Choosing your right hot water heat pump is often a more complicated task than expected - and as it should be. Because of the lack of clear rules and agreements in New Zealand on how to present hot water heat pump specifications, there is no uniformity, which makes it a difficult puzzle for the customer to solve. The only uniformity could be that manufacturers often only share their good qualities and leave out the not-so-great qualities of their systems. Smoke and mirrors…
This article will guide you through key specifications and highlight why understanding them is crucial for choosing the right hot water heat pump for your needs, and helping you make a well-informed decision.
Reading and understanding specifications of heat pump water heating
Capacity of the hot water storage cylinder
An average of 50 litres of 60°C water gives 90 litres of 40°C water (cold water temperature is 15°C), which is a shower of about 9–10 minutes. A bath, depending on the size of course, counts as two or three showers. With this, you can estimate the required capacity for your household. For example, in most cases a family of four can easily manage with a 250-litre cylinder, while a 300-litre cylinder is recommended for larger households.
Recovery time of the system
The time it takes to reheat the storage cylinder. When 4 showers are taken in the morning and 3 showers at night, plus some additional hot water use, you don’t need a 400+ litres storage cylinder as long as your heat pump can reheat the cylinder quickly enough. A 250L or 300L cylinder will do.
Output power related to ambient temperature
When the heat pump output power is not given, you can multiply the input (rated) power with the COP (see COP paragraph below). The maximum power refers to the short power peak that occurs at the start-up of the compressor motor.
Heat pumps, in general, perform less efficiently in colder ambient temperatures. Heat pumps are designed and built with a certain operating range in mind where they perform the best. Check if your proposed system is suitable for your climate zone and how it performs in colder winter temperatures.
Some manufacturers hide or don’t provide specs under different test conditions, making it harder to compare the various models and brands. This is especially true with some All-in-One systems - systems with the heat pump built on top of the hot water cylinder. As the evaporator of the heat pump is rather small, it gives acceptable heating output at summer temperatures but struggles to gain heat at winter temperatures. Manufacturers often ‘solve’ this problem by using the ‘hybrid mode’- a fancy term for using the old-fashioned electric resistance heater together with the heat pump.
For example, a random heat pump may have a heating output of 7 kW and a COP of 5 (= 80% saving) with an output water temperature of 60°C, which is excellent. However, the tested ambient temperature was 35°C- not a typical New Zealand temperature. That same heat pump may have a heating output of 4 kW and a COP of 2.85 (= 65% saving at 60°C water output) at 7°C ambient temperature, which is a much more realistic average New Zealand winter temperature.
Recovery time related to ambient temperature
The time it takes to reheat your hot water cylinder. When these are not provided, you can calculate this by: Cylinder capacity in Litres × 4 × ΔT / 3412 = required power in kW / heat pump output power in kW = recovery time in hours.
For example, an All-in-One system with a given heating output of 2 kW (20°C/15°C, 15°C–55°C): 200L × 4 × 40 / 3412 = 9.37 ÷ 2 (@ 20°C) = 4.7 hr (4 hr and 42 min).
If we assume that the heat pump output reduces to 70% at 7°C = 1.4 kW:
200L × 4 × 40 / 3412 = 9.37 ÷ 1.4 (@ 7°C) = 6.7 hr (6 hr and 42 min).
COP (Coefficient of Performance)
The Coefficient of Performance (COP) shows the efficiency of the heat pump. Heat pumps are designed for a designated purpose, climate zone, etc. It shows the relationship between input power and output power at a given ambient temperature. Always check at which ambient temperature the COP is given - and if it is a realistic New Zealand temperature. (See above: Output power related to ambient temperature).
COP v Savings | ||
COP | Input power (%) | Savings (%) |
1 | 100.00 | 0.00 |
2 | 50.00 | 50.00 |
3 | 33.33 | 66.67 |
4 | 25.00 | 75.00 |
5 | 20.00 | 80.00 |
6 | 16.67 | 83.33 |
7 | 14.29 | 85.71 |
8 | 12.50 | 87.50 |
Is the highest COP always the best?
A COP of 4 means that for 1 kW of input power, you get 4 kW of output power. Logically, this means that the higher the COP, the higher the savings.
However, there is one thing to keep in mind. With a COP of 4, the power usage is 25% (1kW / 4kW), so the savings are 75% (100% - 25%). With a COP of 5, you get 5 kW output power for every 1 kW input power. The power usage is 20% (1kW / 5kW), so the savings are 80% (100% - 20%). The difference between a COP of 4 and a COP of 5 is only 5% extra savings.
When a 7-minute mains pressure shower costs around $1 with an electric hot water cylinder, a heat pump with a COP of 4 will reduce this to 25 cents. A heat pump with a COP of 5 will reduce it to 20 cents—a difference of 5 cents per shower.
If the purchase difference between a COP 4 heat pump and a COP 5 heat pump is $1,000, it will take 20,000 showers (1,000 ÷ 0.05) before you break even on your investment. With 10 showers a day, this takes almost 5.5 years.
COP in different conditions
COP @ 20°C/15°C, 15°C–55°C (ΔT = 40°C) water temperature: This means the COP is measured in different conditions, at 20°C Dry Bulb and 15°C Wet Bulb, and the COP is the average over the water temperature rise from 15°C to 55°C.
The Dry Bulb Temperature is the actual air (ambient) temperature, measured by a regular thermometer. The Wet Bulb Temperature shows how much the moisture level in the air affects the efficiency of the heat pump. For example, with a 100% moisture level, the heat pump performs as if it were 15°C, even with an ambient temperature of 20°C.
The COP is the average efficiency of the hot water heat pump. For example, the COP at the start of heating (from 16°C to 19°C) is 7.66. When heating from 52°C to 55°C, the COP is 2.93. However, the average COP of this unit is 5.
Warranties
Check what is included and excluded with your proposed system. Simply put, parts & labour means the system will be fixed. Parts only excludes labour, which could force you to replace the entire system if the labour cost for repair exceeds the cost of a new system.
Other stuff to consider
Additionally, consider the refrigerant used in the system. Some refrigerants have a higher Global Warming Potential (GWP) than others. Systems that use low-GWP refrigerants (such as R-290 or R-744) are more environmentally friendly and align better with sustainability goals.
Lastly, think about the noise level of the heat pump. Quiet operation is particularly important if the unit is installed near living areas or in urban neighbourhoods. Noise levels are usually measured in decibels (dB), so look for this information in the specifications to ensure a system that suits your household's comfort requirements.
By understanding the key specifications and asking the right questions, you can confidently choose a hot water heat pump that suits your household needs, performs efficiently in your climate, and offers long-term savings and reliability. Feel free to reach out if you have any questions or would like more in-depth information.
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