US Understanding Delta T (Δt) Ratings for Cast Iron Radiators

Understanding Delta T (Δt) in Hydronic Heating Systems

Understanding thermal ratings for cast iron radiators

When sizing a cast iron radiator for your home, it’s essential to understand Delta T (Δt). Delta T is the difference between the average water temperature circulating through your hydronic system and the desired room temperature. This temperature difference determines how much heat (in BTUs or watts) your radiator will deliver.

How Delta T Is Calculated

The formula is straightforward:

Δt = (Flow Temperature + Return Temperature) ÷ 2 – Room Temperature

Example:

  • Flow temperature = 167°F (75°C)

  • Return temperature = 149°F (65°C)

  • Room temperature = 68°F (20°C)

Δt = (167 + 149) ÷ 2 – 68 = 122°F – 68 = 54°F (≈ Δt 50°C)

Common Delta T Ratings

Δt 50°C (≈ Δt 90°F)

  • Standard rating used for most traditional hydronic boiler systems.

  • Assumes water flow of 167°F (75°C), return of 149°F (65°C), and room temperature of 68°F (20°C).

  • Most radiator output charts, including ours, use this standard.

Δt 30°C (≈ Δt 54°F)

  • Typical for low-temperature hydronic systems such as radiant floor heating, air-source heat pumps, and ground-source heat pumps.

  • Assumes water flow/return temperatures around 113°F/95°F (45°C/35°C).

  • At this delta, radiators emit significantly less heat, making correct sizing crucial.

Why Delta T Matters in Hydronic Systems

The same radiator will perform very differently depending on the Delta T:

  • At Δt 50°C (≈ Δt 90°F), a radiator may emit 3,400 BTUs (1000 watts).

  • At Δt 30°C (≈ Δt 54°F), that same radiator may only produce 1,700–2,000 BTUs (500–600 watts).

For low-temperature hydronic systems, you’ll need either larger radiators or more sections to keep the room comfortable.

Using Delta T When Sizing a Radiator

  1. Calculate the heat requirement for your space (in BTUs or watts). You can use online BTU calculators or consult a licensed hydronic heating contractor.

  2. Identify your system’s operating temperature:

    • Traditional boiler hydronic systems: usually Δt 50°C (≈ Δt 90°F).

    • Heat pump or radiant systems: usually Δt 30°C (≈ Δt 54°F).

  3. Refer to the correct radiator output chart:

    • All of our cast iron radiators show performance at both Δt 50°C and Δt 30°C.

    • Always size using the chart that matches your hydronic system.

  4. Slightly oversize for best results: this ensures comfort and efficiency, compensating for furniture placement, insulation quality, and real-world performance.

Need Expert Help?

If you’re unsure which Delta T your hydronic system operates at or how many radiator sections you need, our experts are here to guide you. Call us at 212-814-6888 and we’ll help you select the right radiator for your heating source, room sizes, and design preferences.