Is My Central Heating Sealed or Open Vented? How to Tell

If your boiler or the pipework near it has a pressure gauge, usually reading between 1 and 2 bar, and a filling loop, your central heating is a sealed system. If there is a small feed and expansion tank in the loft, often next to a larger cold water tank, your heating is open vented. Combi boilers always run on a sealed system. For the wider picture, including systems that do not use a wet boiler circuit at all, see our overview of the different types of central heating systems.

The Quick Checks

You can usually tell in a couple of minutes by looking at the boiler and the loft. The signs are clear once you know what to look for.

• Sealed system: a pressure gauge on the boiler front or on nearby pipework, a filling loop (a short braided hose or a valve with a lever or key under the boiler), and no heating tank in the loft. All combi boilers and system boilers work this way.
• Open vented system: a small plastic or metal tank in the loft, usually far smaller than the main cold water tank, with a pipe bending over the top of it. There is no pressure gauge for the heating. These systems use a regular (heat-only) boiler.

A regular boiler can be either type. Some were converted to sealed at some point, and have an expansion vessel and pressure gauge near the cylinder instead of a loft tank. If you have a regular boiler and no small tank in the loft, check near the boiler and the airing cupboard for a gauge.

This Is Not the Same as a Vented or Unvented Cylinder

Whether your heating is sealed or open vented is a separate question from whether your hot water cylinder is vented or unvented. The first is about the water in your radiators; the second is about the water that comes out of your taps.

You can have a sealed heating system with a vented cylinder fed from a loft tank, or an open vented heating system with an unvented cylinder. The loft can contain one tank, two or none. The small tank is for the heating, and the large one feeds the cylinder and sometimes the cold taps.

How Each System Works

The difference is how each system copes with water expanding as it heats. An open vented system lets the water rise into the loft tank; a sealed system absorbs it in an expansion vessel and runs under pressure.

In an open vented system, the feed and expansion tank tops up the heating as water is lost and gives expanding water somewhere to go. A vent pipe rises from the heating circuit and bends over the tank as a safety outlet. The pressure in the system comes only from the height of that tank, so it is low, and there is nothing to top up by hand.

In a sealed system, water is held under pressure, typically around 1 to 1.5 bar when cold. An expansion vessel takes up the extra volume as the water heats, a pressure relief valve lets water out through a pipe if pressure rises too high, and you top up lost water through the filling loop. If you are regularly adding water, something is leaking or the expansion vessel needs attention, and our guide to why a heating system won't hold pressure covers the usual causes.

What Goes Wrong With Each

Open vented systems tend to suffer from air and corrosion. Sealed systems tend to suffer from pressure loss. Knowing which type you have tells you which symptoms to look for.

An open vented system is open to the air at the loft tank, so oxygen gets into the water over time and speeds up the rusting that creates sludge. Common faults include:

• Pumping over: water discharges from the vent pipe into the loft tank while the pump runs, drawing in air and diluting the inhibitor.
• A sticking ball valve in the loft tank, which either stops topping up or overfills and runs out through the overflow pipe.
• A tank full of sludge or debris, which can feed straight back into the heating.
• A tank that freezes in an uninsulated loft.

A sealed system has none of those problems, but a small leak shows up quickly as falling pressure, and the boiler will lock out when the pressure drops too far. A dripping pressure relief pipe outside the house usually means the system is over-pressurised, often because the expansion vessel has lost its charge.

Does It Change How the System Is Maintained?

Yes. Topping up, adding inhibitor and power flushing are all done differently depending on the system type. The good news is that either type can be kept in good condition.

On a sealed system you top up the pressure yourself through the filling loop, and our guide on repressurising a heating system explains when that is fine and when to call someone. On an open vented system you should never need to add water by hand; if the loft tank keeps calling for water, there is a leak somewhere. Inhibitor goes into an open vented system through the loft tank and into a sealed system through a radiator or a filter. A power flush on an open vented system should include cleaning out the loft tank, or the sludge in it goes straight back into the clean system.

Should You Convert an Open Vented System to Sealed?

Converting to a sealed system is often worthwhile, especially when the boiler is being replaced or the loft is being converted. It is not always the right answer, though, and the pipework needs checking first.

The benefits are real: no loft tank to freeze, overflow or collect debris, far less oxygen getting into the water, less sludge over time, and a clear loft. The risk is that a sealed system runs at a higher pressure than the old one ever did. Old joints, corroded radiators and tired valves that were fine at loft-tank pressure can start to weep. An honest conversion starts with a pressure test and a look at the condition of the radiators, and sometimes the right advice is to flush and repair first, or to leave a sound open vented system alone.

Conversion also involves the boiler: a regular boiler has to be suitable for a sealed system, or be replaced with one that is. Any work on a gas boiler must be done by a Gas Safe registered engineer. We cover how conversion fits into wider changes in our central heating system upgrade service.

Not sure what you have, or whether converting makes sense? We check the system type, pipework condition and pressure, then tell you plainly whether converting is worth doing. Book a heating survey.