How to Set a Heat Pump’s Weather Compensation Curve: An Installer’s Guide
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Stefano Fonseca is an energy and environment engineer with over six years of experience in technical building equipment (TGA). He combines technical expertise with a passion for understandable communication. For more than five years, he has been writing as a freelance editor about renewable energy and sustainable living, in particular about photovoltaics and heat pumps.
For any UK installer aiming to deliver an efficient heat pump system, mastering the concept of the heating curve—more commonly known in the UK as weather compensation—is non-negotiable. This crucial setting directly impacts both comfort and running costs. This guide explains why correctly setting the weather compensation curve is far more than a minor technical adjustment and how it is mandated by UK standards.
What is a Weather Compensation Curve in a Heat Pump?
The weather compensation curve is a pre-defined relationship that dictates the required flow temperature of the heating system in response to changes in the outdoor temperature. It is the core intelligence of a modern, efficient heat pump system.
When the outdoor temperature drops, the building loses heat more quickly, so the system must deliver water at a higher temperature to the radiators or underfloor heating to compensate. Conversely, when it is milder outside, a lower flow temperature is sufficient. The weather compensation function automates this process, ensuring the heat pump consistently and efficiently matches its output to the building's real-time heat demand.
Why is the Weather Compensation Curve so Important?
Correctly setting the weather compensation curve is critical because it directly governs the heat pump's efficiency (Coefficient of Performance, or CoP). Heat pumps operate most efficiently when they can produce heat at the lowest possible flow temperature. By continuously adjusting the flow temperature down to the minimum required, the curve ensures the heat pump runs for longer, more stable periods rather than inefficiently cycling on and off.
An incorrectly set curve has immediate negative consequences:
- Curve set too high or too steep: The heat pump will produce unnecessarily high flow temperatures, wasting energy and increasing running costs. This can also lead to the system overshooting the desired room temperature.
- Curve set too low or too flat: The system will fail to provide enough heat during cold weather, leading to uncomfortable rooms and dissatisfied customers.
In the UK, compliance is also a key factor. Both the Microgeneration Certification Scheme (MCS) and Part L of the Building Regulations mandate the use of weather compensation controls for heat pump installations to ensure they meet minimum energy efficiency standards.
How Does the Weather Compensation Curve Work?
The function relies on an outdoor temperature sensor, which is either integrated into the outdoor unit or mounted on a north-facing wall. This sensor constantly feeds temperature data to the heat pump's controller, which then adjusts the flow temperature according to the programmed curve.
Two main parameters define the curve:
- Gradient (or Slope): This determines how significantly the flow temperature reacts to a change in the outdoor temperature. A steep curve means the flow temperature will rise sharply as it gets colder outside, which is typical for poorly insulated buildings with smaller radiators. A flat curve is suitable for well-insulated properties with large emitters (like underfloor heating), where only a small increase in flow temperature is needed.
- Offset (or Parallel Shift): This raises or lowers the entire curve without changing its gradient. It is used to adjust the general temperature level across all conditions. If a property is consistently too cool, even in mild weather, the curve would be shifted upwards.
What Does a Typical Heating Curve Look Like?
A weather compensation curve is a rising line on a graph where the horizontal axis represents the outdoor temperature and the vertical axis represents the flow temperature. The curve starts with a low flow temperature at mild outdoor temperatures and rises as the outdoor temperature falls.
The ideal gradient for a heat pump in the UK typically ranges from 0.4 to 1.2, depending on the building's characteristics.
These values are starting points. The final setting must be based on a detailed heat loss calculation and the specific design of the heating system.
Who Sets the Weather Compensation Curve?
The initial setup of the weather compensation curve is the responsibility of the MCS certified installer during the commissioning phase. The installer will select a starting curve based on the property's heat loss calculation, the type of heat emitters, and the design flow temperature.
However, it is common for fine-tuning to be required after the initial setup. Homeowners can be guided on how to make small adjustments themselves, but it is crucial that they understand the principles and make changes systematically. For any significant issues, the installer should be consulted.
How to Set the Weather Compensation Curve on a Heat Pump
Setting the curve is a systematic process of observation and adjustment. The goal is to find the lowest possible curve that still provides adequate comfort in all conditions.
- Establish Baseline Settings: Record the initial gradient and offset set during commissioning.
- Ensure Full System Operation: Make sure all TRVs (Thermostatic Radiator Valves) are fully open in the main living areas to allow the weather compensation to take full control.
- Adjust the Gradient in Cold Weather: If the property feels cool when the outdoor temperature is low (e.g., below 5°C), the gradient is likely too flat. Increase it in small increments (e.g., by 0.1).
- Adjust the Offset in Mild Weather: If the property feels too warm when it is mild outside (e.g., above 10°C), the entire curve is likely too high. Lower the curve using the parallel shift/offset function.
- Observe and Wait: After making an adjustment, wait at least 24 hours to allow the system to stabilise and to experience different conditions before making further changes.
- Document Changes: Keep a log of all adjustments and the corresponding indoor and outdoor temperatures to track progress.
How to Recognise the Correct Setting
The optimal setting is achieved when:
- All rooms consistently maintain their target temperatures, even as the weather changes.
- The flow temperature remains as low as possible for the given outdoor conditions.
- The heat pump runs for long, stable cycles without frequent starting and stopping (cycling).
- The system operates efficiently, reflected in a good seasonal coefficient of performance (SCoP) and reasonable running costs.
Optimising the Curve: Practical Tips for Installers
- Educate the Customer: Explain the purpose of weather compensation and how it works. Provide simple instructions for minor adjustments they can make.
- Set the Right Expectations: Inform the customer that fine-tuning is a normal part of the process and may take a few weeks during the first heating season.
- Check the Fundamentals: Before adjusting the curve, ensure the system has been properly balanced and that all TRVs are functioning correctly.
- Use Manufacturer Guidance: Always refer to the manufacturer's specific instructions, as the terminology and adjustment methods can vary between brands.
- Review at Annual Service: Use the annual service visit as an opportunity to review the curve settings with the customer and make any necessary adjustments.
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