How to use radiator valve tail extensions

In theory, fitting a new radiator should be a straightforward job. You take the old one off, pop the new one on, reconnect everything, and enjoy a warmer home.

However, in reality, many people discover an unexpected problem, which is that the new radiator is slightly different in width or construction, and the existing pipework no longer lines up with the radiator valves. Even a gap of just a few millimetres is enough to stop the installation running as smoothly as you might have expected.

Fortunately, you don’t usually need to move pipework or call out a plumber straight away. Radiator valve tail extensions offer a simple, reliable way to bridge those small gaps and create a neat, watertight connection.

In this article, we'll explain what radiator valve tail extensions are, why and when you need them, the different types available, and how to install them safely and correctly. Everything is written with a beginner in mind, so even if you’ve never done any plumbing before, you’ll understand what’s involved.

Please note, if you’re not confident with DIY or you’re unsure about anything within this article, it’s always best to call a qualified professional to do the job for you.

• Further reading: How do radiator valves work? A complete guide

Table of Contents

1. What are radiator valve tail extensions?
2. When should you use a tail extension?
3. Preparation and safety
4. How to install a radiator tail extension - step by step guide
5. Important compatibility tips
6. Tail extensions at Trade Radiators

What are radiator valve tail extensions?

Radiator valve tail extensions are small, rigid pieces of pipe designed to extend the reach of your radiator valves. They are most commonly used when a new radiator is slightly shorter than the old one, leaving a small horizontal gap between the radiator’s inlet and the valve.

Most modern radiators in the UK use a ½-inch BSP threaded inlet, and most radiator valves connect using a 15mm compression fitting. A tail extension bridges these two points by having:

• A male ½" BSP thread on one end (which screws directly into the radiator),
• A 15mm compression end on the other (which connects to your valve or pipework using an olive and nut).

Because they use these universal sizes, tail extensions are widely compatible and easy to fit.

Rigid extensions are available in set lengths, such as 20mm, 30mm, 40mm, right up to 100mm. These fixed lengths are the most reliable option for domestic installations, offering a solid, leak-resistant connection.

Where possible, avoid using telescopic or adjustable tail extensions. While they may sound convenient, experienced plumbers often report they are more prone small leaks because they rely on multiple seals internally. A good, solid, chrome-plated fixed extension is almost always the best choice, such as the tail extensions we stock at Trade Radiators.

• Further reading: How to replace a broken radiator valve

When should you use a tail extension?

Many people come across tail extensions when replacing radiators in an older property. Pipework was originally fitted to suit the exact radiator that was in place at the time. If your new model isn’t the same width, the valves may no longer reach.

There are two common scenarios where tail extensions are the perfect solution:

1. Lateral gaps

If your old radiator was, for example, 500mm wide and your new one is 480mm, you'll be left with a 20mm gap. You might use a 10mm tail extension on each side, instantly restoring the correct pipe centre distance without altering the pipework.

This situation accounts for the vast majority of extension tail installations.

2. Changing radiator type or style

Even if the width is identical, different radiator types can have different distances between the bracket faces and the valve inlets. This is particularly common when upgrading to:

• A Type 22 (double panel, double convector) radiator
• Designer radiators
• Column radiators

Different constructions can shift the valve inlets slightly outward or inward, meaning your existing valves no longer sit comfortably in place. So, when planning any radiator installation, measuring and being aware of the the pipe centres is essential when selecting your tail extensions. 

• Further reading: The complete guide to fitting a radiator

Preparation and safety

Working on a radiator means dealing with pressurised water, so safety and preparation are very important. A beginner can absolutely carry out this job, but it must be done carefully.

Switch off your system

1. Turn off your heating system at the boiler.
2. Allow everything to cool completely before you begin.

Depressurise the system

If you have a sealed, or pressurised, system, you’ll need to reduce the pressure before disconnecting anything.

• Place a towel under a radiator bleed valve.
• Open the valve slightly using a radiator key.
• Allow the boiler’s pressure gauge to drop to zero.
• An open-vented (tank-fed) system may require isolating valves or draining down the relevant section.

Protect the area

Lay towels beneath the valves and place a plastic container underneath to catch drips. With the area safe and the pressure reduced, you're ready to get started.

• Further reading: How to refill or repressurise a combi boiler

How to install a radiator tail extension - step by step guide

This is the core of the process and the part most beginners worry about. Don’t be put off as each step is simple when taken slowly. However, if there is any part of the process that you think is beyond your capabilities, don't take the risk and please call in a professional.

Tools you’ll need

• Adjustable spanner or radiator spanner
• PTFE tape
• Tape measure
• Pipe slice (optional, useful if trimming is ever needed)
• Towels and a bucket
• Jointing compound (optional but recommended for extra sealing confidence)

1. Measure the exact gap

Place the radiator up to the brackets and check how far the valve is from reaching the inlet.

Measure the gap accurately. If the gap is 18mm, choose a 20mm extension. You can fine-tune the final position using the compression joint.

Rigid extensions rarely match the gap exactly, but getting within a couple of millimetres is perfectly acceptable.

• Further reading: How to measure a radiator

 2. Disconnect and clean the existing fittings

With the valves closed and pressure reduced:

• Loosen the compression nut connecting the valve to the pipework.
• If necessary, loosen the union nut joining the valve to the radiator and remove the valve entirely for better access.
• Wipe away old paint, rust flakes, or debris. Nuts and olives must move freely for a reliable seal. Even small bits of old paint can prevent nuts tightening properly, so take your time here.

 3. Prepare the extension by sealing the threaded side

This is the most important part of achieving a watertight finish and avoiding leaks.

• Hold the extension so the threads face you.
• Wrap 6–10 turns of PTFE tape clockwise around the male ½" BSP thread. (Doing it clockwise is important because if you wrap it anticlockwise, it will unravel as you screw it in.)
• Press the tape firmly onto the threads so it sits neatly.
• For extra peace of mind, apply a tiny smear of non-setting jointing compound over the PTFE. Beginners often underuse PTFE tape, so if in doubt, use another couple of wraps.

 4. Screw the extension into the radiator

• Screw the newly taped extension tail into the radiator’s inlet and tighten by hand until snug.
• Use a spanner to tighten further, but avoid over-tightening.
• Ensure the extension points squarely towards the valve or pipework. Misaligned fittings create strain and can lead to leaks later on.
• Rigid extensions offer a strong, reassuring feel when tightened, which are much more solid than telescopic versions.

 5. Prepare and tighten the compression connection

• Slide the compression nut and olive onto the pipe or valve spigot.
• Push the extension firmly up to the olive.
• Tighten the compression nut by hand.
• Hold the body of the valve with one hand while tightening the nut with a spanner. Supporting the valve body is important as it prevents twisting forces being transmitted to the pipework behind the wall or floor, which could cause hidden leaks or pipe damage.
• Tighten until firm, but please note that compression joints do not need to be too tight as doing so can deform the olive.

 6. Repressurise the system

• Once everything is back together, close any bleed valves.
• Refill the system slowly using your boiler’s filling loop until the pressure reaches around 1.0–1.5 bar when cold.
• Bleed the radiator to remove air pockets.
• Top up pressure again if needed.
• Turn the heating on and allow the radiator to warm up fully. Heating increases pressure, which helps reveal any small leaks early.

 7. Check for leaks 

• Use a piece of dry kitchen roll to dab around:
  • the threaded joint
  • the compression nut
  • the valve body.
• If you spot a damp patch, turn the system off, let it cool, and tighten the compression joint by a small fraction of a turn.
• Most minor leaks occur because the olive wasn’t seated evenly or the compression nut wasn’t tightened enough. Both are easy fixes.
• Further reading: The complete guide to radiator leaks

Important compatibility tips

Even though radiator valve tail extensions are simple fittings, there are a few important concepts that beginners often overlook. Understanding these will help you buy the right components, avoid leaks, and make sure your installation looks tidy once finished.

Union valves vs. compression valves

Many modern UK radiators use compression valves, but older systems, particularly those from the 1980s and 1990s, often use union valves. The two look similar from the outside, but they work in very different ways.

A compression valve uses a 15 mm compression fitting on the radiator side. It has a nut and a small ring (the “olive”) that clamp tightly around the pipe or extension tail when tightened. This is the type almost all tail extensions are designed for.

A union valve, however, is part of a matched two-piece set. The radiator comes with its own specific “tail”, and the valve has a nut that mates precisely with that tail. Because the two parts are made to work together as a pair, you cannot simply swap the tail or extend it using standard fittings.

If you have union valves and want to use rigid tail extensions, you’ll almost always need to replace the valves with modern 15 mm compression valves. Thankfully, this is straightforward, and it also gives you the opportunity to upgrade to stylish thermostatic radiator valves (TRVs) or modern lockshield valves.

• Further reading: The best thermostatic radiator valves

Microbore systems

Some homes, especially those built or refurbished in the 1970s through the early 2000s, use what’s called a microbore central heating system. Instead of the more familiar 15 mm copper pipes, the radiator pipework is much narrower, usually 8 mm or 10 mm.

Microbore pipework was designed to be compact and easy to route through floors and walls, but the smaller diameter means it can’t directly connect to standard-sized radiator fittings.

Because radiator valve tail extensions are designed for 15 mm compression fittings, you’ll need a microbore-to-15 mm reducer if your system uses narrow pipework. These small adapters simply step up the pipe size so you can attach a normal valve, olive, or extension tail.

It’s important to measure the pipe before buying anything, as many homeowners assume they have 15 mm pipes when they actually have 10 mm. Buying the wrong size fittings can lead to wasted time and unnecessary frustration.

Pipe centres and radiator depth

Before ordering any radiator or extension tail, it helps to understand the term 'pipe centres', because it’s one of the most important measurements in heating installation.

Pipe centres are the distance from the centre of one radiator pipe to the centre of the other. If your existing pipe centres are 600 mm apart and you buy a radiator with 580 mm pipe centres, you’ll have a 20 mm gap to fill, which is exactly the sort of problem a radiator tail extension solves.

Beginners often also wonder whether the depth of the radiator, or the distance it “sticks out” from the wall, actually matters. In most cases, the depth does not affect pipe centres, as these refer to left-to-right spacing. However, a deeper radiator (such as switching from a Type 11 to a Type 22) may subtly alter the way the valve connects because the valve has to reach slightly further forward. This is rarely a major issue, but it can affect how the valve sits or angles.

If the projection from the wall changes significantly, you may need minor adjustments to the valve position or pipework, but for small changes, good-quality compression valves and tail extensions usually compensate without trouble.

• Further reading: A full range of radiators listed by their pipe-centre distances

Pipe sleeves and collars

Once your tail extensions and valves are fitted, adding pipe sleeves and collars is an easy way to make the installation look smart and professional.

A pipe sleeve is a decorative tube that slides over the copper pipe between the floor or wall and the radiator valve. It hides the bare copper, paint splashes, or small imperfections around the pipe entry point.

A collar (also known as a floor plate) is a small disc that sits around the pipe at the point it enters the floor or wall. It neatly hides the rough hole and gives a clean, finished look.

When choosing pipe sleeves and collars, simply match:

• the pipe size (usually 15 mm in standard systems)
• the finish (chrome, brushed steel, brass, black, white, etc.)
• the overall style of your valves and radiator

Trade Radiators stocks a wide range of sleeves (which usually include the floor plates) in different lengths, finishes, and styles, so you can easily find a set that complements your valves and matches the look of your room.

• Further reading: What are pipe sleeves used for?

Tail extensions at Trade Radiators

Radiator valve tail extensions are a simple, budget-friendly fix for one of the most common issues people face when installing or upgrading a radiator; that of pipework that doesn’t quite line up. Rather than moving pipework or replacing valves, a tail extension adds a small, precise amount of extra length so everything connects neatly. They’re ideal for beginners because they’re straightforward to fit. Just remember to take your time, use PTFE tape to seal the threads properly, and always check for leaks once the system is refilled.

If you ever feel out of your depth, especially when draining or repressurising a heating system, it’s perfectly sensible to call in a professional. A leak-free, reliable installation is always the priority.

When you’re ready to go ahead, Trade Radiators has a full range of options to help you get the job done:

• Rigid radiator tail extensions - available in multiple lengths and finishes to suit most radiators and valve types.
• Valves, TRVs, and lockshield sets - so you can upgrade or replace components at the same time if needed.
• Pipe centre tools and guides - to help you measure accurately and avoid common alignment issues.

With the right fittings and a bit of guidance, installing a radiator tail extension is a quick and useful DIY task that makes your heating setup work exactly as it should.