Ideally, domestic hot water should be produced and maintained at temperatures in excess of 55°C, simply because at this temperature harmful pathogens such as Legionella cannot survive.
When deriving domestic hot water from a heat pump powered system, economics always have a major role to play, but the temperature required to produce and store DHW above the deemed safe threshold generally take a heat pump into an uneconomical operational range. However, domestic hot water can still be useful at lower temperatures of say 45/50°C, but regular DHW production at lower temperatures will require an energy-hungry pasteurisation cycle to ensure the DHW is safe and hygienic.
The cost per kW of energy is generally the deciding factor when choosing a suitable heating system, and the production cost of DHW needs to be carefully considered.
I recently came across some data available via this link, http://www.narecde.co.uk/category/blog
The data is derived from a standard air source heat pump, by a leading brand manufacturer, and has been monitored for performance by a test laboratory, specifically during production of DHW.
It can be seen from the published data that a COP significantly below 2:1 is evidenced across two slightly different technologies (conventional ASHP and solar assisted heat pump).
It is interesting to note that an MCS accredited ASHP could not achieve a COP greater than 1.4:1 when dedicated to DHW production. The testing was carried out in September, where average daily conditions were good (winter efficiency can be much worse).
Cost of production: Electrical energy tariffs vary but 12p per kWh is probably a fairly average figure to provide a rough cost comparison.
At an electrical tariff of 12p/kWh the cost of the energy to produce DHW via a heat pump, operating at a COP of 1.4:1 would be 8.57p/kWh, and degrading to over 10p/kWh as weather conditions deteriorate through the winter period.
Under certain adverse conditions, the COP of an ASHP can reach negative figures of 0.9:1 and below, where the cost of running the heat pump can in fact be more expensive than a simple immersion-type heater.
It can be seen from the test results that a standard ASHP, specifically for the production of DHW, is unlikely to offer a more economical solution than natural gas, LPG, or even oil.
Continuous high temperature operation: I occasionally come across retrofit heat pump systems where existing panel radiators have been retained and a heat pump replacing a fossil-fuelled boiler is running at elevated water temperatures to deliver the required amount of energy. It therefore comes as no surprise when the owners of such installations complain about high running costs. Such practices promoting poor heat pump results should be avoided at all costs.
In consideration of the above facts, retrofit systems, dependant upon water circulating temperatures of around 50°C and higher must be avoided due to a fundamental inability to provide an affordable heating solution!
For a heat pump system to produce attractive and affordable operating costs, the DHW production cost must be supported by averaging against a generally economical heating design, and employing circulating water temperatures of 45°C, or even lower.
Fan coil units and underfloor heating matrixes can operate quite successfully at water temperatures as low as 35°C and produce tremendous economical results.