ENERGY EFFICIENCY


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Humidity: The overlooked factor affecting UK heat pump performance


Despite growing awareness and deployment of heat pump technology, conversations around heat pump performance still frequently focus on temperature concerns. Aimee Holloran, business development manager – heating, at Samsung Climate Solutions explains why humidity deserves more attention because of its greater impact on heat pump performance


H


eat pumps are proven to operate well in colder temperatures. In fact, some of Europe’s highest rates of heat pump adoption


can be found in Norway, Sweden, Finland and Estonia, where winter temperatures regularly fall below those experienced across much of the UK. However, while we can look to the likes of Norway


and Sweden as proof that heat pumps work in cold climates, we also need to factor in how Britain’s climate differs. As an island nation, we typically experience humidity levels between 70% to 90% throughout the year. As heat pumps extract energy from the air,


vapour accumulates on the evaporator coil. During colder spells, this can cause it to freeze, prompting the system to enter a defrost cycle so that normal operation can continue. While defrosting is a normal and necessary process for heat pumps, it’s also energy intensive and temporarily reduces heat output to the building. Therefore, the more a unit needs to defrost, the greater the impact on occupant comfort, energy consumption and overall efficiency.


The impact of positioning


One of the most overlooked considerations in UK heat pump design is airflow around the outdoor unit. If a heat pump is positioned incorrectly or installed without sufficient clearance, a localised microclimate can develop around the unit. This causes it to operate in conditions similar to a refrigerator, where cold, moist air becomes trapped around the evaporator, increasing frost formation and preventing the unit from dispensing vapour and condensation.


Get this right and the heat pump can maintain stable airflow, reduce unnecessary defrost cycles and operate far more efficiently through colder conditions. Therefore, it’s essential for designers and


installers to pay close attention to manufacturer guidance around clearances and airflow, factoring them into design from the off and treating them as a requirement rather than a recommendation.


How water volume shapes performance


Greater consideration also needs to be given to water volume. During a defrost cycle, a heat pump draws stored thermal energy from the heating system, and without sufficient water volume, the unit may struggle to complete this process effectively. In some cases, the unit enters a constant defrost cycle that decreases efficiency and limits the system’s ability to provide consistent heat to the property.


Manufacturers already specify a minimum


water volume to support defrost operation, but increasingly, the industry is looking beyond minimum requirements and focusing on what is known as ‘active water volume’. Active water levels ensure there is enough thermal mass within the system for efficient operation and to avoid excessive cycling, typically by limiting compressor start-stops to around four per hour. Frequent starts and stops can reduce efficiency and


place additional strain on key components, whereas longer run times help to deliver the performance and efficiencies that the system was designed to achieve.


Built in resilience


Most heat pumps contain logic that calls upon backup heat under certain operating conditions. These include compressor protection, heating assistance - and crucially in the context of humidity challenges - defrost support. If a heat pump is struggling to complete a defrost cycle, a backup heater can help the system return to normal operation more effectively. Supplementary freeze protection can also include


glycol and antifreeze valves. Ultimately, the right choice depends on the system design and how it will operate in practice.


Designing for the realities of Britain’s climate


As heat pump deployment accelerates, the conversation must move beyond whether the technology works and focus instead on how systems are designed, installed and optimised to perform in Britain’s unique climate conditions. Heat pumps are highly capable technologies, but


like any heating system, they perform best when they’re designed and installed correctly. This requires greater consideration of the interaction between climate, airflow, hydraulics, water volume, controls and operational behaviour from the earliest stages of a project. As MCS reviews MIS 3005 and the industry


continues to refine best practice, there is a valuable opportunity to broaden our understanding of how environmental conditions influence heat pump performance.


18 October 2026


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