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Blog Post 17 November 2021

How to ensure a heat pump runs efficiently

Over the years, our understanding of heat pumps and how they should be controlled has improved. Energy Saving Trust’s field trials in 2010 and 2013 went a long way towards helping us understand this.  

There are often very common issues that can cause poor heat pump running costs, some of which you can address yourself.

Heat pumps are a lot less complex than you might imagine, and usually any problems experienced are because the heat pump was set up incorrectly – not because it’s faulty. This can be daunting if you’re not particularly technically minded, or if you moved into the property and never had the system explained to you properly.  

Issues that may cause increased running costs

Electric boost

Nearly all heat pumps have an additional electric heat element, like the element in your kettle or the electric immersion heater in your hot water cylinder. The heat pump simply asks this additional heater to come on when it’s cold outside to provide extra help, or if you have asked the system to provide extra legionella protection (see below) in your hot water cylinder. 

Heating boost

Usually heat pumps are sized to heat buildings when the outside air is at what is referred to as the ‘outdoor design temperature’. This would typically be around -2°C. 

Heat pumps can perform down to -15°C or more, but there will come a point when the output cannot meet the heating requirement. However, there are very few days a year when we see these sorts of temperatures. For example, in a typical year, Inverness in the northeast of Scotland is below freezing for less  than 700 hours, while in Southampton, this figure reduces to less than 25 hours. Many of those hours would be at night when the heat pump is not operating. 

When it does happen, the heat pump could call on two types of assistance. First, it might switch on another heat source such as a boiler – this would then be referred to as a ‘hybrid heat pump’. If you don’t have a boiler because it was removed at the time of the heat pump installation, the other available option is using what is often referred to in the control panel as ‘heating boost’ or ‘heating comfort’.  

This means the heat pump will temporarily use the additional heater element to add energy to the system, until the temperature outside gets back closer to the design temperature. It’s quite normal for the heat pump to require a little help in the coldest periods. As the length of time it’s required is so short, it shouldn’t add up to much, even over a whole year. 

The heating boost can be set up badly, or inadvertently turned on and forgotten about. It’s worth reading your manual and making sure you have this set to come on only when the temperature outside justifies its use. If you find yourself having to use this function regularly, you should speak to your installer. 

Legionella protection

Under building regulations in the UK, hot water cylinders (but not thermal stores) are expected to reach temperatures of greater than 60°C on a ‘regular’ basis to protect against Legionella. 

This function is set on a timer to be operated either daily, weekly, or permanently. If your bills are high, it may be because the timer has been set to do this job too frequently. For most, it would be perfectly safe to have Legionella protection performed weekly – if the building is in regular use, it’s not necessary to do this daily. 

The heating ‘curve’

Another area that can cause misunderstandings is the ‘heating curve’, which is adjusted and set by the installer at the time of commissioning. It’s quite straightforward but can cause higher running costs if done incorrectly. Let’s look at a heating curve in this graph below.  

  • The upper temperature on the left of the red line defines what temperature water is circulated around the central heating when the air temperature is -5°C outside. 
  • The lower temperature on the far right defines the temperature of water circulated when it’s milder. 

It’s worth checking the curve has been set correctly. Sometimes installers will set these figures very conservatively (high) to make sure your house is warm in winter, but you may be able to reduce them significantly without having much impact on your comfort. 

Lowering the set temperatures at the left-hand side will reduce the reaction time of the heating system, so it will take longer to change the house temperature in cold weather, but it will also have a big impact on running costs.  

Try turning it down in one-degree increments throughout winter until you are confident there is a reduction in comfort levels, then take it back up one degree. 

Issues specific to air source heat pumps

Heat ‘set-back’ and running at night

In the UK, we’re used to turning our heating systems on in the morning, off when we go to work and back on again in the evening when we return home. In Europe, however, they’re more likely to leave heating equipment on for longer, and instead of turning it off, they simply reduce the temperature overnight or during the day when out at work. This is a good strategy for controlling heat pumps, which is known as ‘set-back’. 

If your domestic hot water or central heating is controlled like this, make sure the heat pump is not trying to heat the hot water cylinder or the house overnight when outside air temperatures reduce significantly. It’s not unusual for the air temperature to drop by 5°C or more overnight during winter, and this will make the heat pump run much less efficiently. You should be able to keep the house warm in most circumstances by leaving it on through the day, but not at night – though there will be exceptions in some areas of the UK or when it’s particularly cold. 

Issues specific to ground source heat pumps

Air in the ground loops

The fluid that passes around the ground loops in a ground source heat pump acts just like it does in a radiator system – if there is air in the system, it will reduce efficiency. This fluid passes through a heat exchanger inside the heat pump. Over time, air can get into the system, and if this is left unchecked, then eventually there might be sporadic air locks that need to be ‘bled out’ – just as you may need to bleed radiators. 

If you have a pressure gauge showing the pressure in the ground loops, make a note of what pressure the installer set at commissioning and make sure it doesn’t drop below this level too much. 

Electricity tariffs and usage

Finally, you may want to explore some innovative electricity tariffs offered by some green electricity providers that change the tariff at various times of day. Many of us are familiar with the concept of Economy 7 or Economy 10, which is not that helpful if you want your house warm during the day. However, some providers offer rates that peak at around 5-7pm but are much cheaper during daytime – offering average prices below the normal flat rate. 

Be aware that while normal electricity prices stay flat all the time, these more unusual tariffs will be up to 20-40% more expensive during the peak evening period, so make sure you understand the implications of a switch before taking the plunge. 

Other factors affecting performance and running costs

Undersized heat pump or radiators

Want to understand why radiators are critical for heat pump performance? 

Think of it as being a bit like trying to heat a house with a single radiator. It doesn’t matter how big your boiler or heat pump is, there is no way you are going to heat the house with a single radiator – you just can’t get the heat out quickly enough.  

Now, apply this logic to every room. If the radiator in each room is not big enough, that room just won’t get warm. The radiator might have been adequate when connected to a boiler (typically running at 75°C), but if the temperature of the circulated water drops to 45°C then it may no longer be big enough. 

Problem rooms exhibit common characteristics, including more than two outside walls, large windows or glazed areas, not enough insulation, or very tall ceilings. If you have a couple of rooms like this that also have undersized radiators, the whole house begins to feel less comfortable, as heat drains out of adjoining rooms to compensate. 

Some general points

If you don’t feel comfortable in your house in winter, it’s probably not the heat pump’s fault. If there is something critically wrong with the heat pump, the control screen will tell you – anything else is likely to be a design or control issue. 

If you see warnings on the control screen, don’t ignore them. Keep an occasional eye on the fault log and see if you have recurring fault messages. Speak to the installer and/or manufacturer to try and get to the bottom of what’s causing them. 

Sometimes problems can be simple to resolve and turn out to be a problem with the central heating system rather than the heat pump itself. Faulty pumps, valves or sludge and dirt in the radiators can all cause problems, which at first glance appear to be the heat pump. If you don’t have water treatment and a filter in the central heating system, address this as soon as possible before it causes problems with the heat pump, which could be expensive to fix. 

Neither manufacturers nor competent installers want people to be unhappy with their installations. If you feel your installer is not taking your concerns seriously, speak to the manufacturer. They often have technicians that are more experienced at problem solving, as they’re very familiar with your model.  

Even if it costs a bit more to resolve, it’s better to have it done sooner rather than later, and it will help keep running costs down. Try and make the effort to read your manual and understand the basics of both the control system and the heat pump itself.  

Header image credit: Ben Whittle

Last updated: 16 November 2021