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Specialists in Electric Hot Water Cylinders

Understanding the Parts of Your Unvented Cylinder

19 March 2024

If you have an unvented cylinder in your home, you know just how much of a difference it can make in your life. You get mains pressure hot water out of every tap and don’t have to deal with the issues that arise all too often from vented cylinders. How familiar are you with the parts that make up your unvented cylinder? In this post, we’ll introduce some of the components that help your cylinder produce hot water and protect you.


The cylinder is the largest component and the most obvious. This insulated cylinder holds the water to be heated and also serves as the installation point for the other components that make the system work. The cylinder should be double-walled, with an internal steel surface covered by insulation, and then an external metal surface (and you can install an insulative jacket over this).

Immersion Heater

In electric unvented cylinders, immersion heaters are responsible for heating water. They’re located inside the cylinder and can only be serviced by qualified engineers.


It’s a funny name, but the tundish serves a very serious purpose. In the event of an overpressure situation, it directs water out and down the drain so that it does not damage your home. It also provides a window to see water flowing (if you see water in the tundish, you need to call a plumber).

Pressure Relief Valve

The pressure relief valve is designed to open if the cylinder experiences an overpressure situation to release excess pressure.

Expansion Vessel

Because water cannot be compressed and it expands when heated, it needs somewhere to go when hot. The expansion vessel provides that. Note that some unvented cylinders use an internal bubble in place of an expansion vessel.

Cold Water Inlet

The cold-water inlet is where fresh water enters the cylinder from your home’s plumbing. There’s often a pressure-reducing valve located here.

Pressure Reducing Valve

The pressure-reducing valve does what you might expect it to do – it reduces mains pressure water to prevent damage to the cylinder.

Dip Tube

The dip tube runs from the top of the tank to the bottom and is responsible for carrying cold water into the tank from the cold-water inlet. Used on newer OSO Super SX unvented water cylinders. These cylinders require space above the cylinder, almost as much as the height of the cylinder, to remove the dip tube if there is a fault with the seal or the main valve block needs replacing. Unfortunately for the apartment owner, many Super SX cylinders sit on a supporting table above a washing machine. This then turns a reasonably straight forward job into one requiring the cylinder to be uninstalled, then lifted to the floor by two men before the job can start. The cylinder then needs to be reinstalled after the work has been completed. If there are issues with the o-ring slip seals, not as uncommon a problem as you would hope, the process has to be repeated

Check Valve

Water mustn’t flow back into the piping from the unvented cylinder. A check valve ensures a one-way flow of water and prevents backups from occurring.


The thermostat is usually attached to the cylinder itself. It allows you to adjust the water temperature up or down for comfort, safety, and financial savings. Sometimes thermostats trip and need to be reset

Turn to the Pros for Help

Are you experiencing issues with your unvented cylinder? While they’re highly reliable and much more efficient than old, vented styles, components can and will fail as your cylinder ages. When problems occur, make sure to work with a trusted professional. At EasyFlow, our engineers are G3 qualified, ensuring that they can service, repair, install, or replace your unvented cylinder safely. Get in touch today to schedule a consultation.



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