We regularly review services designs and have been surprised to see many air-to-water heat pump systems designed without consideration of basic energy efficiency.
The Passivhaus Trust has published a guide to help designers understand the impact of their designs on energy performance. Air-to-water heat pumps: The basics of energy efficiency (bit.ly/CJPasHP24) offers a detailed overview of the decisions that have to be made to successfully design air-to-water pump systems.
Despite its title, it includes some complex aspects of design that are often missed by experienced services engineers. Most can be applied to any project irrespective of building energy standards. Topics include flow temperatures, overall system design, distribution losses, heat pump and emitter sizing, system controls, and input into the Passive House Planning Package (PHPP). The paper has been reviewed and endorsed by CIBSE.
The paper only covers design choices that have a big impact on energy performance. While it is published by the Passivhaus Trust, nearly all aspects of the advice can be applied to any project, irrespective of building energy standards.
There are various aspects of systems and heat pumps that influence good performance, but the most critical is flow temperature. The lower the flow temperature, the better the performance. For heating systems, flow temperature should be <40ºC and preferably <30ºC. (See panel, ‘How to get flow temperatures low: the basics’.) These are some other basic considerations when designing a heat pump:
- For non-domestic systems, avoid coupling together systems that run at different time schedules, because this can result in heat pumps and additional components being kept online unnecessarily.
- For heating, review performance during low and typical, as well as peak, conditions; select your heat pump to run at peak efficiency at typical conditions and consider how well it modulates. Domestic heat pumps are generally good at modulating down to lower capacity, but for larger (>30kW) systems it is unusual,and results in poor performance if not managed carefully.
- Reduce distribution losses and improve maintenance for external pipework by keeping external lengths to a minimum and installing all possible valves and components internally.
- PHPP calculates the whole-system performance (as recommended in AM17) and is quick and helpful. Aim for a seasonal performance factor (SPF, or H-1 in PHPP) of >4, with an SPF of 3 as the minimum.
About the author:
Sally Godber is an energy consultant at Warm. She co-authored the guide alongside energy consultant Alan Clarke and Gwilym Still, Passivhaus director at Max Fordham
How to get flow temperatures low: the basics
For space heating, emitters must be sized to deliver the peak heating demand at a reduced temperature. Services engineers need to be aware of this at an early stage to ensure they are large enough.
The flow temperature can be reduced further outside of peak-load conditions, as the required output will be less.
Avoid coupling systems of varying temperatures if this results in the whole system running at the higher temperature. This is usually the case for non-domestic buildings, where systems serve heating and hot water. This is described as ‘bracketing’ and the importance of it is referenced in CPD Module 242: ‘Foundations for transitioning from boilers to air source heat pumps’, CIBSE Journal, November 2024.
For existing buildings, specific constraints need to be considered, which are outlined in the Passivhaus Trust’s guidance ‘The right time for heat pumps in retrofit’, bit.ly/CJPasRet24
