Hot water cylinder questions answered

In short

Short answers to the questions people ask most about hot water cylinders, grouped by topic. Each answer links to the full page, which has the checks, the safety limits and what an engineer does.

Hot water cylinder problems

What are the most common hot water cylinder problems?: No hot water from a failed element, thermostat or zone valve, leaks at valves and joints, and a dripping relief valve on unvented cylinders. A timer or fuse fault often looks like a cylinder fault. Read: Hot Water Cylinder Problems: Find Your Symptom
Why is my hot water cylinder leaking from the bottom?: Water runs downhill, so a wet base can come from a valve, boss or joint higher up. Dry the base, then trace the moisture upwards. Read: Hot Water Cylinder Leaking? Leak Repair in London
What happens after the diagnosis?: If it is an easy fix, the team fixes it straight away. If parts are needed, the team orders them and gives a quotation for the fix, priced by the hour or as a fixed price. Read: Hot Water Cylinder Diagnosis in London
Do plumbers fix hot water cylinders?: They replace the failed parts: elements, thermostats, zone valves, relief valves, expansion vessels and joints. The shell itself cannot be repaired, so a leaking or corroded shell means replacing the cylinder. Read: Hot Water Cylinder Repair in London
How long does it take to change an immersion heater?: It depends on whether the cylinder has to be drained and how accessible the heater is, so no fixed time applies. The testing comes first and decides what is needed. Read: Immersion Heater Replacement in London
Why do I have heating but no hot water?: On a cylinder system the hot water has its own programmer channel, thermostat and valve. If any of them fails the radiators carry on while the cylinder does not heat. Read: No Hot Water but the Heating Works: Check the Cylinder
What should I do if my hot water cylinder is leaking?: Close the cylinder cold supply, switch off the immersion heater and the boiler hot water, then trace the water to its source. Call an engineer if water is coming from the shell or if an unvented cylinder is discharging steam or very hot water. Read: Hot Water Cylinder Leaking: Find the Source First
How do I test a cylinder thermostat?: An engineer isolates the circuit and checks whether its contacts close when the water is cold. You can check the dial setting and whether the cylinder warms on its other heat source, but do not open the cover yourself. Read: Hot Water Cylinder Not Heating Up: Which Heat Source Failed?
Why is my hot water not working but my cold water is?: Cold water comes from the mains or the cold tank. Hot water depends on a heat source warming the cylinder first. Read: Hot Water Not Working? Is It the Cylinder?
Where is the reset button on my hot water cylinder?: It is usually a small red button on the immersion thermostat. It is often behind a cover, in which case it is not a householder job. Read: How to Reset a Hot Water Cylinder (and When Not To)
How do I reset my immersion heater?: On most immersion heaters the high-limit cut-out reset button is behind the cover, which is not a householder job. Only a button reachable without removing any cover may be pressed, once, with the circuit isolated at the consumer unit first. Read: Immersion Heater Not Working: What to Test, in Order

Cylinder replacement

How do I know if my cylinder needs replacing?: A leaking shell or seam, a failed sealed coil, or several worn parts at once. A single failed part on a sound cylinder is repaired. Read: Hot Water Cylinder Replacement: Repair or Replace?
How do I know my cylinder needs replacing and not repairing?: Water coming from the shell or the coil, rather than from a union, valve or element boss, means the cylinder has failed. Anything else is a part that can usually be changed. Read: Hot Water Cylinder Replacement in London, Like for Like
Can you fit an unvented cylinder yourself?: It is not advisable. Approved Document G expects installers with the skills for a sealed, mains-pressure system, with the work notified to building control. Read: Can I Replace a Hot Water Cylinder Myself?
Can a hot water cylinder last 20 years?: Yes, a well-kept cylinder in moderate water can pass 20 years, and manufacturers often back stainless steel vessels with long warranties. A warranty is not a service life, though. Read: Hot Water Cylinder: Repair or Replace?
How long does an unvented cylinder last?: Manufacturers publish warranties, not lifespans. Those cover the vessel under the maker’s conditions, and parts such as valves and heaters are covered for far less. Read: How Long Does a Hot Water Cylinder Last?
Can you use hot water straight after a cylinder is replaced?: Not at once. The new cylinder has to be filled, purged of air and brought up to temperature, and how long that takes depends on its size and heat source. Read: How Long Does It Take to Replace a Hot Water Cylinder?

Unvented cylinders

What are the common problems with unvented cylinders?: A dripping relief valve or tundish, a flat expansion vessel, scale in the strainer or tundish and a tripped thermal cut-out. Read: Unvented Cylinder Problems and the Safety Chain
Why is my TPRV dripping on an unvented cylinder?: Check which valve it is. The discharge may come from the expansion relief valve, lifting because the expansion vessel has lost its charge. Read: TPRV and Expansion Vessel Repair in London
Why is my unvented cylinder not heating?: Check what feeds it first: programmer, timer, fuse and isolator. After that the likely causes are a tripped cut-out, a failed thermostat, a stuck motorised valve or a failed element. Read: Unvented Cylinder Repair in London
Do I need building regulations approval to install an unvented cylinder?: Yes. The work is notifiable under Part G. Read: Unvented Cylinder Installation in London
Do hot water cylinders need servicing?: Unvented cylinders do. They are pressurised and rely on safety valves, an expansion vessel and thermal cut-outs that wear. Read: Unvented Cylinder Service in London
Can an unvented cylinder explode?: Only if its safety devices have been removed, blocked or bypassed so that heat and pressure build with no way out. With the thermal cut-out, relief valves and discharge pipe intact, an overheating cylinder vents through the tundish. Read: Are Unvented Cylinders Safe?
Why is my expansion tank filling with water?: The air side of the vessel is meant to stay dry. Water reaching it means the diaphragm has split, so water is on both sides. Read: Waterlogged Expansion Vessel on an Unvented Cylinder
Is it normal for a TPRV to drip occasionally?: No. A relief valve is meant to open only when something is outside its limits. Read: TPRV Dripping on an Unvented Cylinder
Should there ever be water in a tundish?: No. In normal use the tundish is dry. Read: Tundish Dripping: What It Is Telling You
Do the same rules apply in Scotland and Northern Ireland?: No. This page describes Approved Document G for England. Read: Unvented Cylinder Regulations: Part G and What You Should Hold
Why does my hot water heater keep cutting out?: On an unvented cylinder the usual cause is a thermostat that is not switching the heat off, so the high-limit thermal cut-out operates instead. Read: Unvented Cylinder Cut Out: Which Part Has Tripped?
Is an unvented cylinder better than a vented one?: Not automatically. It gives better pressure where the mains supply is strong, and it saves loft tank space. Read: Vented vs Unvented Cylinder: How Each One Fails

Cylinder costs

Do the fixed prices carry VAT or extra fees?: There is no call-out fee, and no VAT is charged. The fixed prices in the first table are what is charged for each job. Read: Hot Water Cylinder Cost: Fixed Prices and Typical UK Market Ranges
How much does a plumber charge to replace a hot water cylinder?: BookaBuilderUK gives a typical UK market range of £600 to £1,500 for labour on a standard unvented replacement, with the cylinder and valve parts on top. It is a market figure, not an LCS price. Read: Hot Water Cylinder Replacement Cost London (2026 Guide)
How long does it take to fit an unvented cylinder?: It depends on the site check. A swap that keeps the arrangement is shorter than a conversion, which adds pipework, a discharge route and electrical work. Read: Unvented Cylinder Installation Cost UK: Line Items to Expect
How often should a hot water cylinder be serviced?: Follow the manufacturer instructions for your cylinder. Heatrae Sadia, for one, recommends maintenance every 12 months by a competent person for its Megaflo Eco Plus range. Read: Unvented Cylinder Service Cost UK: What the Price Should Cover