Sizing heat pumps
Heat pumps have become the norm in heating system design, offering a low-carbon alternative to gas boilers. However, without careful attention to how they are designed, they can lead to poor performance and increased energy costs for tenants.
We have been working with the Department for Education (DfE) on their updated Output Specification. The previous iteration ensured all new schools are designed to be net zero carbon and preferably use ground source heat pumps (GSHPs) or air source heat pumps (ASHPs) for heat generation.
When reviewing some of these schemes, we often see that the heat pump performance is poorer than predicted. Our research has identified one reason for this: the way these systems are sized.
It’s common within the built environment to size heat pump systems using the same approach applied to gas boilers. This is typically done with little consideration for whether the heat source is oversized or by only considering fictitious ‘design day’ peak scenarios. As a result, there is limited consideration of how the system will perform outside the peak day, during part-load conditions.
We know ASHPs perform best when operating at full load, with their efficiency dropping significantly under part-load conditions. This reduction in performance affects both the overall system efficiency and the ability to achieve net zero carbon. This was less of a concern with gas boilers. Thus, we’ve proposed a different way of sizing the heat generation equipment.
We assessed a typical secondary school heating load profile and tested three approaches, each reflecting a different interpretation of what ‘appropriate’ sizing may look like.
120% oversized system: Three ASHPs sized to meet 120% of the peak co-incidental load (40% each), reflecting the DfE’s previous Output Specification, which inherently oversizes heat pumps.
100% matched capacity: Three ASHPs sized to meet the peak co-incidental load (33% each), aligning the heat pumps with the building demand.
- Reduced heat pump capacity and electrode boiler: ASHPs sized to the 99.6th percentile peak load (roughly two-thirds of the peak demand). The remaining gap between this value and the building’s absolute peak demand is met by an electrode boiler.
This research demonstrates that reducing the size of the installed heat pump increases the Seasonal Coefficient of Performance (SCOP) of the entire heating system, including the electrode boiler, resulting in lower energy use, carbon and operational cost.
We found that:
- Reducing the installed heat pump capacity ensures it operates at full load for a higher number of hours.
- The small number of hours the supplementary electrode boiler is needed doesn't offset the positive effect of the improved heat pump performance.
- Reducing the size of heat pumps can result in lower capital cost, improved operational energy and carbon costs, with expected whole life carbon savings.
- Oversizing reduces performance, increases capital and operational costs, and possibly adds energy costs compared to an efficient gas boiler.
It’s important to note that this analysis was possible due to two key considerations:
- Building demand is modelled dynamically, for every hour of the year and not just on a peak day.
- Manufacturers' data is used for various part load conditions at different external temperatures. A simple SCOP cannot provide an accurate picture of your building.
As heat pumps become central to the UK’s decarbonisation strategy for educational buildings, getting the sizing right is critical. A refined, performance‑led approach delivers the greatest benefits, improving efficiency, cutting primary energy use, and reducing operational and capital expenditure.
Heat pumps are not boilers, and it’s time our design methodologies recognised that.
