Certification Dimensioning Energy adviser Heat pump Heating load Heating system Total heating demand

Dimensioning of heat pump systems

May 4, 2020

Summary

Correct heat pump sizing is essential for efficient, reliable and low-carbon heating. A heat pump that is too large can cycle frequently, increasing wear, noise risk and electricity consumption, while a heat pump that is too small may rely too heavily on back-up electric heating during colder periods. To size a heat pump system correctly, designers need to calculate the building’s design heating load, consider heat distribution temperatures, domestic hot water requirements, climate conditions, and use reliable, independently verified product performance data. As Europe accelerates building decarbonisation, tightens energy performance expectations, and moves towards lower-GWP refrigerants under the revised F-gas framework, accurate dimensioning of heat pump systems becomes even more important.

Why is heat pump sizing so important?

The economic and environmental benefits of generating heat with heat pumps compared to fossil fuel systems are only significant if the systems have been carefully planned, correctly dimensioned and installed, and then properly operated. In particular, the dimensioning of heat pump systems represents a key design challenge. Errors can have a greater impact than with traditional oil or gas boilers, because heat pump efficiency is closely linked to operating temperatures, run times, climate conditions and system design.

What is the total heating demand and heating load?

A poorly designed or incorrectly matched heat distribution system can degrade the efficiency of the heat pump and shorten its service life. Errors are difficult to fix once the system is installed. For dimensioning of the heat pump power, it is therefore important to determine both the annual heating demand and the design heating load of the building. The calculations must be carried out according to the latest valid standards and national methods in each country.

Heating demand and heating load are not the same. Heating demand refers to the amount of energy required over a period of time, typically expressed in kWh per year. Heating load refers to the heat output, or power, the heating system must provide at design outdoor conditions, typically expressed in kW. For heat pump sizing, the design heating load is the key parameter.

How do you calculate heating demand?

The specific heating demand per square metre of living space depends on the construction of the building. Actual values depend on insulation level, airtightness, thermal bridges, ventilation, building geometry, internal design temperatures and local climate.

Note: The values below are indicative only and should not replace a project-specific heat load calculation.

Building type	Indicative specific heating demand
Passive house	0.015 kW/m²
New building with very good insulation	0.04 kW/m²
New building with standard thermal insulation	0.06 kW/m²
Renovated old building or new building with limited insulation	0.08 kW/m²
Old building without thermal insulation	0.12 kW/m²

The formula for calculating the heating demand of the building is:

Living space [m²] × specific heating demand [kW/m²] = indicative building heating demand [kW]

This simplified calculation can be useful for a first estimate, but it should not be used as the final basis for selecting a heat pump. A detailed room-by-room calculation according to the applicable national method is recommended, especially for renovation projects, low-temperature heat emitters, hybrid systems and buildings with varying insulation levels.

How do you calculate heating load?

The heating load of a building is calculated according to the applicable standard or national method and must not be confused with the total heating demand. The heating load indicates the power a heating system must provide to maintain the desired indoor temperature at the relevant design outdoor temperature.

Annual heating demand = energy / consumption over time
Heating load = required heating power at design conditions

To determine the heating load, the climatic situation is taken into account and is included in the form of standard outdoor temperatures for the respective regions.

For example, in many inland or elevated regions of Germany, design outdoor temperatures can be significantly below zero. In milder coastal regions, design outdoor temperatures are typically less severe. The building materials, U-values, thermal bridges, ventilation losses, airtightness and building geometry are also included in the calculation.

Very often, a calculation of the heating load and annual heating demand is omitted and larger heat pumps are chosen to allow for a safety margin. Not infrequently, the heat pump is then significantly larger than needed. This has a negative impact on efficiency, comfort and the life span of the compressor. Particularly in the transitional period, i.e. in spring and autumn, an oversized heat pump can lead to frequent on/off cycling, with short running times that may increase wear and reduce system efficiency. If the power is too low, the back-up heater may need to cover a higher share of the load during cold weather, increasing electricity consumption and running costs.

Correct heat pump dimensioning should also consider:

Heat emitter type and required flow temperature, such as radiators, underfloor heating, fan coils or air-based systems
Building fabric improvements planned before or after installation
Domestic hot water production and storage requirements
Bivalent or hybrid operation, including the temperature at which back-up heating takes over
The heat pump’s performance at part load and at different outdoor temperatures
Sound power levels and location constraints
Refrigerant type and future regulatory requirements, including the transition towards lower-GWP refrigerants under Regulation (EU) 2024/573.

How does certification provide accurate performance data?

Even if the expected annual demand of a building and, more importantly, the exact heating load required have been determined by an energy consultant, design engineer or installer, inaccurate manufacturer information can still be a problem. Certification helps exclude this source of error and enables heat pumps to deliver the greatest possible economic and ecological advantages.

For example, Eurovent Certified Performance for Heat Pumps (Eurovent-HP) and Heat Pumps NF 414 share the same product scope and certified characteristics for residential and light commercial heat pumps with a heating capacity less than or equal to 100 kW. Certified characteristics can include heating and/or cooling capacities, electrical power inputs, COP, design capacity, SCOP, SCOPnet, seasonal efficiency, annual consumption and sound power levels.

Eurovent Certification also offers other heat pump-related schemes, including Eurovent Certified Performance for Chillers and Heat Pumps (LCP-HP), Heat Pump KEYMARK, MCS for Heat Pumps and Hybrid Heat Pumps NF 462. Depending on the product type and target market, these schemes support access to independently verified, comparable performance data for specification, design, regulatory compliance and incentive schemes.

The laboratory tests are performed by third parties under strictly defined, identical conditions, using recognised European and international standards. Factory checks, product surveillance and data verification processes are designed to meet requirements for competence, impartiality and independence.

For designers, installers, specifiers and end users, certified data reduces the risk of relying on unsupported manufacturer claims. It supports accurate sizing, energy modelling, product comparison, tender specification, noise assessment and long-term performance expectations. This is especially important for projects, where heat pumps are central to decarbonisation strategies and where reliable data is essential to avoid underperforming systems.

FAQ

Why is correct heat pump sizing important?

Correct heat pump sizing ensures the system can meet the building’s heating needs efficiently without unnecessary oversizing. An oversized heat pump may cycle frequently, reducing efficiency and increasing component wear. An undersized heat pump may rely too much on back-up heating in cold weather, increasing electricity use and running costs.

What is the difference between heating demand and heating load?

Heating demand is the amount of energy a building needs over time, usually expressed in kWh per year. Heating load is the heating power required at design outdoor conditions, usually expressed in kW. Heating load is the critical figure for selecting and dimensioning a heat pump.

Can I size a heat pump using only floor area?

Floor area can provide a rough initial estimate, but it is not enough for final heat pump selection. A reliable calculation should include insulation level, U-values, airtightness, thermal bridges, ventilation losses, building geometry, domestic hot water needs, heat emitters and local design temperatures.

Why should you choose Eurovent-certified heat pumps?

Eurovent-certified heat pumps provide independently verified performance data, helping users compare products using reliable and consistent information. Certification supports accurate system sizing, energy efficiency, regulatory compliance, sound assessment and confidence that declared performance has been checked through third-party testing and ongoing surveillance.