How Does A Heat Pump Work? Step-By-Step Heating, Cooling Explained

How do heat pumps work? All heat pumps have the same basic principle of operation. Air-source heat pumps use outdoor air as a heat sink, geothermal heat pumps use the ground as a heat sink. Here we will, in the simplest terms, answer this key question:

“What is a heat pump and how does it work?”

A heat pump is an HVAC device that transfers heat from one space to another (outdoor to indoor, indoor to outdoor). Basically, it can work both as an air conditioner (transfers heat from indoor to outdoor) and a furnace (transfers heat from outdoor to indoor).

That makes it very useful (you can use 1 device instead of 2 devices) and extremely energy-efficient. Heating and cooling electricity bills fall significantly (according to the US Department of Energy, “geothermal heat pumps can reduce energy use by 30%-60%“) because you don’t have to use electricity to produce the heating/cooling; you only use electricity to transfer heating/cooling.

Note: Recently, we have seen quite a surge in heat pump sales in the UK due to Domestic Renewable Heat Incentive (RHI) UK Government heat pump grants. You can read more about this incentive here.

Here are two seemingly paradoxical concepts that we immediately think of when looking at how heat pumps work:

1. How can a heat pump heat in the winter? It transfers heat from outdoor air (let’s say 14°F or -10°C) to heat indoor air (+70°F or +20°F):
2. How can a heat pump cool in the summer? The temperature of the outdoor air can be over 86°F or 30°C in the summer; how can this transfer use that high-temperature air to cool indoor air below 68°F or 20°C?

It all has to do with the principle of operation called ‘reverse-cycle conditioning’ facilitated by the refrigerant running from outdoor to the indoor unit. We’ll take a systematic step-by-step approach to explain how all 3 heat pump mechanisms work, namely:

1. How does a heat pump work in winter? (step-by-step heating explanation)
2. How does a heat pump work in summer? (step-by-step cooling explanation)
3. We will also cover the all-important (yet often overlooked) defrost cycle that is essential to protect the heat pump in the winter.

We will see that the operating principle of all heat pumps is based on reverse-cycle conditioning. Understanding this principle helps us to better understand how does a mini-split air-source heat pump work and how does geothermal heating work (basic principle, different heat sink).

How Does A Heat Pump Work In Winter? (Heating Cycle)

The heat pump heating cycle uses outdoor air or ground as a medium-temperature source, extracts heat from it, and transfers this higher-temperature heat to the indoor air. Or simply, the heating cycle heats us in the winter.

Now, before we look at how does a heat pump work step-by-step in winter, let’s clear one thing from our way:

How can a heat pump use cold outdoor air (14°F or -10°C) to heat already warmer indoor air (70°F or 20°F?

The key part here is to understand that a heat pump or a physicist does see air as ‘cold’ or ‘warm’ per se. Warm air is just air with high heat content, and cold air is just air with a little less heat content.

That means that the point of the heat pump heat cycle is to extract available heat from cold outdoor air, and transfer it indoors.

Here’s how the heat cycle in heat pump works in the winter:

Step-By-Step Heating Cycle Explained

1. Refrigerant in the low-pressure and low-temperature liquid state is circulated in outdoor coils.
2. Even cold outdoor air has a higher temperature than this refrigerant. The refrigerant absorbs this heat and turns from liquid to gas (or hot vapor).
3. This refrigerant gas is compressed by the compressor (in the outdoor unit); this further increases its temperature.
4. Hot refrigerant is channeled toward the indoor unit via the refrigerant lines (the lines that go through the wall and connect the outdoor and indoor unit).
5. Hot high-pressure refrigerant is circulated through the indoor coils.
6. When cooler room air circulated around these indoor coils, it causes the refrigerant to cool and condensate back to the liquid state.
7. The refrigerant gas condensation back into a liquid state (an endothermic process) heats up the indoor air around the indoor coil, providing the necessary heat to facilitate heating.
8. The air blower in the indoor handle blows this heated air into our home. We feel that as heating.
9. Refrigerant has done its job; it now passes through a pressure-reducing part (we call it an ‘expansion valve’ that again generates a low-temperature low-pressure liquid-state refrigerant.
10. Refrigerant arrives at the outdoor coil and the heat cycle starts all over again.

You can use this illustration to help you understand the inner working of heat pumps:

In the case of heat pump heating, we see that the outdoor coils work as evaporator coils. Indoor coils in the air handler work as condenser coils.

In the case of heat pump cooling, outdoor coils work as condenser coils, and indoor coils work as evaporator coils.

This is how any heat pump – air-source, geothermal, water-source – generate heat and transfer it indoors with the smart use of a refrigerant liquid-gas transformation.

Note: Heat given off by a heat pump is transferred to what we call a ‘heat sink‘. In the case of heating, indoor air is the recipient of heat or ‘heat sink’; we’re heating our home. In the case of cooling, we expel heat from indoor and transfer it to external heat sinks; outdoor air (air-source), the ground (geothermal), or water (water-source).

Let’s have a look at how does a cooling cycle work in a heat pump:

How Do Heat Pumps Work In Summer? (Cooling Cycle)

As you might imagine, to provide cooling instead of heating, the direction of heat transfer is reversed. Heat pumps have a heating-to-cooling switch that makes this reversal. You can easily control it via the heat pump thermostat.

Here is the step-by-step how does a heat pump work in summer:

1. Switch heat pump thermostat to ‘Cooling’. This activates the reversing valve, switches the direction in which the compressor pump pumps, and reverses the flow of refrigerant.
2. Indoor coils extract heat from indoor air. The liquid refrigerant is turned into a gaseous state by indoor heat.
3. Hot refrigerant gas vapor is channeled into the outdoor unit via refrigerant lines.
4. In the outdoor coils (now acting as condensation coils) the hot refrigerant is cooled, expelling the indoor heat outdoors.
5. The compressor again squeezes the refrigerant into a liquid low-pressure low-temperature liquid state.
6. The liquid refrigerant starts the cooling cycle all over again.

This is the basics of how the heat pump cooling cycle works.

In the summer, it’s hot outside, and we don’t have problems with outdoor coils freezing. However, if you check how a heat pump works in the summer vs. winter, you will notice that in the winter, the outdoor coils can freeze over.

That’s why every heat pump has a third mechanism of operation that defrosts the potentially frozen over outdoor coils. This operating mechanism is known as the ‘defrost cycle’:

Defrost Cycle (Important Feature In The Winter)

In the winter, we have lower outdoor temperatures. If the temperature is too low and if the relative humidity is high enough, the heat pump outdoor coils can freeze. We will notice this as frost forming on the surface of the outdoor coils.

Frozen coils will lower the energy efficiency of a heat pump and can lead to all kinds of negative residual effects. That’s because the ability of the outdoor coils to transfer heat is impeded.

To melt the frost on the outdoor coils in the winter, heat pumps temporarily switch from heating mode to cooling mode (cooling mode heats up the outdoor coil). This is known as the ‘defrost cycle’.

You need to be aware of this defrosting cycle because, for a temporary period of time, the heat pump set to ‘Heating’ in the winter will start to blow cold air indoor. There is nothing wrong with your heat pump when this happens; in fact, this is the standard operating procedure of heat pumps in the winter.

This explains how does a mini-split air-source heat pump work.

Let’s have a look at how does a ground source heat pump work:

How Does A Ground Source Heat Pump Work? (Vs Air-Source And Water-Source)

All heat pumps work on the same principle. What is the difference between how a ground source, air source, and water source heat pump work?

The only difference in how different heat pumps work is where they transfer heat to (summer) and where they transfer heat from (winter). That, in itself, makes quite a difference when deciding which type of heat pump is best for your situation.

Let’s look at the 3 types of heat pumps and their heat transfer:

1. Air-source heat pumps transfer heat to and from the outdoor air. Outdoor air temperature is changing; cold in the winter, hot in the summer; that’s a con. Because of the changing outdoor air temperature, air-source heat pumps work best in areas with above 30ºF or -1ºC winter temperatures. Nonetheless, they are easily to install, cheaper than other types of heat pumps, and are as such the most popular type of heat pumps.
2. Ground-source heat pumps transfer heat to and from the ground. The ground temperature is constant (usually around 55ºF or 13ºC in the UK); that’s excellent news for how does geothermal heating work. That means that in the winter, the geothermal heat pump will be able to extract more heat from the ground than an air-source heat pump would from the cold outdoor air. That also means that ground-source heat pumps are the most energy-efficient and cost-saving type of heat pump. They are expensive to install, however.

   

3. Water-source heat pumps transfer heat to and from the water. The temperature of the water is less constant than the temperature of the ground but much more constant than the temperature of the outdoor air. Keeping the water temperature rather constant, offsets the seasonal temperature changes (during winter, summer) that air-source heat pumps suffer from.

All in all, the way ground-source heat pumps work by using the advantage of constant ground temperature, makes them the most energy-efficient type of heat pumps.

You now, hopefully, understand how heat pumps work. If you have any questions, you can use the comments below and we’ll try to help you out as best we can.