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Correction Factors (Delta T Conversion)

As homeowners shift towards more sustainable heating solutions like heat pumps and are using lower system operating temperatures, understanding how radiators perform, especially in terms of heat output, is essential. The output of a radiator is stated at a specific difference between the design room temperature and the mean system temperature (where the mean is defined as the average of the flow and return temperatures). This difference is referred to as ∆T (or Delta T).

Converting Radiator Outputs to Different System Temperatures

Most radiator outputs are published at ∆T (Delta T) = 50ºC , which is nominally based on a room temperature of 20ºC and a mean system temperature of 70 ºC (flow 80 ºC, return 60 ºC). ​

With heat pumps and other lower water temperature systems, the difference in temperature reduces and the output of the radiator will also reduce. To calculate the correction factors (from an output stated at ∆T = 50ºC) , We have provided a simple tool which requires the published output at ∆T = 50ºC and the actual Delta T you are working at.​

(∆T is calculated as (Flow temp + return temp)/2 – room temp)​

Delta T Correction Value Calculator

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Complying with BS-EN442 Standards

All Zehnder radiators are tested to EN442, the European standard for hydronic radiators. They are also CE marked and compliant with the Construction Product Regulations (CPR). This gives re-assurance to our customers in terms of performance, quality and safety.​

In accordance with EN442 we publish our radiator outputs at ∆T=50ºC and also state the outputs at ∆T=30ºC in our technical data sheets and on CPR compliant labels.

Delta T Correction Value Calculator

Delta T Correction Factor Calculator



Simply input the output of your radiator at ΔT 50°C—usually provided by the manufacturer—into the calculator above. Then, select the required ΔT based on your system’s design parameters. The calculator will apply the correction factor to give you an accurate output for your specific conditions. This allows you to make informed decisions about whether your current radiators are sufficient or if you need to upgrade to models better suited for low-temperature systems.

Please note: This calculator assumes an n coefficient of 1.3, which represents the heat output characteristics of most radiators. Different radiators may have varying n coefficients based on their design and material, affecting how they perform under different temperature differences. While 1.3 is a commonly accepted value for standard panel radiators, always check the manufacturer’s specifications for exact performance data.

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