A thermal store takes a fundamentally different approach to hot water delivery compared to a vented cylinder or a conventional unvented system. The vessel contains a static body of water — heated by immersion, a primary coil, or both — and incoming cold mains water passes through an internal heat exchanger coil, picking up heat and exiting at mains pressure through a blending valve. The stored water never leaves the vessel; the mains supply never enters it. This closed-loop arrangement is what sets thermal stores apart.
The key installation advantage: no G3, no Part G
Because thermal stores are open vented by design, they fall outside the scope of Part G Building Regulations. They are not subject to the discharge requirements associated with traditional unvented cylinders, which means no G3-qualified installer is required, no tundish, no dedicated discharge pipework, and no annual safety inspection. For flats, apartments, and retrofit projects where running discharge pipework is impractical or cost-prohibitive, this is a significant advantage. The output is still mains-pressurised hot water — the regulatory simplicity does not come at the expense of performance.
Who specifies thermal stores?
Thermal stores suit a wide range of applications. They are particularly well suited to flats and apartments where unvented cylinder installations are complex or simply not viable. They also appeal in properties with multiple heat sources — a boiler or district heating primary circuit alongside an immersion — where the indirect (Electric Plus) variants can accept heat from both simultaneously or in sequence. They work well with off-peak electricity tariffs and solar PV, storing heat generated during low-rate periods or surplus generation for use on demand.
Products in this collection
The Harlequin HeatStream range covers both direct electric-only and indirect (Electric Plus) configurations. The indirect models include a primary coil rated for connection to a boiler, district heating, or other heat source alongside the immersion heaters. The inner tank is constructed from non-corrosive polypropylene, making it suitable for areas with poor water quality, and the compact 520 × 520 mm square footprint means it will fit standard airing cupboards and pass through a normal door opening. Where an external overflow connection is not available, internal overfill variants are available that require no external pipework at all.
The Mixergy Cube X brings smart controls and stratification technology to the thermal store format. Rather than heating the full vessel, it uses top-down stratification to heat the upper layer of water first, so hot water is available quickly without wasting energy on the entire stored volume. An embedded solar diverter allows surplus PV generation to be directed to the cylinder, and smart tariff integration via the Mixergy app enables scheduling around cheaper overnight rates.
What to consider when selecting
Capacity sizing follows the same principles as any hot water cylinder: 150 litres suits most one- to two-bathroom properties; 210 litres is appropriate where demand is higher or where simultaneous draw-off from multiple outlets is expected. Unlike a direct unvented cylinder, a thermal store does not require an expansion vessel or T&P relief valve on the cold water inlet, simplifying the installation pipework. A boost immersion can bring 50 litres up to temperature within 20 minutes, providing a useful recovery option where off-peak scheduling is in use. Installers should confirm available electrical supply before specifying immersion wattage, particularly in flats where consumer unit capacity may be constrained.
