PHAM NEWS | NOVEMBER 2026 18 HeatPumps


Why installers might need to rethink heat pump design


A well designed heat pump system should boost effi ciency levels and reduce energy bills, but many installers and contractors continue to oversize their designs. Matthew Henderson highlights some of the problems this can create and advises on some of the best ways to guarantee that an installation meets expectations.


H


eat pump engineers are applying old gas boiler techniques to renewable technology,


routinely installing systems that have double or even triple the capacity the building actually needs – and that needs to change. When installing a heat


pump system, nothing is more important than the design. If installers can get the design right, it would save the end-user signifi cant money by reducing energy consumption. When installing boilers it is


common practice for installers to select a larger output than the property needs. In doing so, the boiler provides plenty of capacity to heat the building quickly, recover from a cold start and meet demand. After all, boilers have been relatively cheap and the fuel for them has been historically aff ordable. But that approach doesn’t necessarily translate to heat pumps.


Understanding effi ciency With heating systems we talk about the coeffi cient of performance (COP), which basically refers to putting in X amount of energy and getting X amount of heat out. With gas boilers, effi ciency


is always under 100%, whereas with heat pumps, the COP can be 3.0 to 4.0 – meaning you eff ectively get 300% to 400% effi ciency. In my opinion, this performance gap is the most attractive aspect of designing with heat pumps over a traditional boiler system. So it stands to reason that just


because a building has a 15kW boiler it does not necessarily need a 15kW heat pump. This fact is still not understood widely enough. Installers will pack in as much capacity as possible rather than considering how it will perform. Oversizing a heat pump


Matthew Henderson BSS ASHP technical manager


on the premise that bigger is always better can create its own problems aside from wasting energy. A unit that is too large for the property may cycle more frequently, operate less effi ciently and cost more to install than necessary. It can also make it harder to achieve the consistent, lower- temperature heating that heat pumps are designed to deliver.


A unit that is too large for the property may cycle more frequently, operate less efficiently and cost more to install than necessary


Getting the design right The key to a successful system is to get the calculations right at the design stage. Heat pumps need to be


selected around the actual heat demand of the building, taking account of insulation, glazing, room sizes, ventilation and the design temperature for the property. Other methods such as


thermal storage need to be considered to get the best out of a heat pump set-up. Unlike a traditional boiler, a


heat pump is generally designed to operate more steadily and effi ciently over longer periods. Thermal storage can help support that approach by allowing heat to be produced when it is needed - or when conditions are favourable - and stored for use later. For domestic installation,


this usually means a hot water cylinder that stores hot water


for when the household needs it. For larger commercial


buildings, a stratifi ed thermal store can be used, which keeps the hottest water at the top of the tank and the coolest at the bottom. The hottest water can then


be drawn off to meet a high heating or hot water demand, while the cooler water remains available to be reheated by the heat pumps. Other solutions can include


a secondary heat source, such as an electric boiler or a gas boiler, which is particularly useful for peak times, such as in the morning in a hotel when everyone wants to shower.


Optimising performance At BSS, we off er an advice service for customers to help them design heat pump systems that perform as well as they can. Our small team will look


over plans and advise on how to increase the COP to keep running costs to a minimum for the end user. Combi boilers were relatively


simple in how they worked. Heat pumps, on the other hand, require good planning and design. Getting the fl ow rate, emitter


sizing and Delta T (ΔT) right is crucial to making the whole system work effi ciently. Our team can off er advice as


an added value service. We’re not the same as everyone else – we don’t just provide products and quotes. People send us their drawings


and we can give feedback. They don’t always take it on board, and that’s absolutely fi ne, but


it’s there as a consideration and inspiration. We can also off er advice on


the industry’s move towards natural refrigerants – and their inherent dangers.


Choosing refrigerants Refrigerants are specialised chemical or natural fl uids that circulate inside a heat pump to absorb, transport and release thermal energy. The global heating industry


is moving away from synthetic chemicals to protect the environment and meet new laws, instead opting for natural alternatives such as propane. Propane off ers high energy


effi ciency and low carbon impact, but it requires diff erent safety considerations due to its fl ammability. Most importantly, installers


need to make sure they account for that in terms of space around the ASHP and drainage to avoid pooling of fl ammable refrigerant in the event of a leak. ◼ phamnews.co.uk/1126/26


CUT TO THE CHASE


 Traditional gas boiler techniques don’t apply when designing a heat pump system


 A unit that is too large for the property may cycle more frequently and operate less efficiently


 Thermal storage can help to get the best out of a heat pump system by allowing heat to be produced only when needed


 BSS offers a design service for customers, advising on how to maximise system performance


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