Higher fan speeds make your air-to-air heat pump more efficient. But why? What is the connection between moving air faster over the indoor coil and the compressor doing less work?
Most people know that lower flow temperatures in air-to-water heat pumps improve their efficiency, but not everyone knows why. The same is true for air-to-air heat pumps; increasing the fan speed results in higher efficiencies, but the reason behind this is rarely explained.
I am going to explain why this is the case through analogies and thought experiments, so you can intuitively understand the physics and how to take advantage of it.
*At lower heat loads fan speed doesn’t really matter it is only at higher heat loads that this becomes of concern*
Part 1: The Intuition (The Bicycle Pump)
To understand why fan speed matters, we need to understand what your heat pump compressor is doing. A good analogy is a simple manual bicycle pump.
Imagine you are pumping up a bicycle tyre. At first, the tyre is empty (low pressure), and the pump moves easily. You can push the plunger down with just one finger. You are moving air into the tyre, but it takes little effort.
As the tyre fills up, the pressure rises and it gets harder to pump. To push the same amount of air into the tyre, you have to use much more effort. Pushing against high pressure requires more energy.
This is the core of why higher condensing temperatures reduce heat pump efficiency. The heat pump compressor (like our bike pump) working against a higher pressure requires more effort, and therefore more electricity.
We will return to this analogy later. But first, we need to understand the journey of the refrigerant.
Part 2: How a Heat Pump Moves Heat (The Refrigeration Cycle)
A heat pump does not create heat; it moves it. It takes heat energy from outside and carries it inside. The refrigerant is the substance that carries this heat on its journey.
Choosing where to start the explanation of the heat pump cycle is difficult. I have chosen at the expansion valve but I could choose anywhere else. It is best to think of all steps happening at the same time, rather than sequentially.
Step 1: Expansion (The Spray Can)
The cycle begins with warm, high-pressure liquid refrigerant arriving at the outdoor unit. It passes through a component called the Expansion Valve.
Think of a can of deodorant or spray paint. Inside the can, the contents are at high pressure. When you hold down the nozzle, the fluid is forced through a tiny hole into the lower-pressure air outside. Two things happen immediately:
- It turns from a liquid into a mist (a mix of tiny droplets and gas).
- It feels incredibly cold.
As liquid deodorant expands it often pulls heat from the rest of the can, cooling it down.
The Expansion Valve acts exactly like that nozzle. It forces high-pressure liquid refrigerant through a tiny restriction into a lower pressure heat exchanger. As the refrigerant sprays out into the larger space of the outdoor heat exchanger, its pressure drops instantly, and its temperature plummets.
Step 2: Evaporation (Absorbing Heat from Outside)
The refrigerant is now an extremely cold, low-pressure mist inside the outdoor heat exchanger. A fan blows outside air over the coil.
Because of the expansion, the refrigerant is colder than the outside air. So heat energy naturally flows from the air into the refrigerant. This happens even on a freezing winter day, as anything warmer than absolute zero (-273.15°C) contains heat energy, so there is always some heat to collect – provided the refrigerant remains colder than the air.
As the refrigerant absorbs this heat, something important happens: it evaporates. It changes from a liquid mist into a pure gas (vapour).
This is where latent heat comes in.
Humans take advantage of latent heat when they sweat. When sweat on your skin evaporates and turns to gas and in doing so pulls heat from your body then floats off into the atmosphere. If it is very humid then sweat won’t evaporate as easily, so you continue to feel hot.
A similar principle applies to refrigerants. To change from liquid to gas, the refrigerant must absorb a large amount of energy. This energy is called latent heat, where “latent” means hidden, because this heat does not raise the temperature. The refrigerant absorbs a huge amount of energy from the outside air, but it does not get noticeably warmer. Instead, the energy is stored in the change of state itself.
By the time the refrigerant leaves the outdoor coil, it has absorbed as much heat as it can from the outside air. It is now a cold, low-pressure gas, perhaps only a few degrees warmer than when it started. Even though it is barely warmer it has absorbed and is carrying a lot of energy, but it is still too cold to heat your home.
Step 3: Compression (Adding Energy and Raising Temperature)
The cold gas now enters the compressor. This is the heart of the heat pump, and the component that uses most of the electricity.
The compressor squeezes the gas into a smaller space. When you compress a gas, it heats up. You may have noticed this if you have ever used a bicycle pump vigorously; the pump gets warm.
A more dramatic example is a diesel engine. Diesel engines have no spark plugs. Instead, they compress air so forcefully that it becomes hot enough to ignite fuel spontaneously.
Why does compression create heat? It comes down to the conservation of energy. Like with the bike pump, to compress the gas, the piston must apply a significant force to push the molecules closer together. It is doing mechanical work on the gas. That energy doesn’t just disappear; it is transferred into the gas itself.
As the volume decreases, the gas molecules move faster and collide more furiously. In physics terms, their kinetic energy increases – due to the work of compressing. Since temperature is essentially a measure of how fast molecules are vibrating and moving, this increase in kinetic energy manifests as intense heat.
In your heat pump, the compressor squeezes the refrigerant until it becomes hot: perhap just 30°C or maybe up to 60°C, or even higher. The cold gas that could not heat anything is now a hot, high-pressure gas that can warm your home.
For an air to air heat pump the compressor has to push the refrigerant against whatever pressure exists in the indoor coil. If that pressure is high, it takes more electricity. If it is low, it takes less. Just like our bicycle pump pushing against a full tyre versus an empty one.
Step 4: Condensation (Releasing Heat Inside)
The hot, high-pressure gas flows into the indoor unit (the condenser).
A fan blows room air over the indoor heat exchanger. Because the refrigerant is hotter than the room air, heat flows out of the refrigerant and into the air, warming your home.
As the refrigerant releases heat, it condenses, changing from a gas back into a liquid. This is the reverse of evaporation, and it releases all that latent heat the refrigerant picked up outside, plus the energy added by compression.
The temperature at which this happens is called the condensing temperature. This temperature is critical to efficiency, as we will see.
Step 5: Back to the Start
The refrigerant is now a warm, high-pressure liquid. It travels back to the outdoor unit, reaches the expansion valve, and the cycle begins again.
Part 3: The Pressure-Temperature Lock
Here is the key insight: for a refrigerant at its boiling or condensing point, pressure and temperature are locked together.
This is just like water. At sea level (1 atmosphere), water boils at 100°C. On top of Mount Everest, where pressure is lower, water boils at around 70°C. Put water in a pressure cooker, raise the pressure, and it boils at a higher temperature.
Refrigerants work the same way. If the condensing temperature is high, the condensing pressure must also be high. If we can lower the condensing temperature, the pressure drops too.
And remember our bicycle pump: lower pressure means less work for the compressor.
Part 4: The Fan Speed Connection (The Puncture Analogy)
Now we can answer our question: why does fan speed matter?
Think of the indoor coil as a bicycle tyre you are pumping up, but with a puncture.
- The compressor is the pump, pushing refrigerant in.
- The heat escaping into your room is the air escaping through the puncture.
- The fan speed represents the size of the puncture.
Low fan speed = Small Puncture.
Heat escapes slowly. To force the heat out through that “small hole”, the system must build up high pressure. High pressure means high temperature. The compressor has to work extremely hard to maintain this high pressure.
High fan speed = Large Puncture.
Heat escapes quickly. The “tyre” does not need to be highly pressurised to let the heat out. Pressure and temperature stay lower. The refrigerant only needs to be moderately warm because the excellent airflow whisks heat away efficiently.
Part 5: Why This Affects Efficiency
The important thing to remember is that, for a given compressor speed, the same amount of heat can be released into the room. But it can be achieved in different ways:
Scenario A: Low Fan Speed (High Condensing Temperature)
- Heat escapes slowly into the room.
- Pressure builds in the coil.
- Compressor pushes hard against high pressure.
- Each “stroke” takes more energy.
- Result: Motor works harder.
Scenario B: High Fan Speed (Low Condensing Temperature)
- Heat escapes quickly into the room.
- Pressure stays lower.
- Compressor pushes against less resistance.
- Each stroke takes less energy.
- Result: Motor works easier.
In both cases, your room receives the same amount of heat. But in Scenario B, the compressor used less electricity to deliver it.
This is the Coefficient of Performance (COP):
Same heat output, less electricity input = Higher COP.