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Fully Vented Gravity Fed Systems Help & Advice

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A fully vented system uses a gravity fed cold water feed leading to a hot water copper cylinder and circulates hot water to radiators, it is vented into a small cistern tank so its self bleeding, which is usually situated in the loft space area. Water is usually heated by a boiler then circulated via 2 in number two port valves sometimes known as zone valves. It can also be circulated via a three port valve depending on the system being an S’ plan or ‘Y’ plan. The hot water cylinders boiler coil or heat exchanger is used to generate hot water and also fed to the radiators for heating.

To control what your boiler will heat is by way of a programmable timer and the zone valves. Because there is no pressurised part whatsoever of this system it is given the term as fully vented. Two water tanks are located usually in the loft space area, one being smaller than the other. The much larger cold-water tank or cistern delivers water via gravity to a copper cylinder to heat up. DHW or domestic hot water system and cylinder vent back through the cistern. The smaller tank or better known as a feed and expansion tank (F&E) vents the radiators so self-bleeding and also keeps the levels topped up. It too works utilizing the force of gravity. Both tank cisterns are fitted with a ball float valve fed by mains supply to maintain the levels within the tanks.

A fully vented gravity fed system is incredibly safe. It is impossible for it to have a dangerous build up of pressure meaning in any part of the system there is no risk of something bursting due to an excessive amount of pressure or worse still a potential explosion. So that’s good! But unfortunately there are always draw backs offering bad news but this is the same with any system. The primary draw back by using a fully vented gravity system is the low direct hot water pressure received at outlet taps. Because the cold water feed to the hot water copper cylinder is utilizing the force of gravity it therefore will always be at relatively low pressure.

To explain this in more detailed way the following should be considered

As a vessel of water is elevated in height and the outlet is kept at ground level it gains potential energy due to the weight of the water. The higher you raise the vessel the pressure at the outlet is increased. So if a vessel is situated at 10 meters high or 33 feet it will produce 1 bar of pressure from the outlet.

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The pressure can vary at different outlets throughout the system due to height differences of the outlets so in turn will effect the pressure at that outlet.

To give you an example of this – A cistern water tank is situated in the loft space of a modern house consisting of two floors and it is feeding on the ground floor a kitchen outlet tap and a on the first floor next to the bedroom it is feeding the bathroom basin outlet tap. The outlet pressure on the ground floor will be greater than the outlet pressure on the first floor even though the are both in the same house and being fed from the same water tank. It’s because of the height difference between the two falls the ground floor outlet by virtue of the height and falls differential; it will be at greater pressure.

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This is the exact reason why a gravity fed shower located in the bathroom of the first floor property is usually rubbish. If you think about it even as you move the shower head up the slide rail depending on your height the higher you raise to the ceiling the closer you are getting to the underside of the cistern tank lowering the pressure even further. Plumb the shower in on the ground floor using the same fed and what a difference you find. It is important when planning your domestic plumbing to realise the height differences where the outlets are situated compared to the cistern water tank will have a direct affect on the outlets water pressure. It has nothing to do with the size of the cistern water tank and adding more water using a bigger tank will not change anything. The position of the hot water cylinder will not affect the outlet pressure by virtue of its situated position to the system.


When the cold-water cistern tank is higher than the tap outlets it is known as a positive head. If an outlet tap was installed higher than the cistern water tank the outlet would be known as a negative head. No water will actually flow from a negative head outlet unless an inline pump is used because as the whole system requires gravity to work water refuses to voluntary run up hill by itself. Funny that.

Gravity open vented systems are very safe and simple. They are low pressure but they possess the advantage of being the only design system you can actually add an inline booster pump to boost a showers performance of for a negative head outlet above the cistern water tank level.

Water Bye Laws do not permit the use of an inline booster pump to a mains pressure system in an attempt to increase the pressure or flow rate. Combi systems are mains fed so will already be functioning at maximum efficiency, attempting to increase the flow would be foolish and pointless. However, if you have a low-pressure gravity fed vented system and sick and tired of the poor pressure of say your shower, it is allowed to add an inline booster pump to enhance the performance. That’s good stuff!

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To Conclude:

Positives – It is an extremely safe system. Suitable for power shower pumps and booster pumps to enhance exceptional flow rates.

Negatives – The system requires water tanks situated in the loft and a copper hot water cylinder so space must be allowed for their installation. A non-boosted, un-pumped system will provide low-pressure water flow rates to taps and outlets.

Economy – Water needs to be stored and heated whether its used or not. So it is important to ensure your storage devices such as cylinders are lagged and fully insulated to minimise heat loss. It’s no less economical to run and produce hot water than any other system.

System Requirements – Hot water cylinder usually in copper, boiler, loft space, tanks, radiators and a cupboard


Please feel free to contact us for help or advice

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If you are interested in booking a survey for any of our services or just a gas safety check, please contact Acorn Plumbing & Heating (APH) and a member of our team will be happy to help or advise you.

CALL – Acorn Plumbing & Heating (APH) now on 01752 201077 or 07779 777965

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