The test in action

Our test was part funded by the Energy Efficiency and Conservation Authority (EECA) under its Innovation Fund. What we devised was a comprehensive test programme that simulates household hot-water use at various temperatures and humidity levels that reflect the country's climate.

Our tests were devised from scratch because, to our knowledge, no one else has done anything similar.

The heat-pump water heaters (HPWHs) were set up in a temperature- and humidity-controlled room. The split models were piped to a new 300-litre A-grade electric hot-water cylinder with the normal element disconnected.

The water tank was filled and the heat pump started up. Electricity consumption and the water temperature were measured continuously. When the HPWH had heated the water to 60°C, a simulated 24-hour controlled hot-water draw-off was started under computer control.

This draw-off included a bath, two showers, a load of laundry, and some dish washing - all at different times and designed to simulate the hot-water use of a typical house of two adults and two children during one day. In total 125 litres of hot water were drawn off during the 24-hour period.

The temperature and volume of the water was measured as it was drawn off. From this test sequence we calculated how much heat was delivered by the heat pump and how much electricity it took to do it.

Two test runs were performed for each model at ambient temperatures of 15°C, 7°C and 2°C (a total of six runs per model). We used three different sets of temperatures to mimic different local climatic conditions.

Performance at low ambient temperature is important, because households tend to use more hot water during the cooler months.

To give us a reference point, the standard electric hot-water cylinder was put through the same test programme. This gave us a performance baseline to compare the heat pumps' energy use against.

In our test results we've used the term "relative performance" to show how much more efficient the heat pumps are compared with a standard electric hot-water cylinder setup.

Hot-water temperatures
 

The building code requires that the water in your hot-water cylinder is kept at not less than 60°C - and delivered to your hot-water taps at not more than 55°C. A special tempering valve that mixes in cold water does the cooling to 55°C. These tempering valves have a reputation for being unreliable and are often removed - which then significantly increases the risk (especially for children) of being scalded by hot water.

Water is stored at 60°C to eliminate the risk of legionella contamination.

All heat-pump water heaters (HPWHs) become less efficient as the water temperature rises, so there have been calls from the industry for the building code to be amended to allow hot-water storage at 55°C or even 50°C. This would make heat pumps even more efficient, eliminate the need for unreliable tempering valves, and reduce standing heat losses from the cylinder. And - more importantly - it would reduce the number of scalding accidents.

We think this is worth the government investigating provided there's no increased risk of legionella contamination.