radiator temperature plus UFH, needs bigger radiators I would say?

9kW is out as it is just the same as as the 11kW, just capable of less power at the top end.

8kW on paper is possible, but I have serious doubts about getting around 7kW out of one of those at -3c, bang in the zone where defrosting is worst for these smaller units.