The Sunamp Thermino xPlus is an indirect heat battery — a compact, mains-pressure hot water system for system boilers, combi pre-heat, air source heat pumps and ground source heat pumps. Using Plentigrade P58 phase change material (PCM), it achieves Energy Class A+ when charged by a heat pump — storing thermal energy far more densely than water and measuring up to 4× smaller than the indirect cylinder it replaces. Available in three sizes: 150, 210 and 300 (litre equivalents). The Optimino key system allows the heat source to be changed — from boiler to heat pump, or from heat pump to heat pump + solar PV — by swapping the key, never the unit. Mains-pressure hot water on demand. No mandatory annual servicing. 10-year warranty on heating element and PCM core.
Energy A+
Class (Heat Pump)
Up to 4×
Smaller
199–402 L
V40 Output
10 yr
Warranty
4 Optimino
Keys
No Annual
Service
Thermino xPlus Model Range — Technical Data
Model
Equiv. Cylinder (L)
V40 Primary (L)
Max HW Flow (L/min)
Max Charge Flow (L/min)
Heat Loss (kWh/24h)
Energy Class
Weight Filled (kg)
Height (mm)
150 xPlus
142
199
15
15
0.67 (28.1W)
A+
136
640
210 xPlus
212
301
20
20
0.77 (32.1W)
A+
187
870
300 xPlus
284
402
25
25
0.84 (34.9W)
A+
233
1050
All models: Width 365mm × Depth 575mm. Minimum supply pressure: 1.5 bar. Maximum working pressure: 5 bar. Recommended TMV: 45–55°C. Max heat source flow temperature: 80°C. V40 = litres of hot water available at 40°C.
⚠ Scald Prevention: In line with UK Building Regulations, a thermostatic mixing valve (TMV) must be fitted at the heat battery outlet, set to deliver hot water at 45–55°C. The PCM stores energy at temperatures that can cause severe burns if delivered unmixed.
Key Features
Indirect Heat Battery — For Boilers and Heat Pumps
The Thermino xPlus is an indirect heat battery that replaces a traditional indirect hot water cylinder. It is charged by an external heat source via an internal heat exchanger — system boiler, combi boiler (pre-heat), air source heat pump (ASHP) or ground source heat pump (GSHP). Grid electricity serves as a backup or boost charge via the built-in immersion element.
System boilerCombi pre-heatASHP / GSHP
Plentigrade PCM — Up to 4× Smaller, Energy Class A+
Uses Plentigrade P58 PCM — Sunamp's patented phase change material. The xPlus achieves Energy Class A+ — higher than the ePlus (Class C) — because the heat pump charging pathway is significantly more efficient than direct electric heating. Up to 4× smaller than the indirect cylinder it replaces.
Plentigrade P58 PCMEnergy Class A+Up to 4× smaller
Optimino Keys — Switch Heat Source Without Changing the Unit
The Optimino key system allows the xPlus to adapt to a new heat source at any time. Start with a system boiler (vf key); later switch to a heat pump (hp key) or heat pump + solar PV (hpPV key) — simply swap the Optimino key. No new heat battery is needed. The xPlus supports a staged transition from gas to fully renewable hot water.
When charged by a system boiler the xPlus reduces gas consumption by storing heat more efficiently than a conventional cylinder. Paired with a combi boiler, the battery pre-heats water reducing the demand placed on the boiler. Switching to a heat pump via the hp key can deliver a 72% reduction in carbon emissions for hot water compared with gas.
Reduces gas use72% carbon reduction (HP)
Mains-Pressure Hot Water On Demand — Three Sizes
Three models: 150, 210 and 300 (litre equivalents). V40 output from 199 litres (150 model) to 402 litres (300 model) at 40°C. Maximum hot water flow rates from 15 l/min to 25 l/min for simultaneous outlets.
199–402 L at 40°C15–25 l/min max flowMains pressure
Ultra-Low Heat Loss — Energy Class A+
Heat loss of 0.67–0.84 kWh/24h across the range — compared with a typical indirect cylinder losing 1.5–3+ kWh/24h. The A+ energy label reflects the combined efficiency of PCM storage and heat pump charging. No mandatory annual servicing required.
A+ energy classUltra-low heat lossNo annual service
Flexible Pipe Access — Three Sides, Modular and Scalable
Pipe and cable entries on three sides. Multiple units can be connected in series or parallel for higher-demand applications. All models share the same 365 × 575mm footprint — only height varies.
3-side accessModular scalable
10-Year Warranty on Heating Element and PCM Core
Market-leading 10-year warranty on the heating element and Plentigrade P58 storage core for products purchased and installed in the UK.
10-year warranty
Technical Specifications
Heat Source Compatibility
Primary Heat Source
System boiler, combi boiler (pre-heat), ASHP or GSHP via heat exchanger
Backup Electric Charging
Yes — built-in immersion element (grid electricity)
10 years (UK purchase and installation, online registration)
Country of Origin
United Kingdom (Macmerry, East Lothian)
ℹ Installation: Must be installed by a qualified and competent person. Always refer to the manufacturer's latest installation manual. In line with UK Building Regulations, a thermostatic mixing valve (TMV) must be fitted at the outlet of all Thermino heat batteries to prevent scalding.
Frequently Asked Questions
What is the difference between the ePlus and the xPlus?
The ePlus is charged directly by electricity (mains, off-peak tariff, or solar PV diverter). The xPlus is charged by an external heat source via an internal heat exchanger — system boiler, combi pre-heat, ASHP or GSHP — with grid electricity as a backup. The xPlus achieves Energy Class A+ (vs C for the ePlus) when charged by a heat pump.
Can I switch from a boiler to a heat pump without replacing the xPlus?
Yes — this is a core design principle of the xPlus. When you install a heat pump, simply replace the vf Optimino key with the hp key. No new heat battery is needed. If you also add solar PV later, use the hpPV key.
Does the xPlus work with a combi boiler?
Yes — in pre-heat mode. The xPlus sits upstream of the combi boiler and pre-heats the cold water supply, reducing the energy the combi needs to raise water temperature. This reduces gas consumption and improves system efficiency.
What is the correct Optimino key for my installation?
vf key — system boiler (volt-free control). vfPV key — system boiler + solar PV. hp key — air or ground source heat pump. hpPV key — heat pump + solar PV. Consult Sunamp's installer resources or the installation manual for the correct key and wiring configuration.
Does the xPlus need annual servicing?
No mandatory annual maintenance is required. There are no user-serviceable parts inside the heat battery. The PCM container must not be accessed in the field — doing so voids the warranty.
Related Products
Sunamp Thermino ePlus Heat Battery
Direct electric — grid, off-peak and solar PV
Sunamp A2057 Installation Kit
PRV, TMV, expansion vessel — required accessories
Solar iBoost+ Immersion Controller
PV diversion compatible with vfPV / hpPV key configurations
BTU & WATT Calculator
Room Dimensions
Room Type
Living Room
Bedroom
Kitchen
Bathroom
Conservatory
Hallway
Glazing Type
Single Glazed
Double Glazed
Triple Glazed
External Walls
0 Wall
1 Wall
2 Walls
3 Walls
4 Walls
Insulation Level
Poor
Average
Good (New Build)
Required Heating
0
Watts
0
BTUs
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