What is a water source heat pump? Why it might be a smart move for your home

A red brick cottage with a large pond in front of the building
Water source heat pumps need to be close to water to draw the heat out of this heat source (Image credit: Getty)

Water source heat pumps are less common than ground source or air source heat pumps, but they are more effective due to the stability of the heat extractable from a water source. 

The major disadvantage is that the building where the heat pump is to be installed needs to be in close proximity to a large water source for the heat pump to operate effectively.

Here we look at what water source heat pumps are, as well as the different types of system that are available, the costs involved in installing and running one as well as how big a body of water your home needs to be beside.

What is a water source heat pump?

Water source heat pumps consist of a series of fluid-filled submerged pipes that extract heat from a river, lake, large pond or borehole. 

They generally need a large amount of water in order to work effectively, which is why ground source and air source heat pumps tend to be more popular, along with the fact that less equipment is needed for those systems. 

They operate by sourcing heat from a body of water and converting it into heat for the home. This is achieved through the circulation of a water and anti-freeze mix (brine) through a collector, submerged in a water resource, that absorbs heat energy from the water. 

The brine is then compressed so that the heat delivered to the building is at a higher temperature and then circulated through the heating system, usually in radiators or underfloor heating and a hot water cylinder for hot water.

A water source heat pump with a white background

Vaillant's FlexoTHERM heat pump works with multiple heat sources, including water, air or ground. (Image credit: Vaillant)

What types of water source heat pumps are available?

There are four different types of water source heat pump – closed loop systems, open loop systems, hybrid systems and solar assisted systems.  

1) Closed loop systems The closed loop systems consist of sealed pipes containing antifreeze fluid which are submerged beneath the water surface. The water heats the fluid in the pipes, which is then returned to the heat pump.

2) Open loop systems In open loop systems, water is extracted from the water resource directly and this then flows through the heat pump which extracts the heat from it. These systems can be more efficient than closed loop water source heat pumps, but permission is required from the Environment Agency (EA) or Scottish Environment Protection Agency (SEPA) to extract the water from the water resource and subsequently discharge it back into the water resource.

3) Hybrid systems Hybrid heat pumps integrate a second heating source operating alongside the water source heat pump. These systems are generally used in older homes that are more difficult to insulate. 

4) Solar assisted systems Solar assisted water source heat pumps add a solar thermal panel to the system alongside the heat pump.

How large a body of water is required for a water source heat pump to work? 

A water source heat pump will need to draw on a large water resource for maximum efficiency. 

The ideal resource for a water source heat pump is a nearby river or lake. The larger the water resource, the more heat the water source heat pump will generate.

A cottage with a river running alongside it

A large body of water is needed for a water source heat pump to be effective. (Image credit: Getty)

How are water source heat pumps installed? 

Water source heat pumps are typically installed inside a building or an outbuilding and pipes are laid from there to the edge of the water source, where they are buried in a trench around 0.5 metres deep and around 40cm wide. 

At the water’s edge the pipes connect to a lake collector or a pond mat, consisting of loops of pipe within which is the water and refrigerant mix. The lake collectors are constructed outside of the water and then sunk to the bottom of the water resource where they are held in situ by weights. Pond mats are coils of pipe attached to a corrosion-resistant steel frame, saving space where the water resource is smaller in size. 

Underfloor heating or larger radiators may increase the cost of installation and the building also needs to be well insulated. 

Most planning authorities will treat a water source heat pump as a ‘permitted development’, meaning that it can be installed without having to obtain planning permission first.

Water source heat pumps could be especially attractive to community energy projects. The government assisted in 2015 by providing a map showing the most suitable water resources, recommending that urban locations near fast-flowing rivers were probably the best placed to install this technology.

What are the disadvantages of water source heat pumps?

The house or other building in which the heat pump is installed needs to be near a large water resource, or the heat pump will not work at maximum efficiency. 

There also needs to be sufficient space to install the pipes between the water resource and the building and there needs to be enough room inside the building for the heat pump’s compressor.

What are the benefits of water source heat pumps?

Heat pumps of all types are more cost-effective than electric or coal heating systems, but not necessarily so when they replace gas heating, although they do of course have lower carbon emissions. 

One of the key benefits of a water source heat pump is that they are more efficient than ground and air source heat pumps. This is due to the heat transfer in water being so much better and also that water temperature tends to be more stable, i.e. an average of between 7 and 12°C, throughout the year. This is higher than the average winter air and ground temperature.

How much does a water source heat pump cost?

The overall price of having a heat pump installed into your home will vary between installers, according to a spokesperson from Vaillant, and underfloor heating and larger radiators may increase the installation cost. However, there are some general guidelines as to the total cost. 

“Typically, the total cost of installing a water source heat pump into your home will reach approximately £10,000,” says Vaillant's representative. “While this seems rather high, it’s important to understand the cost-saving benefits on offer from having a WHSP installed on your property.

"On average, homes with a water source heat pump compared to a traditional heating method stand to save around 15 percent on their heating bills annually. Better yet, with proper use, maintenance and servicing, a water source heat pump will last anywhere from 15 to 50 years. This extends much further than traditional combi-boilers, which are usually recommended for replacement at around the 10 year mark.

"There are grants and incentives on offer for homeowners who look to make the switch through the Domestic Renewable Heat Incentive scheme, though we recommend reviewing government guidance before making a decision.”

You can take a look at heat pump grants in our guide on the topic.

Robin Whitlock
Freelance journalist

Robin is a freelance journalist based in the South West of England, UK. He specialises in environmental issues, climate change and renewable energy, with other interests in transport and green motoring. He is a regular daily correspondent for a renewable energy website, writing news articles and interview pieces on all the main clean energy technologies. He has also written widely for numerous magazines on these topics, as well as writing white papers and web content.