COP at partial load: the real performance of a heat pump

Understanding a heat pump's COP: what the measurement says and what it leaves out

Instantaneous COP, seasonal COP (SCOP): don't mix up the units

COP is a ratio (heat delivered over electricity consumed) measured at one specific point. You often see it at A7/W35 or A2/W35. SCOP, on the other hand, adds up a full heating season, with partial loads and a reference climate. Comparing a "catalogue" COP to annual performance is like comparing a photo to a film. To go further, see also how to interpret the real-world performance of a heat pump.

Outdoor temperature, flow temperature: the two levers that drag COP down

The colder it is outside, the harder the machine has to work. And the higher the hot water temperature you demand (55 °C rather than 35 °C), the lower the COP drops. Well-tuned controls, low-temperature emitters and correct sizing help keep a high COP.

Defrost cycles, short cycling, backup heating: the losses that don't show up on paper

On site, defrost cycles, frequent restarts and backup heating (electric or boiler) eat into performance. Add the consumption of auxiliaries (pumps, fans). Hence the value of thinking in real-world conditions, not a single figure.

Partial load: the most common situation on site

Why a heat pump often runs at 20–60% of its rated output

A heat pump is sized for the coldest peak. The rest of the time, especially in mid-season, needs drop off. Solar gains, occupants, building thermal mass, partial retrofit. As a result, the machine often settles between 20 and 60% to hold the setpoint without overheating.

Inverter, modulation, heating curve: what improves (or worsens) COP at partial load

Inverter-driven controls modulate the compressor and limit restarts. With a well-tuned heating curve, the water temperature drops when it's milder outside. The temperature gap narrows and COP improves. Conversely, a setpoint that's too high, poorly balanced flow rates or on/off cycling drag performance down.

Oversizing: the trap that lowers real-world performance

Too many kW usually means short cycling, more defrost cycles and rising consumption. The heat pump doesn't stay long enough in its efficient zone. It's better to rely on calculated heat losses, check the minimum modulating output and, if needed, add buffer volume rather than oversize.

Reading manufacturer data to estimate real performance at partial load

Performance curves and test points: how to identify the measurement conditions

For a heat pump, a curve only has value if the test points are clear. Identify the standard used (often EN 14511 for instantaneous points, EN 14825 for the seasonal figure). Check the air/water or air/air pairs shown. A common example for air-to-water: A7/W35, A2/W35, even A-7/W35. Also look at the flow rate, the 35 °C or 55 °C regime, and whether electric backup is counted.

COP at partial load vs real-world efficiency: the right analysis habits

A COP at 30% or 50% load is reassuring, but it doesn't tell the whole story. The declared SCOP better summarises actual use, as long as you read the climate zone and the chosen flow temperature. Check the modulation range (to limit cycling), the auxiliary consumption (pumps, standby), and the impact of defrosting in a cold climate. To go further on the calculation, see how a heat pump's SCOP is estimated.

Comparing two heat pumps correctly: same emitters, same regime, same assumptions

To compare two models, align your assumptions. Same type of emitters, same heating curve, same flow temperature. And the same need at the design point. Otherwise, you're comparing numbers, not projects.

• Compare W35 models against each other, and W55 models against each other.
• Check the cold-weather output (A-7) and the bivalent point.
• Keep the same share of domestic hot water in the calculation.

A field method to secure performance in 2026

Sizing: heat losses, emitters, low-temperature regime, a reasonable margin

A heat pump only performs on paper if the sizing is shaky. Start from a heat-loss calculation, check the emitter output, then aim for a consistent low-temperature regime. Keep a reasonable margin, not the oversizing that causes stop-start operation.

Settings and commissioning: heating curve, hysteresis, flow rate, balancing, pipe insulation

On the day, secure the commissioning. Set the heating curve, adjust the hysteresis, check the flow rate and the balancing. Also address pipe insulation to avoid heating crawl spaces.

Monitoring and follow-up: energy meter, readings, client feedback, corrective actions

Install an energy meter where possible. Take simple readings, gather client feedback and schedule visits if consumption drifts. A small, quick fix beats heavy corrective actions.

Impact on your quotes and on client satisfaction

Explaining it simply to the client: "catalogue" COP vs COP at partial load

For a heat pump, the "catalogue" COP often corresponds to one precise test point. In the field, performance varies with outdoor temperature and the required water temperature. Explain that partial load can improve efficiency, but only if the machine is properly sized and the emitters keep up.

Pricing consumption and savings: weather scenarios, usage, temperature setpoints

To avoid disappointment, present 2 or 3 scenarios. Mild, average, cold winter. Setpoint of 19 or 21 °C. Occupied during the day or not. With these assumptions, you can produce an estimate in kWh and euros, and show where the real gain lies without overselling.

Funding and requirements: 2026 watch points (RGE, supporting documents, consistency between study and site)

In 2026, client satisfaction also depends on a clean file. Suitable RGE certification (including QualiPAC), a detailed quote, product references, proof of performance, and end-of-job photos if requested. Keep total consistency between the study, the quote and the equipment installed, otherwise the funding can be challenged. To secure the administrative side, also see securing your process and avoiding disputes.