When it comes to heating your home, most people think about their boiler, radiators, or thermostat—but rarely do they consider the importance of the pipes connecting everything together. In reality, the size of your central heating pipes plays a huge role in how efficiently and effectively your system runs. Choosing the wrong pipe size can lead to slow heating, increased energy bills, or even damage to your boiler. In this blog, we break down everything homeowners need to know about central heating pipe sizes, from the common types used to how pipe diameter affects water flow, pressure, and heat loss.
Whether you’re planning a new heating system or upgrading an old one, understanding the role of pipe sizing will help you make informed decisions and ensure your home stays warm and energy-efficient all year round.
The central heating system in your home works by circulating hot water from the boiler to radiators via a network of pipes. Once the radiators have released the heat into your rooms, the cooler water returns to the boiler to be reheated. This continuous cycle depends on a smooth, unrestricted flow of water—and that’s where pipe size becomes important.
Pipes that are too small will struggle to carry enough hot water, reducing heat output and putting extra pressure on your boiler and pump. On the other hand, pipes that are unnecessarily large can make installation more expensive, take up more space, and result in slower heating due to reduced water velocity. Striking the right balance is key to an efficient and cost-effective heating system.
The most common pipe size used in UK domestic central heating systems is 15mm. These pipes are typically used for radiator circuits, meaning the individual branches that connect each radiator to the system. A 15mm pipe is suitable for most standard radiator setups and can handle a heat output of up to approximately 13,620 BTUs (British Thermal Units).
In practical terms, you should aim to limit each 15mm branch to no more than three radiators. Exceeding this can lead to reduced flow, uneven heating, or excessive noise in the system. For most small to medium-sized homes, 15mm pipework provides the right combination of performance and efficiency at a reasonable cost.
Larger homes or systems with multiple radiators often rely on 22mm pipes for the main supply lines—those that connect the boiler to the rest of the network. These pipes can handle greater flow rates and higher heat demands, making them ideal for feeding several branches or linking to a hot water cylinder.
Using 22mm pipework in your main lines helps reduce pressure drops, ensures even heat distribution across your home, and minimises energy loss. This size is particularly important in gravity-fed systems or where pipe runs are long and resistance is higher.
While not commonly used in standard residential settings, 28mm or larger pipes may be recommended in specialised installations. For example, heat pump systems, large multi-storey homes, or underfloor heating setups may benefit from these larger diameters due to their need for higher water flow.
That said, using oversized pipes where they aren’t necessary can increase installation costs and reduce system responsiveness. Water moves more slowly through larger pipes, which can affect how quickly your radiators heat up. For this reason, pipework should always be sized based on the heating load and system layout—not just “bigger is better.”
Getting your central heating pipe sizes right is about more than just fitting pieces together—it directly impacts the flow of water and the overall efficiency of your system.
Water flowing through your heating pipes must move at the right velocity to maintain efficiency. If the flow is too slow (because the pipes are too large), it can cause sluggish heating and increase the time it takes for your home to warm up. On the other hand, if the pipes are too small, water may rush through too quickly, leading to noise, pressure build-up, and premature wear on components like pumps and valves.
An optimal water velocity is typically between 0.5 and 1.5 metres per second. This range helps to ensure a quiet, efficient, and responsive heating system. Well-sized pipework helps your pump work less hard, reduces electricity usage, and lowers the likelihood of breakdowns.
One of the biggest benefits of correctly sized pipes—especially the 22mm variety—is the reduction of pressure drop across the system. As hot water moves through your pipes, resistance builds up, which slows down the flow and affects performance. Larger pipes offer less resistance, which helps maintain consistent pressure and ensures that all radiators, even the ones farthest from the boiler, receive a steady supply of hot water.
If your radiators are heating unevenly or the system feels sluggish, it could be a sign that your pipework is too narrow or poorly routed. Replacing key sections of 15mm pipe with 22mm pipe can often restore performance without overhauling the entire system.
Another consideration is heat loss during circulation. As water travels from your boiler to your radiators, it naturally loses some heat through the pipes. Pipes that are too small tend to cause more heat loss per metre, especially if they’re not insulated.
Using 22mm pipework for long runs or main lines can reduce heat loss because there is more surface area and less resistance, allowing the water to travel more efficiently. Pairing the right pipe size with quality insulation further improves performance and reduces wasted energy.
Poorly sized pipes—whether too small or too large—can lead to several long-term issues. Undersized pipes can cause noisy radiators, inconsistent heating, pump failure, and higher energy bills. The strain on your system also leads to more frequent repairs and shorter equipment lifespans.
On the flip side, oversized pipes cost more to install, require more space (which can be a concern in modern homes), and slow down system response time. While some homeowners think larger pipes are a future-proofing strategy, in reality, they’re only beneficial when the system’s heat demand justifies their use.
The best way to determine the correct central heating pipe sizes for your home is to assess your system’s total heat requirement. This includes the number of radiators, the heat output of each, the distance between the boiler and the farthest radiator, and the overall layout of your home.
For most homes in the UK:
15mm pipe is suitable for radiator branches and short runs.
22mm pipe should be used for main supply lines from the boiler and for longer pipe runs.
28mm pipe or larger may be required for high-demand systems, such as those using heat pumps or serving large buildings.
If you’re unsure, it’s always best to consult a qualified heating engineer who can assess your needs and create a tailored solution. At Heroes on Hand, we perform a thorough evaluation before every installation to ensure your pipework matches your system and delivers consistent, cost-effective heat.
If you live in an older property, your existing central heating system may use outdated pipe sizes or layouts. In many cases, these systems were designed for lower efficiency boilers and smaller heating demands. Upgrading your boiler without upgrading the pipework can create imbalances in pressure and flow.
Replacing old or narrow pipework with the appropriate modern size can significantly improve your system’s performance. It can also reduce energy bills and extend the life of your boiler and other components. Upgrades don’t always have to mean a full replacement—targeted pipe improvements can yield excellent results.
Choosing the right central heating pipe sizes is a crucial step in building or upgrading an efficient home heating system. Pipes that are too small can choke your system and drive up energy costs, while pipes that are too large add unnecessary expense and slow down performance. The right pipe size ensures water flows at the correct speed, reduces pressure drop, and keeps your entire system running smoothly.
Whether you’re planning a full heating system installation or just making improvements to your existing setup, understanding the role of pipe sizing will help you make smarter decisions. At Heroes on Hand, we take pride in helping homeowners across London choose the most effective heating solutions for their homes. From boiler upgrades to full pipework redesigns, our qualified engineers are here to help you maximise comfort and efficiency.
If you’re unsure about your current pipe sizes or want advice on improving your system’s performance, get in touch with us today for a friendly consultation.
Our expert engineers can help with all kinds of boiler repair and maintenance issues
Boiler Repair
If you need help with a plumbing, heating or gas issue then please contact us by calling 020 8419 8866 or you can book an engineer directly by selecting one of the options below.
Read more posts below:
As the temperature drops, ensuring your heating system is ready to handle extreme cold is crucial.
Cooking gas is an essential utility in kitchens across the globe. The efficiency, control, and availability of gas make it a popular choice for both domestic and professional cooking.
The installation of boiler equipment is a key investment in your home’s comfort, energy efficiency, and long-term value.
If you need help please contact us using the details below or fill out the form and we'll be straight in touch.
