When selecting the right size cast iron radiator for your room or property, understanding Delta T (Δt) is essential. Delta T refers to the difference between the average temperature of the water inside your radiator and the desired room temperature. This temperature difference directly affects how much heat (in BTUs or watts) your radiator will emit.
How Delta T Is Calculated
The formula is simple:
Δt = (Flow Temperature + Return Temperature) / 2 - Room Temperature
So if your flow temperature is 75°C, your return is 65°C, and your room temperature is 20°C:
Δt = (75 + 65) / 2 - 20 = 50°C → Delta 50
Common Delta T Ratings
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Δt 50°C (Delta 50)
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Standard rating used across Europe for most traditional central heating systems.
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Assumes a flow of 75°C, return of 65°C, and room temperature of 20°C.
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Most radiator output charts, including ours, use this standard.
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Δt 30°C (Delta 30)
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Used for low-temperature systems, such as underfloor heating, air source heat pumps, and ground source heat pumps.
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Assumes much lower flow and return temperatures (e.g., 45°C/35°C), meaning radiators emit significantly less heat at this delta.
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Why Delta T Matters When Choosing Your Radiator
The same radiator will emit very different levels of heat depending on the Delta T. For example:
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A radiator that gives off 1000 watts at Δt 50°C may only emit 500–600 watts at Δt 30°C.
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If you're running a low-temperature heating system, you’ll need larger radiators (or more sections) to achieve the same room temperature.
How to Use This When Sizing a Radiator
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Start by calculating the heat requirement for your room (in BTUs or watts). This is often done using online heat calculators or by your heating engineer.
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Identify the operating temperature of your system:
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Traditional boiler systems: likely Δt 50°C
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Low-temperature systems (heat pumps): likely Δt 30°C
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Use the correct output chart for your system’s Delta T:
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All our radiators show outputs at both Δt 50°C and Δt 30°C.
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If using a heat pump, always refer to Δt 30°C figures.
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Always slightly oversize your radiator to ensure comfort and efficiency. This compensates for heat loss, furniture positioning, and real-world performance.
Need Expert Help?
If you're unsure which Delta T your system uses or how many radiator sections you need, our team is here to help. Call us on 01723 321000, and we’ll guide you through the process based on your heating source, room sizes, and design preferences.
