This does not prevent the use of ASHPs as they can be pushed to deliver a higher percentage contribution, generating working temperatures of 45-50°C for preheat, but this at the cost of performance efficiency, requires electrical energy, and that has operating cost implications. However, when compared to an equivalent-sized direct-electric (i.e., from the grid) system, one with an ASHP can achieve carbon reductions of 42-47%, whilst saving 25-35% of the energy costs.
The system will still be required to top up the heat to the necessary 60°C, preferably using an electric boiler. This, combined with the heat pump’s reduced operational efficiency means it will still be much more expensive to run than an equivalent-sized gas-fired system based on a modern and efficient (109% net) water heater. The recommendation in this case is to keep electrical demand down by increasing the size of the hot water storage which is then heated more slowly. This is very different to the high energy input, and low storage seen with gas-fired systems.
A 30kW energy source can heat 750 litres/hour by 34°C, so when the system draws hot water at a faster rate than it can be heated to 44°C for hot showers you start to get complaints that the water is ‘cold’. The larger volume cylinder helps to overcome this under sizing allowing for a two-hour reheat cycle that maintains enough water at 60°C to meet daily demand, whilst slowly heating reserves through the night when demand is minimal to meet the morning peak.
Despite this, carbon savings and costs are no longer aligned. As an example, if we take a building with an average occupancy rate of 23.5 with provision of basins, and shower/wet rooms, typically seen in student accommodations, care homes or boutique hotels, the yearly running costs resulting from a change from gas to direct electric would increase from £1019 to £3019 (based on electricity on average currently costing as much as 3.8 times that of gas). Even with an ASHP operating at optimum efficiency (for 35% recorded reduction in energy) costs would be £2862. Close to three times that of gas alone, so it is inherently important to consider the nominal value of the carbon reduction, especially if planning a refurbishment from gas to electricity.
New build projects, unless exhibiting very large hot water demands, will struggle to receive permission (under Part L of the building regulations) for a new gas connection and as a result will specify electric-based systems. This still should lead to application design that blends ASHP for preheat with other sustainable options that can include solar thermal, but particularly electric boilers.
The simplest approach blends preheat, such as from an Adveco FPi32 ASHP with, for example, the Adveco ARDENT 9-100 kW electric boiler to supply thermal energy to a mains water-fed compact indirect cylinder. Balancing such a hybrid electric system is key to ensuring efficient operation, so consideration needs to be given to controls to assure the water heating remains consistent, and that the two technologies do not fight each other. Working in a balanced combination, enables systems to be sized down, by as much as half in terms of ASHP requirements. This delivers immediate capital savings as electric boilers are far less expensive compared to an equivalent heat pump. You also immediately reduce the physical size of the system embodied carbon and demand from the electric supply.
As a high-temperature heat source, the electric boiler is capable of providing temperatures of up to 75°C and should be used in place of an immersion as these are not designed for primary heating. Immersions are relatively costly to purchase and operate and prone to rapid limescale development and failure in hard water areas, so should only be incorporated as a back-up for additional system resilience. Specifying an electric boiler is far more advantageous, preventing scale deposition, as well as delivering further system redundancy since the boiler will incorporate multiple immersions within its chassis.
Carbon reduction under an all-electric approach is a given, and, as the grid becomes less dependent on gas-fired power stations, carbon emission figures from a system should continue to reduce over time, future-proofing sustainability gains from an implemented DHW application deploying ASHPs.
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