The purpose of a heat pump is to improve the indoor climate by warming or cooling space and saving money on energy costs. But how does a heat pump actually work?
The biggest difference between an ordinary electric radiator (element) and a heat pump is that the radiator heats up the air while the heat pump takes heat from the surroundings. In practice, you can say that a heat pump moves heat from one place to another, for example, from the outside of the house into your living room.
Heat pump efficiency
A heat pump also uses electricity, but it generates more energy than it uses, which means you can save money on heating if you use a heat pump. For every kilowatt the heat pump uses, it creates heat equivalent to approximately 3 to 5 kilowatts, depending on the quality of the heat pump.
Under optimal conditions, a good heat pump can reach a heating factor of 6.0. This means that the heat pump can provide 600 W of heat to a room by using 100 W of electrical energy. A geothermal heat pump normally gives back four times as much energy as it uses itself.
The power factor or efficiency, which is usually called the coefficient of performance (COP), indicates the ratio between how much energy the heat pump uses and the heat it emits.
How a heat pump works
The basic principle behind all types of heat pumps is the same – a heat pump draws heat from the solar energy stored around us in the air, ground, and water.
Inside the heat pump, there is a closed circuit where a liquid circulates. This liquid is called refrigerant and is the heat pump’s so-called working medium. The closed circuit consists of four main components: an evaporator, a throttle valve (expansion), a condenser, and a compressor. The refrigerant condenses and evaporates at different temperatures. In this way, the heat is transported through the circuit.
To understand how the refrigerant circulates in the circuit, one must know three of the laws of physics:
- The law of pressure and temperature
- The law of the boiling point
- The law on liquids and gases
Briefly and simply explained, all the laws will be active if, for example, you are hiking up a mountain and the road you are walking on rises many meters in height. For every kilometer of elevation gain, the temperature decreases by about six degrees. The temperature drops because the air pressure becomes lower the higher you go. The lower the pressure, the lower the temperature, and vice versa.
This is where the law of the boiling point comes in because the lower the pressure, the lower the boiling point. On the stove at home in the kitchen, water usually boils at 100 degrees. At the top of Mount Everest, water boils at 68 degrees.
The law of gas and liquid tells us that liquid evaporates into gas when heat is applied to it. If you turn it around, you can say that liquid takes heat from the surroundings and actually cools the surroundings. For example, sweat cools the body and lowers body temperature.
When gas liquefies, an opposite process occurs, where heat is released and heats up the surroundings. By applying the laws of physics to the heat pump circuit, we can understand what happens in the heat pump.
The physics behind heat pumps
The refrigerant is fed into the compressor in the form of cold gas. The pressure increases in the compressor, and so makes the boiling point. Therefore, the refrigerant is converted from gas to liquid. In the condenser, the refrigerant cools and changes to liquid form, which gives heat to the surroundings.
Then the liquid passes in a thin stream through the expansion valve, also called the throttle valve, and the pressure drops again, and so does the boiling point. This causes the refrigerant to vaporize again, and this process requires heat. The refrigerant is thereby cooled to a mixture of gas and liquid, which flows to the evaporator.
The surroundings heat the refrigerant until everything evaporates, the surroundings get colder, the pressure drops, and the refrigerant gets colder. The cold gas reaches the compressor opening, and we are back to the beginning of the process.
For air conditioning, to cool instead of heat, a reversible heat pump can change the direction of the refrigerant in the circuit. A geothermal heat pump can also draw cold from the bedrock.
How a heat pump works in winter
How does a heat pump work in winter? Can it get too cold for the heat pump? These are common concerns about whether the heat pump will be able to function normally in extreme cold.
In the past, heat pumps had a reputation for performing poorly when winter came in the coldest countries, but now the situation is completely different. The colder it is, the more energy and electricity the heat pump uses to heat the air. Most air source heat pumps operate down to -20 °C (-4 °F), modern heat pumps even -25 °C (-13 °F), but at such low temperatures, the COP value will approach 1 – that is, the heat pump consumes as much energy as it generates.
Fortunately, heat pump technology has advanced enormously since then. Here you get the answers based on different types of heat pumps.
Air-to-air heat pumps can handle really cold
The most common and cheapest variant of a heat pump is called an air heat pump, or air-to-air heat pump, and is mounted on the outside of the house and looks like a normal air conditioner outside the house. While it has low investment costs compared to ground source pumps, it might cause problems in the cold temperatures in winter.
They are environmentally friendly, easy to install, and can reduce heating costs considerably depending on what you had for heating before. Today’s models work very well in cold climates and can withstand really low temperatures. Modern models with extra powerful compressors that use hyper-heating technology. A technology that guarantees a very good heat capacity.
The machines are silent and have a guaranteed heating function down to -25 °C, although with lower efficiency. However, it varies on the manufacturer and model. There are many cases where these newer heat pumps have worked very well in significantly colder temperatures, even down to -35 degrees in test environments. However, they have lower efficiency at colder outdoor temperatures.
Air heat pump efficiency in sub-zero temperatures in winter.
Air source heat pumps have lower efficiency in freezing temperatures. The reason for this is that the efficiency of the heat pump drops in colder temperatures. The so-called efficiency is identified through a COP value or a SCOP value.
Modern air source heat pumps are still very reliable and can perform throughout the winter. To get a more realistic picture of the heat pump’s performance, we recommend looking at the SCOP value, which takes into account the temperature changes between the seasons.
In winter, the heat pump needs more electricity to produce heat energy. This helps to reduce your savings on very cold days. However, it is important to point out that air-source heat pumps are still reliable heat sources that can help you save up to 60% of your energy consumption for heating annually.
For the avoidance of doubt, it should be added that the coldest countries have the highest percentage of installed heat pumps per capita due to the lack of cheap gas and rising electricity bills. And most of them are air-to-air heat pumps.
Improvements, practices, and millions of consumers in cold countries can’t be wrong. Air-to-air heat pumps are a reliable, ecological, energy-efficient, and better heating alternative than traditional fossil fuel heating.
An electric cartridge as backup heat in the winter
If you are worried that the heat will not be enough in the winter, you should make sure that your heat pump has an electric cartridge. This is something that most air-to-water heat pumps have as reserve power. The electric cartridge can supply the household with both heat and hot water – and you can manage to keep the auxiliary heat in the household with the help of your air source heat pump.
The electric cartridge is also good to have in case there is a problem with the heating system so that your household is not completely without heat. The idea with an electric cartridge is that it should be not only able to cover other heating needs but also be able to act as security and emergency heat if something goes wrong unexpectedly for a shorter period.
Air-water heat pumps resistant to cold
In addition to air-to-air heat pumps, air-to-water heat pumps have also gained a solid foothold. Here you get a solution that can handle both heating and hot water production in your home as it connects with an indoor unit, or water heater, for example.
This type of heat pump is also environmentally friendly. Thanks to their water-borne heating system (radiators or underfloor heating), the air-water heat pump provides an even heat distribution. As well as heating the house, it can heat the tap water in an energy-efficient way.
The air source heat pump requires maintenance in the winter
Using the outdoor air source heat pump correctly gives you the best possible effect and energy savings and avoids unnecessary wear and tear. This primarily means that the outside of the air heat pump is taken care of by, among other things, keeping it free of ice and snow.
Tips for keeping an air heat pump clean outdoor
- Check regularly throughout the year that the grille is not clogged by leaves, snow, or anything else. Keep the outdoor area free of snow and ice; carefully brush off it.
- Defrost when the outside of the air heat pump freezes
In the cold season, condensation can freeze and form ice. Condensation forming is a natural part of the process, but ice can cause problems for the device’s operation. Therefore, there is a defrosting process built in. There is timed or demand-controlled defrosting.
- Lead the melted water away
You must ensure that the melt water that occurs from defrosting flows away from the outdoor unit to prevent ice formation inside or outside it. If too much ice forms under or around it can be damaged.
- Place the pump in a sunny place
If possible, the heat pump can also be placed in a sunny place (south-facing gives the most sun) so that the sun naturally helps to keep the outdoor area warmer.
Exhaust air heat pump in winter
Exhaust air heat pumps get their heat from the indoor air and depend on indoor air quality. The air must leave the house through the ventilation system. When that air leaves the house, it has an indoor temperature of about 20 degrees; the warm air contains a lot of energy, which the exhaust air heat pumps make use of and convert into heat and hot water.
An advantage of exhaust air heat pumps is that you get an even temperature all year round; this depends on the indoor temperature being the same throughout the year. It is not dependent on temperature changes in the same way that other pumps can be, especially in winter if it is very cold outside. This means you don’t have to worry about waking up in the morning and it’s cold inside.
However, during the winter, the exhaust air contains less water. It is drier inside, and the energy content of the exhaust air is lower. So there isn’t as much energy to pick up because the air contains less water.
If you have a flow of one cubic meter per second, you can use around 60 kilowatts during the summer. But when it’s really cold weather outside, the heated air contains little water, and then you can only get 40 kilowatts out.
To reduce the energy consumption of the exhaust air heat pump when it is cold and electricity is expensive, it makes sense to install energy recovery from wastewater. Then you can reduce the use of electrical energy and the electrical output of the heat pump.
Ground source cold climate heat pump
Ground source heat pumps are almost independent of outside temperatures as they gather the heat stored very deep in the mountains or less deep underground or in deep water.
Air-independent geothermal heating with boreholes
If you want to install geothermal heating, it almost doesn’t matter what the temperature is outside. Ground source heat pumps draw their thermal energy from the bedrock, where the temperature is more stable all year round.
Vertical closed-loop ground source heat pumps are installed in boreholes 90-200 meters deep (300 to 700 feet), where seasonal changes in soil temperature completely disappear, thus exposed to a constant year-round temperature.
Because of the much greater thermal mass of soils compared to the atmosphere, seasonal variations in the temperatures at depth within the ground are far less than and lag significantly after seasonal variations in the overlying air.
Geothermal heating with ground loops works in winter
On the other hand, horizontal-loop and spiral-loop ground source heat pumps are installed in trenches that usually are less than 3 meters deep (10 feet). It is important to know the expected seasonal changes in the surrounding soil temperature.
Deepening the trench could increase the efficiency of the ground source heating system because deep soils have less pronounced seasonal fluctuations in temperatures than shallower ones.
For example, the maximum soil temperature occurs in late August at a depth of 1.5 meters (5 feet) below the ground surface but occurs in late October (after the heating season has begun) at a depth of 3.5 meters (12 feet) below the surface.
Therefore, installing a deeper ground loop would lower the annual operating costs associated with running electricity for the heat pump, and over time, these accumulated savings cover a lot of the increased initial investment required to install a deeper ground loop.
The temperature at the depth where the loops were buried did not experience any significant fluctuations. Regardless of how cold (or warm) the aboveground ambient air is, the loops will always be able to keep their roots at around 12°C (54°F). It’s more than sufficient for the heat pump to operate efficiently in both cooling and heating mode.
Summarizing the above, we can say with certainty that geothermal pumps with ground loops have a certain dependence on the temperature of the soil in winter, and fortunately, it is more stable than the air temperature. So this heat pump system isn’t afraid of sudden temperature drops.
Ground source heat pump with ground loops is a very effective, reliable heating solution that gives you an advantage in a bit lower cost of installation than bedrock type heat pumps, independence of sharp outdoor temperature changes, savings CO2 emissions, and shorter payback period.
Ground source heat pump underwater in cold air
Unlike geothermal heating, no hole needs to be drilled, but an installation trench must be dug between the house and the water to prevent freezing in the winter. The hose is then placed on the surface of the water, after which it is filled with a liquid which is often a mixture of water and ethanol (or other antifreeze).
The heat source is stable and relatively predictable, and as long as the installation is done correctly, the lifespan is long. You might think that the effect on the water temperature is small, but in stagnant or slowly flowing water, as much as a few kilograms of ice can form on the hose per kWh of heat output during the winter months.
In view of ice formation, it is very important to weigh down the hose considerably. What is enough to keep it at the bottom during the summer is not necessarily enough in the winter. When it comes to ice, it is also important to make sure that the ice next to the land cannot damage the hose where the transition takes place.
The cost of installing a sea heat pump is about the same as an air-water heat pump, but the operations can be cheaper when you have +4 degrees on a lake bed. This means that if you have your water with a flat and fine bottom, a deepwater heat pump is preferable.
How much electricity does a heat pump use in winter?
How many kWh your heat pump needs in winter depends on its type and capacity, the size of your house, your geographical location, and how much heat the house needs. Based on these conditions, you can count on an electricity consumption between approximately 3,840 – 11,280 kWh/year with your air source heat pump.
Let’s take, for example, a Two-storey house with a furnished basement built in 1970 and an area of 110 square meters, can be heated in different ways, from direct-acting electricity that heats electric elements to underfloor heating loops with hot water and requires 25500 kWh/year of electricity consumption.
Then, the geothermal heat pump only needs 7070 kWh (drive energy and supplemental energy) to meet the energy demand of 25500 kWh. The free energy thus corresponds to 18430 kWh. For Air/water heat pump will correspond to 8450 kWh (drive energy and supplementary energy) that needs to be added to meet the energy demand of 25500 kWh. The free energy – 17040 kWh.
How can a heat pump get heat from outside in winter?
Any type of heat pump extracts stored solar energy from surroundings, including outdoor air, and then with a power of physics – pressure and temperature, boiling point, liquids, and gases; this solar energy transmutes into heat that is given off.
What should heat be set at in winter?
It can be good to think about which rooms you want to get the most heat because it is calculated that the temperature drops 1-2 degrees for every threshold that is crossed. Therefore, the temperature setting could be a couple of degrees higher, for example, 22˚C or 71,6°F. With the smart home application, you can do it from your phone.
How does a mini split heat pump work in cold weather?
Mini-splits are a type of air source heat pump that doesn’t require ducts. You have the opportunity to control each mini-split unit independently. It works like any other air-source heat pump system taking solar energy from the outside air and converting it to heat for the house.
How long should a heat pump run in winter?
In winter, a heat pump works hard to maintain a comfortable temperature in your home. To help it last longer, use a timer to turn it on and off instead of leaving it running all day. This is a more economical way to use the heat pump since it only runs when someone is home.
Do heat pumps work in extreme cold?
Heat pumps work in extreme cold because they use refrigerant, a special type of gas that has a very low freezing point (from -78 to -160 ˚C, or -108 and -256 °F depending on the type), that still can absorb the energy/heat in extreme cold. A heat pump uses a refrigerant to transfer heat between two coils – one at a low pressure and one at a high pressure. By evaporating the refrigerant in the first outdoor coil, heat is removed from the environment around it. The refrigerant is then squeezed into the other indoor coil, where it condenses and releases the heat that was stored earlier in the process.
Why does your heat pump work differently in the winter?
A heat pump is a closed-loop system that allows it to work both in winter and summer by extracting heat from the outdoor air and bringing it into your home in winter and doing the reverse in summer.
Do heat pump works in a colder climate?
Definitely. Norway, for example, has the biggest percentage of installed air-to-air heat pumps per capita. Heat pump is able to work in colder climate because they use refrigerant with very low freezing point. The refrigerant works like a heat exchanger by volatilizing in the evaporator coil outside. This process removes heat from the environment around it. The heat transfer is then squeezed into the other indoor coil, where it condenses and releases the heat that was stored earlier in the process.
How can a heat pump possibly pull heat out of the air when it is so cold outside?
A heat pump is a machine that is able to heat a home or building without the use of fossil fuel. It can pull stored solar energy from the air and ground by using a refrigerant which is a chemical compound that absorbs heat. With the right physical environments, including pressure, boiling, and refrigerant characteristics, energy/heat transfers in the close-loop system and reveals in a home.
Will heat pumps work below freezing?
It will because the heat transfer and exchanger – refrigerator has a very low freezing point from -78 to -160 ˚C, or -108 and -256 °F depending on its type.
What is the difference between a heat pump and a furnace?
Furnaces burn oil or gas to generate heat, while heat pumps run on electricity, drawing heat/energy from outside air (even cold air) or ground and transferring it indoors. Because of how they work, the heat pump is more energy-effective with very little CO2 emissions, while an oil or gas furnace could make very high temperatures quickly and easily. But are so inefficient and cause great damage to the environment; we feel the consequences of this every day.