Types of Heat Pumps. Which Type of Heat Pump is the Best for Your Home?

Basic Principle of Heat Pump Operation

We are introducing this topic to explain the basic differences in types of heat pumps in a concise manner before delving into the specifics of each type. Essentially, a heat pump is a device that can extract energy from the outside and, through specific technical processes, convert it into heat that is transferred into your home.

The principle of heat pump operation can be compared to the workings of a regular refrigerator, but in a more advanced form. Heat is taken from one side and released on the other side. Just as a refrigerator cools its interior and releases heat outside (from the back), heat pumps extract heat from outside the building, cooling the specific environment, and then introduce this thermal energy into the building, heating either water or air. The entire process can be succinctly summarized in four steps:

1. ✔️ Evaporation of the Refrigerant: At this stage, the refrigerant (a special type of medium, often colloquially called Freon, although not always strictly accurate) in the heat pump absorbs heat from the outside (e.g., from the air or water). When the refrigerant is cool, it is in a liquid state, but as it warms up, it quickly turns into a gas, thus changing its state to gaseous. This is the evaporation of the refrigerant – the key point being that it absorbs external heat. Now we want to transfer this heat into the house.

2. ✔️ Compression of the Refrigerant: The warm refrigerant then moves to the compressor. Here, the gases are compressed, which causes their temperature to rise even further under high pressure. Through compression, the refrigerant reaches high temperatures, even up to 65-85 °C, depending on the efficiency of the heat pump and the type of refrigerant used. Understanding this helps in choosing an efficient and economical heat pump.

3. ✔️ Condensation: The hot refrigerant reaches a special device called a heat exchanger (or condenser). Cool water (or sometimes air) flows through the heat exchanger and meets the hot refrigerant. They do not mix; the heat exchanger has two channels. The refrigerant transfers heat to the water, and in doing so, condenses on the walls of the exchanger. Entering the condenser, the refrigerant is hot and gaseous, and upon exiting, it is cooled and liquid again. The key point for us is that the water in the exchanger gets heated, which we can now transport into the house, preferably to the central heating system or domestic hot water supply.

4. ✔️ Expansion: The refrigerant expands, then returns to be compressed and heated again.

✅ This process is cyclical and repeats until the building is heated to the desired temperature, at which point the heat pump turns off. It will turn on again when sensors detect the need for more heating.

This cyclical process ensures efficient heating, with the heat pump turning on and off as needed to maintain the desired indoor temperature.

Heat Pump Operation Diagram

Once again, the same knowledge - with the heat pump operation diagram above, in easier way:

1. Evaporation of the Refrigerant: This occurs in the evaporator, where the refrigerant absorbs heat from the heat source (air, water, ground).

2. Compression: The refrigerant is compressed to reach an even higher temperature. This process takes place in the compressor.

3. Condensation of the Refrigerant: The refrigerant releases heat to the receiving medium (usually water) in the heat exchanger (called the condenser). The refrigerant then transitions into a liquid state (condenses).

4. Expansion: The refrigerant expands, and the cycle begins anew.

Coefficient of Performance (COP)

In relation to the above process, when choosing the type of heat pump, you may come across the term Coefficient of Performance (COP), which essentially represents the efficiency of the device. It is the ratio between the heat produced and the energy consumed within your property. The higher the value, the better.

✅ Simply put, the higher the Coefficient of Performance (COP), the less electrical energy will be needed to produce a greater amount of heat.

Possible Heat Sources for Heat Pumps

You already know that heat is extracted from the surroundings. The primary difference between the main types of heat pumps lies in where they specifically obtain their heat energy. You need to consider various external factors and decide from which source you will draw the necessary heat.

There are three basic types of heat pumps because there are essentially three areas from which energy can be sourced:

• ✔️ Air Source Heat Pumps (as well: ASHP): These extract energy from the air. They are the most popular and generally the cheapest.
• ✔️ Water Source Heat Pumps (WSHP): These extract energy from water. They are less common and slightly more expensive than air source heat pumps.
• ✔️ Ground Source Heat Pumps (GSHP): These extract energy from the ground. They are potentially the most economical in terms of usage but the most expensive to install.

From a cost perspective, the cheapest option to purchase will be air source heat pumps. In contrast, ground and water source heat pumps will be more expensive. However, each type of heat pump has its own advantages and disadvantages, and the choice is not solely based on the amount of money you are willing to invest. They differ in the space required (you cannot install a ground source heat pump everywhere), noise levels (air source heat pumps tend to be the loudest), and availability of resources (not every property has access to water). They also vary in terms of reliability, maintenance frequency, and, notably, their Coefficient of Performance (COP).

Air Source Heat Pump - ASHP

An air source heat pump is probably the cheapest in terms of initial investment, which makes it the most popular. Because this type of heat pump does not require any interference with the ground or the surroundings of the property, it can be used almost anywhere. Therefore, despite its shortcomings and not the best economy, you might decide to go with it.

• We have already written a more comprehensive article about air source heat pumps on our blog.

From the perspective of the resulting heat, this device can have two variants:

• ✔️ Air-to-Water Heat Pump: This type heats water in the pipes of our central heating system (or domestic hot water system).
• ✔️ Air-to-Air Heat Pump: This type heats air using special blowers or heaters.

Air-to-Water: The Classic Type of Heat Pump

Air-to-water heat pumps are indeed the most popular type, the leader in sales. Its popularity is due to the price and simplicity of installation. This type of heat pump extracts energy from the air and transfers it to the water, which then goes to our radiators and underfloor heating.

✅ From a functional perspective, air-to-water heat pumps can be divided into two types:

• ✔️ Monobloc Type: The refrigeration circuit is in the external unit (the part outside the building), which is connected to the internal part of the pump via pipes.
• ✔️ Split Type: In this version, the refrigerant is inside the building (which has several implications, such as the split pump not being exposed to freezing).

Disadvantages of Air-to-Water Heat Pumps

Air-to-water heat pumps have their drawbacks adn disadvantages, which must be considered.

• Higher Electricity Consumption: They consume significantly more electricity than pumps that extract heat from water or the ground. Operating costs can be up to 20-25% higher (in terms of usage costs).
• Reduced Heating Efficiency at Low Air Temperatures: Some of these heat pumps stop working at temperatures between -12°C and -15°C or lower. They then switch to a heating element system, but imagine the energy consumption of an 8 kW heating element to heat a house at -20°C.
• Noise Generation: External units of air-to-water heat pumps generate noise. There is an increasing number of court cases between neighbors due to excessive noise generated by this type of heat pump (which not many people know about).
• Compressor Reliability: Some models have issues with compressor reliability.
• Risk of Freezing in Monobloc Types: In areas where there is a risk of power outages, there is a potential vulnerability to the risk of the heat pump freezing.

Fortunately, there are ways to prevent this by using glycol and heat exchangers to separate the heat pump from the water system.

Air-to-Air Heat Pump – An Alternative or a Supplement to Heating System?

In this system, you heat the building with air, not hot water. This means that this type of heat pump cannot be connected to a traditional central heating system, nor can it heat domestic hot water (DHW). The heated air is distributed through special blowers and heaters, resembling air conditioners.

One definite advantage is the ability to cool the air in summer, as it can also function as an air conditioner. In fact, the principles of operation for heat pumps and air conditioners are so similar that in many countries, including ours, people have successfully converted air conditioners into heat pumps.

From our perspective, we recommend considering an air-to-air heat pump primarily as a supplement to heating. It may also be suitable for smaller buildings.

Disadvantages of Air-to-Air Heat Pumps

Unfortunately, air-to-air heat pumps have several significant drawbacks in their application:

• Limited Scope of Application: They only heat the air.
• No Hot Water: They cannot be used to heat domestic hot water (DHW).
• Air Distribution Requirement: Each room requires a blower (which may not always have space or be aesthetically pleasing).
• Air Drying: Heating the air can cause it to dry out.

Ground Source Heat Pumps - GSHP

A ground source heat pump, which extracts energy from the earth, is another efficient way to heat single-family homes as well as large commercial buildings. It has several advantages, particularly in terms of efficiency and operating costs. However, it requires more demanding ground work and a designated space for installation. If you have the resources for this, you gain a highly economical energy source. But you also need to afford it.

Ground source heat pumps draw heat from the ground, but how do they access it? There are generally two options: either you build a surface ground heat exchanger located at a certain depth below the earth's surface (the first type of ground source heat pump) or you dig a geothermal borehole.

Ground Source Heat Pump Using Surface Ground Heat

A ground source heat pump that extracts heat from the earth's surface uses a wide network of plastic pipes laid in the ground, known as a ground heat exchanger. A special antifreeze mixture, typically glycol-based, circulates through these pipes. The system benefits from the stable temperatures usually found below the earth's surface, with this energy further supported by the sun, ensuring it never runs out.

Advantages of this type of heat pump include zero noise levels and lower electricity consumption.
✅ Energy consumption is even 20-30% lower than that of air-to-water heat pumps.