Why sizing decides everything
A collective heat pump is a central heat generator that draws energy from the outdoor air to heat the water in a network serving an entire building. It replaces or supplements a collective boiler in a residential building, a healthcare facility, an educational building or offices.
Its performance does not depend solely on the unit chosen. An air-to-water heat pump sees its available output fall as the outdoor temperature drops, precisely when the building needs heat the most. It also performs less well when asked to produce very hot water for older heat emitters.
Sizing means reconciling these two realities: what the building needs, and what the machine can deliver under the actual conditions of the site. An undersized heat pump leaves the backup system to carry a large share of the demand and worsens the energy balance. An oversized heat pump costs more to buy, runs in short cycles and ages faster.
For operations funded by energy savings certificates (certificats d'économies d'énergie, CEE), this work is not optional. Two standardised operation sheets make it a condition for the issue of certificates.
What sheets BAR-TH-179 and BAT-TH-163 say
Sheet BAR-TH-179 covers the installation of one or more air-to-water heat pumps in existing collective residential buildings. Sheet BAT-TH-163 covers the same operation in existing tertiary buildings. Both sheets are published by the Directorate-General for Energy and Climate (DGEC) in their version A81-2, applicable since 30 April 2026.
| Criterion | BAR-TH-179 | BAT-TH-163 |
|---|---|---|
| Sector | Existing collective residential buildings | Existing tertiary buildings |
| Rated heat output | 400 kW at most | Separate rules up to 400 kW and above |
| Eligible use | Heating, or heating and domestic hot water | Heating, or heating and domestic hot water |
| Required performance | Etas of at least 111% (medium and high temperature) or 126% (low temperature) | Same Etas threshold up to 400 kW; COP of at least 3.4 above |
| Installer qualification | Quality label required | Installation carried out by a professional |
| Prior sizing study | Mandatory | Mandatory |
| Conventional lifetime | 22 years | 22 years |
| Operations committed up to | 31 December 2030 | 31 December 2030 |
In both sheets, only heat pumps sized to cover all or part of the heating demand, or of the heating and domestic hot water (DHW) demand, are eligible. A heat pump that produces only DHW does not qualify for certificates under these sheets.
The residential sheet adds a non-cumulation rule. A BAR-TH-179 operation cannot be combined with sheet BAR-TH-169, which covers collective heat pumps for domestic hot water, when the heat pump installed provides both heating and DHW.
The prior sizing study
The prior sizing study is the document in which the professional justifies the choice and output of the heat pump or heat pumps. Both sheets impose three formal requirements: it is dated and signed by the professional, handed to the beneficiary when the operation is committed, and updated on completion of the works if necessary. The heat pumps installed must comply with its recommendations.
The sheets also set its minimum content. It falls into four groups.
| Group | Content required by the sheets |
|---|---|
| Identification | Beneficiary and location of the operation; professional or engineering consultancy that carried out the sizing |
| Building | Heated floor area and types of heat emitters; flow temperature to the emitters; indoor set-point temperature |
| Demand and output | Design temperature; building heat losses; output delivered by the heat pump at the design temperature and at the flow temperature; annual heating coverage rate |
| Equipment | Make, model reference and performance of the heat pumps; other heating systems retained or installed, with the reasons for retaining them |
Two notions in this table deserve a definition. The design temperature is the lowest reference outdoor temperature adopted for the site, from which the heat losses are calculated. The annual coverage rate is defined by the sheets as the ratio between the energy supplied by the heat pump or heat pumps and the heating demand of the building.
This study is one of the specific supporting documents for the operation. Compliance of the machine actually installed with its recommendations is an explicit condition of both sheets.
Low or medium temperature: a classification that sets the threshold
The seasonal energy efficiency threshold to be achieved depends on the heat pump application. Seasonal space heating energy efficiency, written Etas, is defined by Commission Regulation (EU) No 813/2013 of 2 August 2013, for the average climate conditions of that regulation.
The sheets do not leave the choice of application to the discretion of the project: they derive it from the heat emitters and uses.
| Heat emitters or use | Application under the sheets | Etas considered | Minimum threshold |
|---|---|---|---|
| Underfloor, ceiling or wall heating; water fan coil units | Low temperature | Etas at 35 °C | 126% |
| Radiators, including so-called low-temperature radiators at a 45 °C water regime | Medium or high temperature | Etas at 55 °C | 111% |
| Mixed solutions, for example radiators and underfloor heating | Medium or high temperature | Etas at 55 °C | 111% |
| Heat pump also producing DHW | Medium or high temperature | Etas at 55 °C | 111% |
The Etas taken into account is that of the heat pump alone, excluding any control device. Where a remote system allows the heat pump to produce DHW, the controls must give the heat pump priority for that production.
For a tertiary heat pump above 400 kW, sheet BAT-TH-163 changes criterion. It requires a coefficient of performance (COP) in heating mode of at least 3.4, measured according to standard EN 14511-2 for a temperature of 35 °C at the outlet of the indoor heat exchanger.
The trade-offs that make a good project
The content required by the sheets describes what the study must show. The quality of the project depends on the trade-offs that lead to those figures.
Existing heat emitters and the water regime
In a renovation, heat emitters are rarely changed. Their ability to heat rooms with cooler water determines the efficiency of the heat pump, the applicable Etas threshold and sometimes the choice between a standalone heat pump and a hybrid solution. An accurate survey of the emitters, room by room where necessary, avoids sizing on a flattering assumption.
The role of backup and retained equipment
The sheets require a description of the other heating systems after the works and, where applicable, the reasons why existing heat generation equipment is retained. Keeping a boiler as backup is therefore a choice to be justified, not an oversight. Well balanced, it secures comfort in severe cold without degrading the coverage rate of the heat pump.
Domestic hot water
When the heat pump also produces domestic hot water, it moves into the medium- or high-temperature application, and the controls must give it priority for that production. This choice alters both the performance threshold and the sizing: it is decided at the study stage, not on site.
What this means for you
For a landlord, a property manager or a project owner
The sizing study is the document that protects your investment. It is handed to you when the operation is committed: that is the moment to check that it matches your actual building, its heat emitters and its uses. A generic or copied study exposes the operation to a refusal of certificates and, above all, to an ill-suited installation throughout its lifetime.
For an installer
Compliance of the heat pump installed with the recommendations of the study is a condition for issue. A change of model during the works must therefore be reflected in the updated study handed over on completion of the works. For a BAR-TH-179 residential operation, the qualification or certification decision for the company is also one of the supporting documents.
For an obligated party or a delegated body
The file for a collective heat pump rests on consistency between the study, the proof of completion and the manufacturer's documentation. The proof of completion states, among other things, the rated heat output, the use, the application type and the Etas of each heat pump. An inconsistency between these documents weakens the valuation, even when the installation is technically satisfactory.
For a tertiary facility manager
Above 400 kW, the performance criterion changes and the documentation must follow. The choice between a single heat pump and several heat pumps in cascade deserves to be settled early, as it determines the applicable rule and the documents to be produced.
What we recommend
Robust sizing is built from the building as it is, not from a ratio. We carry out independent sizing studies for collective heat pumps, in residential and tertiary buildings alike. Our studies are designed to meet the content required by the sheets and to withstand an inspection.
Our independence allows us to recommend the solution suited to the site, including keeping a backup system when it is justified. We support the project through to a consistent file, from the technical visit to the study updated after the works.
The sheets change regularly, and a draft order put to consultation in 2026 is a reminder that the catalogue can change quickly. To learn about our energy audit and engineering approach, or to have a collective heat pump sized, contact us.
