The Secret Hero of Radiator Selection: A Guide to Delta T Ratings (T50, T60, & T70)

Choosing a new radiator for your home seems straightforward at first glance. You find a style you like, check the heat output listed in BTU (British Thermal Unit) or Watts, and make your decision based on the size of your room. But what about those small but crucial codes in the product specifications, like ΔT50, ΔT60, or ΔT70?

This "Delta T" rating is the secret hero that determines whether the radiator you buy will actually warm your room effectively. Choosing a radiator based on the wrong Delta T value can leave you with a chilly room and wasted money. Let's demystify this vital technical detail in a way everyone can understand.

What is Delta T (ΔT)?

In the simplest terms, Delta T (ΔT) is the difference between the average temperature of the water circulating inside a radiator and the ambient temperature of the room. The bigger this difference, the more heat the radiator will emit into the room.

The formula is: ΔT = Average Water Temperature - Room Temperature

Here, the "Average Water Temperature" is the mean of the hot water entering the radiator (the flow) and the cooler water leaving it (the return). The room temperature is typically assumed to be a standard 20°C.

What Do the Delta T Ratings Mean? (T50 vs. T60 vs. T70)

You will most commonly encounter three Delta T ratings in the market, each representing a different heating system standard.

ΔT50: The Modern Standard

• What it is: It represents a temperature difference of 50°C. This is the current legal standard (BS EN 442) that all radiator manufacturers in the UK and Europe must adhere to.

• Example Calculation: (75°C flow temp + 65°C return temp) / 2 = 70°C average water temperature. Subtract the 20°C room temperature, and you get 70°C - 20°C = 50°C.

• Why it Matters: Modern central heating systems, especially those with condensing boilers, are designed to be more efficient by running at lower water temperatures. ΔT50 reflects how these modern systems work. When comparing radiators, you should always use the BTU/Watt output stated at ΔT50 to get a true, like-for-like comparison.

ΔT60: The Old Standard

• What it is: This represents a 60°C temperature difference. It was the old UK standard before 2013.

• Why You Still See It: Some retailers might still advertise heat outputs using this outdated standard because it makes their radiators appear more powerful. The same radiator will show a BTU output roughly 25% higher when measured at ΔT60 compared to ΔT50. This can be very misleading for consumers.

ΔT70: The High-Output Specialist

• What it is: This indicates a very high temperature difference of 70°C. It is not a general standard.

• Where it's used: You'll typically see this on "designer" or "high-output" radiators from specialist brands. These are designed for situations where you need to get the maximum possible heat from a small or slimline unit.

• Be Aware: If a radiator's heat output is quoted at ΔT70, it will produce significantly less heat when connected to a standard UK heating system running at ΔT50. Retailers will often provide the converted ΔT50 equivalent to allow for accurate comparison.

The Biggest Risk: An Unfair Comparison

The most common mistake a consumer can make is to directly compare the BTU outputs of two radiators with different Delta T ratings.

Scenario: You see one radiator advertised with a heat output of 1500W (at ΔT60). Another is listed at 1300W (at ΔT50). At first glance, the first one looks more powerful and like a better deal. However, when you convert the ΔT60 figure to the industry standard ΔT50, its true output is only around 1180W. The second radiator is actually far more powerful!

Looking to the Future: Heat Pumps and Lower Delta T

This topic becomes even more critical with the rise of renewable heating technologies like air source heat pumps. Heat pumps operate most efficiently at even lower water temperatures than condensing boilers. This means the Delta T value drops significantly, often to ΔT30 or even lower.

In this case, you will need to install much larger radiators (or alternatives like underfloor heating) to get the same amount of heat into your room. If you are future-proofing your home, considering these low Delta T scenarios when choosing radiators is essential.

What Should You Do? A Quick Checklist

1. Know the Standard: For comparing radiators, the single gold standard is ΔT50.

2. Read the Label: Always check which Delta T rating is being used for the advertised heat output. If it's only listed at ΔT60 or ΔT70, ask the retailer for the ΔT50 equivalent.

3. Use a BTU Calculator: Use an online calculator to determine the heating requirement for your room. These tools will give you a result based on the ΔT50 standard.

4. Consult a Professional: If you're in any doubt, don't hesitate to ask your heating engineer or plumber for advice.

Choosing the right radiator is more than just an aesthetic decision; it's a long-term investment in your home's comfort and energy efficiency. Understanding what Delta T means is the key to making that investment wisely.

Frequently Asked Questions About Delta T and Radiator Heat Output

What does Delta T mean for radiators?

Delta T represents the temperature difference between the average water temperature inside the radiator and the room temperature. It determines how much heat the radiator can release into the room.

What is the difference between Delta T50 and Delta T70?

Delta T50 is the UK and EU standard used for testing radiator output. Delta T70 uses a much higher water temperature, inflating BTU figures and not reflecting real-world performance.

Why is BTU output different at Delta T50 and Delta T60?

Because heat transfer increases with greater temperature difference, BTU values at Delta T60 are about 25% higher than those at Delta T50—but they don’t match the real-world conditions of modern heating systems.

How do I convert BTU from Delta T60 to Delta T50?

Multiply the BTU value by 0.789 to convert from ΔT60 to ΔT50. This gives you a realistic output for comparison.

Why are some radiators advertised with Delta T70?

Some brands use Delta T70 to show exaggerated BTU figures. It’s often a marketing tactic that makes products look more powerful than they actually are at standard operating temperatures.

What is the standard Delta T rating in the UK?

Delta T50 is the official testing standard as per BS EN 442. It reflects typical flow and return temperatures in modern heating systems.

How many BTU do I need for my room?

Multiply your room’s volume (m³) by around 50–100 W/m³ depending on insulation, then convert to BTU (1 W = 3.412 BTU). Use a BTU calculator for accuracy.

Does Delta T affect radiator efficiency?

Yes. A higher Delta T increases heat output. However, efficiency depends on matching the radiator’s rating to your system’s real operating temperature.

How do I know if my radiator is rated at Delta T50?

Check the product specifications. Reputable retailers and manufacturers list BTU outputs at ΔT50 and may also provide correction factors for other ratings.

Is Delta T50 compatible with heat pump systems?

Not fully. Heat pumps operate at lower temperatures, often requiring ΔT30 or lower. This means you’ll need larger radiators to achieve the same heat output.

Are Reina radiators tested at Delta T70?

Yes. Reina typically lists heat outputs based on ΔT70, which is significantly above UK standard and can lead to customer confusion if not converted to ΔT50.

How does Delta T impact radiator sizing for heat pumps?

Lower Delta T values from heat pumps mean less heat per radiator. To compensate, radiators must be much larger than those sized for traditional boilers.