Heat pump vs gas boiler: How do they compare?

In the heat pump vs gas boiler debate, you're looking at very different technologies that are effectively delivering the same result: hot water and central heating. That's pretty much where the similarities end. 

With the government heralding both air source heat pumps and ground source heat pumps as the successors to gas boilers as part of its Boiler Upgrade Scheme, it's the right time to be weighing up your options and how these two systems compare. 

Here we examine the pros and cons of each so that people can make an informed choice about whether or not to replace their gas boiler with a heat pump and what they should be thinking about when considering making the switch to heat pumps.

Heat Pumps vs Gas Boilers: What are the Key Differences?

A heat pump and gas boiler produce different types of heat:

• A boiler will produce a lot of heat with a high temperature difference in a very short space of time. A boiler can thus work with radiators with a very high temperature.
• A heat pump, on the other hand, produces heat more slowly and with a smaller temperature difference. The low temperature of the heat pump means that the radiator (or emitter) needs to have a much larger surface area. In effect, it is no longer a ‘radiator’ but rather a ‘convector’.

This also means that emitters such as underfloor heating (UFH), which run at a lower temperature than standard radiators, are an ideal emitter with heat pumps.

To the homeowner, they create the same result, but the difference in the way the two technologies work means that they need to have a different design setup when it comes to how the heat is delivered to the property.

Let's explore this in a little more detail. A boiler system may be designed to run at an average flow temperature of 70°C, but the temperature difference between the water going into the radiator and the water coming out of the radiator is 10°C. This in effect means the water goes into the radiator at say 75°C and comes out at 65°C, having given up 10°C.

A heat pump system ideally wants to have an average temperature of around 37.5°C and a temperature difference of only 5°C. Therefore the water goes into the UFH or radiator at say 40°C and comes out at 35°C, having given up 5°C.

Ultimately, if you opt for a heat pump rather than a gas boiler, you will need to size the radiators and/or UFH pipe lengths very precisely. You will also need to be able to control the speed of the water in the circuit as well as the delivery of heat to the circuit. The balance is critical and certainly not always possible in older or 'energy hungry' properties.

So, if you ask yourself 'should I swap my gas boiler for a heat pump?', you need to be aware that you'll likely need to update your existing radiators and pipework, as well as your heat source, in an existing home.

What Types of Properties Are Most Suitable for Heat Pumps and for Gas Boilers? 

The slow heat that is required to optimise a heat pump is best suited to a very well insulated building and a gradual build up of the temperature over a long period of time. 

It is generally accepted that a ground source heat pump is better suited to larger properties, while either an air source heat pump or ground source heat pump is suitable for properties under around 300m2. 

Boilers are better suited to producing rapid heat in older, less efficient buildings. They can also be designed into systems where they can deliver low temperature heat in a similar way to a heat pump — meaning they can work alongside underfloor heating.

In a self build scenario, Building Regulations is moving towards low temperature emitters, which will be more compatible with heat pump systems. 

In an existing home, however, things become a little more challenging. The cost of retrofitting a heat pump into an existing home is comparatively more than working with a new build.

Which is More Efficient, a Heat Pump or a Gas Boiler?

A natural gas boiler has a heat exchanger that is in the gas combustion chamber. As water passes through the heat exchanger it absorbs the heat from the burning gas and the heat exchanger is so efficient that around 90% of the energy from the gas is absorbed into the water. Because so much heat is given up to the heat exchanger, water vapour condenses on the heat exchanger and hence it is referred to as a condensing boiler.

Heat pumps do use electricity to run, but due to the refrigeration (vapour compression) cycle the heat delivered to the home can be 300% up to 500% more than the electricity used. 

The efficiency of a heat pump is known as the Coefficient of Performance (CoP) which is the ratio of the amount of electricity used to the amount of heat produced. 

As an example, if the heat pump uses 1kWh of electricity and produces 3 kWh of heat, then the CoP is 3 or can also be referred to as a 300% efficiency. 

There are many contributing factors and many different formulae used to deliver and calculate this efficiency so it is important to know exactly what you should expect from your prospective system when comparing with alternatives.

Is a Heat Pump More Expensive Than a Gas Boiler?

Expect to pay between £8,000 to £16,000 for an air source heat pump installation on a new build property and up to nearly £28,000 on an existing property — that does include upgrading all the radiators and replacing quite a bit of the pipework.

You're looking at around £14,000 to £25,000 for installing a ground source heat pump, and possibly more if you need a large borehole collector. If you happen to have a digger and can dig the horizontal trenches yourself then there could be some good savings to be made on installation.

You can expect a standard combi boiler to cost between £1,000-£2,000 depending on the make and model. Installation will cost approximately another £1,000, making the cost all-in £2,000 to £3,000 for a simple gas boiler replacement. 

What is Cheaper to Run, a Heat Pump or Gas Boiler?

As heat pumps run on electricity and a boiler burns gas, the running cost comparison needs to take into account the unit costs of each fuel. 

Electricity costs between 12p and 24p per kWh depending on which tariff you are on, and natural gas is between 3p and 5p. 

This means that on pure financial cost alone, the heat pump would need a CoP between 3 and 5, or in other words, have an efficiency of between 300% to 500%, to be comparable. 

As the cost of electricity and the cost of gas change, so will this calculation. You also need to be sure that the system will indeed achieve the indicated efficiencies. 

One thing is for sure though. The cost of energy will continue to rise and the gap between the cost of electricity and the cost of gas is sure to close. 

At the moment the switch from gas to a heat pump may not make financial sense, but as the energy cost gap closes, the numbers will certainly become more favourable for heat pumps.

How Much Space Does a Heat Pump Require vs a Gas Boiler?

While you may be more familiar with the size of a standard gas boiler, heat pumps require some different equipment in their home set-ups. 

There are two main types of heat pumps to be considered for our homes: air source heat pumps and ground source heat pumps. As the name suggests, an air source heat pump (ASHP) collects heat from the ambient air and a ground source heat pump (GSHP) gets its heat from the ground. 

An ASHP will usually have a large box (1200mm tall by 1000mm wide by around 600mm deep when you allow some space behind it for air to move) on the outside of your home that has a fan and a compressor in it. This box absorbs heat from the air as it is drawn through it by the fan. The outside unit needs around 2 metres of free space in front of it.

You will also need space inside the home for a hot water cylinder, control box and possibly a second buffer tank which is about a third of the size of a tall hot water cylinder. 

A GSHP has a loop of pipework that is either buried horizontally in the ground outside (not under any buildings or near services or tree roots) or is placed in vertical boreholes. The size of the horizontal loop or the depth of the borehole will vary and will very much depend on the amount of heat required in the home and the conditions of the ground. 

As a generic indication you will need between 50m2 and 100m2 of ground per kW for a horizontal loop and around 10 to 20 metres per kW for a borehole loop. 

Therefore, for an average 8kW GSHP you could need between 400m2 and 800m2 for the horizontal loop, at a cost of between £2,000 and £8,000, and between 80m to 160m of borehole which could cost from £7,000 to £20,000.

The ‘engine' of the GSHP is inside the home and could be around the size of a half-height fridge, up to the size of an American fridge/freezer, depending on the capacity and brand. You will also need the hot water cylinder and buffer cylinder as well.

As such, while a gas boiler may be small enough to be accommodated in a kitchen (some small combi boilers can be hidden within a kitchen unit), the kit associated with running a heat pump can requires much more space.

What Incentives are Available for Heat Pumps vs Gas Boilers?

Currently, the Renewable Heat Incentive (RHI) is available for the installation of heat pumps. The RHI pays an amount of money to the homeowner for seven years for eligible technologies which are installed by Microgeneration Certification Scheme (MCS) registered Installers. 

The RHI is due to end on 31 March 2022 and will be replaced by a one-off grant called the Boiler Upgrade Scheme of £5,000-£6,000 depending on the type of heat pump. 

There is a budget of £450 million, which is effectively 90,000 installations over 3 years. What will happen after that, I’m not sure. 

The grant will not cover the cost of a heat pump installation but it will certainly help and if this leads to a bigger uptake for the technology then the costs will inevitably fall as well.

There are no such incentive schemes for gas boilers.

Can I Still Buy a Gas Boiler?

We have recently learned that the target date of when gas boilers will be phased out is 2035, and that the Government will make a decision on the hydrogen strategy by 2026. 

If we do indeed see an increase of hydrogen in the gas grid alongside low/zero carbon gas such as synthesis gas or anaerobic digestion, then the boilers won’t be a problem as it is currently the fuel that is carbon intensive rather than the appliance.