Guy Cashmore, Technical Director of Kensa Heat Pumps, says: “Using an inverter compressor with an air source heat pump system is almost a no-brainer because of the fluctuating source air temperature. However, we often get asked why Kensa, as the UK’s leading ground source heat pump manufacturer, favours fixed speed compressors in our units, when many international manufacturers mimic the air source heat pump model and use inverter driven controls in their ground source heat pumps.”

The answer is that Kensa believes that fixed speed compressors:

  • achieve better efficiencies in real-life vs test conditions;
  • are more reliable and have a longer product life;
  • are designed to work with refrigerants that are environmentally friendly and future proofed;
  • make maintenance on the heat pump internals easier.

In 2017, Kensa drew on eighteen years of engineering experience designing ground source heat pumps specifically for British properties, to develop the Evo Series. An award-winning and ErP A++ rated range, the Evo has received widespread positive reviews, and also features fixed speed compressors.


What’s the difference between inverter driven and fixed speed compressors?

To better understand the benefits of fixed speed compressors in ground source heat pumps, you first need to understand the difference between the two.

The compressor component of a ground source heat pump is responsible for upgrading the low grade heat energy extracted from ground or water sources to a higher temperature for use within a building’s heating distribution system.

Whilst all compressors are generally of a scroll-type, inverter driven compressors change speed in response to the load or source temperature experienced by a heat pump unit, whereas fixed speed compressors run at one speed which cannot be altered, and are either on or off.

Armed with this knowledge, let’s examine some of the more commonly held misconceptions about inverter-driven compressors in ground source heat pump systems, and the demonstrable benefits of fixed speed compressors.


Are inverters more efficient because they have a lower electricity consumption?

It is true that by altering the speed of the compressor to match demand, an inverter can reduce the amount of electricity used, and so they look like an attractive proposition to the energy conscious. However, this has the effect of making everything else in the system, e.g. water pumps, run for longer, therefore actually increasing consumption.

Due to the way that ErP results are derived, and the fact that inverters can operate at part-load conditions, they can produce better results in test conditions. Kensa believes that fixed speed compressors may actually be more efficient in the majority of real-life scenarios, and some inverter driven ground source heat pumps are actually less efficient due to power losses elsewhere.


Do inverter driven ground source heat pumps perform better?

Based on the compressor rotations per minute (RPM), or speed, an inverter driven compressor has various limits that have to be respected in order to run the system safely.  Whilst they are capable of fluctuating their speed according to demand, inverter compressors are still designed to run at, or as close as possible to, one optimum speed.

Any operation away from this speed will limit the maximum output temperature available and also compromise the efficiency. This ‘sweet spot’ of operation speed means that it is imperative that the inverter compressor is sized correctly for the heat demand – a ‘catch all’ approach does not work efficiently.

As an example, if you purchase an inverter driven ground source heat pump – say 2kW to 12kW – the compressor is sized for the ‘sweet spot’ of around 5kW. The inverter will then ramp up the speed to give the higher output when required; this works of course, but the maximum outlet heating temperature diminishes pro-rata, which is is not ideal in a house with higher heat losses.

Fixed speed compressors are optimised for the best efficiency at the speed they are fixed to run. Therefore, if your ground source heat pump is sized to operate at a maximum output for the majority of its life, a fixed speed compressor will almost certainly prove more efficient than an inverter at the same speed.

This concept is illustrated in the example graphs below:


Are inverter driven compressors more reliable than fixed speed?

Inverters of all types are well known for their reliability issues and have a relatively short working life. A ground source heat pump designed to last 25 years will typically need two replacement inverters in that time; the cost of these alone is likely to exceed any savings made in running costs, based on efficiency projections. Availability of compatible replacement inverters is also questionable over this time period, as electronics of this type are typically only available for around eight years.

Generally the speed variation is controlled by sophisticated and expensive electronics, and whilst these are becoming more robust, they often have a short life compared to fixed speed compressors. They are also vulnerable to damage from voltage spikes and nearby lightning that would not damage a fixed speed unit.


Do inverters allow more control over room temperature?

Whilst this might be true of air conditioning units, with a ground source heat pump, the choice of inverter has no bearing on individual room temperature. This is purely a function of the room thermostat (or whatever heating controls are fitted), and nothing else.


How does the choice of compressor in a ground source heat pump affect ground array sizing and MCS?

Whatever type of compressor a ground source heat pump uses, if it is achieving higher efficiencies, then it will require a larger ground array. Correct system sizing and ground array design is key to the successful operation of any ground source heat pump system.


R410A refrigerant gives Kensa the chills

Currently almost all ground source heat pump compressors designed for use with an inverter are only compatible with R410A refrigerant. This refrigerant has a significantly higher Global Warming Potential (GWP) than the R134a and R407C that Kensa uses, and R410A is already being viewed as ‘obsolete’ by many in the industry because of this.

Quite apart from the environmental considerations due to high GWP, future availability of this refrigerant for service is questionable. R410A operates at very high pressures, approximately double the pressure of R134a. This causes high levels of stress on components potentially leading to reduced reliability in heat pumps using this type of refrigerant. R410A also has more limited maximum hot water temperature, which can cause issues with DHW system; a typical R410A system will be limited to about 10°C less than a R134a system.


More sound advice…

Inverters generate harmonics that feedback along power supply cables, potentially causing disruption and interference to other electrical appliances locally. These can especially affect ‘weak’ rural power supplies, and care therefore has to be taken with the electrical installation.

Inverters also add a great deal of complexity to ground source heat pump internals, making any future repairs significantly more difficult and reliant on specially trained personnel using special tools and equipment. The availability of service over the lifetime of the product is therefore also questionable.


Find out more about the other innovative features of Kensa’s new Evo Series ground source heat pumps.