Design it right, heat it right
Dave Hudson, Managing Director of GTEC Training, explains why heat pump efficiency starts on the drawing board.
When we talk about heat pump efficiency, there’s a tendency to discuss it as though it starts when the system is switched on, but its performance is determined long before that point.
Operational efficiency actually comes down to the design of the building, the emitter specification, the size and route of the pipework and the amount of heat loss from individual rooms. If we get these decisions right at the planning stage, that’s half the battle. If this is done properly, the system is much more likely to deliver the comfort and running costs that customers expect. If it’s done wrong, even the highest quality heat pumps risk having their performance compromised. As heat pumps become a more familiar part of the UK heating landscape, getting this right is particularly important.
We need to avoid treating them as a like-for-like boiler replacement and design a whole heating system based around how a heat pump works. One of the biggest differences between a heat pump and conventional gas boiler is the operating temperature. A boiler can produce high flow temperatures to compensate for an incorrectly sized radiator or poor heat distribution, while a heat pump is most efficient when operating at a lower flow temperature.
A lower flow temperature increases efficiency, which can reduce electricity consumption and strengthen the system’s performance. Flow temperature needs to be a design decision and should be considered right from the start, rather than post-installation. We need to plan and assess which flow temperature suits the building and is optimal for maintaining comfortable conditions. This leads us to emitter design. Bigger emitters can spell better efficiency.
A room that needs a certain amount of heat needs the heat to be transferred into the space somehow. Heating at lower temperatures will naturally reduce the heat output from a radiator, but a larger emitter is one way to address this. Larger radiators, low temperature radiators, fan assisted emitters or underfloor heating are all viable options, depending on the building and application. If this is considered at the design stage, choosing the emitter can be informed by the heat-loss calculation and targeted operating temperature.
A radiator that works perfectly well using higher temperature water for example, will likely not provide the same output at a lower flow temperature. If we only discover this after it’s installed, the choices to rectify it are limited. We can increase the flow temperature, replace the radiator or accept the room may not be the correct temperature, but none are good solutions when the issue could be avoided altogether.
Correct pipe sizing
Pipework is another area that can have a disproportionate impact on system performance. Heat pumps usually need higher flow rates than traditional boiler systems, particularly when they’re operating with a lower temperature difference between flow and return. This needs to be reflected in pipe sizing.
Pipework that’s too small can increase resistance and potentially lead to higher pump energy consumption, reduced flow rates and difficulties balancing the system. And long or complicated pipe runs can cause further losses, making the system harder to commission effectively. Pipe routes and sizing need to be considered alongside all the other elements, as a complete system where components work together. There is no point designing each component in isolation.
Good design also spans to the practicalities of installation and future maintenance. A well planned system that is hard to access, balance or commission risks falling into the category of bad design.
Another important consideration is understanding the building’s heat loss, looking at every individual room to factor in specific requirements caused by things like window size and external walls. This is far more effective than conducting a whole house estimate and allows the system to be designed around those differences.
This is important for most aspects of heat pump efficiency and avoids the common practice of installing unnecessarily oversized equipment to “be on the safe side”. Oversizing can actually increase costs and affect how efficiently the heat pump operates.
Working in tandem
Instead, accurate calculations give designers and installers the information they need to specify appropriately, ensuring that heat pump design works in tandem with the building itself. Insulation levels, airtightness, ventilation and building construction all influence heat demand, and improving the fabric can reduce the heating capacity needed and make lower temperature operation more achievable. The best heat pumps projects involve close collaboration from architects, heating designers, installers, developers and building owners.
Training is equally important. As heat pump installation increases, those fitting them need to understand the wider system design and not just how to install. System design should be at the centre of the conversation, and this approach will go a long way to raise the standard of heat pump installations.
Flow temperature, emitter output, pipework, heat loss, controls and building fabric are interconnected decisions. When they are considered together at the drawing board stage, the result is a heating system that is more likely to deliver the comfort, efficiency and reliability its occupants expect. If we want heat pumps to perform well at scale, that is where we need to start.
