Unvented Cylinder Service: What’s Checked & Why It Matters

Unvented hot water cylinders, such as Megaflo-style systems, provide powerful mains-pressure showers and fast-filling baths, delivering a more comfortable and efficient hot water experience compared to traditional vented systems. However, to ensure they operate safely and comply with building regulations, these cylinders require regular, thorough servicing by a properly qualified engineer. This isn’t just a quick visual check—servicing involves detailed inspections and tests to confirm that all safety devices are working correctly and the system is performing optimally.

Neglecting this maintenance can lead to pressure build-up, leaks, or even dangerous failures, putting your home and family at risk. Regular professional servicing helps prevent costly repairs, maintains water quality, and ensures your hot water supply remains reliable and safe.

How to tell if you have an unvented cylinder

Many homeowners are not sure what type of hot water system they have. Before you think about servicing, it helps to recognise an unvented cylinder and its key safety parts.

An unvented cylinder is usually a white or light-coloured, sealed metal tank, often in an airing cupboard. Unlike older vented systems, it does not have a large feed and expansion tank in the loft and it runs directly from the mains.

• Pressure relief valves: Small brass valves with pipework attached, used to safely release pressure.

• Expansion vessel: A round or oval tank (often red, white or grey) that absorbs expansion as water heats.

• Tundish: A small plastic or metal “viewing window” on a pipe, where you can see water discharge if there is a fault.

If you can see these components around your cylinder, you almost certainly have an unvented system and it should only be worked on by someone with the correct G3 unvented qualification.

Reasons unvented cylinders must be serviced

Unvented cylinders store a large volume of hot water at mains pressure. Without working safety controls, the pressure and temperature could rise to dangerous levels. Servicing checks that all these protections still operate correctly.

UK regulations require unvented cylinders to be installed and maintained by competent people. Regular servicing helps keep you compliant, protects your manufacturer warranty and can extend the life of the system.

It is also about comfort and running costs. A well-serviced cylinder recovers heat efficiently, maintains stable hot water temperature and can help avoid unexpected breakdowns or leaks that cause damage to your home.

What a competent engineer checks during a service

A proper unvented cylinder service is more than a quick look and a pressure test. A qualified engineer should follow a detailed set of checks recommended by the manufacturer and building regulations.

Expansion vessel and system pressure

The expansion vessel absorbs the extra volume as water heats and expands. If it fails, pressure can build up and force water out through the safety valves, or stress the cylinder and pipework.

During servicing the engineer will usually isolate the vessel, check the air pre-charge with a gauge and adjust it if needed. They will also confirm that the system pressure is within the correct range for your particular cylinder.

Pressure and temperature safety devices

Unvented cylinders rely on multiple layers of protection. The pressure relief valve (PRV) and combined pressure and temperature relief valve (PTRV) are crucial to safety.

The engineer will check these valves for correct operation, signs of sticking, and any scale or debris that could stop them seating properly. They will test that they open and close as intended and are not dripping when at rest.

Tundish and discharge pipework

The tundish allows you and the engineer to see safely discharged water from a relief valve. Occasional drips during testing are normal, but continuous flow suggests a fault that needs attention.

The discharge pipework must be correctly sized, supported and run to a safe termination point. The engineer will confirm that it is not capped, restricted or altered in a way that could block discharge in an emergency.

Thermostat, overheat cut-out and immersion heater

Your cylinder has a working thermostat to control water temperature and a separate overheat cut-out as a fail-safe. If the main thermostat fails, the overheat device should shut down the heat source before the water gets too hot.

During service, the engineer will test thermostat settings and operation, check the overheat cut-out and confirm it can be reset correctly. If you have an immersion heater, they will inspect the wiring, check for signs of burning or damage and often test the element.

Visual checks for leaks, corrosion and performance

Finally, the engineer will look for stains, damp patches, or corrosion on pipework, fittings and the cylinder body. Early signs of leaks can often be addressed before they become serious.

They should also ask about your hot water performance. Slow refilling baths, air in the hot water, or fluctuating temperature can indicate issues with valves, filters or controls that need correcting.

Warning signs your unvented cylinder needs attention

Some problems are subtle, but others are clear signals that you should contact a qualified engineer promptly. Do not ignore these warning signs, as they often relate to safety devices doing their job or starting to fail.

• Water dripping or running through the tundish when no test is being carried out.

• Banging, hammering or whistling noises from the cylinder or hot water pipework.

• Inconsistent hot water temperature or running out of hot water faster than normal.

• Hot taps spurting air or pressure that suddenly increases or drops.

• Visible stains, rust marks or damp around the cylinder or nearby fittings.

If you notice any of these, turn off the heat source if it is safe to do so and seek professional help. Do not attempt DIY repairs on an unvented system.

Essential safety do’s and don’ts for homeowners

There are a few simple rules to keep unvented cylinders safe between services. These are things any homeowner can understand and follow without specialised tools.

Never cap or block discharge pipes. These pipes are there to safely remove hot water and steam if a safety valve opens. Blocking them can create a very dangerous situation.

Do not ignore water coming from the tundish. Occasional drips right after a test might be normal, but ongoing discharge is a clear sign that a valve is venting due to a fault. This always needs investigating.

Keep the area around the cylinder accessible and free of clutter, so you and your engineer can see the valves, labels and pipework. If you are unsure about any label or control, ask your engineer to talk it through at your next service.