Vented gravity-fed systems: how older heating layouts work
A vented system uses a cold water storage tank in the loft and an open vent pipe above the boiler rather than a sealed, pressurised circuit. It is common in properties that predate combi and sealed-system installs.
The engineering behind a vented gravity-fed system
Feed and expansion happens naturally through the loft tank and open vent, so there is no pressure gauge to watch and no filling loop to top up. Faults tend to centre on the loft tank itself — a stuck ballcock valve causing overflow, a frozen feed pipe in cold weather, or debris entering the open system through the vent. Converting to a sealed system is often considered during a boiler replacement, but is not always necessary if the existing layout is sound.
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Detail that affects your quote for a vented gravity-fed system
- There is no sealed pressure gauge on a genuinely vented system
- The loft cold water tank should be covered and insulated, not open
- A stuck ballcock valve is a common cause of loft tank overflow
- Vented systems allow debris ingress more easily than sealed ones
- Conversion to sealed is a common upgrade during boiler replacement
Questions we get asked
- Can I keep a vented system when replacing the boiler?
- Yes, many regular boilers still support a vented arrangement, though sealed conversion is worth discussing at the same time.
- Why is my loft tank important?
- It is the feed and expansion point for the whole system, so a fault there — overflow, freezing, or debris — can affect heating throughout the house.
Related help pages
- Cycling faultsShort cycling explained: turning on and off far more than it should
- Unvented systemsUnvented cylinder servicing and the G3 safety requirement
- Oil supply componentsOil supply lines and Tigerloop devices: keeping air out of the fuel line
- Cleaning methodsPower flush vs chemical flush: which one your system needs
- Salon servicingHeating and hot water servicing for salons
- Pipework comparisonPlastic vs copper pipework for heating and hot water
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Straight answers on a vented gravity-fed system
In practice: Vented systems are a proven, older technology that still works well when the loft tank and vent pipe are properly maintained and protected from frost.
Include the appliance make and model in your enquiry so the correct technical information can be checked.
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