What is a Water Source Heat Pump?

A water source heat pump uses submerged pipework to absorb energy from water sources such as lakes, ponds, rivers, aquifers and mine water. It’s the same unit as a ground source heat pump, however, the heat source they use and the way they collect the heat is different.

A house by the water with a water source heat pump

Could I use a water source heat pump for my property?

Any large water source closely situated to a building is an ideal energy source for the heat pump. They can effectively extract heat energy from a variety of water sources to provide heating and hot water to businesses, houses, and boats. In fact, our ground source heat pump factory is heated by a Kensa water source heat pump using mine water!

Water is an excellent source of heat, making water source heat pumps very efficient renewable heating systems.

How does a water source heat pump work?

Submerged pipework in a body of water absorbs heat energy from its surroundings in the same manner as it would if buried in the ground.

The submerged pipes, typically closed loop systems such as pond mats, transfer the water’s heat energy to the heat pump. The water source heat pump then compresses and upgrades this temperature, delivering heating and hot water to radiators or underfloor heating inside the property.

When water is used as the heat source in open loop systems, a secondary heat exchanger is used, so the heat pump itself remains on a closed loop containing glycol. A secondary exchanger can take several forms; loops of pipe or metal panels immersed in the source water, or a gasket plate heat exchanger with the water piped to it.

More on how a heat pump works



Kensa Water Source Heat Pumps Pond Mats - Orkney Islands Council

What are the benefits of using a water source heat pump?

Water source heat pumps have long been regarded as a means to produce affordable and secure heating from a low-carbon source. Water source heating is beneficial for properties because:

  • The heat transfer rate from water is higher than the ground.
  • The return temperature to the heat pump is generally 5-6°C higher than ground collectors, increasing the efficiency of the heat pump.
  • The water is in close contact with all of the pipe at all times, further enhancing its efficiency.
  • The flow or circulation of water provides constant energy replenishment.
  • The use of a water source removes the need for digging or drilling, reducing the cost and duration of the water source heat pump system installation.
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How do water source heat pumps collect heat?

There are many ways to collect heat energy from water, including pond mats and open loop boreholes or straight pipe collectors.

All forms of heat collectors are classed as either closed loop or open loop systems. Closed loop systems are sealed collectors, where transfer fluid circulates the pipework in one continuous loop without touching the water, absorbing and moving the energy from the water to the heat pump. Open loop systems use the water source itself as the transfer fluid.

Heat collector #1: Pond mats

Coils in lake for ground source heat pump system

What is a pond mat?

Pond mats – Kensa’s preferred type of water collector for standing bodies of water or rivers – are slinky pipes attached to corrosion-resistant stainless steel frames. They are sunk to the bottom of the water source, or secured underneath a floating pontoon.

Much like any closed loop system (slinkies or boreholes), the heat pump circulates a cold transfer fluid through the pond mat pipes to attract energy from the surrounding area. The fluid is a mixture of safe food grade glycol and water. It never comes into contact with the water itself – it simply transfers the heat absorbed from the water source to the heat pump.

Why use a pond mat?

Pond mats are specifically designed for easy and quick installation while maximising the energy absorbed from the water. Each pond mat will extract approximately 7kW of energy from the water source.

As pond mats are manufactured by Kensa and are fully compatible with Kensa’s complete range of ground source heat pumps, you can enjoy fast supply to your site, reducing downtime and costs.

What should I consider when using pond mats?

Things you might need to consider if you have pond mats include:

  • Protecting the pond mats from damage (waves, debris, boats, propellers etc.).
  • Physically securing the pond mats against tidal current and waves.
  • Corrosion of the frame, mountings and structure.
  • Marine fouling.

The depth of the water

When using pond mats you must also make sure the water source is deep enough. Assuming the water is relatively still, the minimum recommended depth of water is about 1.2 meters – ideally more if possible. If the water is fast flowing it can be as shallow as 0.5 meters for pond mats. This is to avoid freezing around the pond mat and to prevent any damage from potential boats passing by.

Kensa can help with anything from initial considerations through to sizing and design. Simply submit your plans for more advice.

Heat collector #2: Open loop collectors

What is an open loop collector?

Open loop water collectors take the form of two separate straight pipe collectors. They extract water from one location, filter this through heat exchangers in the heat pump to absorb energy from the water, and then deposit this cooler water into another location of the source.

The majority of open loop collectors for water source heat pumps are boreholes.

How does an open loop collector work?

Water is abstracted and filtered through the heat pump via a borehole or straight pipe. This water is then discharged back to a distant section of the water source or another acceptable discharge area via a second borehole or open loop straight pipe.

Unlike closed loop collectors (slinkies, pond mats or closed loop boreholes), no heat transfer fluid is pumped through open loop collectors. The fluid used is the water source itself.

What should you consider before you get an open loop system?

Preparation & design

You must consider the design of any scheme to ensure its longevity and efficient performance. Design elements that Kensa can help with include:

  • Employing the services of a professional hydrogeologist and/or groundwater engineer to ensure you have a well-designed open loop ground source heat pump system.
  • Making sure there is adequate distance between the abstraction and discharge boreholes. This prevents large quantities of discharged water feeding back into the abstraction borehole, affecting the temperature of the abstracted water and performance of the system.
  • Ensuring the discharge borehole will not clog within a short period of operation. Careful control of the gas content, water chemistry and the particulate content of the water is required, as small bubbles of gas and particulates can result in rapid clogging of the borehole or aquifer. Biofouling with bacterial growth can also become problematic.
  • Avoiding possible freezing and corrosion of the heat pump. The heat pump and water source should be separated by an intermediate plate heat exchanger.
  • Fitting a check valve on the water source side. This ensures that the system does not drain down when the extraction pump is turned off.
  • The signal for the extraction pump can be taken (via a relay) from the 24V supply between the controller and compressor contactor.


Much like pond mat water source collectors, open loop systems can be cheaper to install than digging trenches if a readily available source of water is used.


When extracting or drilling boreholes for water extraction, the Environmental Agency must be consulted for appropriate licences. This includes:

  • In order to drill or test pump a water supply borehole, you will require a consent to investigate a groundwater source under section 32 of the Water Resources Act.
  • To operate an open loop scheme you will need an abstraction licence (if the abstraction is greater than 20m3/d) and an environmental permit to discharge water.
  • There are abstraction limits in place to prevent the removal of too much water. You’re allowed to take 20 cubic metres of water per day from an open loop water source without any licence requirement. If you plan on using more than the limit, you will generally need to apply for permission from the relevant authorities.


To avoid freezing the water around the heat pump, Kensa recommends that open loop systems use an intermediate heat exchanger to separate the circuits. This overcomes risks of corrosion and filtration, so that only a clean water or glycol solution passes through the heat pump itself, protecting the components in the unit.

Filtration & maintenance

With open source collector systems there is a requirement for filtration and additional maintenance issues, which should be considered and addressed.

You may have to clean the open loop pumping equipment and filters from time to time. How often you have to do this will depend on the quality of the water. It is also important that bacterial growth is minimised to avoid fouling. This does mean that a regular maintenance schedule is required.

Remember that the hydraulic efficiency of the system will reduce over time. This can happen even with a good design, especially if thermal breakthrough occurs between the abstraction and discharge boreholes. Background temperature of the groundwater may also change over time if more systems are constructed in the area.

Get Kensa’s full support

As well as offering complete guidance on sizing and design, we provide technical support to every customer for peace of mind.

To alleviate the project loads, installers can even use our MCS service for complete MCS support. This is a mark of quality, and ensures that for domestic projects, the end user or owner of the water source heat pump can apply for the Boiler Upgrade Scheme.

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Closed loop vs. open loop systems

One of the benefits of using a closed loop water system over open loop is the reduced risk of freezing within the heat pump. The closed loop system isn’t exposed to external elements and there is no need for any filtration units, so less maintenance is required.

Disadvantages of open loop water source heat pump systems

The hydraulic efficiency of open loop systems will reduce over time, even with a good design. Clogging can be inevitable, especially if thermal breakthrough occurs between the abstraction and discharge boreholes. Background temperature of the groundwater may also change over time if more schemes are installed in the area.

However, Kensa has successfully supported many projects with open loop systems. If an open loop system is the best solution for your project, we can help to design an efficient system that provides year-round comfort.

Submit your plans for advice

What water sources can be used with a water source heat pump?

•  Lakes

A lake is ideal for pond mats. The number of mats you need will depend on the size of the heat pump, the size of the body of water, and most importantly, how that body of water is fed.

If it’s stagnant water then the only real heat gains are from the sun and rain, so you will need to space your mats correctly to avoid over-extracting energy. If the lake or stream is spring-fed, try and check the average temperature over a couple of seasons.

Pond mats can be placed right next to each other if there’s really good water flow across them. As a general rule, allow around 3 litres per minute per kilowatt of heat pump size.

Having specialised in water source since 1999, our experts can help you with anything from the initial design stages right through to aftersales support.


•  Streams & rivers

If there is a nearby stream or river of sufficient depth, this can be a suitable heat source. If you find that it’s too small or too shallow for pond mats, it’s possible to divert the course of smaller rivers and streams, providing you with a better location to place the mats in.

Again, allowing for around 3 litres per minute per kilowatt of heat pump size will help you check that you’ve got the required flow rate. Kensa can also establish this when helping with sizing and design.

•  Larger rivers

Where a more considerable water source is available such as a large river, you might want to consider an open loop system. Talk to a Kensa expert to discuss your project and see if this is possible.

•  Aquifers

Using an open loop borehole, ground source heat pumps can extract heat energy from the groundwater stored in an aquifer – an underground layer of water-bearing permeable rock.

Open loop systems with aquifers or waterways are excellent ways to extract heat energy. Many farms and rural commercial premises have access to potable water boreholes, which, providing it is deep enough and offers enough yield, can be an ideal source of heat.

Consult a hydrologist to determine the existence and suitability of any subterranean water source and its yield.

•  Mine water

Mine shafts and floodings already hold a fair amount of heat – so why not make the most of it? Mine water is a viable heat source that would otherwise go to waste.

Heat collectors can be lowered into mine water, transforming wasted heat into useful heating and hot water for a building. In fact, we use mine water to heat Kensa’s very own factory and offices, so you can ask us all about it.

Further advice on the use of water sources is available via CIBSE’s Code of Practice for surface water source heat pumps, available to purchase here (free to CIBSE members).

Mine water for a ground source heat pump

•  Seawater

Both open and closed loop systems can be used in seawater. Due to sea water’s corrosive properties, specialist titanium heat exchangers can be used, or pond mats fixed to piers or pontoons.

Having successfully used seawater sources for all sorts of projects, Kensa can support you with anything from specification to sizing and design.

Submit your plans for advice & a quote

Sea water extraction for water source heat pump

Water source heat pump case studies

Mine water: Kensa factory & offices, Cornwall

Situated on the grounds of a 1920s Cornish tin mine, Kensa wanted history to live on. As luck would have it, water inside the old mine shafts proved to be the perfect source of heat.

The mine continues to feed heating and hot water into Kensa’s factory and offices to this day.

See Kensa’s case study
Mount Wellington Mine - before & after

Aquifer: Seven tower blocks, Sunderland

Kensa Contracting, the delivery arm of Kensa Heat Pumps, was tasked with swapping gas for ground source in this record replacement scheme. Using an underground aquifer as the heat source, 364 homes will reap with benefits of safe and affordable heating.

David Broom, Managing Director of Kensa Contracting, reiterated the importance of the project:

Electrification of heat with ground source heat pumps not only makes buildings safer and cheaper to run, but also immediately reduces carbon emissions. More importantly [it] puts UK homes on the pathway to Zero Carbon as grid infrastructure is further decarbonised.

Lake water: Grade II listed property, Cambridge

River House has a large lake on its grounds, which is fed by a natural spring. Thanks to its flow rate, area and depth, sufficient heat energy could be extracted from the lake to provide heat and hot water to the Grade II listed property via a ground source heat pump and pond mats.

The pond mats were connected up to the ground source heat pump unit in an annex building via pipes running under the lawn.

Owner of River House, Keith Clarke, said:

I knew that ground source was a great technology for extracting the natural heat that’s available from the environment to provide a completely green heating system. The good thing about the pipes being in the lake is that water has great conductivity meaning we are able to extract heat sustainably throughout the winter to keep the house perfectly warm.

I know that there’s a big investment involved in getting a ground source heat pump, however we’ve ended up with a system where the yearly cost of electricity is less than we would have been paying for gas.

See the River House case study

Specialising in water source since 1999

Kensa’s very first heat pump in 1999 was designed to be installed onboard a boat. Using seawater as its energy source, each Tamar Class RNLI lifeboat has a Kensa heat pump system on board to keep the crew comfortable in high or low-temperature conditions

Continuing our maritime heritage and long-standing partnership, Kensa supplies the RNLI with sea source heat pumps to heat the lifeboat stations.


Ground Source Review: RNLI Porthdinllaen. Drill Craning

Pricing table small print

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