A heat pump water heater is a domestic hot water tank with a small heat pump attached. In Belgium, the French term ballon thermodynamique is common. The appliance does not create all its heat directly from electricity. It takes low-grade heat from air, upgrades it through a refrigerant cycle, and transfers that heat into water stored in the tank.
This makes it a practical step for households that want lower electricity use for hot water without replacing the whole heating system. It can suit homes in Wallonia, Brussels-Capital and the Flemish Region, provided the technical room, airflow and electrical supply are right.
The cycle inside the appliance
The working principle is similar to a fridge in reverse. A refrigerant absorbs heat from air in the evaporator. The compressor raises the pressure and temperature of that refrigerant. The hot refrigerant then gives its heat to the water through a heat exchanger. Finally, the refrigerant pressure drops and the cycle starts again.
The important point is that the electricity does not only become heat through a resistance. It moves heat that already exists in the surrounding air. This is why the COP can be above 1. A classic electric cylinder is close to COP 1. A heat pump water heater commonly works around COP 2.5 to 3.5 depending on air source, temperature and settings.
Three air sources in Belgian homes
The first option is ambient air. The unit stands in a cellar, garage or utility room and uses the room air. This is simple, but the room must be large and ventilated enough. A minimum technical volume around 20 m3 is a common design reference. If the room is cold or tiny, performance suffers.
The second option is ducted outdoor air. The unit draws and rejects air through ducts. This keeps the room from cooling too much, but Belgian winter temperatures can reduce COP when outdoor air is very cold.
The third option is extracted ventilation air. If the home has mechanical ventilation, the water heater can recover heat from outgoing air. This can be very stable because indoor extracted air is warmer than winter outdoor air. It requires careful airflow design so the ventilation system remains balanced.
Sizing the tank
Storage volume should match the household rather than the largest model in a catalogue. A small household may use a 150 litre unit. Many families sit around 200 or 270 litres. Larger households, frequent baths or high morning demand can require more.
Under-sizing causes the electric back-up resistance to run too often. Over-sizing keeps too much water hot and wastes standing energy. A good survey looks at occupants, bathrooms, baths versus showers, guest patterns, available space and recovery time.
Understanding COP
COP means coefficient of performance. If the COP is 3, the appliance transfers about three units of heat into the water for one unit of electricity consumed by the heat pump components. Laboratory COP is measured under standard conditions. Real COP changes with air temperature, water setpoint, limescale, maintenance and use pattern.
For a Belgian household, the correct question is not just “what is the highest COP on paper?” It is “which configuration will keep a good COP in this building all year?” A stable cellar or extracted air system may beat a high-spec model installed in a poor location.
Grants, VAT and paperwork
In Wallonia, a heat pump water heater may qualify for the Prime Habitation grant. The 2026 temporary regime requires an energy audit before the quote, a compliant dwelling, and an installer with the required heat pump certification. The maximum R1 amount is 1,680€ under the current table.
At federal level, Belgium applies a 6% VAT measure for heat pump water heaters from 2026 to 2030. This is separate from regional support. Brussels and Flanders use different regional systems, so a rule from Wallonia should never be copied into a Brussels-Capital or Flemish Region file without checking the local source.
Maintenance and reliability
A heat pump water heater needs maintenance because it contains air filters, a refrigerant circuit, a fan, a tank and corrosion protection. The filter should stay clean, the condensate path should remain open, and the tank anode should be checked according to water hardness. Hard water areas in Belgium can require closer attention to limescale.
The appliance also needs a safe anti-legionella cycle. This may use a higher temperature periodically, sometimes with electric back-up. That is normal. What is not normal is a system that relies on resistance heating every day because the tank is too small or the air source is badly chosen.
How EcoChaleur selects a setup
EcoChaleur starts with the building: room volume, ventilation, drainage, electrical panel, hot water habits and grant region. The proposal then explains the suitable tank size and the documents needed if a Walloon grant file is possible.
For English-speaking owners, the value is often in the translation between Belgian administrative language and practical decisions: what the audit means, why RESCert appears in the file, what the BCE proves, and which documents you should keep from quote to final invoice.