The ABCs of ASHPs: A Jargon-Free Introduction to Heat Pump Basics

In some ways, a heat pump is like a traditional boiler. You install it, and it heats your home and, usually, your hot water. However, in other ways, it couldn’t be more different. This guide is here to help you understand the key differences and what they mean in practice.

It is aimed at complete beginners who want to understand what a heat pump is and how it works in a heating system, either because they are considering getting one or have recently acquired one. It starts with some basics and then follows the timeline of a typical installation, from the initial design stage through to the installation and the normal everyday use of a heat pump to heat your home.

One of the key things about heat pump-based heating systems is that, at their core, they are extremely simple. The heat pump industry, of course, likes to make them sound as complicated as possible to justify high price tags. But the reality couldn’t be simpler: they use a technology that has been around for over a hundred years to pump warmth around your home in the simplest way possible. Sometimes the technical details are complicated, but the basics are straightforward. It will help to remember that if you find yourself feeling overwhelmed. Instead of being overwhelmed, look for the simple, solid ideas that underpin heat pumps.

Although there are various types of heat pumps, this introduction will focus on air-to-water heat pumps, commonly referred to as air source heat pumps or ASHPs, because they are by far the most common in domestic situations.

What is a Heat Pump?

At its most basic, a heat pump is a device that does exactly what it says on the tin: it pumps heat from one place to another. Almost certainly, you already have one – your fridge. It pumps heat from inside the fridge (causing it to cool) and dumps it on the outside. You may have noticed the warmer air around the back – that is the heat that has been pumped out of your fridge. A heat pump for heating does the same thing: it pumps heat from one place to another. In this case, it takes heat from the outside air (hence ‘air source’) and pumps it into your home. Even when it is cold outside, the outside air still contains enough heat to make this possible.

Fridges and heat pumps use the same technology, which has been around for over a hundred years. This is why heat pumps tend to be very reliable. Just as your fridge usually runs without a hitch year in and year out, you can expect your heat pump to do the same.

Heat pumps use the same technology as domestic fridges, which have been around for over a century. They have the same basic core components, and in many ways, they even look similar. You will find the same components inside your heat pump as those that can be seen in this domestic fridge advert from 1917. Image source: wikipedia 

Let There Be Light – Understanding Power and Energy

If you want to understand heating, you need to grasp the basics of power and energy. These are terms we use in everyday life, but they have specific meanings when it comes to domestic appliances. Although related, power and energy are not the same thing.

Power, measured in watts (W) and kilowatts (kW, where 1 kilowatt equals 1,000 watts), tells you how powerful something is. The scale is linear, meaning something with twice the kW rating is twice as powerful. For example, a 10kW heat pump is twice as powerful as a 5kW heat pump. You will come across power ratings primarily for heat pumps, but also for radiators. All other things being equal, a 1,000W radiator is twice as powerful and will emit twice as much heat as a 500W radiator.

Energy, on the other hand, is what gets things done – and, for that matter, costs money. We measure energy use and production (for example, by a heat pump) by multiplying together (a) the power of the device (in watts or kilowatts, as above) and (b) the duration of use (in hours). Thus, a 10kW heat pump running at full capacity for one hour will use 10 kilowatt-hours (kWh) of energy over that hour. The same heat pump running at 50% capacity (5kW, perhaps because the weather is milder and there is less demand for heat) for one hour will use 5kWh of energy. Running at 5kW for 10 hours will use 50kWh of energy, and so on. It is energy, measured in kWh, that we see on our electricity bills, and it is energy that costs us money. Unsurprisingly, much of what we do with heat pumps is aimed at keeping energy (kWh) use as low as possible.

Heat Pump Magic – (Almost) Getting Something for Nothing

Conventional electrical heaters, such as storage heaters or electric fan heaters, convert all the electrical energy you put into them into heat. So, if you feed 2kWh of electrical energy from the mains into such a heater, it will output 2kWh of heat. We say that these heaters are 100% efficient.

With gas or oil boilers, some of the energy produced is lost up the flue. For every 10kWh of energy you put in, you might, for example, get back 9.0 to 9.5kWh of energy to heat your home. Such a boiler is said to be 90 to 95% efficient.

Heat pumps are completely different. They extract heat (thermal energy) from the air around them and, because they are highly efficient, they can often output significantly more energy than the electrical energy required to run them. A typical heat pump might easily produce 30 to 40kWh of energy to heat your home for every 10kWh of electrical energy consumed.

To describe this, we divide the heat (in kWh) output by the heat pump by the electrical energy input, resulting in a number called the coefficient of performance (or COP). A heat pump that outputs 30kWh of heat for every 10kWh of electrical energy input is said to have a COP of 3. The higher the COP, the better. Much of what we do with heat pumps is aimed at keeping the COP as high as possible to maximise the return on our investment.

A heat pump’s COP is not fixed; it varies depending on a range of factors. The two most important are:

• The outside air temperature (OAT) – and there’s not much we can do about that! Lower OATs mean lower COP values, while higher OATs mean better COP values. This characteristic of heat pumps is significant because it means that while heating bills may rise, sometimes alarmingly, in cold weather, they will also decrease considerably in moderate and mild weather. Over time, these variations tend to balance each other out.
• The temperature of the water leaving the heat pump to supply heat to your home. Here, the opposite applies: lower water temperatures mean higher COP values. This leaving water temperature (LWT, also called flow temperature or FT) is something we can control. The lower the LWT, the higher the COP values will be, and the more efficient the system will be, leading to lower heating bills.

Because COP changes constantly, we need to consider it over longer periods, such as hours, days, weeks, months, or even an entire heating season, to get an accurate measure of performance. The SCOP (seasonal coefficient of performance) is a measure of the average COP over a whole heating season.

Heat pump manufacturers do publish SCOP values for their products, but be aware that these are derived under standardised conditions. The idea is to allow comparisons between different heat pumps. However, your heat pump, in your specific circumstances, may not achieve the same SCOP. That said, in a well-designed and well-operated system, it should come reasonably close.

The Heat Pump Timeline

We have now covered enough of the basics to move on to the practicalities of the three stages of your heat pump timeline: the design stage, the installation stage, and the everyday running stage.

Stage 1: Design

We’ll assume that if you are reading this, you are at least considering installing (or have recently installed) a heat pump. However, while heat pumps are remarkably capable and adaptable, and work very well in a very wide variety of circumstances, including ‘old leaky buildings’ (see here for a fuller discussion of this), there are inevitably rare occasions when they may not be the right solution. Typically these happen when a heat pump is not used as recommended (the recommendations are covered later in this guide). If you believe such circumstances might apply to you, please do ask on the forum for advice. 

Assuming these circumstances don’t apply to you, let’s move on to the details of the design stage.

1.1 Heat Loss

Whenever your home is warmer than its surroundings, it will lose heat. You can think of your home as a giant radiator heating the outside air.

The rate at which your home loses heat is called its heat loss. This is measured in kW, which describes how quickly your home loses heat – or, to put it another way, how powerful a radiator your home is. For example, a home with a 10kW heat loss will lose heat twice as fast as one with a 5kW heat loss. As we noted earlier, we can convert power (in kW) to energy (in kWh) by multiplying the power by the duration. A home with a 5kW heat loss will lose 5kWh of energy over one hour, 10kWh over two hours, and so on.

Your home is, in effect, one huge radiator heating the air around it.

A home’s heat loss depends on various factors. The four main ones are:

1. The size of your home. All other things being equal, larger homes lose more heat than smaller homes.
2. The level of insulation in your home. Better-insulated homes lose less heat.
3. The difference between the inside air temperature (IAT) and the outside air temperature (OAT).
4. Other environmental factors, such as solar gain and wind chill.

The first two factors – size and insulation – are mostly fixed, while the fourth is usually a relatively small influence. This means that, day to day, the primary factor affecting heat loss is the difference between the OAT and your home’s IAT. When it is very cold outside, creating a large temperature difference, the heat loss is high. Similarly, if you choose to maintain a higher IAT, the heat loss will increase. In warmer weather, or if you choose a lower IAT, the heat loss will be lower. Since most people keep the IAT fairly constant, in practice, the OAT is the main determinant of a home’s current heat loss.

Because heat loss varies, we must choose a set of design conditions during the design phase. It makes sense to select conditions that represent the most demanding scenarios your home and heat pump will encounter – but not the absolute extremes. We avoid the most extreme conditions because they occur so rarely and can distort the design, leading to an unnecessarily oversized system. The goal is to ensure your heat pump can heat your home 99% of the time. For the remaining 1%, you can use supplementary heating – or simply put on an extra layer!

The design conditions used are set by an organisation called MCS (Microgeneration Certification Scheme), which oversees the heat pump industry and acts as the gateway to grant access. The design OAT depends on your location. For example, in the London area, it is around -2°C, while in Glasgow, being further north, it is closer to -4°C. The design IAT varies by room, with living rooms typically set at 21°C and bedrooms at 18°C. If your installation is funded in full or in part by a grant, these conditions are generally the minimum standard to which the installation must be designed.

1.2 Determining Your Actual Heat Loss at Design Conditions

Almost invariably, this will be done by your designer or installer. They will use one of the many spreadsheet- or web-based tools available to perform the calculations. Detailed measurements of every surface in all rooms of your home will be entered into the tool, along with assumptions about heat loss through each surface and ventilation (air changes). If you have added insulation that isn’t immediately obvious, make sure to inform your surveyor so it can be included in the assessment. Once all the data is collected, everything is totalled up, and voilà! You have your home’s heat loss. Typical heat losses at design conditions range from 4 to 16kW.

Make a note of your home’s heat loss, as it is one of the key numbers. If you post on this forum, you will almost invariably be asked what it is!

1.3 Sizing the Heat Pump

The purpose of determining the heat loss at design conditions is to enable you and your installer to select a heat pump with sufficient heating capacity (power) to heat your home under those conditions. This process is often referred to as ‘sizing’, and we frequently describe the heat pump’s power as its ‘size’.

In principle, sizing your heat pump couldn’t be simpler. You simply need a heat pump powerful enough to match the heat loss at design conditions. For example, if your home has a heat loss of 8kW (at design conditions), you need a heat pump that can deliver 8kW under those conditions (see ‘Another Eureka (Light Bulb) Moment’ below for a useful analogy).

In practice, however, it’s a little more complicated. The maximum output of a heat pump varies depending on the flow temperature and outdoor temperature, so the ‘badge’ power rating is only indicative. Your installer should take this into account and choose a heat pump based on the more detailed figures provided by the manufacturer for the specific conditions applicable to your home. If you are in any doubt, feel free to ask your installer whether they have checked the heat pump’s output at lower outside air temperatures.

It’s also important that the heat pump is not significantly oversized. A heat pump with more capacity than necessary will cost more, may be physically larger and noisier, and could trigger additional complications, such as planning permission requirements, without providing any real benefit. Furthermore, an oversized heat pump will have a higher minimum output setting, which can cause problems in milder weather.

For all these reasons, getting the right size heat pump is crucial. Better installers will take into account all available evidence, including, for example, any measurements of your home’s heat loss based on prior fuel consumption. If you have any doubts about what you are being told, feel free to ask questions on the forum.

As a very rough guideline for reasonably well-insulated homes:

• 3-4 bedroom homes typically need a heat pump with a capacity of around 8kW.
• 2-bedroom homes may need only around 6kW.
• Larger homes may require up to 12 or even 16kW.

Very well-insulated homes will need less than these figures, while poorly insulated homes will need more.

You can perform a very rough sense-check using your total annual gas or oil consumption in kWh. Dividing this figure by 2,000 will give you a very rough upper limit for the heat loss, while dividing by 3,000 will give you a very rough lower limit.

If you are being quoted figures much higher than these, your installer may be oversizing the heat pump. Unfortunately, this does happen, as it reduces risk for the installer, but it will result in extra and unnecessary costs for you. This is a complex area, so if you are in any doubt, feel free to post your questions on this forum.

Another Eureka (Light Bulb) Moment: Energy In vs. Energy Out

Imagine your home is like a bath full of water (representing heat). Unfortunately, the bath (your home) has a leak (heat loss) that you can’t fix, no matter how hard you try. The only way to keep the bath full of water (your home warm) is to replenish the water (heat) lost by using a tap (your heat pump). The key point here is that the flow of water (heat) into the bath must exactly match the water (heat) lost if the bath (your home) is to stay at the right level. Too much water (heat) in, and the bath overflows (your home overheats); too little water (heat) in, and the bath (room temperature) level drops.

How to keep the Archimedes duck happy (and you warm): Match the water (heat) loss with an equal amount of water (heat) flowing into the bath (your home).

This is why a heat pump’s output needs to match the heat loss – no more (to avoid pointless expense) and no less (to prevent your home from getting cold).

1.4 Sizing the Emitters (Radiators and/or Underfloor Heating)

The emitters, whether they are radiators or underfloor heating, must also be correctly sized. Because heat pumps operate at lower temperatures than traditional boilers, the radiators usually need to be larger than those designed for boiler temperatures in order to emit the same amount of heat into the room. Sometimes, particularly if you have added insulation since the existing radiators were installed, you may be lucky – an existing radiator may have sufficient output even at the lower heat pump temperature.

However, often the radiators will need upgrading, sometimes by as much as a factor of two. This can be achieved by choosing a radiator with more panels, increasing its dimensions (height and width), or a combination of both. Yes, upgrading radiators can be expensive and highly disruptive, but if you set up your heat pump system to fail by using radiators that are too small, you have only yourself to blame. Again, there is plenty of experience and expertise available on the forum if you have specific questions about radiator sizing.

Underfloor heating is similar in principle but is usually designed at the build stage for low-temperature operation, even if it is intended to be driven by a boiler. As a result, it is unlikely that underfloor heating will need to be upgraded when a heat pump is installed.

1.5 The Pipework

Because heat pumps operate at relatively low temperatures, the pipework needs to be wide and unobstructed to allow as much water as possible to flow through the system. It is the water that carries the heat around the system, and there needs to be enough of it moving to deliver the heat you require.

The ‘primary’ pipework from your heat pump to the inside of your home will be the widest bore of all, typically 28mm (outside diameter) if the heat pump is 7kW or above, or perhaps 22mm if it is 6kW or less. The main pipework that distributes water to the radiators will often need to be 22mm and may require upgrading in some circumstances. The individual feeds to the radiators themselves are often, but not always, adequate.

In retrofits, where a new heat pump is fitted to an existing system to replace a boiler, your installer should handle this for you. However, if they suggest what seems like excessive replacement of existing pipework, feel free to challenge them and/or seek alternative quotes. While some pipework upgrades may be necessary, some installers overdo this, which increases costs and disruption.

Because the heat pump itself and some of the pipework are outside, your system will need some form of frost protection for those rare occasions when it is both frosty outside and your heat pump cannot protect itself, typically due to a long power cut. There are two common ways to achieve this protection:

1. Adding antifreeze to the circulating fluid (this comes with a small performance penalty).
2. Adding antifreeze valves that activate just before your expensive heat pump is at risk (these come with a nuisance penalty).

There are several forum threads on antifreeze protection if you wish to investigate this further.

1.6 Domestic Hot Water Heating

Because of the way a heat pump works – low and slow – it requires a domestic hot water tank to store sufficient hot water for your needs. This tank must be designed for use with a heat pump (it will have a larger heating coil inside, reflecting the need for space heating radiators to be larger). Most people heat their hot water on a timer. When the timer activates, the system diverts hot water from the space heating circuit to the hot water tank and then switches back to space heating when the domestic hot water heating cycle ends.

1.7 Things to Avoid

Heat pumps are simple devices and work best when connected to a simple central heating system.

Unfortunately, installers sometimes add unnecessary components such as buffers, low-loss headers, or plate heat exchangers. These are very rarely needed and can significantly worsen performance if not correctly installed. This is a complicated area, but the key takeaway from this guide should be: don’t include these components unless there is a compelling reason to do so. If your installer proposes fitting one, question why – don’t accept ‘because we always fit them’. Again, feel free to ask on this forum and/or find another installer. Unless you have an unusual circumstance, you will almost certainly be better off without these components.

There is a variant of the buffer tank called a volumiser, or sometimes a 2-port buffer – unfortunately, the terminology can be confusing, which doesn’t help. The key feature is that this component has only two connections (one in and one out) and simply adds water volume to your system without introducing other complexities. This can be useful, even necessary, in smaller heating systems and has no particular disadvantage. As ever, the forum has many threads devoted to this subject.

Generally, you will want to avoid all external controls. At best, these are expensive and ineffective; at worst, they can harm your system’s performance. This guide covers this in more detail below in the section ‘3.1 KISS 1: Run Your Heat Pump Low and Slow’.

Any obstruction in your heating system – whether it’s pipes or radiators that are too small or external controls that ‘fight’ your heat pump’s internal controls – can cause major issues. Image source

1.8 Choosing an Installer

It goes without saying that you should obtain several quotes. During this process, you will get a feel for each installer. Competence is clearly one essential component, but just as important – and frankly easier to identify – is an open and engaging manner, with a willingness to explain details and answer questions. You are new to heat pumps, and they are different from other forms of heating. Your installer should be an expert, and part of their job is to impart sufficient understanding of what heat pumps are, how they work, and what you need to do to operate them effectively. If all heat pump installers did this, there would be no need for this guide, but unfortunately, not all installers are up to scratch!

As always, read and re-read the small print. If something is unclear, ask. If the answer is evasive or dismissive, give serious consideration to finding another installer.

Stage 2: Installation

There’s not a lot to say about this stage! If you have done your homework, the installation should proceed smoothly without any unexpected shocks or surprises. If things don’t go according to plan, feel free to introduce yourself on the forum and describe the problem you’re facing. It is not unlikely that others will have encountered similar issues, and solutions will (usually) have been found.

Stage 3: Installed at Last – How to Run Your Heat Pump Efficiently and Effectively

Recall from the introduction that heat pumps are not complicated. Running them is all about the KISS principle – Keep It Simple, Dear Friend. You do not need complicated controls, gizmos, or gadgets to make them work. Instead, just KISS.

3.1 KISS 1: Run Your Heat Pump Low and Slow

By now, you should have more than just an inkling that heat pump-based systems are not the same as boiler-based systems and should not be run in the same way. If there is one essential takeaway from this guide, it is that heat pumps should be run low and slow.

What we mean by this is that they should be on all the time, running at as low a temperature as possible while still keeping your home warm. They are the slow cookers of home heating – simple, one-pot designs that slowly and steadily keep your home warm. Just as it would make no sense to turn off your slow cooker while it’s cooking your supper, it also makes no sense to turn off your slow-cooker home heating system when it’s warming your home. Both slow cookers and heat pump heating systems are designed to be run low and slow.

The reason for running low and slow lies in the relationship between the temperature of your heating system and its performance. Recall that the higher the leaving water temperature (LWT) – the temperature of the water as it leaves your heat pump on its way to heat your home – the lower the coefficient of performance (COP), and thus the lower the efficiency of your system. This relationship is quite stark. For example, doubling the LWT can more than halve the COP, which can effectively double your heating bill for the same level of comfort. To achieve the best performance, you must run your heat pump with as low an LWT as possible while still keeping your home warm.

It follows that if you deliberately keep your LWT as low as possible, the heat pump will need to run for longer – and in practice, this usually means running all the time. Recall Aesop’s fable of the tortoise (a heat pump) and the hare (a traditional boiler). Both reached their destination in the end, but they did so in different ways. Tellingly, the tortoise got there first.

As an aside, Aesop’s tortoise and hare fable can shed light on why stop/start running (via timers or room thermostats) can be less effective. The tortoise rumbles along low and slow, reaching its destination without much fuss. The hare, on the other hand, has to burn lots of extra energy to recover from its rest periods. It can even be the case that the extra energy needed by a heat pump to recover from an off period can equal or exceed the energy saved during the off period.

The physics behind this is complicated and somewhat counterintuitive. It has to do with the fact that your home continues to lose heat even when the heating is off. By the end of the off period, the heat pump has to replace that lost heat, which it would not need to do if it had been running continuously. It is this ‘recovery boost’ that upsets the applecart, sometimes enough to wipe out the energy savings from the off period entirely.

Because we run heat pumps low and slow, we generally avoid using controls that interfere with this principle, as they effectively fight the heat pump’s natural way of working. Most of the time (though there are always exceptions), heat pumps should not be run using:

• Timers (concession: small overnight setbacks may sometimes make sense. There has been extensive discussion about this on the forum if you wish to explore the topic in greater depth).
• On/off room thermostats, thermostatic radiator valves (TRVs), and other smart or not-so-smart controls. These end up fighting the heat pump’s own controls, harming performance (concession: some smarter control systems, such as Homely, work with rather than against a heat pump’s controls, but they are complex and, in many homes, simply unnecessary – remember, KISS).

3.2 KISS 2: Use Weather Compensation

If we don’t use timers and thermostats to control heat pumps, how do we control them? We use something rather neat called Weather Compensation (some manufacturers give this a different name, e.g., Samsung calls it ‘Water Law’, but it is the same thing).

Weather compensation works by adjusting a heating system’s leaving water temperature (LWT) according to the weather, specifically the outside air temperature (OAT). Given that the major day-to-day determinant of heat loss, and therefore heat demand, is the OAT, why not simply regulate the LWT according to the OAT? When it is warm outside, heat loss (and therefore demand) is lower, so the LWT can be dialled down. When it is cool outside, heat loss (and therefore demand) increases, so the LWT is dialled up.

By adjusting the heat pump’s output according to the OAT, we closely match the actual output to the demand. It does take a bit of trial and error to get the settings right (because all homes vary, there are no absolute guidelines – see below), but once set up, weather compensation will seamlessly ramp the heat pump’s output up and down as needed, keeping the inside of your home at a steady, comfortable temperature. It acts as a sort of autopilot for your heat pump, automatically adjusting the throttle according to demand. KISS – what could be simpler than that? It cannot be emphasised too strongly that, for perhaps 90% of homes, weather compensation is all they will need to control their heating system.

Because all homes vary, setting up weather compensation, as noted above, does require some trial and error to get it spot on. Competent installers will normally apply a best-guess set of settings (called the weather compensation curve, or WCC for short) at installation but will leave it to the homeowner to do the final tweaking. This typically involves setting the endpoints (the extreme left-hand and right-hand ends) of the WCC to match the home’s heat demands. Your installer, your heat pump manual, or even the forum are good places to look for instructions on how to access these settings.

Typically, this involves setting the low OAT (left-hand) end and the high OAT (right-hand) end, and then, for each of these OATs, setting the LWT (the temperature of the water coming out of the heat pump) to a suitable level. For example, in a typical installation with radiators, the low OAT end of the weather compensation curve may have the OAT set to -2°C, with the LWT set to 45°C, while the high OAT end of the curve may have the OAT set to 15°C, with the corresponding LWT set to 30°C. This means that at -2°C OAT, the LWT will be 45°C, and it will then steadily decrease as the OAT increases, until the OAT reaches 15°C, when the LWT will be 30°C. A WCC can be shown as a graph, and one with the above settings will look like this:

A typical weather compensation curve. Your heat pump constantly monitors the outside air temperature, and then sets the leaving water temperature depending on the outside air temperature

Once you have mastered how to set the weather compensation curve (WCC), run the heating for a few days and observe how the indoor air temperature behaves. If the home is running a little too warm, lower the WCC leaving water temperature by a degree at each end of the curve and observe again for a few days. Over time, through trial and error, you will gradually tweak your WCC until it keeps your home at a comfortable temperature most of the time, without needing daily adjustments.

Weather compensation, and only weather compensation (no other controls were in play) in action during a week in February 2025. Notice how despite the changing outside air temperature, the indoor air temperature stays stable. Weather compensation achieves this by adjusting the leaving water temperature according to the outside air temperature, the lower the outside air temperature (top chart), the higher the set leaving water temperature (bottom chart)

3.3 KISS 3: Avoid Over-Complicating Things

Most heat pumps will run extremely well when left to operate low and slow using a correctly set-up weather compensation curve. However, human nature being what it is, once you’ve achieved this state, you may be tempted to add further tweaks. Be warned: very few of these adjustments will provide significant benefits. The reason is simple – a heat pump running low and slow with a properly configured weather compensation curve is already close to optimal performance, delivering the required comfort level at the lowest possible cost.

Some of the better in-built or bolt-on additional controls may offer marginal benefits, but many will not. Instead, they may interfere with the heat pump’s own controls, reducing overall efficiency. If you are in any doubt, search the forum for discussions about the control add-on you are considering, and read about other people’s experiences (or start your own thread if the control hasn’t been discussed before).

Similarly, setbacks – such as setting a lower indoor air temperature (IAT) for, say, six hours overnight – may appear to save money at first glance. However, the reality can be quite different. As noted earlier, a home must recover after a setback, and this recovery requires extra energy that would not have been needed without the setback. In simple terms, you might save 6kWh over a six-hour setback but then use an extra 3kWh during the recovery period, reducing your net saving by half. Worse still, this apparent saving may not translate into lower running costs, as your heat pump will have to work harder (and thus less efficiently) during the recovery phase. It could even end up costing you more!

This is a complex area, and in some cases, it can be counterintuitive. While it is beyond the scope of this ABC guide to delve into the details, there are plenty of threads on the forum that discuss these subjects in greater depth, should you wish to explore them further. For now, the guide offers one simple message for running your heat pump: KISS! In summary:

• Run your heat pump low and slow.
• Use weather compensation.
• Avoid gimmicks and gizmos.

Job done!

Glossary of Terms and Abbreviations

ASHP (Air Source Heat Pump) – A heat pump that extracts heat from the outside air and pumps it into the home, usually using warm water.

Coefficient of Performance (COP) – A measure of how well a heat pump performs over the short term (hours, days). Compare with Seasonal Coefficient of Performance (SCOP).

Cycling – A process used by a heat pump to reduce its output below its minimum steady output. Can be long cycling (less than around five times an hour, acceptable) or short cycling (six or more times an hour, undesirable).

Defrost, Defrost Cycle – A cycle that occurs in cold weather to defrost the external components of a heat pump.

Delta T (ΔT or dt) – The difference between two temperatures, e.g., the difference between the flow temperature (FT) and return temperature (RT).

DHW (Domestic Hot Water) – The heated water used in your home for washing, bathing, etc.

Emitters – A generic term for devices that emit heat, e.g., radiators or underfloor heating.

Flow Rate – If not otherwise specified, the flow rate of the water pumped by the heat pump.

Flow Temperature (FT), Leaving Water Temperature (LWT) – The temperature of the heated water as it leaves the heat pump.

Hydraulic Separation – A practice, usually unnecessary and discouraged in domestic situations, of separating the primary heat pump circuit from a secondary radiator circuit using devices such as buffers, low-loss headers, or plate heat exchangers.

Indoor Air Temperature (IAT) – The temperature inside a room.

Kilowatt (kW) – A unit used to describe a device’s power (how powerful it is). It can refer to either input or output.

Kilowatt-hour (kWh) – A unit used to describe how much energy a device either uses (energy in) or produces (energy out).

Open Circuit – An arrangement where anything that restricts the flow rate is removed as far as possible.

Outside Air Temperature (OAT) – The ambient temperature outside.

Return Temperature (RT), Returning Water Temperature (RWT) – The temperature of the water when it returns to the heat pump.

Seasonal Coefficient of Performance (SCOP) – An average measure of COP over an entire heating season, allowing the performance of one heat pump to be compared with another in theory.

Weather Compensation – Setting a heat pump’s output based on the outside air temperature (OAT), usually without using room thermostats or timer controls.

Weather Compensation Curve (WCC) – A description of the weather compensation settings for a particular heat pump at a given time.

Footnote: edited 2025-02-09 1900 to (1) improve image caption layout and (2) remove unnecessary complexity from Stage 1: Design about rare circumstances

Footnote: edited 2025-02-22 1000 to add weather compensation in action chart