Hot Water Cylinder London — Repair & Replacement

A cylinder that runs out of hot water quickly has one of these causes: (1) The thermostat is set too low — below 50°C reduces usable capacity because hot and cold water mix in the cylinder. Set it to 60°C. (2) The cylinder is undersized for your household's demand. (3) The boiler isn't firing the cylinder circuit because the motorised valve is stuck or the programmer is incorrect. (4) In a vented system, the cold feed is gravity-fed and the pressure is too low to push hot water around the circuit effectively. An engineer can check all these in 30–60 minutes.

A vented cylinder gets its cold water supply from a cold water tank in the loft — the height difference provides water pressure (gravity pressure, typically 0.2–0.5 bar). An unvented cylinder connects directly to the mains cold water supply, providing hot water at mains pressure (1.5–3 bar in most of London). Unvented cylinders must be installed by a G3-qualified engineer and require safety components including pressure relief valve, expansion vessel and temperature/pressure relief valve.

A vented cylinder replacement (like-for-like) typically costs £600–£1,200 including labour, the new cylinder and connections. An unvented cylinder installation costs £1,200–£2,500 depending on cylinder size and complexity of pipework modifications. These prices include Gas Safe certification where gas work is involved and Building Control notification for unvented systems. Converting from a vented to an unvented system also involves removing the cold water tank from the loft.

Extremely hot hot water from a cylinder usually means the thermostat has failed in the open position — the boiler keeps heating the cylinder beyond its normal shutoff point. This is a safety issue: turn off the immersion heater and call us. The cylinder should also have a thermal cut-out that prevents dangerous overheating — we check this is functioning during any thermostat replacement.

No. Building Regulations Part G3 prohibits unvented cylinder repairs (other than simple controls like timer adjustments) by anyone other than a G3-qualified engineer. The consequences of incorrect repairs include cylinder failure at full mains pressure — potentially catastrophic. If you have an unvented cylinder (Megaflo, Telford, Heatrae Sadia, OSO, etc.) and suspect a fault, call a qualified engineer.

Legionella pneumophila bacteria can colonise hot water storage systems where water is held at temperatures below 55°C. The risk is primarily in commercial or institutional settings but exists in domestic cylinders that are set too low. Set your cylinder thermostat to 60°C — at this temperature, Legionella is killed within 2 minutes. If your cylinder has been at a low temperature for extended periods, run all taps and shower heads at maximum temperature for 5 minutes to flush the distribution pipework.

A dripping pressure relief valve or temperature/pressure relief valve (the safety valves on an unvented cylinder) indicates the cylinder pressure or temperature is exceeding normal parameters. This is a warning sign, not something to ignore. The valve is doing its job — preventing dangerous over-pressurisation — but the underlying cause (expansion vessel failure, thermostat fault, or excessive mains pressure) needs diagnosing and fixing. Call us — don't block or cap the dripping valve.

A vented cylinder replacement takes 3–5 hours. Converting to an unvented system (including removing the cold water tank from the loft and modifying pipework) takes 6–10 hours — usually one day. If there is significant pipework work involved or the loft tank is hard to access, allow 1–2 days. We give you a specific time estimate at the survey stage.

Yes — many London flats have unvented cylinders in airing cupboards. The mains water supply in most London flats is adequate for unvented cylinder installation. Key considerations: the freeholder may require notification for installation of a new cylinder; the expansion pipework must be able to discharge safely; and the mains stopcock supplying the flat must be accessible and functional. We check all these during a site survey.

If the cylinder body is sound (no corrosion, no pinhole leaks) and the fault is a thermostat, immersion heater or motorised valve, repair is almost always better value. Cylinder body replacement should be considered if: the cylinder is over 40 years old with signs of external corrosion; you've had two or more internal component failures in 3 years (indicating general deterioration); or you want to upgrade to unvented for better pressure. A good copper vented cylinder will last 40–60 years if the water chemistry is managed.