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Impact of Radiator Material on Heating Efficiency

Impact of Radiator Material on Heating Efficiency

The reality is, the choice of radiator will impact the efficiency of your system, this has been proven in laboratory settings and the last 200 years or so of science contains all the information we need to work this out without a laboratory. We aren’t reinventing the wheel here, all this information exists, we just have to know how to use it. Before we can dive straight into this, we need to cover some background information around how the product (radiator) works in our environment (home), along the way we will build a few concepts to help us understand the science behind this.


Room Heating Requirement

An individual room will have a specific energy requirement to maintain a certain temperature internally, depending on how cold it is outside. This is based on numerous factors such as the room construction, number of external walls, what’s above it, what’s below it, insulation etc. There will be an energy requirement for the room to maintain this temperature. Further to this, your geographical location will determine what outside temperature to use, for example, Brighton could be -2°C while Scottish Highlands could be -6°C. Calculating this is the first step in choosing a radiator, you can check out RADSIZER® if you want to try this for one of your rooms to see the heat requirement it gives you, it will also pick out radiators that will fit your room.

RadSizer® App

 How energy movement works is based on a temperature gradient, this is the difference between the inside and outside temperatures. For our room, we will say that the following was calculated: While it is -1°C outside and we want an internal temperature of 20°C, the room heating requirement is 1000 Watts. This gives us a 21°C difference between the inside and outside world. That means that our heat loss per degree between the inside and outside is 1000 Watts / 21 degrees: 1000 / 21 = 47.6 Watts per degree Taking the same room, if it were say 5°C outside and we wanted the room to be 20°C, which is a difference of 15°C, would require: 47.6 Watts X 15 = 714 Watts of energy to maintain the room temperature. This is not the energy required to raise the temperature of the room, just the amount of energy to maintain it. This is a simplified explanation of the second law of thermodynamics, it has been refined by people far smarter than you and I since about the 1850s, this is how it works, we accept this. Which we will sum up as: Concept 1:- heat energy will always go from the hotter atmosphere to the colder atmosphere, the energy transfer rate is defined by what is in the way and the temperature difference of the two atmospheres. Now to meet this energy requirement, we need a radiator that can deliver that heat energy, but over different atmospheres. Which is the internal radiator atmosphere (or heating system), and the room atmosphere.

Radiator Heat Outputs

When shopping for a radiator you will see its heat outputs in Watts, but they must show a Delta number next to them to mean anything. On our website, we show Delta 50, 40, and 30. This roughly translates to:
  • Delta 50 = Gas or oil boiler with a flow temperature of around 75°C and a return temperature of around 65°C.
  • Delta 40 = Enhanced efficiency of a heating system with condensing boiler. Flow temperature of around 70°C, return of 50°C.
  • Delta 30 = Low temperature solutions, like heat pumps (we wont go into this now, but you can read more about these with our heat pump blogs on this link).

 But what does this actually mean? In simple terms, they represent the next concept: Concept 2:- If you heat the water in the radiator to a certain temperature, and the room is at a certain temperature, a certain amount of energy will transfer from the radiator to the room represented by its Delta heat output. This is the same law of physics that dictates how our room energy/heat loss occurs. So let's say we have a Radiator that says Delta 40 = 1000 Watts. If your boiler flow temperature is say 70°C and the return temperature from the radiator is say 50°C, there is a mean water temperature across the radiator. That mean water temperature is (70+50)/2 120/2 60°C The Heat output shown on a radiator then assumes that the room is 20°C, so we need to find the temperature difference between the radiator and the room, which is no different from the room heating requirement, we are just comparing the radiator internal atmosphere with the room atmosphere: Mean Water Temperature in radiator – Room temperature For our example: 60-20 = 40°C Or Delta 40. This leads us to Concept 3.  Concept 3:- In order to gain the heat output listed on a radiator, the water must be heated, therefore, the water content of the emitter is an important factor in its ability to function, as the more water there is, the longer it will take for your heating system to raise the temperature of the total amount of water.  Now we have one more atmosphere to understand, this is one of the points where most guidance online lacks accuracy.

The material of the radiator heating up

You may come across the terms “thermal conductivity” and “specific heat capacity” mentioned around radiator efficiency. Usually this is a lot of jargon that is misinterpreted and basically boils down to: The radiator material doesn’t matter, as Thermal Conductivity is the movement of heat from the water, through the material into the room. But we now know because of concept 1 that this is very silly. If this were true, in our example where the water in the radiator is 60°C, the room is 20°C, and the material of the radiator is say 10°C, where is the heat energy going? The heat energy is going from the water AND the room into the radiator metal, which makes absolutely no sense. So at the very least, the radiator material would need to be above the room temperature to transfer heat via thermal conductivity, but at the rate we need, it will need to be close to the water temperature, because the heat from the water cannot ignore the metal it is touching, the lower the temperature of the metal, the less heat would go into the room. Concept 4:- In order for the radiator to transfer heat from the water to the room, we must raise the temperature of the material of the radiator. Now, we need to understand one final thing before we can show you which radiator material is more efficient.

A certain amount of energy is required for anything to simply function

The easiest way to understand this is with an analogy as simple as driving a car: You put your foot on the pedal. Your car speeds up to 60 miles per hour. What happens if you take your foot off the pedal? It does not remain at 60 miles per hour. So there is a certain amount of energy required to get the car to 60 miles per hour, then a certain amount of energy to maintain that. In our case, for radiators, we must raise the temperature of the radiator, the water within it and the material, as well as the room temperature, after which, there is a certain amount of energy required to maintain it, represented by our room heating requirement and our radiator Delta. Concept 5:- In order to output energy at a certain level, a radiator requires energy stored within it to function as desired, this is the energy that has raised the water temperature and the material mass of the radiator, this must be achieved in order to get the required heat into the room.   Before we go on to compare radiators and how we would expect them to function in the real world, let's just recap those concepts: Concept 1:- heat energy will always go from the hotter atmosphere to the colder atmosphere, the energy transfer rate is defined by what is in the way and the temperature difference of the two atmospheres. Concept 2:- If you heat the water in the radiator to a certain temperature, and the room is at a certain temperature, a certain amount of energy will transfer from the radiator to the room represented by its Delta heat output. Concept 3:- In order to gain the heat output listed on a radiator, the water must be heated, therefore, the water content of the emitter is an important factor in its ability to function, as the more water there is, the longer it will take for your heating system to raise the temperature of the total amount of water. Concept 4:- In order for the radiator to transfer heat from the water to the room, we must raise the temperature of the material of the radiator. Concept 5:- In order to output energy at a certain level, a radiator requires energy stored within it to function as desired, this is the energy that has raised the water temperature and the material mass of the radiator, this must be achieved in order to get the required heat into the room. So, we need a larger amount of energy to bring the radiator and water up to a temperature high enough to allow this heat energy to transfer into the room, this energy is stored in the radiator and is essentially wasted energy, because it will only be released once you have turned your heating off and the room starts to lose heat. But, by turning your heating off you are declaring that you no longer require the heating and therefore the release of this stored heat isn't required. The efficiency of a radiator is not defined by how it stores heat, but by how it transfers heat into the room. We now know that the structure, material and water content of the radiator will impact the rate at which it does this. The quicker your system can raise the temperature of the radiator and its water to begin to transfer heat, the more efficient it is.    Now, hopefully, it is apparent that I wouldn’t have mentioned any of this unless radiators were actually different in ways that would impact their efficiency. So, we need to start comparing some radiators and materials.

Radiator Material Attributes

There are 4 attributes that we need to understand but 2 are fairly self-explanatory:
  1. Water Content – This is the amount of water in Litres that the radiator holds, and therefore, the amount of water that must be heated to our desired temperature in line with our concepts above.
  2. Mass – The mass of the radiator, in KG, is again the amount of mass or metal we must heat in line with our concepts. We know that this must be hotter than the room temperature at least, but likely close to the water temperature.
  3. Thermal Conductivity – This requires its own section below.
  4. Specific heat Capacity – This requires its own section below.

Thermal Conductivity

This is the rate that a material can transfer heat from one atmosphere to another, in our case from the water to the room. There are various different subcategories of materials, but to keep this simple, we will give a rough idea as the difference is quite significant, so I won’t focus on what the numbers mean at a technical level, just what the difference represents to us: Aluminium – Around 200 W/m K Mild Steel – Around 50 W/m K Cast Iron – Around 50 W/m K Cast Iron and Mild Steel are not too different, they are both Iron based materials used to manufacture consumer products, so we wouldn’t expect them to be different. Whereas, the heat transfer rate of Aluminium is nearly 4 times Mild Steel and Cast Iron, which in turn means that the surface area doesn't need to be as large to achieve that same rate of transfer.  But what does this mean to us when we have radiators all with the same heat output in Watts but with different materials? Well, the first question would be, what is the thickness of the material? As much like our room, where the wall is in the way of the internal room atmosphere and the outside room atmosphere, the radiator material is between the water in the radiator and the room, so how thick that is is going to determine how well the thermal conductivity works for our specific product. So what do the radiator standards (which are law under the construction products regulations) say about this? Aluminium – 1.1mm thick or 1.5mm thick dependent on type Steel – 1.11mm thick Cast Iron – 2.5mm thick So for radiators made from these materials to achieve the same heat output, this tells us the following: Aluminium – Lowest amount of material and water content (the material is thinner - in some cases, the required surface area is lower because of the heat transfer rate and a smaller radiator has a lower water content)  Steel – Higher than Aluminium Cast Iron – Significantly higher than Aluminium. Because when we think about this topic, we have to consider all variables, including standards that impact what high level physics comments tell us. The next attribute we need to look at is Specific Heat Capacity.

Specific Heat Capacity

What this means is the amount of energy to raise 1kg of a given mass by 1°C. Water is commonly accepted as having a specific heat capacity of 4.18KJ (kilojoules), which is the amount of energy it takes to raise 1kg of water (which is the same as 1 litre) by 1°C. For our purposes:
  • 1 Watt = 1 Joule
  • 1 Kilowatt (KW) = 1 Kilojoule (KJ)

So let's start with water and how we work this out: If we have 1 litre of water that is at say 10°C, for our example heating system, we want to raise it to 70°C (boiler flow temperature), that is a raise of 60°C. 60 x 4.18 = 250.8 KJ If there are 10 litres of water in the radiator, that would be 2508 KJ to raise 1 litre of water to 70°C. The materials also have different specific heat capacities: Aluminium = Around 1KJ Mild Steel = Around 0.5KJ Cast Iron = Around 0.5KJ So if the radiator material is at 10°C, and I want to raise 1KG of that mass up to 70°C, the energy required is:  Aluminium - 60 x 1 = 60 KJ Mild Steel - 60 x 0.5 = 30 KJ Cast Iron - 60 x 0.5 = 30 KJ Mild Steel and Cast Iron are half that of Aluminium, which is why you often see Cast Iron and Steel focused retailers stating that because of its lower specific heat capacity, it is more efficient than Aluminium as it stores less energy. However, we have already established under very basic principles of physics coupled with the requirements of UK law, that in order for a Aluminium, Mild Steel and Cast Iron radiator to have the same heat output, Aluminium requires a lot less mass and water content than the other two, followed by Mild Steel, followed by Cast Iron So each radiator will have a unique mass which we need to complete the calculation above. But now we have an understanding of the basic physics principles along with the UK law requirements, we can now compare radiators in practical application and I can show to you as clear as day the energy wastage of chosen models.

Comparing the Radiator materials for efficiency

First we need to establish some room and radiator requirements.

Room and Radiator Requirements

The industry will say that you can’t compare radiators as all rooms are different, however, your room is the constant, and therefore, we can compare radiators if we use a constant room. We have a square room which is 4m in length by 4m in width and 2.5m in height. The heat required to maintain 20°C inside the room is 1000 Watts. You have a Condensing boiler which is Delta 40. This may not be your room or system, but these are our constants, so imagine it is. We then have the following radiators with similar heat outputs, while the design and the orientation are not the same, don’t worry, because the difference is not justified by any nuance like that. Cast Iron Example Plain - Cast Iron Radiator H620mm x W1098mm 2 Column 1009 Watts 118kg 34 Litres Steel Example Sherwood - White Round Top Column Radiator H500mm x W785mm 2 Column 1030 Watts 45.2kg 28.4 Litres Aluminium Example

Temple - White Vertical Square Tube Column Radiator H1800mm x W390mm

Watts 1064 Mass 22.7 kg Water Content 3.5 We can use this information to calculate the radiator specific heat capacity including the internal water, which is basically our stored energy requirement to maintain the room temperature at 20°C. 

Radiator specific heat capacity

In our table below, we are going to be raising the temperature of the water content and the dry mass (metal) from 10°C to 70°C, so a difference of 60°C, but now we will be using the actual water capacity and dry mass of the above radiators, to work out how much energy is required to make our emitter function and transfer the advertised heat output.
Aluminium Steel Cast Iron
Delta 40 Watts 1064 1030 1009
Mass (KG) 22.7 45.2 118
Water Content (L) 3.5 28.4 34
Material Specific Heat Capacity (KJ) 1 0.5 0.5
Water Specific Heat Capacity 4.18 4.18 4.18
Energy required to raise water temperature for initial heating 3.5 * 4.18 = 14.63 KJ 14.63 x 60°C = 877.8 KJ 28.4 * 4.18 = 118.7 KJ 118.7 x 60°C = 7122 KJ 34 * 4.18 = 142 KJ 142 x 60°C = 8520 KJ
Energy required to raise material mass for initial heating 22.7 * 1 = 22.7 KJ 22.7 x 60°C = 1362 KJ 45.2 * 0.5 = 22.6 KJ 22.6 x 60°C = 1356 KJ 118 * 0.5 = 59 KJ 59 x 60°C = 3540 KJ
Total Energy for water and material mass 2239.3 KJ 8478 KJ 12060 KJ


This energy is what is required at the very minimum to get our radiator to achieve its heat output. From this point onwards, all the radiators will require roughly the same amount of energy per second to provide the heat output required. In other words, when we put our foot down on the pedal, this much energy was used to get us to 60 miles per hour, the energy required to maintain that speed is now equal to the output at Delta 40. This is roughly our stored energy within the radiator, give or take. So, if you heat your room for 4 hours, this energy is only released when the heating system is turned off. This is the wastage of each radiator. The industry argument is that all that energy will be heat energy, and therefore it is not wastage, but, we have to consider our actual product not just quote a simple law of physics that suits us, by virtue, because this energy is only released after our heating turns off, it is not required energy for heating our room.  As mentioned earlier, we turned the heating off, we no longer need this heat energy. In reality while this is happening, energy is leaving from the radiator to the room, but if we consider that this does not happen, we can quantify this into a minimum amount of time and cost per heating period. For this we need a sample boiler, let’s say it is a 20KW boiler and this is the only radiator on the boiler. A 20KW boiler is 20KJ per second of energy. So to find out how long it would take the boiler to generate the required amount of energy for the radiator to function i.e. the stored energy, we divide those totals by 20KJ   

Total Energy for water and material mass Aluminium: 2239.3 KJ Steel: 8478 KJ Cast Iron: 12060 KJ
Time in seconds 2744.3 / 20 = 112 seconds 8478 / 20 = 423 seconds 12060 / 20 = 603 seconds
Time in minutes 1 minute 52 seconds 7 minutes 3 seconds 10 minutes 3 seconds


Note: this is for a single radiator. Which we can quantify into a cost per heating period. If it is a gas boiler, which would be around 10p per KWh, the boiler is burning at 20KW for the times stated above to get 1 radiator to functional temperature for our room requirement, which would make our running cost of the boiler when it is maxed out £2 per hour, per minute it would be 0.033p and per seconds it would be 0.00055p If we consider that we have 2 heating periods per day and say 45 days of cold winter every year Approximate costs just to get the radiator and water up to required temperature would therefore be, for our example:

Aluminium Steel Cast Iron
Total Energy for water and material mass 2239.3 KJ 8478 KJ 12060 KJ
Time in seconds 2744.3 / 20 = 112 seconds 8478 / 20 = 423 seconds 12060 / 20 = 603 seconds
Time in minutes 1 minute 52 seconds 7 minutes 3 seconds 10 minutes 3 seconds
Cost per heating period 0.00055 x 112 = £0.06 0.00055 x 423 = £0.23 0.00055 x 603 = £0.33
Cost per year with 2 heating periods and 45 days of cold winter £0.06 x 2 x 45 = £5.40 £0.23 x 2 x 45 = £20.70 £0.33 x 2 x 45 = £29.70

How realistic are these prices to your actual home

A 1000W radiator is pretty big, so it's unlikely you would have one of these in every room, the age of your property would also dictate how likely this is. So, we aren’t claiming Aluminium will save you that much over Cast Iron, simply that there is a significant difference between Aluminium, Steel and Cast Iron and there is going to be a noticeable impact to energy wastage. However, with regards to the industry argument of “well all that energy becomes heat energy, therefore it is not wastage”, as we have established, that disregards a lot of how the world works, specifically:
  1. When we turn our heating off, or make it a lower temperature, it would be clearly implied we no longer need that level of energy, therefore, the energy wastage directly relates to the programming we have in place.
  2. Everything in our world of technology, whether a radiator, a car, a phone, requires a certain amount of energy to simply be, further energy is then required to fulfil its task. This concept is covered by the laws of thermodynamics.

So once our radiators have heated up, we then have the room heating energy requirement, which will be fulfilled by Aluminium a lot quicker, as it reaches its required level of performance a lot quicker and therefore achieves a greater transference of heat quicker. So I wouldn’t suggest you are wasting that much per room in your home, but I would suggest that the material has a significant impact on your wastage. After the room has heated up, and before you turn your heating off, the usage is the same regardless of the radiator material, they all provide a similar amount of heat energy to the room and meet the requirement of your thermostat, but there is further differences in functionality that can also be a cost burden.

Eco Design Concepts

One of the concepts of eco design is a device's ability to respond to controls. While it isn’t generally accepted by the industry, the material of radiators directly impacts your ability to lower and raise the temperature of your room. Raising the temperature just incurs the same issue as already covered, but lowering the temperature, that will be wastage as well. The wastage in theory shouldn’t be beyond the above scenario, but it will take longer for your room to lower in temperature from the Cast Iron example than the Aluminium. Where this would usually incur additional wastage is if you were to open an internal door or external window to let heat out, the reheat cycle will then start again for the room air. That consideration is a concept of Eco Design, how a user responds to controls and the choice of radiator material will impact the user with how well the controls’ function. The next eco design concept we should consider is how this time impacts the user, this is an area that can be quite a large cost for people. Due to the time that it can take for say Cast Iron vs Aluminium to heat your room, where Cast Iron takes quite some time to get to temperature when we consider the home as a whole, the previous time stated just being for a single room of that size, there would be a higher chance of you being disappointed with your heating system and the solution being to raise the boiler temperature to compensate for the time it takes for the home as a whole to heat up. Just to highlight the contrast, if you have 10 rooms of the above example, that is the difference of 18 minutes or so for aluminium compared to 100 minutes or so for cast iron just to get the radiator and water to the correct temperature, then on top of this  there is the energy required to heat the air in the room, which aluminium would cater for faster as well. As you can see from the calculations, it is the water content that has the highest impact, so even if we live in the fantasy world where thermal conductivity is all that matters and the heat energy passes straight through the metal, the difference is still significant. There is also the response from users in the home to turn up TRVs on radiators and the boiler room thermostat, which won’t speed up the time it takes to heat your home, but will cost you more, it is however a common response to your home heating up slowly. These concepts are not hearsay and conjecture, in fact, you can look at various research studies carried out by Government departments like BEIS (department for Business, Energy and Industrial Strategy) where installers quite clearly inform us that a common scenario they come across after installation is raising boiler temperature due to customer complaints about how long the home takes to heat up. It is so common, that we don’t feel the need to provide the lists of specific references, it is known as khaleesi, this is the way Mandalorian (I have to fit at least one nerd quote in every blog).

Accounting for everything, what material is better?

This blog is only looking at one aspect of efficiency and functionality, there are other benefits to different types of radiators and materials. For example, we compared a steel column radiator as structurally it is similar to the other radiators chosen, however, if we considered steel radiators with convector fins, we would find a significant improvement. Convector fins increase the heat output of a radiator by adding thin fin shaped rows of metal to the inside of the panels. Now we know, based on our results and general knowledge, that the bigger the gap between the water content and the material mass, generally speaking, the less wasted energy. There is also the upfront cost, so aluminium and cast iron radiators are generally quite expensive, whereas mild steel radiators are quite a bit cheaper for the same heat output. There will be variances of space taken up in the room for the heat output required for different materials. We cover this in more detail in our ultimate guide to radiator materials blog, but as a summary, we consider Aluminium to be overall best for most categories with Steel being a close second. Cast Iron for numerous reasons, water content, mass, lack of design choice, high prices compared to heat output, space occupied in the room compared to heat output, we consider to be quite a bit lower in general usability across the UK property market, there is a requirement for it in certain instances but generally speaking it is not for the vast majority of us.

Summed up in a nutshell

When we hear statements made about radiators we have to consider those statements alongside practical use. Cast Iron is often boasted as “due to its design, it will keep your room warmer for longer after the heating has turned off, saving you money” If we break that down, that gives us two significant points:
  1. The energy has to be generated and stored in the emitter, therefore it cannot be free.
  2. That energy is being released after I have turned my heating off or to a lower set back temperature.

So is that energy efficient and useful? If we then consider that we have a desired temperature for our room, if we plan for this and turn our heating off earlier to capitalise on that energy, that energy is not released at the same rate, we took our foot off the pedal, so the energy is released slower as the temperature of the radiator and water inside starts to get closer to the room temperature, which means the room starts to lose its heat and it won’t maintain your desired temperature for very long. We may hear that “aluminium only provides a quick burst of heat”, but if we consider this with how it functions, it is great at passing heat from the water into the room. Where do we want the heat energy? In the room.  We can see it will heat up quickly given the lower water content and mass and your boiler is controlled by a room thermostat, which will call for more hot water if the room falls below your set temperature, regardless of the radiator in place. So your boiler is on for shorter bursts, as opposed to long uphill climbs. If we hear that Cast Iron is most efficient due to its lower specific heat capacity, we have to compare that with the mass, so it is true that its specific heat capacity is half of aluminium, but the mass for our examples was 5-6 times higher, which makes aluminium radiators 1/3 the heat capacity of a cast iron radiator, so what is the benefit of Cast Iron claims vs Aluminium when we consider what specific heat capacity actually means.  Often Steel makes a wide sweeping claim that “due to convector fins, steel is most efficient”, while we can see it would improve efficiency, not all steel radiators have convector fins and where they do, the amount of convector fins can change.

How do I use this to choose a radiator for efficiency

Like we say, there’s numerous reasons why you may choose one radiator over another, but if you have a stalemate that can be broken with efficiency, then compare the following attributes:
  1. Make sure the heat output is similar, but most importantly that it meets your rooms requirements
  2. Compare the water content
  3. Compare the Dry mass

If the heat outputs are suitable for the room, then having a lower water content and dry mass will mean less energy is wasted. It is really that easy to compare radiators for efficiency, even those of the same material, but for a more accurate but quick to calculate formula, you can try this: Heat Output / (Water Content x 4.18) + (Dry Mass x specific heat capacity) You can use the approximate specific heat capacities below if you cannot identify the specific heat capacity for the exact grade of metal used, as water is the biggest cause of stored energy wastage, but here are our example radiators:

Aluminium Steel Cast Iron
Delta 40 Watts 1064 1030 1009
Mass (KG) 22.7 45.2 118
Water Content (L) 3.5 28.4 34
Specific Heat Capacity 1 0.5 0.5
Water Content x 4.18 14.63 118.72 142.12
Dry Mass x specific heat capacity 22.7 22.6 59
Combined Mass (above 2 rows added together) 37.33 141.32 201.12
Heat Output / combined mass 1064 / 37.33 = 28.5 1030 / 141.32 = 7.22 1009 / 201.12 = 5


The bigger the number from this formula, the less energy wastage there would be, this then works for comparing models of the same material, as you may find water content is the same, along with a similar dry mass, but a significantly different heat output, like you find with Steel radiators where convector fins are added.   If you have made it this far, well done! That wasn't the easiest of reads (or writes!), but we hope this knowledge will help you choose the best possible radiator for your system and room and hopefully save some money in the long run. If you are at the start of your radiator purchase, we recommend you begin by using RADSIZER® to find your room's heat requirement. It will suggest some radiators that match this heat requirement but if you want to utilise your new material efficiency knowledge you can use the heat requirement value to search our wide range of designer and column radiators in various materials to find your perfect match! 

RadSizer® App

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