The wonders of water source

I talk a lot about air source and ground source heat pump installations, because these are by far the most common installations of heat pumps in this country. Yet using water as a source of environmental heat for a heat pump, is in fact, far more efficient. Put simply, heat conducts better through wet media than dry. Ground loops placed in damp ground are more efficient that those placed in dry, sandy locations and it follows that those placed directly in water can outperform those on land. Moreover, the temperature of a water collector should be higher year round than a ground collector, because water absorbs and holds the sun's energy better than the ground.

The resulting gains in efficiency can be impressive (see graph below). A typical GSHP can achieve 20 watts per square metre with a horizontal closed collector. A water source heat pump can achieve 60 watts per square metre of surface area covered by pipe. This gives a huge impact on COP and the KW output of the system. A 20kw system on the ground loop should deliver the specified 20kw; but water can give a 25-6 kw for the same input. This is, of course, just a rule of thumb and installers should always carry out a decent assessment of the ground conditions as per MIS 3005 (see footnote).

Moreover as output is higher it is often possible to choose a smaller rated heat pump (because they are rated at a source tempertaure of zero - see graph on increases relative to source temperature) than you would otherwise do for a given situation. This can save on install costs for the client. One top tip here is to remember to to consider a different hot water tank to match this higher output than you would normally choose for that size of heat pump.

Graph showing how COP increases with warmer source temperature
kW output increases relative to source temperature

There are two types of water project:

  1. A sealed pipe installation. These have a sealed loop as for a normal ground loop installation with ethylene glycol in the pipes as a heat transfer medium.
  2. An open loop system. These take water directly from the water source and pass it directly through the heat exchanger before discharging it back into the environment.

Open loop systems

Pros Cons
offer the greatest heat exchange efficiency because heat does not need to be transferred into a carrier medium, but is extracted directly from source need for filtration to keep debris out of your heat exchanger; this can increase power consumption and significantly increase maintenance. Not therefore ideal for muddy/turbid watercourses
Cheaper up- front costs because there is no loop to install More energy is also required to pump the “loop” compared to a closed loop system where the system pressure is constant.
Less pipework to be damaged or dislodged by traffic, debris and water flow You are removing water from the source and passing it directly through a heat exchanger which means that you are having a more direct impact on the environment (e.g. living organisms can be sucked into the system) than in an the inert closed loop system
  Requires an extraction license from the environment agency

Closed loop system

Closed loops must be installed on “pond mats” or lattice frameworks to keep them held in place, spaced out across the bottom of the available area to prevent them drifting away or together and touching (which ruins efficiency).

Pros Cons
Only an inert plastic pipe comes into contact with the environment Should the loop be damaged the ethylene glycol can escape
Less maintenance than an open loop system In moving water loops need to be secured to prevent them being washed away
  Where waterways have high traffic there is a risk of damage to the loop by passing boats etc.

Installations can be made using a specially designed water to water heat pump (open loop), or using a ground source (GS) pump either as it is for closed loop, or with a product such our GWS groundwater module, which allows a standard GS heat pump to be connected to a groundwater source.

So what water sources are suitable? Well, most, but each has its own challenges.

  • Lakes are perhaps the most common and straightforward installations. Typically still or with a steady, slow throughput of water they can be suited to open systems. Owners are sometimes able to drain lakes for the installation of a closed loop, or indeed we have been involved on projects where the lake is built for the project.
  • Rivers and streams pose the challenges of dramatic seasonal variations in flow and water levels which can expose or block pipes and can carry a great deal of debris. It is often impractical to install closed loop systems because pipework can become displaced by the fast flow (although more rigid pipes are available and a closed loop would be my preferred option in a river due to the lower maintenance). Other considerations include the traffic on the waterway, which might damage pipework
  • Wells offer an option where the water levels remain stable (which will need confirmation). Pumps for wells have to be submersible and have to pump a big head of water, so are more expensive than for other systems. Accessibility is also an issue. The quality of water can be aggressive and this needs to be accounted for when selecting kit
  • The sea provides tides and tidal drift to contend with if using a closed loop system. Salty water demands that the heat exchangers are titanium or stainless steel 323. Again traffic and moorings must be taken into consideration when planning placement of pipework

Doing a risk assessment

It’s clear that any water source installation requires an extra level of planning and forethought. To this end, any water source installation should have a formal risk assessment carried out before work commences. This can take the form of an FMEA (failure mode effects analysis) procedure. This might sound a bit complicated, but essentially just means getting all the key parties around a table to brainstorm what could go wrong, assign each failure mode a level of risk and then plan measures to reduce the risks to an acceptable level. So for example, the table below shows how one risk identified suring this process could be documented and planned for -

Closed loop system risk assessment example for a lake

Item Failure mode Risks Risk level Recommended actions Responsibility and deadlines for action
Ground loop Loop is damaged during dredging Ethylene glycol leak harms wildlife 8 1.Use food grade glycol 2. Use device to stop automatically flow if leak detected, 3. Tie floating markers to lattice to show position of loop DW to feed back on cost implications by 1/12/12

There are lots of online resources to help with this kind of process – try, as a starting point this useful guide to FMEA.

In conclusion then, despite a number of challenges presented by water source installations, effciency and cost saving benefits for clients are many. So next time you visit a site where there is a source of water, remember that it is a fantastic resource that should be taken into consideration when planning a heating system.


1. MIS 3005 4.2.3 “When designing Closed-Loop Heat Exchangers the thermal conductivity of the local ground conditions, loop configuration, local climate and landscaping shall be taken into consideration. If thermal conductivity data is not available then appropriate in-situ thermal conductivity testing should be performed. Such testing shall be performed by a competent company."