When heat output claims don't add up: why compliance matters for installers
The heating industry relies on a straightforward assumption: that the product data used to design a system is accurate. Installers depend on it to carry out heat loss calculations, select appropriately sized radiators and deliver systems that perform as expected. Homeowners, in turn, depend on installers to translate that data into comfortable, efficient heating.
Recent concerns raised by the Manufacturers’ Association of Radiators and Convectors (MARC), in collaboration with the Office for Product Safety and Standards (OPSS), have highlighted cases where some radiators are being marketed using non-compliant heat output figures. This results in products appearing to deliver higher outputs than they would under recognised testing standards.
For installers, the implications extend beyond compliance, affecting specification decisions, customer satisfaction and ultimately the trust that underpins long-term customer relationships.
Why this matters for installers
Radiator selection is only as reliable as the data behind it.
Where heat output figures are overstated, installers risk specifying products that do not meet the actual heat demand of the space. The impact often won’t become apparent until after commissioning, when rooms fail to reach design temperatures and customers begin to question system performance.
That creates avoidable pressure on installers, even where design work and installation have been carried out correctly.
What is driving concern in the sector?
Under BS EN 442 and the Construction Products Regulation 2011, radiator outputs must be independently tested and declared using standardised conditions, including Delta T50 and Delta T30, and supplied with a Declaration of Performance (DoP) showing verified heat output values.
MARC has raised concerns that some suppliers are promoting outputs using non-standard conditions, such as Delta T60, which can inflate perceived performance and distort product comparisons.
The Office for Product Safety and Standards (OPSS) has supported MARC’s work in highlighting compliance expectations across the sector, reinforcing the importance of accurate, standardised product information.
The impact of different Delta T ratings becomes clearer when you look at a typical specification example. A radiator advertised at Delta T60 might be rated at 85/75 °C and deliver 1000W under those conditions. However, if the system operates at Delta T50 (for example, 75/65 °C), the same radiator may deliver around 800W. This means a system designed around a 1000W heat demand could be underperforming if the output was based on the higher Delta T rating. In practice, it is easy to focus on headline wattage figures without checking the flow and return temperatures behind them, which can lead to misalignment between calculated demand and real-world output.
Inaccurate heat output figures can influence heat loss calculations, radiator sizing and overall system performance. For installers, this undermines one of the most important parts of the specification process: confidence in the product data being used.
System performance and the shift to low-temperature heating
The issue becomes more significant as heating systems continue to evolve.
Heat pumps typically operate at lower flow temperatures than traditional boilers, meaning radiators must be correctly sized to deliver the required output under those conditions.
Where radiator performance has been overstated, systems may appear underpowered once installed, even if the heat pump and design have been specified correctly.
That creates unnecessary troubleshooting for installers and risks misdirecting attention towards the heat source, rather than the emitters.
Vulnerable customers and system performance
While this is a technical and regulatory issue, the outcomes are felt in real homes.
Households that are most vulnerable to cold conditions, including older residents and those with health conditions, are disproportionately affected when heating systems underperform.
Accurate radiator specification therefore plays a direct role in ensuring comfort, safety and wellbeing, particularly where homes rely on efficient, low-temperature heating systems.
How can installers source accurate performance data?
When specifying radiators, installers can take a few simple steps to ensure performance claims are based on compliant data:
Check that heat outputs are declared at Delta T50 and Delta T30.
Ask for the Declaration of Performance (DoP), which should be available directly from the supplier or via their website.
Confirm that heat output figures are supported by independent testing in accordance with BS EN 442.
Avoid comparing products using different Delta T values, as this can create misleading performance comparisons.
Use compliant heat output figures when carrying out heat loss calculations and system design.
Taking these steps can help reduce specification risk, improve system performance and protect both installers and their customers from avoidable issues further down the line.
Promoting clarity: Myson’s leadership on trust and compliance
For installers, confidence in a specification starts with confidence in the data behind it.
As part of Purmo Group, Myson provides independently verified performance data, clear technical documentation and products tested to recognised industry standards.
That commitment reflects a wider Group focus on transparency, compliance and supporting the transition to low-temperature, energy-efficient heating systems.
By providing accurate product information and technical support, Myson helps installers specify with confidence and deliver systems that perform as intended.
Industry response and ongoing discussion
The issue of non-compliant heat output claims is now a key focus across the sector.
To support installers and contribute to industry-wide understanding of the issue, MARC has invited Chris Hone, Sales Director GB at Purmo Group, to join an industry panel at InstallerSHOW 2026.
The session, Illegal radiators – What Installers Need to Know, takes place on 24 June 2026 from 12:00–13:00 at InstallerPLAZA. It will explore the scope of the issue, ongoing regulatory work and the practical steps installers can take when specifying radiators.
As a Group, Purmo continues to work closely with industry bodies and standards organisations to promote clarity, compliance and confidence in product performance data. That work is reflected across its brands, including Myson, and in ongoing engagement with installers who rely on accurate information every day.
More about Myson
Myson has carefully created a complete range of low temperature heating solutions that caters to all needs. Ranging from hydronic and electric radiators to underfloor heating, heating valves and electronic controls, all our products are designed to offer the highest levels of comfort and efficiency.
To find out how Myson’s technologies can be deployed on your projects, contact our team.