How to Fix a Hot Water Cylinder Leaking
Last updated: 30 September 2026
Turn off the water and heating, identify the leak source, and call a plumber for cylinder replacement.
Overview
A leaking hot water cylinder is a serious issue that can cause water damage. This guide covers diagnosing and responding to a leaking cylinder.
Step-by-Step Fix
Before beginning any repair, ensure you have isolated power or water supplies. Clear your workspace, gather standard hand tools, and wear appropriate personal protective equipment (PPE).
- Turn off the water supply to the cylinder at the isolation valve or the main stopcock. Switch off the boiler or immersion heater to prevent the cylinder from heating up further and increasing pressure. This is essential for your safety.
- Identify where the water is leaking from. The most common leak points are the immersion heater boss the pipe connections and the cylinder body itself. Dry the cylinder with a cloth and observe where water reappears.
- If the leak is from the immersion heater boss the gasket may have perished. Turn off the power to the immersion heater and drain the cylinder to below the level of the immersion boss. Remove the immersion heater and replace the gasket.
- If the leak is from a pipe connection tighten the compression nut carefully with an adjustable spanner. If the leak persists the olive inside the fitting may need replacing. Turn off the water and drain the pipe section before replacing the olive.
- If the leak is from the cylinder body itself the cylinder has corroded and must be replaced. Do not attempt to repair a leaking cylinder body with sealant. The corrosion will continue and the cylinder may burst.
- Place buckets and towels under the leak to catch water. If the leak is significant turn off the water supply completely to minimise the flow. Contact a qualified plumber for cylinder replacement.
- Do not use the boiler or immersion heater while the cylinder is leaking. The leak may cause the system pressure to drop which can damage the boiler. Turn off the heating until the cylinder is repaired.
- A new cylinder must be installed by a qualified plumber. The plumber will drain the system remove the old cylinder install the new one and reconnect all pipework. This typically takes a full day.
- After the new cylinder is installed check the pressure in the system and bleed any air from the radiators. Run the heating and hot water to test that everything is working correctly.
- Monitor the new cylinder over the next few weeks for any signs of leakage. If the problem persists contact the plumber who installed the cylinder. Regular maintenance including an annual service helps prevent cylinder problems.
Common Symptoms & Diagnostic Resolutions
Review the common error codes and operating symptoms below to quickly identify the root cause of the fault before ordering replacement parts.
| Symptom / Code | Likely Root Cause | Recommended Resolution |
|---|---|---|
| Leak from cylinder body | Cylinder has corroded and failed | The cylinder must be replaced by a qualified plumber. Do not attempt to repair a corroded cylinder. |
| Leak from immersion boss | Perished gasket around the immersion heater | Drain the cylinder replace the gasket and reinstall the immersion heater. |
| Leak from pipe connection | Loose compression fitting or worn olive | Tighten the fitting or replace the olive. Drain the pipe section if needed. |
| Water dripping from pressure relief valve | System overpressure or faulty valve | Check the system pressure. If the valve continues to leak replace the pressure relief valve. |
Frequently Asked Questions
Why is my hot water cylinder leaking?
Common causes include a corroded cylinder a perished immersion gasket a loose pipe connection or a faulty pressure relief valve. The leak location determines the fix.
Can I fix a leaking hot water cylinder myself?
You can tighten pipe connections and replace the immersion gasket. If the cylinder body is leaking it must be replaced by a qualified plumber. Do not attempt to repair a corroded cylinder with sealant.
How long does a hot water cylinder last?
A well maintained hot water cylinder typically lasts ten to fifteen years. Corrosion is the most common cause of failure. Regular servicing and checking the pressure relief valve extends the life of the cylinder.
How much does it cost to replace a hot water cylinder?
A replacement hot water cylinder costs between five hundred and one thousand five hundred pounds including labour. The cost depends on the size and type of cylinder and the complexity of the installation.