What is Weather Compensation and is it Important?

It appears that several owners of air source heat pumps (ASHPs), specifically those with Grant Aerona units, have realised that their weather compensation was not activated during the installation and commissioning process. This raises the question: is this significant? I aim to explain what weather compensation entails, its operational mechanism and the reasons for its activation when feasible. Although the primary focus is on ASHPs, the information is equally pertinent to GSHPs and boilers.

Weather compensation is defined by the Microgeneration Certification Scheme (MCS) as ‘Ambient Air Temperature Load Correction’. This process seeks to enhance the heat pump’s efficiency during different seasons by acknowledging that the output of most heat emitters does not need to be at its highest in low load conditions. Consequently, the flow temperature can be lowered, typically leading to increased unit efficiency.

However, the concept is not straightforward. Weather Compensation adjusts the heating flow temperature in accordance with the estimated heat loss from the house as the outdoor temperature changes. When the external temperature drops, the heating flow temperature rises, and conversely, it decreases when the external temperature climbs.

Is it essential to enable Weather Compensation?

The short answer is yes, unless there are valid reasons against it. The MCS guidelines are somewhat vague, stopping short of mandating the activation of Weather Compensation upon installation. They suggest that it should be activated barring significant reasons. For most heat pump systems, Weather Compensation is an adjustable feature through the system controller, with the control parameters subject to fine-tuning. The default settings provided by the manufacturer are typically a good starting point. However, optimising weather compensation usually involves some trial and error, as no two houses share identical heating space load, desired indoor temperature or thermal response and performance.

What occurs when Weather Compensation is active?

With rising outdoor temperatures, the leaving water temperature (LWT) from the heat source (assuming an ASHP) decreases, and the opposite happens when it’s cooler outside. This results in an automatic, self-regulating reduction in the ASHP’s heating output. In homes with radiator emitters, the radiators will feel cooler when it’s warmer outside, but not cold.

Graphic from MCS ‘Domestic Heat Pumps – Best Practice Guide’ – typical weather compensation control

If Weather Compensation is not activated, the heat source (again, assuming an ASHP) will strive to reach a preset maximum LWT, regardless of the outdoor temperature. This might seem beneficial, as the radiators will feel warmer, potentially heating a below-par indoor space more rapidly. However, there’s a significant drawback. The ASHP will exert more effort to heat the LWT to the maximum temperature, thus consuming more electricity. The efficiency of a refrigerant-based heat pump largely hinges on the delta between the LWT and the outdoor air, which approximately correlates to the refrigerant’s evaporating and condensing temperatures and pressures. The smaller this delta, the more efficient and cost-effective the ASHP operation.

Will a heat pump without Weather Compensation be more expensive to operate?

Generally, yes. During winter’s peak, the savings might be negligible. An ASHP, for instance, will work intensively to provide a LWT close to the system’s maximum design temperature, set to counterbalance the maximum heat loss in winter. However, during spring and autumn, the ASHP could operate at a lower output, or modulate, with a reduced LWT. ASHPs excel in modulation, a design feature. The variation between the maximum winter LWT and the minimum LWT in milder seasons can be up to 20°C for systems with radiator emitters. For underfloor heating systems, this difference is usually less, as they are designed for a lower maximum LWT of about 35°C. A commonly used industry benchmark suggests that every 1°C rise in LWT increases electricity consumption by approximately 2.5%. As the flow temperature escalates, the heat pump’s efficiency diminishes.

Graphic from MCS ‘Domestic Heat Pumps – Best Practice Guide’ – SCoP vs Flow Temperature (LWT)

Manufacturers present efficiency in terms of Seasonal Co-Efficient of Performance (SCoP). These estimates assume a weather-compensated, modulated seasonal output with a lower LWT during milder months. Most published SCoPs tend to be optimistic, but a system running without Weather Compensation will experience a yearly reduction in SCoP, as the heat pump often operates at a higher-than-necessary flow temperature.

What determines the maximum system LWT?

Heating systems are typically sized following industry guidelines, such as those from CIBSE, incorporated into the UK Building Regulations Part L standards, NHBC standards and others. In mainland UK, the heating load required for a house is calculated based on a minimum winter temperature ranging from -1°C in Southern England to -5°C in Scotland and The Isles, with -3°C being the average often cited in heat loss calculations. New houses must have a heat source with a maximum heating design temperature not exceeding 55°C. However, for all heating systems, especially heat pumps, a lower design temperature is preferable. The capacity of the emitters (radiators, underfloor heating pipes, etc.) primarily dictates the maximum LWT. Larger emitters allow for a lower design temperature, but considerations like available space and the cost of upgrading emitters are factors in retrofit conversions.

Why wasn’t Weather Compensation enabled by my installer?

It may have been an oversight in the rush of commissioning, or a more cynical view suggests it could be an intentional act to make the system reach the maximum design LWT immediately, regardless of outdoor temperature. This prevents homeowners from querying why their radiators are not as hot as they were with a boiler. Effective homeowner hand-over is crucial to explain the differences between high and lower temperature renewable heating systems.

Another reason might be that Weather Compensation is rarely optimal ‘out of the box’. Every home is different, requiring adjustments to the WC parameters to suit the house during the first heating season. A too high maximum LWT reduces system efficiency, while a too low one may not achieve target room temperatures.

The Grant Aerona control panel has a weather compensation setting parameter 21 00 – ‘1’ indicates WC is enabled. Is yours activated?

Who is responsible for fine-tuning Weather Compensation?

Ideally, in this era, it should be entirely automatic and self-adjusting. However, most heat pumps come with less sophisticated controls. Third-party intelligent control systems like Homely add some smartness to the process, albeit at an additional cost over a basic ASHP control.

The homeowner is best suited for fine-tuning, as they experience the house’s climate firsthand. They can judge whether adjustments to the minimum LWT and coinciding maximum outdoor temperature settings are needed if it’s too warm in milder conditions, or adjustments to the maximum LWT and coinciding minimum outdoor temperature settings if it’s too cold in winter.

Reducing the maximum LWT can lead to significant energy and cost savings. Heat loss calculations often overestimate, and the system design temperature/maximum LWT is likely set higher than necessary. Lowering the maximum flow rate by several degrees Celsius might not affect comfort and room temperature but can save about 2.5% in electricity consumption for every 1°C reduction. Thus, a decrease of just 4°C could result in approximately 10% savings in heat pump running costs.