Open Vented Hot Water Cylinders

Vented or open-vent hot water cylinders and systems are one of the most common installations in the UK. They differ from combi systems because they have a store of water (usually about 100 litres) and they differ from unvented systems because of their open vented nature.


Please watch the instructional video below on vented hot water cylinders for a basic guide as to how they work.

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Principles of the system

Vented hot water systems are a way of heating up a large store of water for user delivery at outlets throughout a house. They also have the capacity to automatically feed more water into their system using a cold water feed tank above the hot water cylinder. When cold water is heated up it expands and therefore needs somewhere to expand to. This is when the open vented system uses an expansion pipe back up to the cold water feed tank. Take a look at the diagram below for a general system layout:

Cold Water Supply Tank

This tank should be of adequate size to feed the hot water cylinder and any cold taps required on the system (generally the bath tap). If any taps are opened in the house gravity will force the cold water into the bottom of the hot water tank for heating up and eventual use. The cold water supply tank is supplied by a mains pressure cold water feed that uses a ball valve to automatically keep the system topped-up. The expansion pipe should have its outlet overhanging into the cold water supply tank but not below the lip of the tank. The cold water tank should have an overflow pipe leading to a safe spot outside that is visible in case the expansion runs too much or the ball valve fails and doesn’t shut off.


Cold Water Feed Pipe

This pipe supplies cold water to the hot water cylinder for re-heating and eventual use. It is prudent to fit an isolating valve on this pipe whenever possible.


Hot Water Feed Pipe

When the cold water has been heated up by either the immersion heater of heating coil, this pipe supplies the outlets (or taps) with the hot water supply. This pipe also tees off up to the expansion.


Expansion Pipe/Vent pipe

The most important pipe on the system and the reason it is open vented. Any water being heated up can expand up this pipe. In the event of overheat the pipe will drip safely into the cold water feed tank.


Immersion Heater

This is usually seen as the electric backup heater for the hot water cylinder. It acts in the same way as your kitchen kettle.


Heating Coil

The heating soil is used to transfer heat from heating system water (from the boiler) to hot water for the taps. Usually controlled by a programmer time switch and a thermostat on the cylinder which signals either a  3 port or 2 port valve to open and close.


Open Vented Tips

If you find the expansion pipe is running into the cold water storage tank there are a few things you can try before calling a plumber. Firstly adjust the temperature thermostat down a little. This might reduce the amount of expansion. If that doesn’t work try raising the height of the expansion crook above the cold water tank.