Heat pumps in renovation projects: radiators become a key factor in performance

In many renovation projects, the focus is largely on replacing the boiler. The boiler is replaced by a heat pump, the energy-saving objective is clearly stated, and the existing radiators are often retained as a matter of course. This approach can work, but it should not be taken for granted. In a hot-water heating system, actual performance depends on the combination of the heat generator, the distribution network and the radiators.

This issue becomes particularly sensitive with a heat pump. This type of heat generator achieves its best performance levels when the water temperature required by the circuit remains limited. However, a radiator is sized to compensate for heat loss in a room according to a given temperature regime. If the existing radiator is unable to provide the necessary output with cooler water, the system may suffer in terms of comfort and efficiency.

The consequences are well known in practice: slow temperature rise, inadequately heated rooms, operation at a higher temperature than anticipated, higher-than-expected energy consumption, and even corrective work following commissioning. For the installer, the issue of radiators is therefore not a minor detail. It determines customer satisfaction and whether the advertised performance levels are achieved.

The first step is to assess the existing system. Available heat output, water flow rate, and the hydraulic condition of the system – including balancing, the presence of air, sludge build-up, positioning within the room, thermostatic valves and installation conditions – must all be taken into account in the assessment. A radiator positioned too close to the floor, too close to the wall or recessed may lose a significant portion of its heat output due to inadequate convection. These factors directly influence the result perceived by the occupant.

Where the existing radiators are unsuitable, several solutions are possible: adjusting the sizing, replacing certain heat emitters, or using radiators better suited to low-temperature operation. The aim is not to systematically replace all radiators, but to ensure that the heat emitters allow the heat pump to operate under optimal conditions.

In this analysis, the use of NF-certified water-filled radiators provides a useful benchmark. NF certification covers declared and verified characteristic parameters, notably thermal output, maximum operating pressure and acoustic power, depending on the products in question. For the installer, these figures make it easier to select a heat emitter that is consistent with the intended water flow rate and the actual needs of the room.

Modern radiators have evolved significantly in terms of power output relative to their size, compactness, aesthetics and ease of integration. Low-temperature solutions, designed to operate at lower water temperatures, can help maintain comfort whilst promoting more efficient operation of the heat pump. In certain configurations, very low-temperature radiators with dynamic boost can also provide additional heat output when demand increases.

For installers, the message is simple: a heat pump cannot be assessed in isolation. The quality of the installation also depends on the choice and sizing of the emitters. Checking the radiators before the installation begins, clearly explaining the issues to the customer and relying on certified data helps to reduce the risk of poor performance, limit after-sales service calls and ensure the expected outcome is achieved.

Search for certified radiators

Discover more about the radiators programme