Waste Heat as a Heat Source for Heat Pumps

Waste heat is thermal energy produced by technical systems and processes that is not used for its primary purpose. Instead of discharging it to air or water, this heat can be captured, transported, and used as a heat source for a heat pump.

This page explains waste heat as the heat source. It does not cover system sizing, installation design, costs, or product selection.

If you want a full overview of environmental heat sources, see Heat sources for heat pumps

Key terms and boundaries

To avoid confusion, it helps to separate the “source” from the “method”:

• Waste heat: heat that would otherwise be released unused.
• Waste heat recovery: the process of capturing waste heat (usually via a heat exchanger).
• Recovered heat: waste heat after it has been captured into a usable circuit.
• Waste heat as a heat source: using recovered heat as the input energy for a heat pump.

In policy and accounting contexts, “waste heat” is often treated as valid for decarbonization claims only if it is truly waste, meaning it could not reasonably be avoided and could not reasonably be used on-site first.

Facts and figures

These figures help anchor why waste heat is discussed as a major heat source option:

• Industrial heat dominates energy use: European Heat Pump Association states that more than 60% of energy used by European industry is for providing heat.
• Industrial heat pumps can upgrade waste heat: EHPA highlights industrial heat pumps as deployable today and able to reach up to ~180–200°C in some applications.
• Temperature bands used in EU reporting: An European Commission report splits industrial heat into <100°C (low), 100–400°C (medium), and >400°C (high).
• Low-grade heat often needs upgrading: The Scottish Government guidance defines low-grade heat as below 100°C and notes it is often too low to use directly without upgrading.
• Real-world heat recovery efficiency varies: The Australian Government Department of Climate Change, Energy, the Environment and Water notes typical average efficiency ranges around 50–80%, with higher “peak” claims sometimes quoted by manufacturers.
• Waste heat can be a large share of industrial losses: The U.S. Department of Energy estimates 20–50% of industrial energy input can be lost as waste heat.

What “waste heat as a heat source” means

A waste heat source is thermal energy that already exists in a technical system (or process) and would otherwise be discharged (for example, through exhaust air, cooling water, or equipment losses). A heat pump can use this available heat, often by collecting it through a heat exchanger and then upgrading it to a usable temperature level.

What this is

• A heat source created by operation of technical systems
• Often predictable and continuous when processes run steadily
• A source that can be very local (building, facility, site)

What this is not

• A different heat pump technology
• A guaranteed efficiency result (temperature levels and system design still matter)
• A replacement for “reduce first” thinking (waste heat is most valuable when avoidable losses are already minimised)

Waste heat source attributes

Waste heat sources differ widely. These attributes help describe any source consistently:

• Heat carrier: air, water, exhaust gas, refrigerant loop, equipment surface losses
• Temperature band: low (<100°C), medium (100–400°C), high (>400°C)
• Continuity: continuous, batch, seasonal, intermittent
• Quantity and stability: steady flow vs fluctuating flow
• Cleanliness / fouling risk: particles, oils, biological growth, scaling
• Distance to demand: on-site, near-site, or network-connected
• Controllability: how predictable the source is over time

Main waste heat source categories

Waste heat sources are typically classified by where the heat originates:

Industrial processes and production systems

Heat may leave the process through exhaust streams, cooling loops, and equipment losses. The main feasibility question is not only “how much heat exists,” but whether it is available at a stable temperature and stable flow that matches the demand profile.

Exhaust air from buildings and infrastructure

Ventilation exhaust air can carry usable heat. Heat recovery devices and heat exchangers are commonly used to transfer heat from exhaust air to a usable circuit, and testing/certification practices exist for ventilation heat recovery equipment in Europe (often referencing EN 308 test principles).

Technical infrastructure heat loads

Many technical systems reject heat as part of normal operation (for example, cooling systems, refrigeration processes, or equipment rooms). Whether this becomes a practical heat source depends on continuity and how easily heat can be captured.

Temperature level: why it changes what is possible

A simple way to understand waste heat is by temperature band:

• Low temperature (<100°C): often needs temperature upgrading for many heating uses
• Medium temperature (100–400°C): more energy-dense, sometimes usable directly in process contexts, often still needs adaptation to match a specific demand
• High temperature (>400°C): process-specific, typically found in heavy industrial contexts

This is why heat pumps are frequently discussed for upgrading lower-temperature heat sources, including waste heat, into a usable temperature level.

The interface between waste heat and the heat pump

Waste heat is rarely “plug-and-play.” A controlled interface is usually required.

Heat exchanger (capture)

A heat exchanger transfers heat from the waste heat stream into a controlled circuit. This separates:

• the source side (process/exhaust conditions), and
• the technical side (heat pump + building heating system)

Heat transfer circuit (transport)

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

• the expected temperature range
• the stability of flow/availability
• the cleanliness and fouling risk of the source stream

Temperature upgrading (use)

The heat pump upgrades the recovered heat to the temperature level needed by the building (or the process demand, in industrial contexts)

What influences how usable waste heat is

Waste heat potential depends on both physics and practical constraints:

1. Temperature level (how much upgrading is needed)
2. Continuity (is it available when heating is needed?)
3. Quantity and flow stability (steady vs intermittent)
4. Contamination and fouling risk (can the interface stay reliable over time?)
5. Distance to the heat demand (shorter distances reduce complexity and losses)
6. Rules and site constraints (especially when interfacing with external networks or regulated systems)

When waste heat is a suitable heat source

Waste heat is typically considered when:

• the source is regular and predictable (processes, continuous exhaust streams, stable technical loads)
• the heat can be captured through a reliable interface
• there is a nearby heat demand that aligns with the source profile
• the project aims to improve overall system efficiency by reusing unavoidable heat losses

When waste heat is a suitable heat source

Waste heat is typically considered when:

• the source is regular and predictable (processes, continuous exhaust streams, stable technical loads)
• the heat can be captured through a reliable interface
• there is a nearby heat demand that aligns with the source profile
• the project aims to improve overall system efficiency by reusing unavoidable heat losses

When another heat source may be more practical

Another heat source may be preferred when:

• the source is intermittent or poorly matched to heating demand
• temperature is very low and upgrading would be inefficient for the target use
• fouling/maintenance risk is high for the available interface
• site rules or complexity outweigh the benefits

For comparison, also see:

• Air as a heat source
• Ground as a heat source
• Water as a heat source