35 or 55 degrees? What a heat-pump energy label really tells you

A heat-pump energy label may show two efficiency classes for the same appliance: one at a 55°C flow temperature and another at 35°C. This is not a minor technical footnote. It is a reminder that the appliance must be assessed together with the home's heat emitters, heat loss and controls.

The European Commission's label guide explains that a heat-pump space heater can carry separate efficiency classes for 55°C and 35°C applications, along with indoor and outdoor sound-power levels and energy information. Its consumer guide adds that performance is usually better at a lower flow temperature, but that advantage can only be achieved when the heating system is capable of keeping the building warm with low-temperature water.

1. The 35°C figure is not a universal promise

The 35°C rating is the comparison point for a low-temperature application. Systems designed for these conditions often use underfloor, wall or ceiling heating, fan coils, or radiators with enough emitter area for low-temperature operation.

That does not mean every heat pump automatically suits every underfloor-heating system, nor that every radiator home is unsuitable. The decision requires a room-by-room heat-loss calculation and confirmation of how much heat the existing emitters can deliver at the proposed flow and return temperatures. The label compares products; it does not size the system for you.

2. The 55°C figure represents a more demanding operating point

The 55°C value is the rating for a medium-temperature application. It may be more relevant to systems that require hotter water in cold weather, including some existing radiator networks.

If the 35°C class is visibly better than the 55°C class, that is neither a defect nor an exceptional promise by itself. It indicates that the machine works under more favourable conditions at a lower flow temperature. Before buying, establish what water temperature the home needs on the coldest design day. Do not treat the current boiler setting as an automatic answer: poor balancing, oversizing and intermittent operation can make it different from the temperature that is genuinely required.

3. Size the building first, then the machine

An oversized heat pump is not automatically the safer choice. In milder weather it may cycle on and off in short runs, while an undersized unit may rely heavily on supplementary heat at peak load. The output stated on a label or product sheet must therefore be matched to the building's calculated heat demand and the local design temperature.

Ask for a written sizing summary that includes:

1. room-by-room, or at least whole-building, heat loss;
2. the selected outdoor design temperature;
3. the required flow and return water temperatures;
4. emitter output at those temperatures;
5. heat-pump output and electrical input at the relevant operating points;
6. the conditions under which supplementary electric heat starts.

This is not a performance guarantee. It is a check that the installer and client are discussing the same building and the same operating point.

4. Do not confuse the product label with the package label

EU rules distinguish between an individual appliance and a package. A package label may rate a space or combination heater together with temperature controls, solar thermal equipment, a storage tank or a supplementary heater. Manufacturers and installers can assemble such packages from components listed in EPREL.

A higher package class therefore does not automatically mean that the heat pump itself has the same class. Ask for the appliance label, product fiche, package label and package calculation separately. Verify that the exact model, controller, tank and supplementary component in those documents are the ones listed in the quotation.

5. Read the noise figure as sound power

The label may show indoor and outdoor sound-power levels. Sound power is a comparative measure of noise emitted by the equipment; it is not the same as the sound-pressure level that would be measured at a particular window or property boundary.

What you hear depends on outdoor-unit placement, reflection from walls and corners, the base, vibration isolation, fan speed and any night mode. Ask for a placement drawing, the manufacturer's clearance requirements and a noise assessment for the nearest protected façade or boundary. An inspirational photograph does not prove that the same placement is appropriate for your home.

6. Domestic hot water is a separate question

Combination units that provide both space heating and domestic hot water can carry a separate water-heating class and load profile. A good space-heating class does not automatically promise fast or economical hot-water production.

Ask about cylinder volume, reheating time, hygiene-cycle operation, circulation losses and when electric backup is used. A large bath, several consecutive showers or permanent circulation can create a very different demand from a small household's short morning peak.

7. Use EPREL as a verification point

Match the model identifier on the label with the appliance rating plate, the quotation and the public EPREL product sheet. Do not settle for a family brochure: output, noise, integrated electric heat and storage can differ within the same range.

Seven questions before accepting a quotation

1. What is the home's calculated heat loss at the outdoor design temperature?
2. What flow temperature do the existing emitters require?
3. What does the exact model's label show at 35°C and 55°C?
4. What heat output does it provide at the relevant cold operating point?
5. When does electric backup start, and at what capacity?
6. Where will the outdoor unit sit, and has noise and vibration been assessed?
7. Do the product label, EPREL sheet and package label describe the same components?

The best class letter cannot design a heating system on its own. A sound decision begins by reading the 35°C and 55°C data together with the home's real heat demand, emitters, noise-sensitive locations and hot-water routine.