Image 1 - inverter driven air source heat pumps vs on off

Applying inverter driven air source heat pump technology


The air source heat pump market is moving from on-off machines to inverter driven pumps. Inverter driven air source pumps are more efficient and have a number of key benefits:


  • Inverter driven heat pumps speed up and slow down the motor compressor to suit energy demand and use a lot less energy because they follow the needs of the property more closely (see Image 1)
  • Because they don’t use a surge of energy to restart all the time they put less strain on the electrical supply making them better for single phase supply.
  • Inverter driven heat pumps can be ‘tuned’ to suit the climate in which they are operating – for example our WPL 15/25 is at it’s most efficient when the outside temperatures mean demand for heating is at it's peak in the UK (see image 2 for how the Stiebel Eltron WPL 15/25 has a sweet spot for operation within these parameters)
  • As they maintain a far more stable set-point they don’t need such a big buffer to maintain the temperature of a property, saving space and installation costs


image 2 - Stiebel Eltron WPL 15/25 air source heat pump is tuned for the UK climate

With this all-round better performance you would be forgiven for thinking that an inverter driven heat pump must therefore be better for every situation where you would previously have installed an on/off air source. And, generally, for situations where the main demand on the heat pump is for heating in winter, you would be right.

The mistake that I have seen made on a few recent installations (not of Stiebel Eltron kit, I hasten to add), is to use inverters as a stand-alone solution where the main demand on them is hot water or for output during the summer months . In one notable instance I saw a (broken) inverter which had been installed to provide hot water for a camping site in summer. In these situations an on/off machines remains the correct choice. Standard on/off air source pumps get more efficient as the ambient temperature rises but inverters get less efficient (look again at the graph above and you’ll see an ineffective range for inverters when the temperature and demand rise above a certain point together). This makes on/off pumps great for summer hot water production, with the caveat that any heat pump running hot is always under more strain and can wear out quickly (unless of course you choose one of our High Temperature Heat Pumps, which are designed to operate at higher temperatures) so should be used to pre-heat water for a solar system (or other high temperature system) rather than producing all the heat itself. In this situation the better choice would have been an on/off heat pump specifically designed for hot water, such as our WWK hot water heat pump.

A stand alone on/off machine is also often the correct choice for heating swimming pools because the temperature of the water required is relatively modest, and puts little strain on the pump (and there is no need for a large buffer as the pool itself acts as one). As outdoor pools in particular often get more use and are more often heated in summer, the on/off pump may be doubly the right choice here. This is not to say that there may not be an inverter tuned for summer use (because the point is they can be tuned for efficiency at different climate points), but that you need to be aware of what conditions a particular pump is optimised for.

Inverter driven heat pumps, on the other hand are designed to be highly efficient at cold temperatures. They typically have a smaller evaporator than on/off machines. This makes them great partners for solar thermal systems too where demand for hot water is modest as you can use the solar in summer and the heat pump in winter for maximum efficiency.

At this point I have to the praises of our new WPL 15/25 air source heat pump which has a bespoke inverter compressor, tuned for maximum efficiency in UK climatic conditions. For the first in the UK market all the component parts of the machine have been optimised to suit UK demands. It has the best seasonal performance on the market in its class with a COP of 2.90.





WPL 15/25 inverter driven air source heat pump

The unit includes a blue fined evaporator as standard protecting against aggressive salt water conditions, perfect for coastal areas.

Domestic hot water is also generally limited to around 50°C from heat pump systems – however, the WPL15/25 can run at a much higher flow temperature of 65°C and 60°C in the domestic hot water tank. This means there is no need for the unit to purge once a week to destroy Legionella bacteria, saving up to £120 each year in energy costs. Also the output for DHW production can be reduced so that it can be connected to existing hot water tanks (in a retrofit situation).

So in short, inverter driven air source heat pumps are better for most applications. However, the key to selecting the right heat pump for your project is to look closely at the conditions in which you need a pump to work hardest and make sure that you choose a pump optimised for those conditions. Oh, and do try the WPL15/25 - you won't be disappointed!