Advantages of Inverter Heat Pumps over Fixed Output Single Speed

Deciding to install a heat pump is a big decision to make for homeowner. Replacing the traditional fossil fuel heating system like a gas boiler with a renewable alternative is one that people spend a lot of time researching before committing to.

People will want to know things like what it’s like to live with a renewable heating system, how much they cost to install and operate, and whether to install an air source or ground source heat pump.



When carrying out research, the question may come up with to install a heat pump with an inverter drive unit or a fixed output. Depending on which manufacturer you’re researching, companies will have a preference one way or the other – usually influenced by the range of products they offer.

At Thermal Earth, we supply both fixed output and inverter heat pumps. We have over a decade’s worth of experience in the design, installation and maintenance of both types of heat pump systems, and we’ve seen first-hand how both inverter and fixed output heat pumps perform.

This knowledge and experience has confirmed to us, without doubt, that an inverter heat pump offers significant advantages in terms of:

  • Higher overall annual energy efficiency
  • Less likely to have issues with connection to the electrical network
  • Spatial requirements
  • The lifespan of a heat pump
  • Overall comfort

But what is it about inverter heat pumps that makes them the heat pump of choice? In this article we will explain in detail the differences between them and fixed output heat pumps two units and why they are our unit of choice.


What is the difference between the two heat pumps?

The difference between a fixed output and an inverter heat pump lies in how they deliver the energy needed from the heat pump to meet the heating demands of a property.

A fixed output heat pump works by continuously either being turned on or off. When turned on, the fixed output heat pump works at 100% capacity to meet the heating demand of the property. It will continue to do this until the heat demand is met and will then cycle between on and off heating a large buffer in a balancing act to maintain the requested temperature.

An inverter heat pump, however, uses a variable speed compressor which modulates its output increasing or decreasing its speed to match exactly the heat demand requirements of the building as the outdoor air temperature changes.

When the demand is low the heat pump will reduces its output, limiting the electricity usage and the exertion placed on the heat pump’s components, limiting the start cycles. 



The importance of correctly sizing a heat pump

In essence, the output of a heat pump system and how it delivers its capacity is central to the inverter vs fixed output debate. To understand and appreciate the performance benefits offered by an inverter heat pump, it’s important to understand how a heat pump is sized.

To determine the size of the heat pump needed, heat pump system designers calculate how much heat the property loses and how much energy is required from the heat pump to replace this lost heat through fabric or ventilation losses in a building. Using measurements taken from the property, engineers can determine the heat demand of the property at outside temperatures of -3oC. This value is calculated in kilowatts, and it’s this calculation that determines the size of the heat pump.

For example, if the calculations determine the heat demand is 15kW, a heat pump producing a maximum output of 15kW is necessary to provide heating and hot water to the property year-round, based on the current room temperatures required by BS EN 12831 and the projected minimum temperature for the area, nominally -3oC.

The size of the heat pump is significant to the inverters vs fixed output heat pump debate because when a fixed output unit is installed, it will be running at its maximum capacity when switched on, regardless of the external temperature. This is an inefficient use of energy because 15 kW at -3oC may only need 10 kW at 2oC. There will be more start – stop cycles.

An inverter drive unit, however, modulates its output across a range between 30% and 100% of its maximum capacity. If the heat loss of the property determines a 15kW heat pump is needed, an inverter heat pump ranging from 5kW to 15kW is installed. This would mean that when heat demand from the property is at its lowest, the heat pump will work at 30% of its maximum capability (5kW) rather than the 15kW used by a fixed output unit.  


Inverter driven units offer far greater efficiency

When compared to traditional fossil fuel-burning heating systems, both fixed output and inverter heat pumps offer far greater levels of energy efficiency.

A well designed heat pump system will provide a coefficient of performance (CoP) between 3 and 5 (depends whether ASHP or GSHP). For every 1kW of electrical energy used to power the heat pump it will return 3-5kW of heat energy. Whereas a natural gas boiler will provide an average efficiency of around 90 – 95%. Heat pump will provide approximately 300%+ greater efficiency than burning fossil fuels for heat.

To get the maximum efficiency from a heat pump, homeowners are advised to leave the heat pump running continuously in the background. Leaving the heat pump switched on will keep a steady continuous temperature in the property, reducing the ‘peak’ heating demand and this most suits inverter units.  

An inverter heat pump will continuously modulate its output in the background to provide the consistent temperature. It reacts to changes in heat demand to make sure the fluctuation in temperature is kept to a minimum. Whereas a fixed output heat pump will continuously cycle between maximum capacity and zero, finding the right balance to supply the temperature needed cycling more often.


Less wear and tear with an inverter unit

With a fixed output unit, cycling between on and off and running at maximum capacity puts not only the heat pump unit under strain but also the electrical supply network. Creating surges on each start cycle. This can be reduced by using soft starts but these are prone to fail after only a few years operation.

As the fixed output heat pump cycles on, the heat pump will draw a surge in current to make it start. This places the power supply under stress as well as the mechanical parts of the heat pump – and the process of cycling on/off takes places multiple times a day in order to meet the heat loss demands of the property.

An inverter unit, on the other hand, utilises Brushless DC compressors which have no real start spike during a start cycle. The heat pump starts with a zero amp starting current and continues to build until it reaches the capacity needed to meet the demands of the building. This places both the heat pump unit and the electrical supply under less stress whilst being easier and smoother to control than an on/off unit. It’s often the case that where multiple start/stop units are connected onto the grid, this can cause issues and the grid provider may decline a connected without network upgrades.


Save money and space

One of the other appealing aspects of installing an inverter-driven unit is both the money and spatial requirements that can be saved by eliminating the need to fit a buffer tank or it can be much smaller if underfloor heating full zone control is used.

When installing a fixed output unit into a property, space needs to be left to install a buffer tank alongside it, approximately 15 litres per 1kW of heat pump capacity. The purpose of the buffer tank is to store pre-heated water in the system that is ready to be circulated around the central heating system on demand, limiting the on/off cycles.

For example, say you have a spare room in your home that you rarely use which is set to a lower temperature than other rooms in the house. But now you want to use that room and decide to turn up the thermostat. You adjust the temperature but now the heating system has to meet the new heat demand for that room.

We know that a fixed output heat pump can only run at maximum capacity, so it will begin to work at maximum capacity to meet what is actually a fraction of the maximum heat demand – wasting a lot of electrical energy. To bypass this, the buffer tank will send pre-heated water to the radiators or underfloor heating of the spare room to warm it up, and use the maximum output of the heat pump to re-heat the buffer tank and likely overheating of the buffer tank in the process ready for the next time it’s called upon.

With an inverter-driven unit installed, the heat pump will be adjusting itself to a lower output in the background and will recognise the change in demand and adjust its output according to the low change in water temperature. This capability, then, allows property owners to save on the money and space required to install a large buffer tank.


MasterTherm: specialists in efficiency

Thermal Earth are the UK-exclusive distributor of MasterTherm heat pumps. The specialist Czech manufacturer has been at the forefront of the heat pump industry for over 25 years, and their range of air source and ground source inverter heat pumps prove to be enormously popular with property owners in Britain.

As well as offering significant advantages in energy efficiency, MasterTherm’s inverter heat pumps come complete with online functionality – allowing homeowners to control their heating remotely – and a 7-year warranty on all units.

Whether it be an air source heat pump for a domestic home or a ground source heating system capable of heating an entire office block, at Thermal Earth and MasterTherm we have the experience of designing and installing inverter heat pump systems in any setting.

View the MasterTherm range


Interested in converting to a heat pump system? You can find out more information on heat pumps by visiting our dedicated pages for air source and ground source heat pumps. Or, if you’d like to speak to us directly, click here to get in touch with a member of our team.

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