Heating jargon can feel a little intimidating. Some people happily disappear down an internet rabbit hole of calculations and technical guides, while others would rather skip straight to choosing between anthracite and antique brass. We get it.
But when it comes to choosing the right radiator, these numbers really do matter.
In fact, one of the most important things to consider when choosing your heating is how much warmth a radiator or heated towel rail will provide for your room. Too little heat and the space may never feel comfortable. Too much and you could be using more energy than you need.
This heat output is officially measured in Watts, though many homeowners are more familiar with BTUs (British Thermal Units). That's why you'll often see both displayed when comparing radiators, as they both tell you how much heat a radiator produces.
Thankfully, we've made this bit easy. Our BTU calculator does the hard work for you, leaving you free to get back to choosing the radiator that catches your eye.
But heat output is only part of the story. There's another number worth paying attention to: Delta T.
What is Delta T?
Don't worry — you don't need an engineering degree for this one.
Delta T doesn't tell you how much heat a radiator produces. Instead, it tells you the conditions under which that heat output was measured. In other words, the same radiator can have very different heat outputs depending on which Delta T rating is used.
Delta T is simply the difference between the average temperature of a radiator and the temperature of the room it's heating.
Because a radiator's heat output changes depending on how hot the water running through it is, manufacturers and retailers need a standard way of measuring performance. Delta T provides that common point of comparison.
Think of Delta T as the testing conditions behind a radiator's heat output. Once you know which Delta T rating is being used, comparing radiators becomes much easier.
Today, radiator heat outputs are stated at Delta T30 and Delta T50. But what do these ratings actually mean?
Understanding Delta T50
For many UK households, Delta T50 remains the most familiar and widely used standard. You'll notice Delta T50 is one of the heat output figures displayed across our product pages and used within our BTU calculations.
T50 is typically used for traditional central heating systems running at standard flow temperatures, making it a useful benchmark when comparing radiators.
The UK previously used Delta T60 as its standard until 2013, and you may occasionally come across T60 or T70 ratings in older literature or from some retailers. Today, radiator thermal outputs are stated at Delta T30 and Delta T50, which better reflect modern heating systems and provide a consistent basis for comparison.
So how does Delta T50 work?
Imagine water enters your radiator at 80°C and leaves at 60°C. The average radiator temperature would therefore be 70°C.
If we assume a room temperature of 20°C, the difference between the radiator and the room is 50°C - giving us Delta T50.
Why T30 matters in modern heating
You may have also noticed that we display radiator and heated towel rail outputs at both Delta T30 and Delta T50 in Watts and BTUs.
This isn't just us being extra nerdy.
Delta T30 is becoming increasingly important as the UK moves towards lower-temperature heating systems designed to improve energy efficiency and reduce running costs.
Heat pumps typically operate at lower flow temperatures than traditional boilers, which means radiators don't get as hot. Many modern condensing boilers are also being set to run at lower temperatures to improve efficiency and reduce energy use.
Heat pumps aren't the only reason T30 matters. Many gas boilers can be operated at lower flow temperatures too. In fact, condensing boilers are generally most efficient when running at lower temperatures, which can help reduce energy use and lower bills.
That's where T30 comes in.
Delta T30 provides a more realistic picture of how a radiator is likely to perform in lower-temperature heating systems. As homes continue to become more energy efficient, and low-carbon heating becomes more common, T30 will only become more important.
By displaying both T30 and T50 outputs, we give you the information you need to choose the right radiator for your heating system today — and potentially for the future too.
Which Delta rating should I use?
The good news is that you don't need to become a heating expert to choose the right radiator. If your heating system runs at more traditional temperatures, Delta T50 remains a useful guide and is still widely used across the industry.
However, if you have a heat pump or run your boiler at lower flow temperatures to improve efficiency, Delta T30 may provide a more realistic indication of how a radiator will perform in your home.
The most important thing is to compare like for like. Whether you're looking at Watts or BTUs, always check that the heat output is based on the same Delta T rating when comparing radiators across different retailers.
How do I find out what temperature my heating system runs at?
The easiest place to check is your boiler settings. Many modern boilers display the central heating flow temperature on the control panel or within the settings menu.
As a rough guide:
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Around 70 - 80°C flow temperature: Often associated with older or traditionally run boiler systems, where Delta T50 is usually a useful guide.
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Around 45 - 55°C flow temperature: Common with heat pumps and boilers set up for greater efficiency, where Delta T30 may provide a more realistic indication of radiator performance.
If you're unsure, don't worry. Comparing radiators using both Delta T30 and T50 figures can help futureproof your heating choices, whatever system you have today.
Remember, T50 remains useful for many homes today, while T30 is increasingly relevant as lower-temperature heating becomes more common.
Why comparing Delta ratings matters
Understanding Delta T means you can shop with confidence, compare products fairly and choose heating that performs as expected in your home. As lower-temperature heating becomes more common, understanding the difference between T30 and T50 will only become more valuable.
Still deciding on your heating? Take a look at our useful buying guides: Find the Right Radiator for Your Space and Everything You Need to Know About Heated Towel rails.
Alternatively, browse our full range of radiators, heated towel rails and underfloor heating.
