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Below: Viessmann partner Michael Paini, director and lead heating engineer at Option Energy Solutions Ltd. Here he shares his hints and tips for getting heat loss calculations right first time.
Left: Site constraints, such as locations for the outdoor unit or limits on noise, space or airflow can influence design temperatures and emitter choices.
Below: For heat pumps, such as the Viessmann Vitocal 151-A (pictured), a detailed room-by-room HLC is non-negotiable, affecting performance, compliance and running costs.
Raising the bar on heat loss calculations
As heat pumps gain ground in the UK, room-by-room heat loss calculations (HLCs) are becoming routine practice in all types of installations. Viessmann partner Michael Paini, director and lead heating engineer at Option Energy Solutions Ltd, explains how to approach them efficiently and accurately
W
hether a job involves a heat pump or boiler, everything should start with one key question: how much heating power
does this property genuinely need? For heat pumps, a detailed room-by-room HLC is non-negotiable. It affects performance, compliance and the customer’s running costs. However, it’s also good practice for boiler installations, particularly in larger properties, to ensure precise range-rating and avoid oversizing.
Too big
Unfortunately, overspecification is an all-too-common issue. I see systems with double the required output simply because no one took the time to calculate heat loss. I recently helped a customer who’d been told by two companies – neither of whom had carried out an HLC – that she needed either a commercial boiler or a pair of domestic boilers for her sizeable property. My assessment showed the true heating requirement was actually under 32kW, meaning a single Viessmann Vitodens 200-W gas condensing boiler was ideal. Thankfully, as more installers move into
renewables, the discipline of carrying out HLCs is spreading. The process isn’t overly complicated, but there is an art to getting it right.
Clarity
As a full HLC typically takes three to five hours, it’s important customers understand why it’s necessary and how they can help. It’s not simply a regulatory tick- box for Boiler Upgrade Scheme (BUS) funding, but a
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crucial step in good system design, to keep installation and running costs down and avoid remedial work later. Gather as much detail as possible from your client about the building fabric including floors, windows, walls, doors, roofs and insulation. Always agree room names together before you begin - you’d be surprised how many misunderstandings this avoids! If the property has recently been renovated, there
may be detailed plans containing all the information you need. It’s always worth checking the latest EPC certificate too, making sure to factor in any subsequent or planned building alterations.
Reading the building
Understanding something about the history of UK building methods and materials can help ascertain what’s hidden behind plasterboard or render without invasive investigations. For example, lining outer brick walls with concrete blockwork has been the norm since the mid-20th century and increases insulation by around 50% compared to purely brick structures.
CIBSE guidance provides standardised air change
rates (ACRs) by construction era, yet these are often inaccurate. The biggest factor is airtightness. Two houses of the same age may have dramatically different ACRs depending on window replacements, draught-proofing, floorboard gaps or ventilation.
Additional checks for heat pump operation Pipe sizing is important. Heat pumps can operate
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effectively on microbore, but only when the system is designed around it. Alongside the HLC, heat pump installations
require some additional checks including measuring water pressure and assessing existing heating and electrical systems. Ask the customer for their Meter Point Administration Number (MPAN), fuse cut-out rating, and ideally their maximum power demand. If there’s a smart meter installed, this information is usually straightforward to obtain. Finally, consider any other site constraints, such as locations for the outdoor unit or limits on noise, space or airflow. These can influence design temperatures and emitter choices.
Ongoing adjustments
Even a well-executed HLC is only an informed estimate. There are always variables you can’t precisely measure. Once the system is up and running, it can be attuned to the actual heat loss of the building. This allows scope to optimise flow temperatures for maximum efficiency and comfort. Larger discrepancies, however, indicate miscalculations or installation issues. Guidance
Getting heat loss right has a big impact on
outcomes for customers, which means it’s very good for business. A variety of HLC software packages and training programmes are available to help installers build confidence. Viessmann’s Academy centres in Telford, Stockport and London Colney offer the BPEC Low Temperature Hot Water Heating Systems courses, while the Viessmann Access scheme, which supports installers with MCS accreditation, can also produce HLCs as part of a wider design and BUS-application service. Peer insights on social media can also be great for learning from other professionals’ real-world heat loss experiences. As the market continues to move towards low- temperature systems, there’s never been a better time to dive more deeply into the topic.
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