Unvented Cylinder Installation & Replacement in North West London

What does Part G3 require of an unvented cylinder?

Part G3 of the Building Regulations requires a hot water storage system to stop the stored water ever exceeding 100°C. Any discharge from its safety devices must also be carried to where it is visible but cannot harm anyone. Approved Document G explains how an unvented cylinder meets that. It needs at least two independent safety devices, such as a temperature relief valve and a thermal cut-out, in addition to the normal thermostat. The temperature relief valve should be fitted by the manufacturer, directly on the vessel, and never moved or disconnected except to replace it.

The cylinder should carry a permanent warning label telling the user not to adjust any component. It should also say to switch off and call the installer if hot water comes from the discharge pipe. If your cylinder has no label and no record of who fitted it, assume it needs checking.

How should the tundish and discharge pipe be installed?

The discharge pipework is the part of an unvented system most often done badly. It is also the part that protects people if everything else fails. Approved Document G sets out the arrangement.

• D1, from the valve to the tundish: a short metal pipe, no smaller than the valve outlet
• The tundish: vertical, in the same space as the cylinder, lower than the valve, with no more than 600 mm of pipe between them, and with an air gap
• D2, from the tundish: at least 300 mm of vertical pipe before any bend, then a continuous fall of at least 1 in 200
• D2 is metal, or another material proven and marked for the temperature, and at least one size larger than the valve outlet, larger again for longer runs
• It ends somewhere safe and visible, such as a trapped gully below a fixed grating, or low level outside with a guard

Should I choose a vented cylinder, an unvented cylinder or a combi?

The choice depends on your mains supply, how many bathrooms run at once and how much space you have. An unvented cylinder gives mains-pressure hot water to several outlets at once and frees the loft of tanks. It only works well if the incoming mains can deliver the pressure and flow the cylinder maker specifies, so we measure first. A vented cylinder fed from a loft tank is simple, tolerant of low mains pressure and cheaper to maintain. Pressure, though, depends on the height of the tank. A combi heats water on demand and needs no cylinder at all.

It suits a flat or a one-bathroom home, but struggles when two outlets run together. We do not install or replace boilers. If the answer is a new combi, we will tell you and refer you to a Gas Safe registered engineer. In an older house a cylinder upgrade can expose weak points. Old pipework and fittings that were fine at gravity pressure now see mains pressure, so we check them before we commit to a price.

How often should an unvented cylinder be serviced?

Check your cylinder's manual: many manufacturers ask for a yearly service by a qualified engineer, and some make it a condition of the warranty. A service checks that the safety valves open and reseat, and that the expansion vessel holds its charge. It also checks the incoming pressure reducing valve and strainer, the tundish for any sign of discharge, and the immersion heater and thermostats.

The Benchmark scheme, which covers domestic heating and hot water products, asks engineers to record each service in the service record on the Benchmark checklist. Keep that record with the cylinder paperwork. In a hard-water area such as Harrow, scale builds up on immersion elements and valve seats, and a yearly check catches a valve that has started to let by.

What should a landlord know about an unvented cylinder?

An unvented cylinder in a rental falls under the landlord's general repairing duty, and under the legionella duty. The gas safety check does not cover it. The HSE's legionella guidance for landlords suggests storing hot water in the cylinder at 60°C, and says tenants should be told not to change the temperature setting. That makes the cylinder thermostat part of your legionella control, and the cylinder part of your risk assessment.

Keep the installation certificate, the Benchmark checklist and the service records together. If an incident ever happens, those papers show the system was installed and maintained properly. Tell tenants what normal looks like, and that water running from the tundish means switching off and calling. A cylinder that has stood unused through a void should be heated and the outlets flushed before the new tenant moves in.

My unvented cylinder is leaking. What do I do now?

First, work out where the water is coming from. Water running into the tundish or out of the discharge pipe outside is a safety device operating, often because the expansion vessel has lost its charge or a valve is failing. Water dripping from a joint or the cylinder body is a leak. In either case, switch off the immersion heater and the boiler or programmer so the cylinder stops heating.

If water is escaping from the pipework or the vessel, close the cold supply valve feeding the cylinder, or the main stopcock if you cannot find it. Do not open, adjust or remove any of the safety valves, and keep clear of hot water at the tundish. Then call us. Leaking cylinders are attended same day in opening hours, Monday to Saturday.

What affects the cost of cylinder work?

For a replacement, the cylinder's size and type (direct for electric heating, indirect for a boiler) and whether it is like for like. Converting from a vented system costs more, because the loft tanks come out, the pipework is upgraded and a new discharge route is needed.

Access matters too: a cylinder in a cramped airing cupboard upstairs is slower to swap than one in a utility room. Servicing is a defined job. Repairs, such as a failed valve or an expansion vessel that will not hold its charge, depend on the part.

Certificates and defined jobs are a fixed price confirmed before we attend; repairs are charged by the hour, with the rate and an estimate confirmed before we attend.

Where we work on cylinders

We install and service cylinders from Kenton Lane, across Harrow, Kenton, Stanmore and Edgware, and in Wembley, Pinner, Mill Hill, Northwood and the nearby areas. Much of the local housing is interwar and still has a vented cylinder and loft tank, so most of our unvented installs are conversions. Affinity Water's supply here is very hard, which is why servicing and descaling matter.

Plumbing by area: Plumbers in Harrow, Plumbers in Kenton, Plumbers in Stanmore, Plumbers in Edgware, Plumbers in Bushey, Plumbers in Colindale, Plumbers in Hendon, Plumbers in Mill Hill, Plumbers in Northolt, Plumbers in Northwood, Plumbers in Pinner, Plumbers in Ruislip and Plumbers in Wembley.

What we do not do

We do not install, service or repair gas boilers, including the boiler side of a cylinder swap. That needs a Gas Safe registered engineer, and we will coordinate with yours. We do not certify commercial or large-volume hot water plant. Where a leak has caused damp in walls or ceilings, we repair the leak, but we do not carry out damp investigations.

Why people trust us

• Repairs tested before we leave, and the area left clean
• A walk-in office on Kenton Lane, not a serviced address
• We diagnose first and tell you what we have found before doing more
• A time slot rather than a morning-or-afternoon window
• Work photographed before and after, and reported
• We tell you when you do not need something we sell

We are building a review profile under this name and would rather show you none than borrow someone else's. Ask us for references from landlords and we will put you in touch.

Official sources

• GOV.UK: Approved Document G, sanitation, hot water safety and water efficiency (2015 edition, 2024 amendments incorporated)
• legislation.gov.uk: Building Regulations 2010, Schedule 1 (Part G3 hot water)
• HSE: Legionnaires' disease, landlords' responsibilities
• Benchmark: What is Benchmark?

Last reviewed October 2026 by EliteShires Property Services.