Octopus Cosy vs Grant Aerona3: Which Air Source Heat Pump Is Better? (2026)
Quick Verdict
Grant Aerona3 is the more efficient choice on its MCS-certified figures, with lower estimated running costs, while Octopus Cosy has the longer standard warranty (8 yr).
Side-by-Side Comparison
| Feature | Octopus Cosy | Grant Aerona3 |
|---|---|---|
| Installed price (before grant) | £8,000 - £12,000 | £8,000 - £13,000 |
| After £7,500 grant (illustrative, England and Wales) | £500 - £4,500 | £500 - £5,500 |
| SCOP at 35°C (MCS) | 4.36 | 5.2Higher |
| SCOP at 55°C (MCS) | 3.43 | 3.47Higher |
| Largest size | 9kW | 10kWLarger homes |
| Standard warranty | 8 yearsLonger | 1 year |
| Longest warranty (conditions apply) | 8 yearsLonger | 7 years |
| Sound power (declared) | 56 dB(A)Lower | 64 dB(A) |
| Annual running cost (est., 45°C flow) | ~£675/yr | ~£605/yrLower |
| Refrigerant and max flow | R290, up to 70°C flowHotter flow for radiators | R32, up to 60°C flow |
| Country | UK | Ireland |
| UK brand | Yes | No |
| Best for | Price-led switchers happy to buy and install through Octopus | Off-gas homes replacing oil or LPG heating, where the £9,000 grant can apply in England and Wales |
Specifications and pricing are indicative for the UK market as at June 2026. Post-grant figures illustrate the £7,500 Boiler Upgrade Scheme in England and Wales (Scotland uses the Home Energy Scotland grant and loan; Northern Ireland has no equivalent). Running costs assume ~10,000 kWh of heat a year at 26.32p/kWh (Ofgem cap average, October to December 2026; changes quarterly). Confirm current models and exact pricing with your MCS-certified installer.
Model by Model, Size for Size
Each Octopus Cosy model against the nearest Grant Aerona3 size. Better figure in green.
| 9 kW vs 10 kW | Cosy 9R290 | Aerona3 R32 10kWR32 · HPID10R32 |
|---|---|---|
| SCOP at 35°C (MCS) | 4.36 | 5.20 |
| SCOP at 55°C (MCS) | 3.43 | 3.47 |
| Sound power (declared) | 56 dB(A) | 64 dB(A) |
| Maximum flow temperature | 70°C | 60°C |
| Running cost (est.) | ~£675/yr | ~£605/yr |
| Installed price from (before grant) | £9,000 | £8,500 |
| Outdoor unit weight | 125 kg | 70 kg |
| Suits | 3-4 bed homes | rural 3-4 bed homes replacing oil |
Sizes are the makers' labels; where a label is not the rated output, the brand page says what is. Prices are PumpSwap's indicative estimates. Running costs use the same basis as the table above.
Price Comparison
Octopus Cosy starts from £8,000 installed before any support (indicative, June 2026), while Grant Aerona3 starts from £8,000. Both are installed prices in GBP with 0% VAT (until 31 March 2027) already applied. In England and Wales the £7,500 Boiler Upgrade Scheme grant comes off both equally, so the pre-grant difference carries straight through: illustratively £500 - £4,500 for Octopus Cosy and £500 - £5,500 for Grant Aerona3 out of pocket. Your MCS-certified installer claims the grant for you.
Efficiency Comparison
SCOP (seasonal coefficient of performance) is the key efficiency measure for a heating heat pump: the kilowatt-hours of heat you get per kilowatt-hour of electricity across a whole heating season. The figures here are MCS-certified, at 35°C flow (underfloor heating or large radiators) and 55°C (hotter radiators). Octopus Cosy is certified at 4.36 at 35°C and 3.43 at 55°C, while Grant Aerona3 is certified at 5.2 and 3.47. On our estimates at a 45°C design flow temperature, the Grant Aerona3 costs about £70 a year less to run at Ofgem cap electricity prices, or approximately £700 over ten years. Real-world efficiency depends heavily on system design and flow temperature, which is what the MCS heat loss survey is for.
Warranty Comparison
Octopus Cosy: 8-year manufacturer warranty on the hardware as standard, plus 2 years on workmanship. Cosy Care (paid, from £12 a month) extends cover on Octopus installs. Grant Aerona3: 12 months as standard; 2 years if registered within 30 days and serviced annually; 7 years when a Grant-trained G1 installer registers it within 30 days, with annual servicing. On standard terms, the Octopus Cosy has the longer cover; the longer terms on either side depend on registration, servicing and the installer. Workmanship cover for the installation itself comes separately from your installer and should be in writing.
Noise Comparison
Noise matters in UK homes because the outdoor unit often sits close to a neighbour's boundary or a bedroom window. The like-for-like figure is the maker's declared sound power: the noise the unit itself emits. Octopus Cosy: 56 dB(A). Grant Aerona3: 64 dB(A). On paper the Octopus Cosy is quieter by 8 dB(A). What a neighbour hears is much lower and falls with distance, and permitted development rules require the installation to meet the MCS noise standard at their window, so siting matters as much as the unit.
Cold-Weather and Retrofit Suitability
Two things decide how well a unit suits an older UK home: cold-weather output and flow temperature. Octopus Cosy uses R290 refrigerant (up to 70°C flow), while Grant Aerona3 uses R32 (up to 60°C flow). The Octopus Cosy's higher flow temperature helps existing radiators carry the load without a full re-pipe. Modern units hold useful output well below freezing. In the colder parts of the UK (Scotland and northern England in particular), frosty mornings trigger short defrost cycles that trim efficiency, so ask each installer for low-temperature output figures and take the MCS heat loss survey seriously; radiator upsizing is more commonly recommended there so the system can run at efficient flow temperatures.
Pros and Cons
Octopus Cosy
Pros
- Octopus median quote £4,064 after the grant (June to September 2026)
- Designed and built in the UK
- 8-year hardware warranty as standard
- Your installer handles the grant paperwork
Cons
- Only available through Octopus or its accredited installers
- Newer hardware: Cosy 6 launched in late 2023, Cosy 9 in 2025
Grant Aerona3
Pros
- Certified SCOP of 5.2 at 35°C (MCS)
- Compact and light (70 kg)
- 7-year warranty through a G1 installer
- Rated to work down to -20°C
Cons
- High sound power (64 dB(A)), so siting matters
- Maximum flow temperature 60°C, lower than R290 units
- Standard warranty only 12 months unless registered
Which Should You Choose?
Choose Octopus Cosy if...
- You want a longer standard warranty (8 yr vs 1 yr (up to 7))
- A lower declared sound power matters to you (56 vs 64 dB(A))
- You want a UK brand with local manufacturing or headquarters behind it
- You want natural R290 refrigerant
- You are keeping existing radiators and want a higher maximum flow temperature (70°C vs 60°C)
Choose Grant Aerona3 if...
- You want the higher certified efficiency (SCOP 5.2 vs 4.36 at 35°C, MCS)
- You want lower estimated annual running costs (~£605/yr vs ~£675/yr)
The grant is the same, whichever brand you pick
Every air-to-water unit on this page qualifies for the £7,500 Boiler Upgrade Scheme grant in England and Wales (£9,000 for oil and LPG heated homes until March 2027) when fitted by an MCS-certified installer, who claims it on your behalf so it comes straight off the quote. Scotland uses the Home Energy Scotland grant of up to £7,500 (£9,000 rural and island) plus an optional interest-free loan; Northern Ireland currently has no equivalent scheme. 0% VAT on residential installations applies UK-wide until 31 March 2027. Accurate as at July 2026.
Heat pump guides
- Air source heat pump installation cost in the UK
- The best air source heat pumps in the UK, compared
- The £7,500 Boiler Upgrade Scheme, explained
- Air source heat pumps: costs, grants and brands
- Heat pump grants in London
- Heat pump grants in South East England
- Heat pump grants in South West England
- Heat pump grants in East of England
- Heat pump grants in East Midlands
- Heat pump grants in West Midlands
- Heat pump grants in Yorkshire and the Humber
- Heat pump grants in North West England
- Heat pump grants in North East England
- Heat pump grants in Scotland
- Heat pump grants in Wales
- Heat pump grants in Northern Ireland
Where are you?
See indicative costs and local installers for your area