Heat Pump Basics

Welcome to Kensa's "Heat Pump Basics"

online learning. This course is intended for industry professionals (plumbers, heating engineers) who may not be familiar with heat pumps, and want a basic introduction to the technology itself, and where to apply it. It will also give an overview of the technology to householders, architects and specifiers.

By the end of this module you will understand what a heat pump is and be able to explain how a ground source heat pump works. You will get an understanding of why they are so efficient, and how that efficiency is measured. You'll also learn about the industry standards relating to heat pumps, and about Government subsidies available.

Study of this module is a requirement under Kensa's MCS Umbrella Scheme.

Part 1. So what is a Heat Pump?

What is a Heat Pump?

A Heat Pump is an alternative form of generating heat, which can be used for space heating or hot water. They are used instead of fossil-fuel burning boilers (gas or oil). A heat pump is a more environmentally friendly alternative - even though they use electricity as their main source of power, heat pumps use that power to generate a lot of useful heat.

Heat pumps take low-grade heat from the ground, water, or even the air, and then use a compressor and refrigerant circuit to "upgrade" that heat so that it's usable in a conventional heating system (such as radiators or underfloor heating).

There are several types of heat pump - air-source, ground-source and water-source, and air-to-air. Kensa manufacture ground source heat pumps, which can also be used with water as a heat source.

Kensa only make Ground Source Heat Pumps. This is because we firmly believe they are the most environmentally friendly, efficient way to generate heat! We make a wide range of units that suit many applications, and we also supply system components and accessories. To view the full range, visit the Kensa webpage.

Part 2. How do they work?

So how does a Heat Pump work?

Ground Source heat pumps use pipework buried in the soil to extract heat from the ground itself. During the winter when everything seems cold, there is still energy stored in the ground - even if it's only a small amount, it's enough for a heat pump to take advantage of. This is what makes a heat pump so efficient - it can take a lot of low grade energy, and concentrate it into a usable form.

The refrigerant circuit, showing ground loops

Circulating around the underground pipework (known as the Ground Array) is a mixture of water and antifreeze. This is called the Thermal Transfer Fluid. This transfer fluid is pumped around the ground array, and then through a heat exchanger. In this heat exchanger, the transfer fluid gives up the energy it has gleaned from the ground.

On the other side of that heat exchanger is a closed circuit of refrigerant gas, almost identical to the refrigerant circuit found in a normal domestic fridge.

So the energy from the ground is transferred into the refrigerant circuit, via this heat exchanger. This component is called the Evaporator - it's named after what happens to the refrigerant inside it.

The refrigerant used in this closed circuit starts off as liquid - very cold liquid! When a couple of degrees of heat are added though, it boils, and turns into a gas. It evaporates - just like water turns into steam when boiled, only at a far lower temperature.

The refrigerant, in its gaseous state, passes into a compressor. The compressor is powered by electricity.  The compressor "squeezes" the refrigerant gas.

Now you'll need to remember some high school physics! When a gas is compressed, the same number of molecules have to occupy a smaller physical space. They rub against each other far more, generating more heat. For a working example of how this happens, try using a bicycle pump to inflate a football - touch the nozzle and you will find it warm.

So now we've got higher-grade, usable heat! But it's in the refrigerant circuit still, so we have to get it into the heating system. This is done by means of another heat exchanger, which does the opposite job of the Evaporator. This one is the Condenser. On the other side of the Condenser is the heating circuit fluid (the water that flows around radiators or underfloor heating).

As the refrigerant gives up its heat via the Condenser, it does just that - begins to condense back into liquid, transferring its energy (heat) into the heating system fluid. It's still under high pressure though - so to get the refrigerant back to the start of the cycle, it has to return to its natural, uncompressed state.

This is where the final component in the refrigerant ciruit comes in. The Expansion Valve allows the refrigerant to pass through to the low pressure side of the circuit, and the cycle begins again.

What happens once the refrigerant circuit has done its job?

We've seen how the compressor helps change low-grade heat from the ground into usable heat, ready to circulate around the heating system.

There is a circulating pump inside the heat pump which circulates water around the heating system. There is also a pump that circulates the thermal transfer fluid around the ground array.

Heat pumps by their nature give much lower flow temperatures than traditional fossil-fuelled boilers. Modern homes though are much better insulated, so using low-temperature heat emitters is easy - especially when the property is newly built or being refurbished to meet current standards.

Heat pumps are best partnered with underfloor heating, because underfloor heating works on lower flow temperatures anyway. The lower the flow temperature, the less work the heat pump has to do, so the more efficient it is.

It's possible to use radiators with a heat pump - the lower flow temperature just means that the radiators have to be a lot bigger, in order to heat the room to the desired temperature.

A heat pump will work for domestic hot water too, and can heat water by means of a cylinder - similar to a traditional boiler based system. The key difference is the heat pump operating temperature, which makes a difference to the cylinder itself - in the same way radiators have to have a larger surface area to achieve the same result, the coil in the hot water cylinder must also be larger.

Place the text on the correct component

  • Ground Array
  • Compressor
  • Expansion Valve
  • Condenser
  • Evaporator
  • Heating circuit

Part 3. What is an ideal application for a Ground Source Heat Pump?

Where heat pumps work well

Because a heat pump is at its most efficient where a low flow temperature is used, they work really well with underfloor heating or large radiators. 

This means that well-insulated properties, such as new build homes or those undergoing reurbishments, are well suited.

The first step for anyone considering fitting a heat pump is to ensure that the heat losses of the building are as low as they can practically be - so double glazing, draught proofing and insulating cavity walls and lofts are basic steps that should be taken before even considering a heat pump.

Retro-fits and older properties

It's still possible to fit a heat pump in an older home. But there are many things that need to be considered, and because the capital cost of a heat pump is comparitively high, more care needs to be taken to ensure it is operating as efficiently as possible.

The same rule applies - if you can cut the heat losses by means of improving the fabric of the building, do so. If the house is being completely gutted then there is a good chance that it will be well insulated.

Take care when asked to fit a heat pump to an existing heating system. Radiators will almost certainly need upgrading - often they may have been oversized to start with, but don't count on it. You will also need to replace the existing hot water cylinder.

High Temperature heat pumps are available which will give flow temperatures of up to 60°C - these can be really useful for radiator based systems.

You should also consider the amount of space available for ground arrays.

Part 4. Can anyone fit heat pumps? What qualifications do I need?

Do I need special qualifications to fit a heat pump?

Fitting a heat pump isn't any harder than installing a boiler.

So, if you are a qualified plumbing and heating professional, you'll be fine. If you are project-managing a self-build, you  will need some help from a plumber and probably an electrician.

The differences? Well, you need to find someone who can do the groundwork for you - whether  this is digging trenches and laying the slinkies, or drilling boreholes. Kensa can help you find a borehole driller. Groundworkers are easy to get hold  of, especially if you are looking at a new build - there may already be plant and workers on site. Kensa are well used to supplying the ground arrays long before the heat pump is required!

Kensa heat pumps can work for heating and hot water, but when used for hot water, a 3-port diverter valve is supplied which you must use. System schematics and wiring details are available online and on request.

If you use a subterranean (underground) manifold, you will have to carry out electro-fusion welding on all joints. Electro-fusion machines can be hired easily and make exceptionally reliable joints.

Core plumbing competencies are required to plumb the heat pump into the heating system - for example NVQ3, City & Guilds. Be prepared to present proof of your qualifications.

If you fit an unvented cylinder, you must of course have the relevant qualification (often referred to as your G3 ticket).

You will also need to have basic electrical qualifications - the same as for a boiler (Part P). Alternatively get a qualified electrician to do the wiring for you. The heat pump needs a permanent live, and for controls, all it needs is a switched live for heating, and one for hot water (if applicable).

You should ensure that the heat pump is properly sized for the house. This means doing full room-by-room heat loss calculations. If you don't know how, Kensa can carry these out for you for a small fee. We need a full set of plans and elevations, and details of the construction.

It's wise to check with the householder that the electrical supply to the house is capable of supporting a heat pump. They will have to check with their distribution network operator ("DNO") and show you evidence that the DNO is happy with the heat pump connection.

Actually fitting the heat pump is no harder than piping up a boiler. If you need help, our Technical team are here at the end of a phone, and are always happy to advise you.

Part 5. What makes a heat pump so efficient?

How can a heat pump be over 100% efficient?

You've probably heard a lot about how heat pumps can be really efficient.

You might have heard of instances where they haven't been.

Well, let's look at how efficient they actually are!

We've seen how a heat pump "concentrates" energy from the ground.  The actual amount it can "concentrate" by depends on the compressor power.

A heat pump's basic efficiency is measured in COP - Co-efficient Of Performance. This is a "point measurement" meaning it's only correct for one specific set of circumstances.

In the example above, you could express the COP of 5 as a percentage - this heat pump is 500% efficient. For every unit of power you put in, you get 5 times that much out! Of course this is an example - realistically, heat pump COPs are around 3 to 4.  To keep things fair and help people compare, COP is generally measured with an inlet temperature of 0°C, and an outlet temperature of 35°C.

Consider boiler efficiency - you put in a unit of power (gas), and you get heat out of the other side. But even with a new boiler, some heat energy is lost, and the boiler is around 90% efficient.

How is it measured?

Simple COP measurements don't really give you a good idea of how efficient a heat pump is.  Perhaps the single most important message we can give you, throughout all of our training, is this:

Any heat pump's efficiency depends on the heating system you attach it to.

It also depends on the temperature of the heat transfer fluid coming in.

So instead of COP,  the heat pump industry uses something called SCOP - Seasonal Co-Efficient of Performance. After March 2016 all manufacturers had to publish their SCOP data.

(The heat pump industry really love their acronyms. Don't panic, we'll go over them in a Jargon Buster later on!)

SCOP applies to all types of heat pumps, and it's measured in the same way. The problem with this is that it makes GSHPs look like they don't compare favourably against ASHPs (Air source heat pumps). 

The efficiency of a heat pump will vary depending on the season, because the inlet and outlet temperatures might vary.

In fact, ground temperatures tend to be quite constant a metre down (which is where your slinkies go). It sits at about 10°C all year round - give or take a degree in the depths of winter. Of course if you use water as your heat source there would be more variation.

Of course, from season to season, the air temperature varies a lot! SCOP takes a range of efficiencies, measured at -7°C to +12°C for air source units, and averages them out. In the UK, we don't often get as cold as -7°, but we do spend a fair chunk of the year between 0° and 12°. So the average efficiency for an air source unit looks pretty good.

SCOP for ground source is measured at a range of flow temperatures, but always with the incoming temperature at 0°C. Given that the ground temperature in the UK is around 10°C, this makes GSHPs look less efficient !

For a comprehensive explanation, see Kensa's fact sheet on the subject.

What is SPF then?

SPF stands for Seasonal Performance Factor. It's very similar to SCOP,  and was used prior to March 2016 to estimate the performance of a heat pump system via the Heat Emitter Guide.

The two terms are now virtually interchangeable, but take care that any quoted SPF takes into account power used by circulating pumps, boost heaters, controls etc.

Why is it so important anyway?

"These acronyms make my head hurt. Why is it so important?"

Look at it this way:

  • The better the heating system is designed, the lower the flow temperature is needed from the heat pump.
  • The lower the flow temperature, the more efficient the heat pump.
  • The more efficient the heat pump, the better the SCOP.
  • The better the SCOP, the more the householder gets paid under the RHI.

You can see that it pays to be careful with system design. Of course, you can go overboard - there is no point having wall-sized radiators that have cost a small fortune, just to net an extra couple of hundred pounds a year from the RHI.

But it could be that this is the difference between quotes from one installer to another - this small detail could win or lose business for an installer.




Part 6. What guidelines and legislation apply to Ground Source Heat Pumps?

Can anyone get Government payments? Do I have to abide by any standards?

The sizing, design and installation of any heat pump is governed by The Microgeneration Certification Scheme.  It's there to make sure that all systems are properly sold, designed, sized, and fitted. The MCS standard that relates to ground source heat pumps is known as MIS3005.

MIS3005 sets down methods for sizing the heat pump, sizing the ground array, and designing the heating system.  This is because getting one of these 3 bits wrong can result in the entire system under-performing or even failing. So arguably more attention is paid to each element of the system than with a boiler-based heating system.

If your customer wants to claim goverment subsidies (under the Renewable Heat Incentive, or RHI) for their heat pump, then the heat pump itself, the installation, and the installer must all be MCS approved.

If you are not an MCS approved installer, this doesn't mean that you can't fit a heat pump.

It does mean that Kensa can take on the responsibility of covering the MCS Accreditation, under our MCS Umbrella Scheme. You can read more about the Scheme in the "Standards Applying to Heat Pumps" module.

We do charge for this, and because we are essentially putting our name to your workmanship, we will ask for evidence that the system is designed and installed correctly.

However, when you come to us for an estimate, we will ensure that the heat pump and ground arrays are properly sized. We calculate ground array sizing for you, and we can undertake the room by room heat loss calculations which are required to determine the size of the heat pump.

When you, as the installer, prepare a quote for your customer, it is important that the main body of the proposal including running costs are provided to the client in order to comply with MCS.

Your responsibility is the design and installation of the heating system itself (and hot water cylinder if required). We will ask for photographs and proof of your competence (copies of your qualifications). There is a checklist for the required items here.

As part of our MCS Umbrella service, we will provide a "handover pack" for your client, containing all the design work, calculations, manuals and evidence required, as well as the MCS Certificate for the installation - this certificate is the key to their RHI claim.

Information on claiming domestic RHI payments can be found here.

So much to remember - let's see how you're doing.

Kensa can help!

Kensa can help you make sure that your installation is MCS-compliant. Our sizing calculations comply with so you know that whatever you install is more than suitable! We will carry out sizing of the heat pump and for you. With Kensa's scheme, we aim to take as much of the hassle out of the design and certification process as we can, leaving you to concentrate on a high quality installation.

What will I need to provide to get Kensa to cover the MCS on my project?

  • Proof of qualification to fit unvented hot water cylinder
  • Proof of competency for electrical works
  • Details of heating system
  • Driving licence
  • Bank details
  • A copy of the EPC
  • Plans and elevations (if you need us to do heat losses)
  • Proof of plumbing qualifications

Is ground source right for this job?

What about this project? Could GSHP work?

Jargon Buster! Match the jargon to the definition.

  • MIS3005
    The rules that govern how heat pumps and ground arrays are sized and installed
  • MCS
    A Government scheme that covers all renewable technologies
  • RHI
    Renewable Heat Incentive - government-backed payment scheme for renewable energy sources
  • MCS Umbrella
    Kensa's scheme for covering non-MCS Accredited installers
  • Heat Loss Calculations
    A detailed calculation which proves how much heat is needed to warm a house
  • SAP
    Standard Assessment Protocol - a way of checking how carbon-efficient new houses are
  • EPC
    Energy Performance Certificate - a certificate that states how energy efficient the house is, and what improvements can be made to it
  • SCOP
    A measurement of heat pump efficiency that considers how well the system runs year-round
  • COP
    A simple measure of heat pump efficiency
  • SPF
    A measurement of system performance that is basically the same as SCOP