Your Boiler Cupboard Called. It Wants Its Space Back.
Traditional hot water cylinders are enormous, inefficient, and increasingly incompatible with the low-carbon systems — heat pumps, solar PV — that modern UK homes are transitioning to. They leak heat around the clock, require G3 certification, annual servicing, and careful Legionella management.
Sunamp — the Scottish engineering company that earned the King's Award for Enterprise Innovation (the first ever in thermal energy) — built the Thermino heat battery to fix this in one product. Instead of storing energy as a large tankful of hot water, Thermino stores it in a phase-change material (PCM): a compact, ultra-dense substance that absorbs and releases enormous amounts of heat as it melts and re-solidifies. The result is a unit no larger than a tall under-counter appliance that delivers the equivalent of a 300-litre cylinder's worth of hot water — instantly, at mains pressure, with no G3 certification required and zero mandatory servicing on the unit itself.
At AIZO Quality Heating, we stock the full Sunamp Thermino range — Thermino ePlus, Thermino xPlus, all Optimino Keys, and the Installation Kit — ready to ship.
"Thermino is not a better hot water tank. It is a fundamentally different way to store domestic heat — one that shrinks the footprint, eliminates compliance overhead, and works with every low-carbon energy source."
Sunamp — World Leaders in Thermal Energy Storage, EdinburghHow Does a Heat Battery Actually Work?
At the heart of every Thermino is Plentigrade P58 — a patented phase-change material developed with the University of Edinburgh and tested at the Diamond Light Source particle accelerator. Plentigrade stores energy as latent heat: the enormous amount of energy locked inside a material as it transitions between solid and liquid — the same principle as a pocket hand warmer, but engineered for 50 years of daily domestic use.
Three-Stage Operation
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1
Charging
A heat source — grid electricity, off-peak tariff, heat pump, boiler, or surplus solar PV — heats the Plentigrade PCM via an internal element or heat exchanger coil. The material melts and stores the energy as latent heat at a stable 58°C.
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2
Storage
The PCM holds heat with ultra-low standby losses — under 0.77 kWh per day, less than a quarter of that of a comparable hot water cylinder. Under 15 litres of water is held internally, eliminating Legionella risk by design.
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3
Hot Water On Demand
When you open a tap, cold mains water flows through the high-power internal heat exchanger, instantly drawing heat from the PCM. You get mains-pressure hot water at a constant temperature — like a combi boiler, but from stored thermal energy.
Why Plentigrade Is Different
4× More Energy Dense Than Water
Plentigrade P58 stores four times more thermal energy per litre than water — the core reason a Thermino can be dramatically smaller than any equivalent cylinder.
RAL Certified — World's Only
The only thermal battery globally to hold RAL PCM certification. No noticeable degradation confirmed to 40,000 cycles — over 50 years of daily use.
University of Edinburgh Partnership
Co-developed with one of the world's top 20 universities. Royal Society of Chemistry Industry-Academia Award winner 2022.
G3 Exempt — No Discharge Pipework
Under 15L internal water volume means no unvented (G3) certification, no tundish, and no external discharge pipework.
Non-Toxic, Non-Flammable & Recyclable
Salt-based, no heavy metals or hazardous chemicals. All components fully recyclable at end of life.
Up to 4× Smaller Than a Traditional Cylinder
A Thermino 300-litre equivalent sits at around 60cm tall — the footprint of a large under-counter appliance. A traditional unvented cylinder demands over 1.8 metres of ceiling height and an entire airing cupboard.
Unvented Cylinder
~1.8m tall
Sunamp Thermino xPlus 300
~0.6m tall
Fits Under Stairs, in Kitchens & Tight Spaces
Opens install locations that conventional cylinders simply cannot reach. Over 30,000 installed across UK homes, new builds and retrofit projects.
No Discharge Pipework Needed
G3 exempt means no tundish, no external pipework to outside, and no compliance overhead around pressure relief routing.
Perfect for Combi Replacements & New Builds
Ideal where no hot water cylinder currently exists. Installs in spaces too small for any conventional cylinder, including flats and apartments.
Phase Change Materials: The Future of Hot Water
Independent technology channel Undecided with Matt Ferrell (1M+ subscribers) covers how phase-change materials are reshaping domestic hot water heating, with the Sunamp Thermino as the centrepiece. The video is queued directly to the Thermino section — no skipping required.
▶ "You've Never Seen a Water Heater Like This" — Undecided with Matt Ferrell · Featured by Sunamp on their official website
For Sunamp's own installation walkthroughs and product videos, visit @SunampLtd on YouTube.
ePlus vs xPlus: Which Thermino Do You Need?
Two Thermino families. Same Plentigrade PCM core. Same mains-pressure performance. The difference is how they're charged — direct electric or indirect (heat pump / boiler).
Thermino ePlus Range
Charged by grid electricity, off-peak tariffs (Economy 7, Octopus), 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.
From £1,595 inc. VAT → Indirect · xPlusThermino xPlus Range
Charged via heat exchanger coil by a heat pump, boiler, or solar thermal — with optional grid or solar PV backup. The natural choice for any heat pump installation replacing an indirect unvented cylinder. Available in 150L, 210L and 300L equivalents.
From £2,090 inc. VAT →Quick guide: ePlus = direct electric / solar PV only. xPlus = heat pump or boiler as primary source. Both support the Optimino key ecosystem — add new energy sources at any time without replacing the battery.
Sunamp Thermino vs Heat Pump Water Heaters
Heat pump water heaters are often positioned as the go-to low-carbon hot water solution. They work well in some scenarios — but for most UK homes switching from a combi boiler, the Thermino offers a more space-efficient, cost-effective, and compliance-friendly path.
| Criteria | ✦ Sunamp Thermino | Heat Pump Water Heater |
|---|---|---|
| Installed Cost | ✓ ~£4,300 (Thermino xPlus 300 typical) | £8,750–£12,500 typical installed |
| Physical Size | ✓ Up to 4× smaller — 60cm tall equivalent | Large combined cylinder + compressor unit |
| G3 / Unvented Cert | ✓ Fully exempt — under 15L internal water | ✗ Full G3 compliance required for installation |
| Annual Maintenance | ✓ None mandatory on the heat battery itself | ✗ Annual servicing recommended on compressor |
| Legionella Risk | ✓ Inherently safe — no bulk stored water | Requires periodic thermal disinfection cycles |
| Standby Heat Loss | ✓ ~0.77 kWh/day — less than a quarter of old cylinders | Varies; larger water volume = higher standing loss |
| Solar PV Integration | ✓ Via plug-in Optimino PV Key (simple add-on) | Possible, but typically requires additional setup |
| Heat Pump Compatibility | ✓ Wide brand compatibility via Optimino keys | Has own built-in heat pump compressor |
| Boiler Upgrade Scheme | ✓ Qualifies as ancillary equipment alongside HP grant | — Separate product category, different process |
| Product Lifespan | ✓ 50+ years / 40,000 cycles (RAL certified) | Typically 10–15 years for compressor components |
| Flow & Pressure | ✓ Instant, mains pressure, high flow rate | Mains pressure (cylinder-fed) |
| Running Costs | ≈ Comparable when charged off-peak or via solar PV | ~£139/yr via COP 3–4 vs ~£418 direct electric |
Cost data from Sunamp published pricing and GreenMatch / Eco Experts analysis (2025). Actual costs vary by property size, tariff, and installer. Read Sunamp's full technical comparison →
The Keys That Unlock Every Energy Source
One of the smartest aspects of the Thermino system is its modular Optimino key architecture. These small control modules plug directly into the Thermino and allow it to intelligently draw from different energy sources — without ever replacing the heat battery itself. Start with grid electricity; add solar PV or switch to a heat pump later. All three key types are stocked at AIZO.
Optimino PV Key for Solar
Captures surplus solar PV generation and charges the Thermino ePlus with free renewable electricity. Works alongside a solar diverter (e.g. myenergi Eddi) 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. 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 air source heat pumps (Vaillant, Daikin, Samsung, Bosch and more). Available in multiple variants to match your heat pump model.
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
- Installation Documentation
⚠️ We strongly recommend ordering the Installation Kit alongside your Thermino. These components are required for a compliant install and will save your installer significant time on site.
A Technology That Has Earned Its Reputation
Sunamp have been refining phase-change thermal storage for nearly two decades. Their track record is recognised by the highest institutions in UK industry, science and government.
King's Award for Enterprise — First-ever in Thermal Storage (2023)
Deloitte UK Tech Fast 50 — Only Environmental Tech Manufacturer Listed
Royal Society of Chemistry — Industry-Academia Collaboration Award 2022
RAL Certification — World's Only PCM Product with Global Phase Change Standard
£9.25M Government Grant — Advanced Thermal Storage Trial Across 100 UK Homes
Everything You Need — In Stock Now
Thermino heat batteries, Optimino keys, and the Installation Kit — all stocked at AIZO Quality Heating and ready to ship. Our technical team can help you specify the right model and key for your project.
AIZO Quality Heating · service@qualityheating.co.uk · 020 4577 3096 · qualityheating.co.uk