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A guide to radiators

Radiators serve as your main source of heat by regulating the temperature in your home. When choosing a radiator, you have to consider a wide variety of factors partnered with an extensive selection of different designs, types and styles now available on the market. At Hometree, we have compiled a short but comprehensive guide to assist you in choosing the right radiator and get you on the road to a suitable and energy-efficient purchase.

How to calculate the radiator size you need

The best place to start with choosing a radiator is to calculate the BTU output required in order to heat the space you’re planning to heat. Sizing radiators for your chosen room is based on heat output measured using BTU/h (British Thermal Units per hour), which is calculated using the volume of the room and taking into account any eventual heat loss within that room. First, calculate the volume of the room with the following formula:

Length of the room (m) x Width of the room (m) x Height of the room (m) = Volume of the room (m3)

In a second step, you multiply the volume of the room with 153. The result is the BTU output required to heat the room.

However, as mentioned before, there are a few factors that play a role in increasing or decreasing the required BTU output. For instance, if your home is not insulated, you will require a radiator with a slightly higher BTU output. See the table below for details and adjust the BTU output accordingly.

FactorAdjust by
For solid floor-10%
For non-insulated cavity walls+10%
For foam-filled cavity walls-20%
For upstairs bedrooms-25%
For double glazing-5%
For two outside walls+15%
For three outside walls+40%
For northern aspects+10%
For missing loft insulation+15%
For high ceilings – 3m+20%

The term radiator is slightly misleading, especially since they convect a lot more heat. Most radiators produce around 80% of their heat through convection leaving 20% through radiation. Radiators work when air is introduced through the bottom of the radiator and over the convection fins, causing atoms in the air to vibrate and create thermal energy. Convection currents are formed continuously when the air above your radiator is heated and then cooled. The currents being created in this process move heat around your room.

Usually the best place to install a radiator would be under a window, as the cold air would push more hot air into the room through heat conduction. The reason for choosing an area under a window is because it tends to be the coldest part of the room, unless your windows are double-glazed.

Types of radiators

As far as types of radiators are concerned, you typically have the choice between your everyday common radiators or a convector radiator (if you need help choosing a radiator, check out our guide here). In common radiators, the hot water will run top down through its components, which are all made from various metals. Convector radiators, however, circulate hot water through a tube surrounded by small fins, each increasing contact with the surrounding air and therefore amplifying the heat exchange between the radiator and the surrounding air. The advantage of choosing a convector radiator is that you can choose a smaller model than if you were to choose a common radiator that requires a larger surface area, therefore taking up more room.

Moving onto more technical terms, you may or may not have come across the following names for radiators: P1, K1, P+ and so on. When choosing a radiator and taking into consideration your calculations, you may need to look at single or double panel radiators as well as whether they come with a certain number of convection fins. Below is a brief guide to each type of radiator and what they offer:

NameAlternative NameTypeRadiator PanelsConvection Fins
P1Single panelType 1010
K1Single panel one convectorType 1110
P2Double panelType 2020
P+Double panel one convectorType 2121
K2Double panel double convectorType 2222
K3Double panel triple convectorType 3333

The radiator panels are simply “tanks” filled with hot water to emit heat around your room. Where the more panels, the more heat it will emit (as long as the surface area is the same as a single panel radiator). The decision to invest into one, two or even three panels can be based on a number of factors although space is often the determining factor.

In addition to panels, convector fins are the zig-zagging metal strips behind a single radiator panel or between two radiator panels. These were introduced into everyday radiators as a means to emit more heat introduced by the main panel tank and conducted through these fins. When it comes to single panel radiators, if they lack these convection fins they will not emit as much heat as a radiator with convection fins. Below is a graphical overview of the different types of radiators:

Design, Material & Efficiency of Radiators

The material of a radiator determines how quickly a radiator can heat up and cool down, while different metals and coatings may radiate more or less heat. Here’s a quick summary of each radiator material:

  • Cast iron radiators came before modern insulation and offer a “Victorian” feel. If a radiator is built from cast iron, it will take a lot longer to heat up and ages to cool down. If you do prefer the old, “bulkier” Victorian look, then stainless steel versions are available for this design.
  • Mild steel is the most common material used for radiators nationwide. They’re low cost, and you’ll find plenty of designs as well as a range of colors. Mild steel is the middle ground between other radiator materials since it heats and cools at a steady pace.
  • Stainless steel doesn’t rust and will stay warm for a long time after you’ve switched off your heating. Stainless steel radiators are more expensive and offer better quality than the other types of radiators mentioned here.
  • Aluminum is lightweight and acts as a superconductor. When you switch on your heating, the radiator will almost immediately start to heat your home. They’re also lightweight and easy to install (so lower installation fees). The only issue with aluminum is that it cools quickly after you switch off your heating which might not be ideal in the winter.
  • When it comes to finish, a typical chrome-finished radiator can be way less efficient and radiate less heat due to this coating, which provides insulation. Choosing the correct material for your radiator is again dependent on the BTU output required for your room.

While some radiators have different heat-to-touch levels, with young children as well as pets around it may be an option to buy a cover, which again comes in a variety of colors and styles to suit your home. However, it is also important to remember that with covers you also run into issues with your radiator performing less effectively due to trapped heat. This would be the same if you had a sofa or any other furniture in front of your radiator. You can also purchase a wide range of LST (= Low Surface Temperature) radiators, which provide outstanding heat performance as well as safety in domestic and safety-critical environments.

With the ever-expanding choice and advances in radiators, you can now browse through a wide range of designer radiators. Designers radiators come in many different shapes, sizes, styles and looks, each giving your room a stylish and slick finish to fit in with your room layout. Design can also be a factor when choosing the right radiator for you, each design and style coming at varying prices through different manufacturers.

How energy efficient will my radiator be?

To answer this, there are many ways to ensure your radiator is at the most efficient it can be, to prevent heat loss and to ensure your radiator is giving out the most heat it possibly can. For example, you can install foil sheets which stick to the wall behind the radiator, thus stopping any heat escaping through the connected wall.

It is also essential to bleed your existing radiators regularly. Bleeding is the process of venting trapped air from a radiator, which over time can cause your radiators to stop heating evenly and correctly. Even if a small amount of air enters your central heating system through the mains, the air will start to collect at the top of your radiators, which will start to reduce their heating capability. If you’d like to check your radiators, wait for your heating to turn on until your home is fully heated and then start to feel each radiator. If you notice that they’re only heating towards the bottom of the radiator and there is a cold spot towards the top, you will need to bleed it. If you discover that one of your radiators is not heating at all, then you may need to bleed it completely so that it can be functional again. Maintaining your radiators with regular bleeds as well as checking for any corrosion will keep your heating system in check. Head over to ours bleeding guide  to find out more about bleeding your radiators.

Ensuring there are no blockages in your radiator will lead to less energy being used to produce heat in your home. If bleeding doesn’t do the trick, then powerflushing your radiators may be your next option. A powerflush is carried out by professionals and involves cleaning out your central heating system of any sludge and debris that has built up over a few years. It’s worth doing as your boiler could be working harder than it needs to be and you will find that your home will be heated more efficiently after. Powerflushes are a more complicated procedure and normally cost a few hundred pounds. You should be aware that older radiators may not be able to handle the intense pressure and flow of water from the powerflush, which could eventually cause them to leak.

Post-powerflush, your engineer may recommend a replacement magnetic filter (or install one) to slow the debris and sludge build-up. They will also add anti-corrosion fluid to the now clean water to prevent rust or corrosion forming on any pipework. For more information on how a powerflush works, click  here .

Which radiator valves would I need?

Manual valve  – The manual valve is the easiest to use out of all other types of valves. All you need to do is twist the top with your hand and it will alter the flow of hot water into your radiator, almost as if you’re switching a tap on or off. Manual valves tend to be a lot smaller than TRVs, as well as a lot simpler.

TRV  – Thermostatic Valves (also known as TRVs), are similar in design to manual valves with the key difference being that they feature a temperature sensor. Although the sensor might sound quite high-tech (unless you already have or looking to purchase an electronic version), TRVs typically feature a bit of wax or liquid that reacts to the surrounding air temperature and regulates the radiator’s output. Therefore, a TRV will give you the basic control necessary to start saving energy.

Lockshield valve – Lockshield valves are supplied with your valve purchase to regulate the flow of water leaving your radiator. This type of valve is used to balance your system, and ensures that all your radiators heat up at the same rate. So, when you do purchase a pair of valves, one of them will be a lockshield.

If you’d like to find out more about radiator valves, check out our guide here .

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