The Traditional Hot Water Cylinder Has Met Its Match
For decades, storing domestic hot water meant one thing: a large, inefficient cylinder sitting in your airing cupboard, radiating heat you paid for into the air, demanding annual servicing, and making heat pump retrofits near-impossible. Sunamp built the Thermino to fix all of that.
The Thermino heat battery is the first domestic hot water solution built around a phase-change material (PCM) — a substance that stores thermal energy not as a large volume of hot water, but as latent heat locked inside a compact crystalline material. The result is a unit up to four times smaller than an equivalent cylinder, delivering instant mains-pressure hot water with no mandatory servicing, no G3 certification required, and no Legionella risk by design.
Available in full at AIZO Quality Heating, the Sunamp Thermino range — Thermino ePlus, Thermino xPlus, all Optimino keys, and the Installation Kit — is in stock and ready to ship.
"The Thermino doesn't just replace a hot water cylinder. It replaces the concept of one — storing energy in matter rather than water, and delivering it on demand, instantly, at mains pressure."
Sunamp — World Leaders in Thermal Energy Storage, EdinburghThree Stages, Zero Compromise
The Thermino's operation is elegantly simple: charge it from any energy source, hold the heat with minimal loss, then release it as instant mains-pressure hot water on demand. The Plentigrade PCM core handles all three stages with physics that water simply cannot match.
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Charging — From Any Energy Source
A heat source heats the Plentigrade PCM inside the Thermino — via a direct immersion element (ePlus) or heat exchanger coil (xPlus). Compatible sources include grid electricity, Economy 7 and off-peak tariffs, surplus solar PV via the Optimino PV Key, air and ground source heat pumps, and gas or electric boilers. The PCM melts at 58°C, storing the energy as latent heat in typically 30–60 minutes.
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Storage — Ultra-Low Heat Loss, No Legionella Risk
The charged Thermino retains heat with class-leading efficiency. Vacuum insulation panels limit standby heat loss to under 0.77 kWh per 24 hours — less than a quarter of a comparable unvented cylinder. Critically, the Thermino holds less than 15 litres of water internally — far below the threshold that creates Legionella risk. No disinfection cycles. No risk protocols.
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Delivery — Instant Hot Water at Mains Pressure
When a tap opens, cold mains water flows through the Thermino's high-powered internal heat exchanger, extracting heat from the PCM and emerging as hot water at full mains pressure. No stored cylinder to wait for, no pressure-balancing vessel. The Thermino behaves like a combi boiler for hot water — instantly on, constant temperature, mains-pressure throughout.
Why Plentigrade PCM Is Unlike Anything Else
Every Thermino is powered by Plentigrade P58 — a patented phase-change material co-developed with the University of Edinburgh and validated at the Diamond Light Source particle accelerator. It is the most rigorously tested PCM in domestic thermal storage, and the only one in the world to hold RAL certification.
4× More Energy Dense Than Water
Plentigrade P58 stores four times more thermal energy per litre than water — why a Thermino can be dramatically smaller than any equivalent cylinder while delivering the same volume of hot water.
RAL Certified — World's Only
The only thermal battery manufacturer globally to hold RAL PCM certification. No noticeable degradation confirmed to 40,000 cycles — the equivalent of over 50 years of daily domestic use.
University of Edinburgh Partnership
Co-developed with one of the world's top 20 universities and validated at the Diamond Light Source synchrotron. Royal Society of Chemistry Industry-Academia Award winner 2022.
G3 Exempt — No Discharge Pipework
Under 15L internal water volume means no G3 unvented certification, no tundish, and no mandatory external pressure relief discharge pipework. Simpler installation, wider installer pool.
Constant 58°C Output Temperature
Unlike water cylinders that cool from the top down, Plentigrade releases heat at a consistent temperature throughout the entire discharge cycle. No first-draw penalty, no temperature drift.
Non-Toxic, Non-Flammable, Recyclable
Salt-hydrate based with no heavy metals or hazardous chemicals. Every component is fully recyclable at end of life, and the PCM itself is refillable — not landfilled.
Up to 4× Smaller Than a Traditional Cylinder
A Thermino 300-litre equivalent stands roughly 60 cm tall — the footprint of a large under-counter appliance. A traditional unvented cylinder of equivalent capacity demands over 1.8 metres of ceiling clearance and a full airing cupboard.
Fits Under Stairs, Kitchens & Tight Spots
Opens installation locations conventional cylinders cannot reach. Over 30,000 units installed across UK homes, housing associations, new builds and retrofits.
No Discharge Pipework Required
G3 exempt — no tundish, no compliance overhead, no pipework to outside. One of the biggest hidden installation cost reductions of any modern hot water solution.
Enables Heat Pump Retrofits Everywhere
The biggest barrier to heat pump retrofits is cylinder space. A Thermino eliminates that barrier — allowing a heat pump to be installed where a cylinder simply wouldn't fit.
See Why the Thermino Is Unlike Any Hot Water System
Independent technology channel Undecided with Matt Ferrell (1M+ subscribers) covers how phase-change materials are reshaping domestic hot water, with the Sunamp Thermino as the centrepiece. Video queued directly to the Thermino section at 3:58 — no skipping needed.
▶ "You've Never Seen a Water Heater Like This" — Undecided with Matt Ferrell · Officially featured by Sunamp on sunamp.com · Queued to Thermino section at 3:58
For Sunamp's own installation guides and product walkthroughs visit @SunampLtd on YouTube →
ePlus vs xPlus: Which Thermino Do You Need?
Two Thermino families. Same Plentigrade PCM core. Same mains-pressure hot water performance. The difference is how they are charged — directly by electricity and solar, or indirectly by a heat pump or boiler.
Thermino ePlus Range
Charged by grid electricity, off-peak tariffs (Economy 7, Octopus Agile), or surplus solar PV via the Optimino PV Key. The direct replacement for an electric hot water cylinder. Available in 70L, 150L, 210L and 300L equivalents. A+ energy efficiency rated on the 150/210/300 models.
From £1,595 inc. VAT — View Range →Thermino xPlus Range
Charged via its internal heat exchanger by a heat pump, boiler, or solar thermal — with optional grid or solar PV top-up via Optimino keys. The natural choice for heat pump installations replacing an unvented indirect cylinder. Available in 150L, 210L and 300L equivalents. A+ rated across the range.
From £2,090 inc. VAT — View Range →Quick guide: ePlus = grid electricity, off-peak tariffs, solar PV. xPlus = heat pump or boiler as primary source. Both support the modular Optimino key system — add new energy sources at any time without replacing the battery.
Sunamp Thermino vs Heat Pump Water Heaters
Heat pump water heaters are widely marketed as the go-to low-carbon hot water solution. For most UK homes switching from a combi boiler or replacing an existing cylinder, the Thermino delivers a more space-efficient, cost-effective and compliance-friendly outcome.
| Criterion | ✦ Sunamp Thermino | Heat Pump Water Heater |
|---|---|---|
| Installed Cost | ✓ From ~£4,300 (Thermino xPlus 300) | £8,750–£12,500 typical fully installed |
| Physical Size | ✓ Up to 4× smaller — approx. 60 cm tall | Large cylinder + compressor unit |
| G3 / Unvented Cert | ✓ Fully exempt — under 15L internal water | ✗ Full G3 compliance required |
| Annual Maintenance | ✓ None mandatory on the heat battery | ✗ Annual compressor service recommended |
| Legionella Risk | ✓ None by design — no bulk stored water | Requires periodic thermal disinfection |
| Standby Heat Loss | ✓ ~0.77 kWh/day — best in class | Higher — larger water volume = more loss |
| Solar PV Integration | ✓ Plug-in Optimino PV Key — simple add-on | Possible, but needs additional setup |
| Heat Pump Compatible | ✓ Wide brand compatibility via Optimino keys | Has its own built-in heat pump compressor |
| Boiler Upgrade Scheme | ✓ Qualifies as ancillary equipment with BUS | — Separate grant pathway |
| Product Lifespan | ✓ 50+ years / 40,000+ cycles (RAL certified) | Typically 10–15 years for compressor parts |
| Mains Pressure Flow | ✓ Instant, constant temperature, mains pressure | Mains pressure (cylinder-fed) |
Cost data from Sunamp published pricing and GreenMatch / Eco Experts analysis (2025). Actual costs vary by property, tariff, and installer. Read Sunamp's full technical comparison →
The Keys That Unlock Every Energy Source
These small control modules plug directly into the Thermino and allow it to draw from different energy sources — without ever replacing the heat battery itself. Start on grid electricity, add solar PV, switch to a heat pump. The battery you buy today is future-proofed for the energy system you'll build tomorrow.
Optimino PV Key for Solar
Captures surplus solar PV generation and charges the Thermino ePlus with free renewable electricity. Works alongside solar diverters such as myenergi Eddi and Immersun for maximum self-consumption.
£59.00 inc. VAT → 🔥Optimino xPlus Boiler + PV Key
Enables the Thermino xPlus to work with a gas or electric boiler as primary heat source, with optional solar PV top-up. The smart transition path for homes not yet ready for a heat pump.
£123.00 inc. VAT → 🌡️Optimino Keys for Heat Pumps
Enables the Thermino xPlus to work with leading heat pumps — Vaillant, Daikin, Samsung, Bosch, Kensa and more. Multiple variants to match your specific heat pump model and control protocol.
From £79.00 inc. VAT →Don't Forget the Installation Kit
Everything your installer needs to commission a Thermino in a single kit — one order, one delivery, no last-minute supply runs.
- Expansion Vessel
- Pressure Relief Valve
- Brass Push-Fit Connectors
- Check Valves
- Isolation Valves
- Tempering Valve
- Installation Manual
⚠️ We strongly recommend ordering the Installation Kit with your Thermino. These components are required for a compliant install and save significant time on site.
A Technology That Has Earned Its Reputation
Sunamp has been refining phase-change thermal storage for nearly twenty years. The recognition they have received comes from the highest institutions in UK industry, science and government.
King's Award for Enterprise in Innovation — first ever awarded in thermal energy storage (2023)
Deloitte UK Technology Fast 50 — only environmental tech manufacturer on the list
Royal Society of Chemistry Industry-Academia Collaboration Award 2022 with University of Edinburgh
RAL PCM Certification — world's only thermal battery with the global phase-change material quality standard
£9.25M UK Government Innovate UK grant for advanced thermal storage trial across 100 UK homes
30,000+ heat batteries installed across UK homes, housing associations, holiday lets & commercial sites
Everything You Need — In Stock at AIZO
The full Sunamp Thermino range, all Optimino keys, and the Installation Kit — stocked and ready to ship. Our technical team can help specify the right Thermino and key combination for your project.
AIZO Quality Heating · service@qualityheating.co.uk · 020 4577 3096 · qualityheating.co.uk