A conventional unvented cylinder is a single thermal mass. Its immersion element or coil heats the whole tank to setpoint, and cold mains water entering at the base mixes with the stored charge as soon as it is drawn. Every litre is heated whether or not it gets used, and the full hot surface area keeps leaking heat into the airing cupboard. A standard or poorly insulated cylinder typically loses ~0.65 to 0.9 kWh/day this way, paid for again with each reheat.
Mixergy reverses the flow. A pump draws cold water from the base, passes it over the heat source and returns it to the top of the vessel. This builds a hot layer downward from the crown, with a sharp thermal boundary layer (a stratification shield) between the hot and cold zones that stops cold water diluting the usable charge. Embedded sensors report state-of-charge as a percentage, so the controller can heat 30% of the tank instead of 100%.
Sunamp removes water storage altogether. Its cells hold sodium acetate trihydrate, a PCM that stores large amounts of latent heat as it melts at a steady 58°C, then releases that heat to mains water through an internal heat exchanger. Because energy density is far higher than sensible-heat water storage, and the core is wrapped in vacuum insulation panels (VIPs), standby leakage falls to a small fraction of a cylinder’s.
| Metric | Mixergy (stratified cylinder) | Sunamp (PCM battery) |
|---|---|---|
| Mechanical footprint | Same as a conventional cylinder; drop-in replacement | Ultra-compact; fits kitchen units and tight plant spaces |
| Standby degradation | Reduced by part-charging; losses scale with the heated volume | VIP-insulated latent store; a minor fraction of cylinder losses |
| Storage physics | Sensible heat, stratified water column | Latent heat, phase change at 58°C |
| Time-of-use integration | Percentage-based charging, tariff-aware scheduling | Charges to full latent capacity during cheap windows |
Both platforms turn hot water into a flexible grid asset. Automated machine-learning schedules learn household draw patterns and pre-charge thermal reserves during cheap overnight windows or Agile negative-pricing events, then coast through the evening peak. For specifiers, the choice comes down to geometry and retrofit constraints. Mixergy keeps a familiar cylinder footprint and adds fine-grained control; Sunamp gives up the water volume entirely in exchange for density and very low standby losses.