Why Are Heat Pumps More Efficient at Lower Hot Water Temperatures?
Why Are Heat Pumps More Efficient at Lower Hot Water Temperatures? Heat pumps are generally more efficient at lower hot water temperatures because the compressor doesn’t need to work as hard to transfer heat from the air into the water. The smaller the difference in temperature of the heat source and the temperature you need the water to be, the higher the heat pump’s efficiency will be, usually. Therefore, an air source heat pump can usually heat water up to 45°C to 50°C more efficiently than water at 60°C or 65°C under the same conditions.
What’s this business about temperature lift?
Temperature lift is a massive factor in determining just how efficient a heat pump is going to be.
An air source heat pump sucks thermal energy out of the outside air using an evaporator. The refrigerant grabs this energy before getting squeezed by the compressor, which increases its pressure and temperature.
The higher you need to heat the water, the bigger the pressure difference the compressor has to create within the refrigeration cycle.
For example, heating water to 45°C when the air is 10°C outside is a whole lot easier than producing 60°C water from the same cold air.
The compressor therefore tends to use more electricity as the required water temperature goes up.
View our dedicated range pages for further information on air source heat pump domestic hot water production:
Wall mounted DHW heat pump water heaters
Domestic DHW heat pump water heaters
Commercial DHW heat pump water heaters
How does COP behave at higher water temperatures?
The Coefficient of Performance, usually just called COP, describes how much useful heat you get out of the electricity you put in.
For example, a COP of 3 means that about 3 kWh of heat is delivered for every 1 kWh of electricity used under the right conditions.
COP isn’t a fixed number, though – it changes depending on different things, including:
What temperature the air is when it enters the evaporator
The temperature you need the hot water to be
How cold the cold water is
The design of the refrigerant and compressor
How well the air is flowing through the evaporator
How well the heat exchanger is working
Whether the heat pump is in defrost mode, and so on
As the required water temperature goes up, COP will generally go down because the compressor needs to work harder to achieve that temperature.
Shouldn’t you just run the hot water as cool as possible?
Lower temperatures do improve heat pump efficiency, but domestic hot water temperature isn’t just a matter of choosing the lowest temperature you can get away with.
The system also needs to provide enough usable hot water while keeping an eye on water hygiene and Legionella control.
Many heat pump water heaters therefore use a normal operating temperature somewhere around 50°C to 55°C, depending on the appliance and the job in hand. Sometimes, you’ll need to raise the cylinder temperature to around 60°C or higher to satisfy the hygiene requirements.
Some appliances can do this using the heat pump’s compressor and refrigerant circuit alone, while others use an electric immersion heater to give a bit of extra heat.
You should always follow the manufacturer’s temperature and hygiene control guidelines.
Does an immersion heater really reduce efficiency?
An immersion heater uses direct electric resistance heating rather than transferring heat from the surrounding air. So, for example, 1 kWh of electricity fed into an immersion heater will give you about 1 kWh of heat right where you need it; however, a heat pump operating at a COP of 3 can theoretically give you around 3 kWh of heat for the same 1 kWh of electricity under the right conditions.
This is why overusing an immersion heater can really increase the electricity consumption of a heat pump water heater.
However, immersion heaters do have their uses – for example, they’re great for backup heating, rapid recovery, and high-temperature hygiene cycles.
Are R290 heat pumps really made for higher temperatures?
Modern heat pumps that use the R290 refrigerant, another name for refrigerant-grade propane, are really suitable for higher-temperature applications.
Some of these R290 heat pumps can even produce water at 60°C, 65°C, or even higher just using the refrigeration cycle alone. This can make it possible to go without using the electric backup at all when producing high-temperature water. However, the basic principle still applies – the higher the temperature you need the water to be, the harder the compressor has to work, and the less efficient the heat pump will be.
What temperature gives the best heat pump efficiency?
From a purely theoretical point of view, the lowest practical water temperature that satisfies the system’s requirements will generally give the best heat pump efficiency.
For domestic hot water, though, the optimum temperature has to balance COP, usable cylinder capacity, hot water recovery, standing heat loss, Legionella control, and the risk of scalding.
This is why getting the size of the hot water cylinder right is just as important as choosing the right heat pump in the first place. Having enough stored water capacity can help you avoid the need to operate the whole system at unnecessarily high temperatures just to get more hot water.
In the end, heat pumps work best when the temperature lift is kept as low as possible. A well-designed domestic hot water system therefore aims to get the right level of comfort and water hygiene without unnecessarily high operating temperatures that increase compressor workload and electricity consumption.