Understanding heat pump weather compensation without staying theoretical
What the heating curve actually does: outdoor temperature → heating flow temperature link
On a heat pump, the heating curve sets the target flow temperature from the outdoor temperature. In practice, the control calculates a flow setpoint and modulates the compressor and circulator to hold it, rather than waiting for a thermostat to cut out. That is the core of a well-tuned heat pump weather compensation setup: stable and with little cycling.
The 2 settings you need to know on the control: slope and parallel shift (offset)
The slope defines how quickly the flow setpoint rises as outdoor temperature falls. The parallel shift moves the whole curve without changing its shape. During commissioning, correct the offset first if the error is uniform, then adjust the slope if the fault only appears in deep cold or in mid-season.
What you must check before touching the curve: sensors, flow rates, balancing, hysteresis
- Sensors: consistency of outdoor, flow and return temperatures. Location, insulation, calibration.
- Flow rates: filters, air removal, circulator. Flow/return ΔT consistent with the emitter.
- Balancing: valves, manifolds, pressure losses, end-of-loop emitters.
- Hysteresis: dead band and time delays, to avoid hunting and short restarts.
Choosing a coherent heat pump heating curve based on your emitters and the building fabric
High-temperature radiators vs low-temperature radiators: direct impact on the curve
| Emitter | Typical flow temperature in design cold weather (°C) | Curve setup |
|---|---|---|
| High-temperature radiators | 55 to 70 | Steeper slope, higher base |
| Low-temperature radiators | 35 to 50 | Moderate slope, lower base |
On a heat pump weather compensation setup, the emitter sets the realistic flow-temperature ceiling. If you target a flow temperature that is too high for an undersized radiator, you increase the slope, lose COP, and end up relying on backup heat.
Underfloor heating: limiting flow temperature and protecting comfort (and the screed)
Aim for a low, stable flow temperature. In practice, 25 to 35 °C covers most cases. Fit a flow-temperature limiter (control or mixing valve) and keep a low slope; otherwise you will quickly overheat the screed’s thermal mass and the system will start hunting.
Insulation level and thermal inertia: how they change slope and responsiveness
The better insulated the building, the flatter the curve can be. You reduce the slope and fine-tune the base of the curve. In a lightweight house, make small corrections and record your readings (outdoor temperature, flow temperature, return temperature, room temperature) over 24 hours before touching the slope. For more detail, see the heating curve as a function of outdoor temperature.
Heat pump weather-compensation setup: step-by-step commissioning procedure on the control
Quick starting point: the manufacturer’s “standard” curve + flow-temperature limits
Start from the manufacturer’s “standard” curve. For heat pump weather compensation, set the limits straight away. Set a max flow temperature that matches the emitter (underfloor heating, radiators) and a minimum flow temperature that prevents short cycling in mid-season. Also check that the outdoor sensor is enabled and that there is no parasitic compensation from an overly aggressive room setpoint.
Adjustment method over 48 to 72 hours: one change at a time, at the right time
Work with two levers. First the slope, to correct cold-weather performance. Then the parallel shift, to reset the overall level. Change only one parameter at a time, then let the system run for 48 to 72 hours with comparable weather. Make changes during stable conditions, outside domestic hot water recovery periods, and monitor compressor cycles and run time.
How to log your trials (2026): outdoor temp, flow temp, room temp, cycles and customer feedback
Keep a single log sheet. Record date, time, outdoor temperature (sensor), target and measured flow temperature, return temperature, room temperature (one reference point), number of starts, run time, events (defrost, domestic hot water). Add a daily note of customer feedback (too hot, too cold) without changing the setpoint on the fly.
Correcting an installation that is overheating: diagnose first, then adjust the curve
Site symptoms: rooms too warm, surging, short cycling, water noise
On site, overheating shows up as a flow temperature that stays too high for the current outdoor temperature. Record outdoor temperature, flow temperature, return temperature, pressure, circulator speed, emitter condition, and start frequency. Surging and water noise often point to a control setting that is too “high” or unstable flow in heat pump weather compensation.
Priority actions: lower the parallel shift before reducing the slope
Start by lowering the parallel shift. You move the whole curve down without changing how it reacts to cold weather. Wait for stabilisation before touching the slope, otherwise you stack two effects and lose track of the diagnosis.
| Setting | Change per trial (unit) | Wait before re-measurement (h) |
|---|---|---|
| Parallel shift | -1 to -2 (K) | 12 to 24 |
| Slope | -0.05 to -0.10 | 24 |
| Anti-cycle delay | +5 to +10 (min) | 0 to 1 |
Common cases: oversized radiators, closed thermostatic valves, flow too low
With oversized radiators, keep a moderate slope and look for the lowest stable flow temperature. With thermostatic valves closed, leave one “reference” circuit open, or a bypass, otherwise the heat pump will cycle. With flow too low, check the filter, venting, isolation valves and balancing, then verify the flow rate via the flow/return ΔT. A very high ΔT often indicates insufficient flow.
Correcting an installation that is not reaching temperature: restore comfort without killing COP
Identify whether the issue comes from the heat pump weather-compensation curve or from a limit (max flow temp, power curve)
Start by getting objective data. Compare the flow temperature requested by the controller and the temperature actually achieved. If the calculated setpoint rises but the heat pump tops out, look for an active limit. Max flow temperature set too low, power curve limiting compressor output at low outdoor temperatures, or insufficient flow causing the delta T to rise and triggering safety cut-outs.
- Record over 30 minutes. Outdoor temperature, flow, return, compressor %.
- Check whether the demand matches the heat pump weather-compensation curve.
Raising the slope or the offset: choosing the right lever based on weather and inertia
If the lack of heat only appears when it is cold, adjust the slope. If it is constant, work on the offset. Make short changes, then let the system stabilise according to its inertia. Underfloor heating, be patient. Radiators, response is quicker. Keep the objective in mind: the lowest possible flow temperature to preserve the real COP.
Other points to check in parallel: defrost, backup heat, DHW curve, scheduling and setpoint
Before blaming the curve, check what is stealing kilowatts. Too-frequent defrost cycles, domestic hot water priority that stretches heating phases, backup heat disabled or, conversely, overactive, and time schedules that drop the setpoint at the wrong time. Also validate the room sensor, outdoor sensor and reduced-mode setpoints.
