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central heating radiators & systems – expert guide

This guide draws on over 18 years of industry experience to explain everything you need to know about central heating systems and radiators, from basic principles to practical details. Learn how two-pipe and one-pipe systems work, how to position radiators for maximum efficiency, and how water pressure, diverters, and balancing affect heat distribution.

We also cover thermostats, radiator pipework, system pressures, and why bleeding and flushing radiators are essential for maintaining optimal performance. With clear explanations, step-by-step guidance, and supporting diagrams, this guide is designed to help you understand your central heating system and make informed decisions about radiator selection, installation, and maintenance — all based on real-world, hands-on expertise.

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1. Central Heating Systems - The Newer 2 Pipe System
  • There are two main types of central heating system: the older one-pipe system and the more modern two-pipe system.
  • In a two-pipe system, multiple pipes flow to and from the boiler:
  • >> The flow side (red pipes in the image below) carries hot water from the boiler into the radiator.
  • >> The return side (blue pipes in the image below) sends the cooler water back to the boiler to be reheated and recirculated.
  • Depending on how the system is piped, the flow and return can be on either side of the radiator. Compare the top section of the diagram with the bottom — you’ll see how the flow and return are reversed.
  • Installation Note: When installing or replacing radiators, it’s important to check the flow and return orientation to ensure they match your system piping for optimal performance.
2. Central Heating Systems - The Older 1 Pipe System
  • In a single-pipe system, hot water is directed through each radiator in turn along the heating circuit. The main drawback of this design is a progressive drop in water temperature as it travels through the system.
  • As hot water passes through the first radiator, it gives up heat to the room. This cooler water then continues along the single pipe, lowering the overall temperature before reaching the next radiator. Water in a one-pipe system can only be reheated after it has passed through the last radiator.
  • By contrast, in a two-pipe system, the water in the primary flow pipe is used in only one radiator before returning to the boiler for reheating. This allows all radiators to achieve a more consistent temperature, eliminating the uneven heating common in older single-pipe systems.
  • In the diagram below, you can see how water flows sequentially through each radiator in a one-pipe system, compared with the independent flow and return in a two-pipe system.
Open or Vented Systems and when to use Inhibitor / Silencer
  • three main types commonly found in homes today:
  1. >> Open Vented System – using a hot water cylinder
  2. >> Sealed System – using a hot water cylinder
  3. >> Sealed System – using a combination boiler
  • An open (vented) system draws in fresh water from the mains supply, which makes the radiators more prone to internal rusting. Most modern homes use sealed systems (types 2 and 3), where the water circulating through the radiators is retained and recycled.

  • Sealed systems require two key additives:
  1. >> Inhibitor: Prevents mild steel radiators from rusting internally when in contact with water.
  2. >> Silencer: Reduces the noise caused by boiling water inside the system, commonly referred to as kettling.
  • There are many brands of inhibitors and silencers available, and they can be added through any radiator in the system. This task should always be carried out by a qualified plumber.
Best Place to Locate a Radiator in the Room
  • The ideal position for a radiator is on an outside wall, and most commonly directly under a window. The images below show why this works so well:
  • >> Cold air naturally falls.
  • Even with a well-fitted window, a small amount of cooler air will enter the room. This cold air drops downwards, creating a downward airflow directly below the window.
  • >> Warm air from a radiator rises.
  • When a radiator is placed under the window, the heat rising from it moves upward at exactly the same point the cold air is falling.
  • >> The two air movements mix at source.
  • As shown in the three diagrams below, the falling cold air and rising warm air meet and mix immediately. This blending of temperatures helps eliminate cold spots and produces a more stable, comfortable ambient temperature across the whole room.
  • Positioning the radiator under the window ensures the natural airflow in the room works with the radiator, not against it — which is why this placement is recommended wherever practical.
air flow when a radiator is placed under a window
Thermostats Control Room Temperature on a Central Heating System
  • A room thermostat measures the air temperature in the room. When the room becomes too cold, it signals the central heating system to turn on. Once the room reaches the set temperature, it tells the system to switch off again.
  • For a thermostat to work accurately, it needs a free flow of air around it. It shouldn’t be blocked by curtains, furniture, or placed near heat sources such as radiators or direct sunlight, as these can give false readings. Most homes have one main thermostat, with fine-tuning in each room handled by Thermostatic Radiator Valves (TRVs) — which we cover later.
  • As shown in the image below, the Terma SMART system uses wall-mounted thermostats (VTS) that communicate with both Terma SMART TRVs (for central heating radiators) and Terma SMART heating elements (for electric models on the same system). The system also features unique open-window sensors (VOS), which automatically detect rapid drops in temperature and reduce heating waste.
Room Diagram showing the Terma SMART system with its therostat
Radiator Pipework
  • Central heating pipework can be installed in three different ways, as shown in the images below:
  1. Through the wall – This is the most common method in modern installations. The pipes enter the radiator horizontally through the wall, giving a clean, uncluttered finish.
  2. Through the floor – Often seen with traditional or cast-iron radiators, as it suits a more period look. The pipes rise vertically from the floor directly into the radiator valves.
  3. Along the wall – This is typically used when a central heating system is added after the room has already been plastered or finished. The pipes run visibly along the wall surface before connecting to the radiator.
Three options for how pipework can be installed, shown with arrows pointing to different installation methods.
Pressures Within the System
  • The pressure in a central heating system is measured in bars, and the way this pressure is created depends on the type of system you have.
  • There are two main methods:
  • 1. Tank System / Tun (Tank) Fed System – Gravity Fed
  • In this older style of system, pressure is created naturally by the weight of the water held in the header tank (usually in the loft). The higher the tank, the greater the pressure. This type of system is often referred to as gravity fed.
  • 2. Combi Boiler System – Pump Generated Pressure
  • In a modern combination boiler system, there is no header tank. Instead, the boiler uses an internal pump to create and maintain the pressure throughout the system. This is why combi systems typically run at higher and more stable pressures than gravity-fed setups.
  • The images below show how each system produces pressure differently.
Diagram showing a water system with pressure determination explanation
Balancing the System After Installing a New Radiator
  • When removing or replacing radiators, your plumber should also balance the system in addition to physically fitting the radiator.
  • In any central heating system, the pressure (or force) of the water naturally decreases the further it travels from the boiler. As a result, the heat output from radiators further down the circuit can be lower. See the supporting diagram below.
  • Balancing ensures that water pressure is distributed evenly, so all radiators get adequately hot.
  • Balancing is especially important when replacing a horizontal radiator with a taller vertical model, as the water now has to push upward against gravity to circulate around the new radiator.
  • If you notice your new radiator isn’t heating as well as the others, you can attempt to balance the system yourself:
  • Close the valves on all other radiators (turn clockwise to close), leaving the new radiator fully open.
  • Switch on the boiler so the full system pressure goes to the new radiator. Wait until it is hot all over.
  • Open the valves on the other radiators one by one, allowing the pressure to balance throughout the system.
  • Once complete, your new radiator should heat evenly, keeping your room warm and comfortable.
Diagram of a heating system with text 'balancing the system after installing a new radiator' and a pressure gauge icon.
What is a Diverter (or Baffle) Used for in a Radiator?
  • Higher-capacity radiators often come with pre-installed diverters (also known as baffles) to help water circulate fully inside the radiator.
  • To understand why, imagine the bottom horizontal bar of a radiator as a simple “sewerage pipe,” as shown in the image below. When the boiler is on, water naturally seeks the quickest path from the flow side to the return side. Without a diverter, most water flows straight across the bottom bar, meaning only the bottom of the radiator heats up.
  • With a diverter, the incoming water hits an internal barrier, forcing it up the first bar, across the top, and down through all the other bars. This ensures the entire radiator heats evenly, rather than just the bottom section.
  • The Rolo Room model comes with an optional insertable diverter. Since this model can be installed vertically or horizontally, the internal position of the diverter plug varies depending on the orientation of the radiator.
Diagram showing how a diverter works in a radiator to aid water flow.
How Radiators Are Pressure Tested for Leaks During Manufacturing
  • During the manufacturing process, all radiators undergo a pressure test to check for leaks, as shown in the image below.
  • The radiator is filled with pressurised air (similar to a car tyre) and then submerged in a water bath. The operator watches for bubbles rising from the bath — any bubbles indicate a potential leak. Depending on the manufacturer, the radiator is either repaired by welding or scrapped if it cannot be fixed.
  • Just like a central heating system, where pressure is measured in bars, radiators are typically tested at around 13 bars during this process to ensure durability and safety before leaving the factory.
Radiators being pressure tested during manufacturing process
Bleeding Radiators
  • Bleeding is the process of venting trapped air from a radiator. Over time, small amounts of air can enter the central heating system whenever fresh mains water flows through the boiler. This air collects at the top of radiators, preventing the top section from filling with warm water and reducing heating efficiency.
  • Regularly bleeding your radiators can dramatically improve the efficiency of your central heating. It’s a simple task that can usually be done yourself, but if you’re unsure or encounter problems, it’s best to call a plumber.
  • How to check if a radiator needs bleeding:
  • Turn on your heating and allow radiators to warm up.
  • Look for cool spots, especially near the top.
  • If the radiator remains completely cold, it may be full of trapped air and needs bleeding before it can heat the room properly.
  • How to bleed a radiator safely
  • Turn off the heating – you don’t want to burn yourself with hot water.
  • Gather your tools: a radiator bleeding key or a flat-headed screwdriver for modern systems.
  • Locate the bleed valve at the top corner of the radiator. Place a cloth underneath to catch any drips.
  • Slowly turn the valve anti-clockwise. You should hear a hissing sound as the trapped air escapes.
  • Once water starts to flow steadily, close the valve promptly to avoid excess water spilling out.
  • Turn the heating back on and check that cold spots have disappeared.
  • Regular bleeding ensures even heating throughout the radiator and helps maintain overall system efficiency.
Flushing Radiators
  • Flushing radiators is a professional job and not something Terma carries out or routinely discusses, but it’s useful to understand what it involves.
  • The process completely cleans the radiator, removing sludge and debris that can build up over time. This can significantly improve the efficiency and warmth of your heating system — in some cases, you may find your rooms heat so well that you won’t need to change your radiators at all.
  • Flushing is a more complex procedure and must be done by a qualified plumber. Typical costs run into a few hundred pounds. On older radiators, there is a small risk of revealing leaks, which may require a replacement radiator. Despite this, the improvement in system performance usually makes it worthwhile.
  • It’s also an ideal time to install a magnetic filter, such as Magnaclean, which helps prevent future sludge buildup. This is often done when a new boiler is installed, but it can also be retrofitted during a radiator flush to maintain a cleaner, more efficient system.
diagram explaining how you flush radiators

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