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Introduction – How Does A Heat Pump Work?
There’s been a lot of buzz around heat pumps recently. They can be cheaper to run, safer, lower maintenance and they’re a lot better for the environment. We’ve been installing them on one of our new build development sites, so it looks like this technology is here to stay. But how does a heat pump work?
Let’s start simple. A heat pump takes heat from the outside and brings it inside. It does this by using electricity, but the amount of heat it sends into your home is much greater than the amount of electricity it uses.
Since a heat pump uses the heat that is already in the environment, it doesn’t burn any fuel and doesn’t give off any carbon dioxide.
How does a heat pump work – the detailed version
Everything around us has heat, which is a form of thermal energy. Heat moves from places that are warmer to places that are cooler. When it’s cold outside, we need heat to move in the opposite direction, from a cooler place to a warmer one, so that we can heat our homes.
Five key parts make up a standard “monoblock” heat pump system: a compressor, an expansion valve, a fan, an external radiator, a heat exchanger, in order to feed interior radiators. These parts are all connected by a pipe that is filled with refrigerant, a liquid with a low-temperature boiling point. This refrigerant is essentially a gas while it’s warm, and when it’s cooled, it transforms back into a liquid.
A heat pump’s compressor kicks on first when the system is turned on. This raises the pressure of the refrigerant gas on one side of the system while being restrained by the expansion valve.
This gas is compressed, which causes the molecules to crowd in close, clash, and heat up right away. Think about a deodorant container; you can feel the deodorant cooling in your hand as you release it. It would heat up if you attempted to force it back in.
By heating the heat exchanger, we heat the water that circulates around our radiators. This plate cools down when the water flows through the radiators and returns cooler, condensing the refrigerant gas into a liquid.
Following the expansion valve, the compressed refrigerant decompresses. As a result, its temperature falls below that of the ambient air.
With the aid of a fan, this liquid, which is now extremely cold and low pressure, passes through an outdoor radiator to absorb heat from the outdoor air. It gets warm, boils, turns back into gas, and is then immediately fed back to the compressor.
Phase transition and the application of latent heat helps to accelerate everything. Any gas that transforms into a liquid releases this latent energy as heat and heats things up without cooling itself. Any liquid that transforms into a gas absorbs what is known as latent energy (that is, energy being absorbed without temperature change).
A boiler’s efficiency is increased by “condensing” when the flow temperature is lowered using the exact same idea as latent energy, which is also the basis for phase change batteries.
Since we derive our thermal energy from the outside air, a heat pump’s usage of electricity merely allows us to transport (or “pump”) that heat from the outside air and concentrate it into our radiators. The air is the source of the heat energy, not the electricity.
There are now several types of heat pumps, including air-to-air, ground-source, and water-source models, and they all operate on the same fundamental concept.
Calculating heat pump effectiveness – COP
With gas boilers, we measure the amount of gas we put in, say 1 kWh, and the amount of heat we extract, say 0.8 kWh. We would then achieve 80% efficiency. You’re probably familiar with this measurement being displayed on the graph on the left.
Gas energy is converted to heat by burning it. In our case, take the 1 kWh of gas, light it on fire, and the 0.8 kwh of heat will be released. Water vapour absorbs the 0.2kWh that was lost as latent energy that wasn’t caught. However, you still pay for the power you consume, not the heat produced.
Now heat pumps are far more efficient – using the same metrics as gas boilers and only accounting for the power you pay for rather than the heat gained, most heat pumps are in the region 200-500% efficient. The efficiency will be 400% if we measure the electricity input as 1 kw and the heat output as 4kw for heat pumps. This means that for every kW of power used by the pump’s compressor, 4 kW of heating power is produced.
The coefficient of performance, or COP, is what is commonly used to describe this; a 400% efficiency would have a COP of 4.
Heat pump effectiveness over time – SCOP
Efficiency changes constantly throughout the day and year, so measuring it at a single point in time is a rather unfair benchmark.
In the instance of a lower outside temperature, the compressor has to work harder to provide a bigger pressure differential between the high and low pressure sides of the heat pump, which results in a wider temperature differential to raise the temperature of the radiator. This would result in a lower COP.
By installing bigger radiators, we can reduce the amount of energy required to heat the building or employ weather compensation. But we must also take it into consideration heat pump efficiency now that we are aware of the reasons why it differs.
Therefore, instead of using the COP when discussing a heat pump’s efficiency, we typically use the SCOP or SPF.
The SPF specific performance factor is the actual on-site measurement, while the SCOP (seasonal coefficient of performance) is an estimate of the average COP over the course of the entire year.
As the average efficiency throughout the year does not accurately reflect where the majority of heat is produced, in the winter, you might now be concerned that the SCOP or SPF may not be a realistic evaluation of year round efficiency.
The SCOP and SPF are weighted toward the winter efficiency since they measure the total kWh of heat produced by the heat pump over a year divided by the total kWh of energy consumed over the same time period, and the unit must produce more heat in winter when it is less efficient.
We can help you work out your SCOP performance during the survey process so you can determine your heat pump’s annual operating expenses accurately before installation, despite its decreased winter efficiency. We can also assist with advising other factors that may improve the SCOP, such as insulation.
Last updated: January 2023.
For independent advice view more information on The Energy Saving Trust website.
