The Sunamp uses a food-grade material called Plentigrade. When this material is heated, temperature rises in the usual way. But at 58 degrees, something different happens: it continues absorbing heat energy without increasing in temperature. That energy is used to melt the material from solid to liquid. Once fully melted, the temperature rises again.
This is phase change. The result is that the Sunamp stores far more energy per litre than a water-filled cylinder. Sunamp claim their units are up to four times smaller than a standard hot water cylinder for equivalent storage. When you call for hot water, cold mains water passes through the store and is heated as the Plentigrade material transitions back from liquid to solid at 58 degrees. The Sunamp also stores at 58 degrees, which gives a naturally low risk of legionella without needing boosted temperatures.
Space-constrained homes are the Sunamp's strongest use case. If a property does not have room for a standard cylinder, no airing cupboard, no utility room, no garage, the Sunamp opens up a heat pump installation that would otherwise not be feasible. In this situation, the efficiency trade-off is worth taking. Hot water accounts for roughly 2,500 kWh per year in a typical home. The additional cost from lower efficiency is real but not enormous, particularly if you are using time-of-use tariffs or solar PV to keep energy costs low.
When a heat pump heats a standard cylinder, there is one point of heat exchange: the coil in the cylinder. The flow temperature from the heat pump needs to be slightly higher than the target store temperature to drive that exchange, a gap called distortion. A well-designed cylinder with a large coil minimises this gap, allowing the heat pump to run at low flow temperatures and high efficiency.
With the Sunamp, there are two points of heat exchange. The heat pump has to heat the Plentigrade material to above 58 degrees, and then the stored heat has to transfer to the cold water at the point of use. This double distortion means the heat pump needs to run at a higher flow temperature to charge the Sunamp.
In our monitoring data, flow temperatures required to charge a Sunamp regularly reach 70 degrees. At those temperatures, heat pump efficiency averages around 230 to 250%. A well-designed standard cylinder with a properly commissioned heat pump will average over 400% efficiency for hot water, and regularly exceeds 500% in good conditions.
The Sunamp also does not provide the heat pump with a store temperature reading. It uses a resistor switch that signals either cold or hot rather than a continuous temperature. This limits the heat pump's ability to modulate and optimise its output. These are not reasons to dismiss the Sunamp. They are reasons to go in with clear expectations about what you are trading away.
If space is the constraint but a full Sunamp installation is not the preference, there are alternatives worth considering. A smaller standard cylinder at a higher store temperature stores more usable hot water in a smaller tank, though the heat pump runs at higher flow temperatures for those charges. The Heat Geek Mini Store is a compact shell and tube heat exchanger designed for space-constrained installations, with better efficiency characteristics than the Sunamp in most configurations.
The Sunamp is a genuine and well-engineered product. It solves the problem of space. If that is your problem, it solves it well. If space is not your problem, a standard cylinder designed for heat pump use will deliver significantly better efficiency and lower running costs.
