Intro
On most jobs, a small drip can turn into a big mess fast. Fixing a leaking radiator valve is simple when you follow a clear plan. In this guide, we explain what causes the leak, the tools you need, and how to stop it for good. We cover packing gland fixes, union weeps, and full valve swaps. You’ll see pressure targets, PTFE tips, and timeframes. If you’ve got water on the floor and a client on your phone, this shows you how to act fast. It’s straight talk and real steps you can do today.
Quick Answer
To stop a leaking radiator valve, isolate the valve, drain 2 buckets (about 10–20 liters) if needed, then tighten or repack the gland, or reseal the union with 10–15 PTFE wraps. Refill to 1.0–1.5 bar, bleed, and recheck after 30 minutes. If corrosion is heavy, replace the valve.
Table of Contents
Key Takeaways
- Most leaks stop with a quarter-turn on the gland or fresh PTFE.
- Typical repairs take 30–60 minutes, plus 10–15 minutes for testing.
- Common sizes: 15 mm pipe, 1/2" BSP radiator tail, 24–30 mm union nut.
- Refill sealed systems to 1.0–1.5 bar and bleed top-down.
- Replace the valve if the body is pitted, cracked, or seized.
Why Radiator Valves Leak
Small leaks have a few common sources. Knowing which one saves you time.
- Packing gland under the handle: The small nut around the spindle dries out. You see a slow drip down the stem. A quarter-turn tighten or a bit of PTFE string usually stops it.
- Union joint to the radiator tail: The big nut by the radiator weeps. Often from disturbed joints, old fiber washers, or poor PTFE on the tail.
- Compression joint on the pipe: The olive didn’t bite well or the pipe has a scratch. It seeps under pressure.
- Valve body damage: Corrosion, cracks, or stress from poor alignment. If the body is pitted, replace, don’t repair.
Spot the source first. Dry the area with a rag. Use tissue to track the drip. Work from top to bottom.
Have the right kit ready before you shut water off.
- Adjustable spanner (200–300 mm) and grips
- Open-ended/box spanners for union nuts (commonly 24–30 mm)
- PTFE tape (gas-grade is fine) and PTFE string
- New fiber washer(s) and olives (15 mm)
- Radiator valve (15 mm, angled or straight) if replacing
- Buckets (2), towels, and a wet vac if you have one
- Radiator bleed key and hose for draining
- Inhibitor for refill if system is drained
- Pressure gauge on combi/pressurized systems
Prep and Safety Before You Start
Get the system safe and the work area ready.
- Cool down: Let the heating cool for 1–2 hours. Hot water burns and expands, which makes leaks worse.
- Isolate: Turn the boiler off. Close the radiator valve you’re fixing. Close the lockshield on the opposite side (note its turns to reset later, often 1–2 turns open).
- Protect: Lay towels and a tray under the valve. Place 2 buckets nearby.
- Drain smart: If the leak is at the gland, you can often fix it wet. If at the union or compression, drain the radiator only. Use the bleed screw and the union nut to control flow.
Most contractors skip photos. Don’t make that mistake. Take quick photos of settings before you start.
Step-by-Step: Fixing a Leaking Radiator Valve
Follow these steps in order. The goal is to stop the leak and leave the system balanced.
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Identify the leak point
Dry everything. Wrap tissue around the gland, union, and compression. Watch where it wets first.
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Try a gentle gland tighten
On the spindle gland nut, tighten 1/8–1/4 turn. Don’t overdo it. Open the valve and check. If it still weeps, proceed.
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Repack the gland
Close both sides. Crack the gland nut and lift it slightly. Wind PTFE string once or twice around the spindle (not too thick). Reseat the nut and tighten gently. Reopen the valve and test.
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Reseal the union to the radiator
If the union leaks, close both sides. Place a bucket. Loosen the big union nut (commonly 24–30 mm). Drain 2–5 liters. Remove the nut and check the radiator tail. If it uses a fiber washer, replace it. If it’s a tapered tail, add 10–15 wraps of PTFE tape on clean threads. Refit and tighten snug, not brutal.
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Reseat a compression joint
If the pipe compression joint weeps, back off the nut. Inspect the olive. If it’s scored or oval, replace it. Add a smear of jointing compound if you use it, or 3–4 wraps of PTFE on the olive. Reassemble and tighten firmly while supporting the valve body.
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Align and support
Misalignment causes repeat leaks. Ensure the valve, tail, and pipe sit square. Support the opposite side with a second spanner to avoid twisting the radiator.
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Partial valve replacement (if needed)
If the body is damaged or seized, plan a full swap. Drain the radiator fully (5–10 liters for a typical panel). Replace like-for-like: 15 mm, 1/2" BSP tail. Use 10–15 PTFE wraps on the new tail. Fit a fresh olive on the pipe. Tighten both ends while supporting the valve.
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Reopen and pre-check
Close the bleed screw. Open the lockshield and valve 1 turn each to pre-fill the radiator. Check for drips for 2–3 minutes.
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Bleed air
Open the bleed screw a quarter turn with a key. Catch air and water. When water runs steady, close it.
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Restore settings
Return the lockshield to its original position (e.g., 1–2 turns). Set the TRV or manual valve as requested.
Testing, Refilling, and Bleeding
Testing is where many call-backs start. Slow down and verify.
- Pressure targets: On sealed systems, refill to 1.0–1.5 bar cold. On open vented, fill until header tank level is correct.
- Leak check: Wipe all joints dry. Watch for 5 minutes. Tissue test again. Recheck after 30 minutes of heat-on.
- Heat cycle: Run the system for 15–20 minutes. Warm metal expands and will show a bad seal.
- Final bleed: Bleed highest radiators first. Then recheck pressure and top up to 1.2–1.5 bar.
- Add inhibitor: If you drained more than one radiator, add inhibitor per label (often 250–500 ml for small systems).
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Repair vs. Replace: Make the Call
Not every leak needs a new valve. Use these simple rules on site.
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Repair it if:
- The gland seeps and responds to 1/8–1/4 turn or repacking.
- The union washer is flat or the tail PTFE was poor.
- The compression olive was mis-seated and now seals after reset.
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Replace it if:
- The valve body is pitted, cracked, or twisted.
- The spindle is loose with side play, or the TRV head failed.
- The leak returns after heat-up and two proper reseals.
Typical replacement takes 45–90 minutes including drain, swap, refill, and bleed. Quote clearly for parts, inhibitor, and testing time. If you need help with pricing structures, check our advice on invoice templates and clear scope notes.
FAQ
Do I need to drain the whole system?
Usually no. For fixing a leaking radiator valve at the gland or union, you can drain just that radiator. Close the valve and lockshield, loosen the union, and catch 2–5 liters. Drain the whole system only for widespread valve swaps or major corrosion.
How many PTFE wraps should I use on the radiator tail?
For most 1/2" BSP tails, 10–15 wraps of PTFE tape works well. Keep wraps neat, tight, and in the thread direction. On the gland, PTFE string used sparingly seals better than thick tape. Too much material makes assembly hard and can crack parts.
What pressure should I refill to after the repair?
On sealed systems, refill to 1.0–1.5 bar when cold. Many contractors aim for about 1.2 bar. After bleeding air, top back up to the same level. On vented systems, just restore the header tank level and ensure the feed and expansion pipe are wet.
Can I fix a leaking valve without turning the boiler off?
Turn the boiler off. Working with hot, pressurized water is risky and makes leaks worse. Let it cool 1–2 hours. Close the radiator valves, control the drain, and repair safely. You’ll do a better job and avoid burns.
How long should a repair take?
Most gland or union fixes take 30–60 minutes, plus 10–15 minutes of testing and bleeding. Full valve swaps run 45–90 minutes depending on access, paint, and alignment. Add time if you’re replacing multiple valves or dosing inhibitor.
Conclusion
Fixing a leaking radiator valve is about three things: identify the leak point, seal it correctly, and test under heat. Use 10–15 PTFE wraps on tails, refill to 1.0–1.5 bar, and recheck after a full heat cycle. Next steps you can take today:
- Photograph valve settings before you start.
- Try a 1/8–1/4 turn on the gland, then repack if needed.
- Reseat unions with fresh washers and neat PTFE.
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