In the light of the recently released EST field trials, it is clear that many existing heat pump installations fall short of the energy efficiency that they could achieve. The data collected from this on-going test of a wide variety of installed heat pumps could give an invaluable insight towards ‘how to get it right'.

Heat pumps are different to the conventional central heating systems that we are used to. The energy efficiency of a heat pump system is affected greatly by the working temperatures that it is operating at.The variation in energy efficiency can be dramatic, and can be caused by a number of issues.

Sadly, we do not have access to the EST data, but we do know some potential pitfalls. The settings are one area where considerable savings may be possible at no cost other than a bit of time fathoming it out.

Note: - due to the large number of different heat pump types with many possible configurations, it is impossible to give suggestions that would suit everyone. The integral central electronic controller (fitted to most models) has a host of parameters that could be adapted to suit a particular installation in a specific house. Unfortunately, these are not always set to their optimum for energy efficiency, and usually require a product-specific trained expert to figure them out.

The notes below could be helpful, but there is a risk that they may confuse or worry. My apologies if they do. Always seek expert advice.

Keeping the heated water temperature low.

It seems counter intuitive to think that a heating system is more efficient if it’s heating at a low temperature level. However this is the case with heat pumps. A heat pump is happiest if the radiators or under-floor heating are taking the heat away as 'quickly' and efficiently as possible. This is why radiators should be as big as possible. This reduces the circulating water temperature, and increases the COP(energy efficiency).

Heat pumps usually have an integrated digital controller that has a multitude of functions. However, some have a simple knob or digital control. Whatever control you have, it should be set so as to achieve the lowest circulating water temperature that you can get away with. i.e. the minimum temperature that gives adequate room heating. Try a lower temperature, and see if the rooms are still warm enough. Adjust frequently if you have the inclination. This will give you the lowest running costs. If you are using a time control, you can achieve lower temperatures by increasing the time that the system is 'enabled'. i.e. reduce the 'off' periods.
For every two degrees reduction, you may save around 5 % in running costs, for the same heat output. This shows how important the settings are.

Weather compensation

This is the automatic adjustment of the heated water temperature dependant on outside conditions. The system automatically adapts to outside conditions, and potentially saves a significant amount of energy. Interestingly, this control alone, if set correctly, can maintain a reasonably constant room temperature with no actual room thermostat.
Most heat pumps have a weather compensation function, and it is generally set-up by the installation engineer. It usually has a ‘virtual’ room temperature setting that control is aiming to achieve, and also a ‘heating curve'. Other weather-compensation controls ave various heated water temperature settings that depend on outside temperatures. Whilst the initial setting is usually estimated by the installing engineer, the final setting is best made by the occupier using the method of trial and error.

If you are uncertain about the setting, you may need to sit down with your user instructions, and a cup of tea. (Many user/installer manuals are available on-line.)
The setting for weather-compensation can be adjusted down until the heating is just inadequate, then turned back up a little. This has to be done in various weather conditions, and could potentially save you a significant amount of energy.

If you need to operate with a high setting, then you may need to investigate why: - is your under floor or radiator system circulating water adequately?
Are there unwanted restrictions in your system?

The anomaly or weather-compensation and air source heat pumps.

On frosty nights, your weather-compensation control will automatically increase the heated water temperature in an attempt to match heat demand. If your system is air source, then it will by nature be less energy-efficient during this frosty night. If the following day is warm, the water temperature setting will automatically reduce. In essence, the control is encouraging the system to run more during the frosty night, and less in the day.
If your house has a reasonable amount of internal brick or stone, it is said to have a high 'thermal mass'. Such buildings will retain heat during the frosty night, and could actually take advantage of the day's heat by storing some of it. Ideally, the air source heat pump would be encouraged to operate for some of the day at high efficiency, and with limited running at night.
Sadly, it seems that no controllers are advanced enough to deal with this intelligently yet.
Suggestion: Use the set-back facility in the controller to partly discourage night time running. This might help. i.e. reduce the required room temperature by 1.5 to 2 degrees (°C).
Warning - Make sure this action does not increase the use of any electric back-up heater (see below). i.e. if you reduce the hours the heat pump runs per night, then it might call on the electric back-up at some other time in the day, so its always important to keep an eye on this.

Back-up/ auxiliary/ supplementary heaters
Many systems have an internal electric auxiliary heater; this is designed to only operate occasionally when the outside temperature is particularly cold. High running costs could be due this heater operating too often. There are many reasons why this may happen, and it is more likely to be a problem if the heat pump is undersized.

All controllers have ‘system information’. This should include a record of the total hours that the heat pump compressor and the auxiliary heater have clocked-up. This is worth keeping an eye on. Note down the hours and the date and time.

Some heat pumps allow you to switch off the electric heater. However, care should be taken to ensure that any legionella protection for the hot water cylinder is still satisfied.

Worcester Bosch have a nice facility where you can fit either a time clock of manual switch to their ‘external input’. This can be configured to inhibit the electric heater at times when you don’t want it.
Most heat pumps allow you to limit or minimise the amount that an electric heater is used, but its rarely simple to configure.
Remember, it is the cold spells like early 2010 when this heater is likely to be over used.
I repeat – by limiting compressor use (too many degrees of night setback), there is the possibility of increasing the electric back-up use. Compressor running is always preferable to the electric back-up heater.

Top Tip - If you are wasting energy and money by your internal boost heater coming on when it should not, it would be worth fitting a 'whole house energy monitor' to your heat pump's supply (not to the whole house). These often have an alarm function, so they could alert you when the boost heater is coming on due to a sudden increase in the input power. You could then investigate the settings if you want to avoid it.
(note, these devices often indicate inaccurately high compressor power. However, they will inform you when the expensive boost heaters come on)

Room temperature control

Whilst weather compensation alone could maintain reasonable room temperatures, it is normal to have room thermostats, or sometimes, radiator TRV’s. (thermostatic valves)
If TRV’s are fitted to your radiators, your heat pump will not like it if too many of these start to turn off.
Tip:- turn up TRV’s in main and central rooms, and control the room temperature in these rooms by adjusting down the heating curve, or the electronic room thermostat.
Use TRV’s to limit the temperatures in the extremity rooms/bedrooms only etc.

If you run with a timer, try running for longer/ start heating earlier. This can allow you to drop the heated water temperature.

If you run continuously, consider using 1 or 2 degrees setback. This may save you a little, and may or may not be worth the trouble

If your heat pump switches on and off many times per hour, then investigate why. Heat pumps ideally would operate for periods of 15mins. or much more.

In Summary

Keep the temperature of the water passing through your heat pump low
Don’t restrict the water flow through the heat pump.
Try more continuous-heating with a low water temperature.
Try 1.5 or 2° setback overnight, especially with air source.
Minimise the use of auxiliary electric heaters.
Record the run hours for both compressor and electric heater. Keep a close eye during very cold weather.