Water as a Heat Source for Heat Pumps

Water can act as an environmental heat source for a heat pump system. It stores thermal energy well and can transfer heat efficiently. Depending on the water body, source temperatures may be relatively stable over the year.

For a general overview of heat source categories, see Heat sources for heat pumps.

Facts and figures (reference values)

These values help explain why water is often described as a “strong” heat source. They are reference values measured under defined conditions:

• High heat storage: liquid water has a specific heat capacity of about 4.20 kJ/(kg·K) around 10°C.
• Heat transfer: thermal conductivity of water is about 0.6065 W/(m·K) at 25°C and 0.1 MPa (reference condition).
• Example groundwater temperatures (Switzerland, 2022): mean groundwater temperature at monitoring stations was reported between 5 and 15°C, with 10–13°C mentioned for parts of the Central Plateau and the southern side of the Alps (example regional values).
• Groundwater can change over time (Austria example): a study of 227 wells reported an average change of +0.7 ± 0.8 K from 1994 to 2013, with strong local differences by site.
• Common rating notation: W10/W35 describes a test/rating condition (10°C source-side water and 35°C heating water outlet). It is used in industry testing contexts to compare performance at defined temperatures.

What “water as a heat source” means

A water heat source is thermal energy stored in a water body outside the building. The heat pump does not create this heat. It collects existing heat through a heat exchange interface and then upgrades it to the temperature level the building needs.

What this is

• A thermal reservoir outside the building
• A heat source that can be stable (especially groundwater), depending on the site
• A source that is always site-dependent and needs local feasibility checks

What this is not

• A different heat pump technology
• A guarantee of efficiency on its own (efficiency depends mainly on temperature levels and system design)
• A substitute for local environmental rules and permits

If you want the system category page, see Water source heat pump

Main water heat source types

Water heat sources are classified by one question: Where does the heat come from?

Groundwater (subsurface water)

Groundwater temperatures are often more stable than outdoor air because subsurface conditions change more slowly.

Example monitoring data from Switzerland shows station means reported between 5 and 15°C (with regional examples around 10–13°C).

Groundwater can also warm over time and vary strongly by location. An Austrian well study (227 wells) reported an average change of +0.7 ± 0.8 K from 1994 to 2013, with local factors influencing individual wells.

Lakes and rivers (surface water)

Surface water is exposed to the atmosphere, so it often varies more across seasons. A review of surface-water systems notes that river water temperature tends to track average daily air temperature, which can affect suitability at certain times of year.

In practice, very low water temperatures can also create operational constraints (for example, near-freezing conditions can increase icing/freeze risk in some configurations).

Seawater (coastal regions)

Seawater can be used where it is available, but feasibility depends on local conditions (temperature levels, intake/outfall constraints, and environmental requirements). The core concept stays the same: the water body is the heat source, and a controlled interface transfers heat into the system.

The interface between water and the heat pump

The water body is the heat source, but the heat pump needs a controlled interface to use it.

Water-side heat exchange

A heat exchanger transfers heat from the environmental water into a circuit connected to the heat pump. This separation helps protect the technical system from site conditions (such as fouling tendencies) while keeping heat transfer predictable.

Heat transfer circuit

A circulating circuit transports collected heat to the heat pump. The circuit must match:

• Expected source temperatures over the season
• Operating conditions on the building side (especially heating water temperature)

What influences how much heat water can provide

Water heat availability depends on both physics and constraints:

• Water temperature level and seasonal range
  Groundwater is often more stable; surface water often changes more with weather.
• Source continuity (flow and availability)
  A usable source must remain available under normal operating conditions.
• Water quality and fouling tendency
  Particles, biological growth, and minerals can affect heat exchange surfaces over time.
• Building-side temperature level
  Lower heating water temperatures generally improve heat pump efficiency, which is why comparison points often use outlet temperatures such as 35°C.
• Rules and environmental constraints
  Local requirements can define what is permitted and how the source can be used.

Understanding test-point labels like W10/W35

When comparing performance data, the operating point matters.

• W10/W35 means: 10°C source-side water, and 35°C heating water outlet at the defined test condition.

These labels help make values more comparable because they describe the temperature conditions used for testing.

When water is a suitable heat source

Water is typically considered when:

• a usable water body exists near the building
• the source can provide stable conditions over the heating season
• a reliable heat exchange interface is feasible under local constraints
• the building can operate with suitable heating distribution temperatures

When another heat source may be more practical

Another heat source may be preferred when:

• no suitable water source is available or accessible
• water conditions increase complexity (seasonal variation, fouling risk, restrictions)
• local constraints make water use difficult to permit or operate
• a more universally available source is needed (such as outdoor air)

For comparison, see Air as a heat source and Ground as a heat source.