How Do Compressors and Frequency Affect Heat Pump Efficiency?


At the heart of every heat pump is a critical component known as the compressor. A heat pump works on the principle of transferring heat from one place to another. It can extract heat from the air, ground or water outside a building and transfer it inside for heating. Conversely, it can remove heat from inside a building and release it outside for cooling. The compressor in a heat pump is akin to the heart in a human body; it pumps the refrigerant, the blood of the system, through the heat pump.

The compressor’s primary role is to increase the pressure of the refrigerant. When the refrigerant is compressed, its temperature rises. This high-pressure, high-temperature refrigerant then travels through the heat pump, where it either releases heat for warming or absorbs heat for cooling. The compressor, thus, plays a pivotal role in enabling the heat pump to move heat from one place to another, enabling heating or cooling operation.

The Heat Pump Compressor as a Car Engine

To better understand the compressor, let’s compare it to a car engine. A car engine compresses an air and fuel mixture which, when burned, produces power or rotational force, which drives the car forward. Similarly, the compressor consumes electrical energy to pump and compress the refrigerant, propelling it through the system. Just as an engine’s efficiency and power output are crucial for the car’s performance, the efficiency and capacity of a compressor directly impact the heat pump’s effectiveness in heating or cooling a space.

Frequency and Speed

Frequency, usually measured in Hertz (Hz), plays a vital role in the operation of a heat pump compressor, especially in models with variable-speed compressors. The frequency correlates to the speed at which the compressor operates or rotates. A higher frequency means the compressor runs faster, pumping more refrigerant and increasing the heat transfer rate. On the flip side, a lower frequency results in a slower compressor speed, reducing the heat transfer rate.

This concept of frequency controlling the speed is crucial for the efficiency and adaptability of the heat pump. Modern heat pumps often come with inverters that adjust the frequency based on the heating or cooling demand. This not only optimises energy consumption but also enhances comfort by maintaining more consistent indoor temperatures. The sophisticated electronic control strategies within modern, high-quality heat pumps manage compressor speed, maintaining efficient and reliable operation.

A controller may choose to switch off a compressor if it is operating outside a specific operating zone in part-load conditions for efficiency and reliability. This may look like cycling, which it may be in poorer quality units, but in high-quality units, it is to maintain optimal efficiency due to the characteristics of their compressor operation and performance.

Frequency as the Accelerator Pedal

Imagine the frequency of a heat pump compressor as the accelerator pedal in a car. When you press down on the accelerator, the car speeds up, and when you lift your foot, it slows down. Similarly, increasing the frequency makes the compressor run faster, ramping up the heating or cooling output. Reducing the frequency slows down the compressor, decreasing the output. Just as a driver adjusts the accelerator to maintain an optimal speed based on road conditions, the heat pump adjusts the frequency to efficiently meet the heating or cooling needs of the building.

The compressor in a heat pump is a vital component, much like an engine in a car, driving the system’s functionality. The frequency at which the compressor operates can be thought of as the accelerator pedal, dictating the speed and efficiency of the heat transfer process. Understanding these aspects of a heat pump not only demystifies its operation but also highlights the sophisticated engineering that goes into creating comfortable, energy-efficient environments in our homes and workplaces.

Related posts

How much energy does an ASHP use in freezing vs. mild weather?


What is a heat loss calculation?


How to DIY Fix a Heat Pump Flow Error?

Notify of

Newest Most Voted
Inline Feedbacks
View all comments
1333 kWhs
3 months ago

Because I have a lot of thermal mass in my concrete floor, I only run my heating during the 7 hours of cheap[er] electricity [when OAT is higher and my floor will keep the rooms warm the whole day].  It seems to take about 3.5 hours before my compressor speed stabilizes.  I have considered “soft starting" my heat pump, and did this manually a couple of times.  So set the target water out temp to 25, then once it reaches 25, increase in .5 deg steps until it gets to the target flow temperature.  This significantly reduced my max compressor speed during startup. Not sure this will help longevity, but I will automate this at some point and see if it gets to a stable compressor speed quicker.Last nights “automatic switch on" [WITHOUT a “soft start"] in chart below.
For a fair/scientific comparison with a “soft start" I need to automate it, so this make take a while.  A basic test shows my peak compressor speed is much lower with a “soft start".  Will report back once I have coded the “soft start" in home assistant and run some tests.


Below is data from one of my manual attempts [ at a “soft start"], though not at .5 deg steps, a bit larger than that.  about 18Hz less peak.  Need to collect a whole bunch of data in a more controlled manner.



Derek M
13785 kWhs
Reply to  william1066
3 months ago


It would be useful to include the leaving water temperature, indoor air temperature and power consumption on the chart.

1333 kWhs
3 months ago

You state that you are running the heat pump at the lower tariff rate to reduce cost. I have my doubts if you are actually achieving what you want, I may be wrong.

I suspect you will probably be correct, just need to code something to process the data. 

On most under floor heating, I will set the weather compensation to 40 at -4 and 23 at 15.

Thank you, that has been my next challenge, is finding a good range for weather compensation, given the thermal mass in the floor.  My strategy is [lower] cost over comfort so my max flow temp is 34 [@-2] and my min around 25 [@15], but have not proved this will work continuously given I am on a schedule most of the time.
I will report back once I have had the opportunity to test and crunch the numbers.

Please leave a comment.x
x  Powerful Protection for WordPress, from Shield Security
This Site Is Protected By
Shield Security