Heat Pump Weather Compensation: How to Set the Heating Curve

Introduction

A heat pump designed for 45 °C flow on the coldest day does not need 45 °C on a mild October afternoon. Weather compensation lets the controller lower the flow temperature as the outdoor temperature rises, which may help efficiency because the heat pump works against a smaller temperature lift for most of the season.

The benefit depends on one setting that is often left at factory default: the heating curve. Set too high, the system may overheat rooms and lose efficiency. Set too low, rooms may struggle on cold days and users turn the curve up by guesswork.

This guide is for installers, commissioning engineers and technically minded owners. It explains how a heating curve works, how to set a starting curve from the design point, and how to adjust it step by step on site. Controllers differ in how they define slope, offset and curve number, so the values here are illustrative and should be checked against the manufacturer's instructions.

What is weather compensation?

Weather compensation is a control strategy that sets the target flow temperature from the outdoor temperature. An outdoor sensor feeds the controller, and the controller reads the target flow temperature off a heating curve.

The logic follows heat loss. A building loses heat roughly in proportion to the difference between indoor and outdoor temperature. When it is colder outside, the emitters need to deliver more heat, so they need warmer water. When it is milder, less heat is needed and cooler water may be enough.

Compared with fixed-flow-temperature control, weather compensation can:

- reduce the average flow temperature across the season, which typically improves heat pump COP

- give steadier room temperatures, because output follows demand more closely

- reduce on/off cycling caused by water that is hotter than the load requires

Why the design flow temperature matters so much for efficiency is covered in [45 °C vs 55 °C vs 65 °C: what flow temperature should your heat pump run at?](https://nordthermglobal.com/blog/45-c-vs-55-c-vs-65-c-what-flow-temperature-should-your-heat-pump-run-at) This article focuses on how to set the curve.

How a heating curve works

A heating curve is a line that links outdoor temperature to target flow temperature. Most controllers describe it with four elements.

- **Design point.** The flow temperature needed at the local design outdoor temperature, the coldest condition the system is sized for. It anchors the cold end of the curve.

- **Slope (gradient).** How much the flow temperature rises for each degree the outdoor temperature falls. A steeper slope adds more heat as it gets colder.

- **Offset (parallel shift).** Moves the whole curve up or down by the same amount at every outdoor temperature.

- **Limits.** A maximum flow temperature protects the emitters, the floor or the heat pump's operating envelope. A minimum, and often a heating cut-off outdoor temperature, stop the system running when no heating is needed.

*Illustrative values for explanation only: linear curves from the design point to a 20 °C warm-end reference point.*

The two lines show why emitter type matters: both reach their design point on the coldest day, but radiators need a higher, steeper curve than underfloor heating. Some controllers use a curve number, others a ratio or two set points, so the same physical curve may be entered in different ways.

How to set the initial heating curve

A good starting curve comes from the system design, not from the factory default. It needs two points and a set of limits.

1. **Confirm the design outdoor temperature.** Use the value from the heat loss calculation, normally taken from the national design data for the site (for example, under EN 12831 and its national annex).

2. **Confirm the design flow temperature.** This is the flow temperature at which the emitters meet the room heat loss on the design day. The worst room sets it: if one room needs 50 °C while the rest need 45 °C, the curve must serve that room or the room's emitter must be upgraded. Our guide [R290 heat pump vs gas boiler: can you keep your radiators?](https://nordthermglobal.com/blog/r290-heat-pump-replace-gas-boiler-keep-radiators) shows how this is worked out.

3. **Set the warm end.** Many controllers anchor the warm end of the curve at a reference point near the room set point. Real buildings usually reach zero heating demand somewhat below the set point, because internal and solar gains cover part of the heat loss, and heating normally stops at the heating cut-off temperature. The controller's minimum flow temperature and compressor operating limits also apply, so set the warm end with the cut-off temperature, emitter requirements and these limits in mind.

4. **Translate the two points into the controller's terms.** Depending on the controller, this may be a slope and offset, a curve number, or two set points entered directly.

5. **Set the limits.** A maximum flow temperature that respects the emitter and floor manufacturers' limits, and a heating cut-off outdoor temperature (often in the mid-teens °C, depending on the building).

As an illustration, take a radiator system needing 50 °C at −10 °C, with the curve drawn towards 25 °C at an illustrative warm-end reference point of 20 °C outdoor. The slope is roughly:

```latex

\frac{50 - 25}{20 - (-10)} \approx 0.83\ \text{K per K}

```

| Outdoor temperature | Illustrative target flow temperature |

| --- | --- |

| −10 °C | 50.0 °C |

| −5 °C | 45.8 °C |

| 0 °C | 41.7 °C |

| 5 °C | 37.5 °C |

| 10 °C | 33.3 °C |

| 15 °C | 29.2 °C |

Values are linear interpolation between the two example points, not settings for any specific controller.

These numbers are only an example. Real emitter output is not linear with temperature, so many controllers apply a slightly curved line rather than a straight one.

One common habit is worth avoiding: adding a few degrees "just in case" at the start. Every degree above what the building needs typically costs efficiency for the whole season. It is usually better to start at the calculated curve and adjust from observed results.

Radiators vs underfloor heating: how the curve differs

The emitter type changes the shape of the curve more than any other single factor.

| | Radiators | Underfloor heating (screed) |

| --- | --- | --- |

| Typical curve | Higher and steeper | Lower and flatter |

| Why | Smaller heat-emitting surface needs warmer water to deliver the same output | Large surface delivers output at low water temperature |

| Response to a change | Hours | Often 24 hours or more, due to screed thermal mass |

| Main limit to watch | Worst-room radiator output | Maximum floor surface temperature and covering limits |

In mixed-emitter systems, the heat pump's primary flow temperature may need to serve the higher-temperature circuit, while a separately controlled mixing circuit can supply lower-temperature water to the underfloor heating, sometimes on its own compensation curve. The actual arrangement depends on the hydraulic design and control strategy. Specific flow temperature ranges for each emitter are covered in the [45 °C vs 55 °C vs 65 °C guide](https://nordthermglobal.com/blog/45-c-vs-55-c-vs-65-c-what-flow-temperature-should-your-heat-pump-run-at)

Step-by-step commissioning

The initial curve is a calculated starting point. Commissioning turns it into a curve that fits the real building. The routine below is general practice; follow the controller manufacturer's procedure where it differs.

1. **Prepare the system.** Confirm the system is filled, vented and balanced. Open thermostatic radiator valves (TRVs) fully in the main living areas, or at least in a chosen reference room. A partly closed TRV or an unbalanced circuit can look like a curve problem when it is not.

2. **Pick a reference room.** Choose a room with no strong extra heat sources (fireplace, large south glazing, cooking) and no unusual heat loss. Its temperature is your main feedback.

3. **Run on the starting curve and record.** For each day, log outdoor temperature, target and actual flow temperature, and reference room temperature at a similar time. Many controllers store this data.

4. **Compare across outdoor conditions.** A single day tells you little. Look for a pattern: is the room too cold or too warm on cold days, mild days, or both? The next section shows how to read the pattern.

5. **Change one setting at a time, in small steps.** Adjust slope or offset, not both, by a small amount (often 1–2 K equivalent at the affected end).

6. **Wait before judging.** Radiator systems may settle within hours. Underfloor heating in screed may need 24–48 hours or longer. Adjusting several times a day means chasing the system's own lag.

7. **Repeat across the season.** A curve can only be checked at the outdoor temperatures that actually occur. A curve tuned in autumn should be reviewed during the first cold spell.

8. **Record the final settings.** Note the curve values, limits and date in the commissioning record so later service visits start from the known state.

Common heating curve problems and how to correct them

The pattern of the problem across outdoor temperatures tells you which setting to change. Raising or lowering the whole curve only fixes problems that occur at every outdoor temperature.

Heating curve adjustments are controller-specific. Depending on the controller's reference point, changing the slope may move both the cold and the warm end. Before applying a change, check the resulting target flow temperature at two outdoor conditions, one cold and one mild. The table below describes the intended adjustment; how it is entered (slope, offset or two set points) should follow the controller manufacturer's instructions.

| Symptom | Likely cause | Typical adjustment |

| --- | --- | --- |

| Too cold at all outdoor temperatures | Curve too low overall | Raise the offset |

| Too warm at all outdoor temperatures | Curve too high overall | Lower the offset |

| Too cold on cold days, fine on mild days | Slope too flat | Increase the slope |

| Too warm on cold days, fine on mild days | Slope too steep | Reduce the slope |

| Too warm on mild days, fine on cold days | Warm end too high | Lower the warm-end target while keeping the cold design point |

| Too cold on mild days, fine on cold days | Warm end too low | Raise the warm-end target while keeping the cold design point |

| One room cold, others fine | Emitter, balancing or TRV issue | Check that room before touching the curve |

Two problems are often blamed on the curve when the cause lies elsewhere.

**Room temperature swings.** These may come from a room thermostat or room-influence setting acting too strongly, from large solar or internal gains, or from a flow temperature well above need. Check the control interaction before changing the curve.

**Frequent on/off cycling in mild weather.** A curve that is too high can add to cycling. A common cause is that the building's heat demand falls below the heat pump's minimum modulating capacity, which no curve setting can fully remove. Insufficient system water volume or flow, narrow control hysteresis, and frequent thermostat or zone-valve interruptions can also contribute, so the cause should be identified before choosing a fix. Where system volume is the issue, see [do you need a buffer tank with an air-to-water heat pump?](https://nordthermglobal.com/blog/do-you-need-a-buffer-tank-with-an-air-to-water-heat-pump)

Weather compensation and room thermostats: working together

Weather compensation and room control answer different questions. The curve decides how warm the water should be. Room control decides whether, and how much, a room needs heat right now. They work best as partners, not alternatives.

- **Outdoor sensor only.** The curve does all the work. This can give very steady operation in a well-balanced building, but it cannot react to solar gains, occupancy or open windows.

- **Room influence (load compensation).** Many controllers let a room sensor nudge the target flow temperature up or down. Used moderately, it corrects for gains the outdoor sensor cannot see. Set too strongly, it can make flow temperature swing and mask a poorly set curve.

- **Room thermostat as on/off.** A thermostat that switches the heat pump off when a room reaches set point may cause cycling and pull the system away from steady low-temperature operation. Where used, it is usually set as a high limit a little above the comfort temperature, so the curve does the day-to-day work.

- **TRVs and zone valves.** These can fight the curve. If the reference room's TRV is turned down, the room may stay cool even with a correct curve, which can lead to the curve being raised for the whole house.

A useful rule during commissioning: tune the curve first with room influence low or off, then add room control as fine adjustment.

FAQ

**Does weather compensation save energy?** It can, because it lowers the average flow temperature across the heating season, and heat pump efficiency generally improves at lower flow temperature. The size of the saving depends on the building, the emitters and how well the curve is set.

**What heating curve setting should I use?** There is no universal value. The right curve depends on the design heat loss, emitter sizing and design outdoor temperature, and controllers define slope and offset differently. Start from the design point and adjust from observed room temperatures.

**How long should I wait between curve adjustments?** For radiator systems, often several hours to a day. For underfloor heating in screed, often 24–48 hours or more. One small change at a time gives a clear result.

**Should I adjust the slope or the offset?** If the room is wrong by about the same amount in all weather, adjust the offset. If the error grows as it gets colder, or appears only in mild weather, adjust the slope or the warm end.

**Can I use weather compensation with radiators?** Yes. Radiator systems typically need a higher, steeper curve than underfloor heating, and the worst-performing room sets the design point. Check that each room's radiator can meet its heat loss at the chosen design flow temperature.

**Why does my heat pump still cycle with weather compensation?** In mild weather the heat demand may fall below the unit's minimum modulating capacity, which the curve cannot fully solve. Low system volume or flow, narrow control hysteresis, and thermostats or zone valves switching off the circuit can also cause cycling.

Continue exploring

**Related guides**

- [45 °C vs 55 °C vs 65 °C: what flow temperature should your heat pump run at?](https://nordthermglobal.com/blog/45-c-vs-55-c-vs-65-c-what-flow-temperature-should-your-heat-pump-run-at)

- [R290 heat pump vs gas boiler: can you keep your radiators?](https://nordthermglobal.com/blog/r290-heat-pump-vs-gas-boiler-can-you-keep-your-radiators)

- [Do you need a buffer tank with an air-to-water heat pump?](https://nordthermglobal.com/blog/do-you-need-a-buffer-tank-with-an-air-to-water-heat-pump)

- [Underfloor heating under tiles vs wood vs carpet](https://nordthermglobal.com/blog/underfloor-heating-under-tiles-vs-wood-vs-carpet-which-flooring-works-best)

**Products**

- [R290 air-to-water split heat pump for heating, cooling and hot water](https://nordthermglobal.com/products/r290-air-to-water-split-heat-pump-for-heating-cooling-and-hot-water)

**Planning a heat pump system?**

Share your design heat loss, emitter type, target flow temperature and design outdoor temperature. We can help you identify suitable heat pump specifications and discuss the control features available for your project. \[Contact us\+86-13727963335,Email:[email protected]] (routing: Hazel)