Q&A: Heat Pumps Explained

Looking to cut your carbon footprint whilst lowering your heating bills? Read on to find out why a heat pump might be right for your project
by Build It
8th August 2013

Installing a piece of renewable technology into your home is a bold move. But it’s one that could save you a lot of money in the future, as well as providing you with the peace of mind that your own carbon footprint has been reduced.

One of the most popular options for self-builders is to install a heat pump – either a ground or air source version.

Before you take the plunge and invest in this type of kit, you need to ensure it’s right for your project. Here, we will look at some common queries to ensure you’ve got the essential knowledge required to select the right pump for your property.

What are heat pumps and how do they work?

The simplest way to describe this renewable heating appliance is that it works like a fridge in reverse.

Using a vapour compression cycle, pumps take low grade warmth and concentrate it to a higher temperature that can be used for your underfloor heating or hot water supply, for example.

Ground source heat pumps (GSHPs) extract warmth from the earth via coils of pipe – the ground loop – buried in the soil.

“All GSHPs have two parts: a circuit of underground piping outside the house and a heat pump unit inside,” says Martyn Bridges from Worcester, Bosch Group. “The outdoor piping can be specified either in a continuous loop, as compact collectors or in a bore hole depending on the space and location.”

The loop is filled with a water/antifreeze, which absorbs heat from the earth. This is fed through a heat exchanger in the pump to concentrate it.

The ground stays at a fairly constant temperature under the surface, so the heat pump can be used throughout the year – even in winter. Longer loops can draw more heat from the ground, but need more garden space or deeper boreholes.

Air source heat pumps (ASHPs) take warmth from the outside air (and can work in temperatures as low as -15C). Air-to-air systems produce hot air that is circulated around your home by fans. Air-to-water systems are used to heat water in a storage tank, which is then fed into your central heating system.

As the warmth produced is low grade, these technologies work best with low-flow radiators, underfloor systems or warm air heating systems (for ASHPs).

What is a COP?

COP stands for co-efficient of performance and is generally accepted as the method of measuring heat pump efficiency. It indicates the amount of usable energy extracted from the ground or air for each unit of energy used to run the pump.

“A COP of three means that for every 1kW of electricity used to run the heat pump, three units of output are achieved, so it’s 300% efficient,” says Paul Watson from Ice Energy.

“The COP varies depending on the external temperature and what the water outlet temperature required is – the general rule is that the higher the temperature you want heat delivered at, the lower the COP.”

Are heat pumps a brand new technology?

Central heating as we know it – which is usually a boiler and radiator set up – became widespread in the UK in the late 1960s.

In comparison, the emergence of domestic heat pumps in this country is still relatively new, but it has been pioneered by forward thinking and eco-aware self-builders over the past five to10 years.

Looking slightly further afield, heat pumps have been manufactured in Germany and Scandinavia for approximately 30 years – so they are a very much tried and tested technology, especially ground source versions.

Whilst the efficiency of products may have improved over recent years, the core technology used in heat pumps is basically refrigeration, which has been around for decades.

Do heat pumps require any maintenance?

Heat pumps require minimal maintenance in comparison to conventional boilers – and are often referred to as a ‘fit and forget’ technology.

However, a general rule of thumb is that an annual check should take place to ensure the filters remain clear.

For ASHPs, it’s recommended that the areas around the unit are cleared regularly to ensure there is no build up of debris, such as leaves or moss, which can affect the air flow.

What is the lifespan of a heat pump?

Heat pumps have an expected lifespan of around 20-25 years, which is far beyond that of a conventional boiler.

“Not only do they last longer, they also maintain much higher efficiency rates, so pound for pound, they are a great investment,” says Paul Watson.

“For example, while an oil boiler could in theory last just as long, it is likely that the efficiency will decline to, say, 80% of that achieved on the day it was installed, if not less. In the same period, a heat pump would not have lost any efficiency, meaning your running cost savings actually grow each year.”

Can a heat pump be retrofitted?

There are a lot of things to consider before installing a GSHP to an existing property. For example, the garden will need to be dug out to install the ground collectors. Also, the outdoor space will need to be accessible for excavation machinery.

“It is very important to make sure the current radiator system is sized correctly, or replaced if necessary, to ensure it can work at the same water temperature that the heat pumps produce. Otherwise there will be a loss of efficiency,” says Chris Davies from Dimplex’s renewables division.

“ASHPs are easier to install than ground source versions, particularly when retrofitting, as they are fixed above ground.”

As with all heat pump systems, it is important to accurately calculate your property’s heat loss in order for the pump to be correctly sized (in relation to demand and the expected heat loss).

For example, the output of an ASHP changes as the external temperature gets colder, so it is important that a pump is chosen that can provide all the heat needed, even in the depths of winter.

“The location of an air source heat pump is very important. The unit requires a firm base to support its weight and needs to be fitted in a position where it’s protected from bad weather,” says Chris.

How much does a heat pump cost?

It is difficult to be too specific about the total cost of a ground source heat pump because a lot depends on the type of ground collector chosen and conditions on site. A typical system will cost between £8,000 and £15,000.

“Dimplex has a handy running cost and savings calculator on its My Green Heating website (www.mygreenheating.co.uk), as well as other handy information on the application of heat pumps and the costs,” says Chris.

“A typical air source heat pump system will cost between £6,000 and £10,000. The running costs will vary depending on the size of the property, the current insulation and the amount of radiator upgrades required.”

Can a pump be integrated with solar tech?

Heat pumps can easily be integrated with both solar thermal and photovoltaic (PV) technology.

While it’s true that the electrical requirement of a heat pump can’t be met with PV panels overnight or in the winter (when heating demand is at its greatest) it is possible that they can produce more energy over the course of a year than a heat pump would use annually – plus such an installation would be eligible for both the RHI and Feed In Tariff payments.

“In terms of solar thermal, Worcester’s range of Greenskies solar cylinders can be used in conjunction with its ground source heat pumps. Together these two technologies have an even more beneficial impact on hot water production efficiencies in the home,” says Martyn Bridges from Worcester, Bosch Group.

Why do I need to use a MCS installer for a heat pump?

When installing any type of heat pump it is essential that you use a Microgeneration Certification Scheme (MCS) approved installer.

This is a government backed quality assurance scheme, aimed at providing high quality products and installers, so you can be safe in the knowledge that your heat pump has been installed correctly and will have maximum efficiency.

MCS approved installers and products must be used to qualify for any government funding scheme, such as the Renewable Heat Premium Payment (RHPP) or Renewable Heat Incentive (RHI).

Photo: This diagram from Nu-Heat shows a GSHP set-up with both horizontal and vertical collecting systems

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