An open vented central heating system is a traditional heating setup where the water circuit is open to the air through a small feed and expansion tank, usually in the loft. The tank keeps the system topped up and lets excess pressure and air escape safely.
- It uses a small tank in the loft to top up water and absorb expansion.
- It normally works alongside a separate hot water cylinder.
- It runs at low pressure, fed by gravity rather than the mains.
- It is common in older UK homes, but sealed systems have largely replaced it in new builds.
- Regular checks on the tank and the water quality keep it healthy.
How Does It Actually Work?
Think of it as a closed loop of water with a “breathing hole”.
The boiler heats the water. A pump pushes it through pipes to your radiators, then back to the boiler. The same boiler usually heats a hot water cylinder too, so you get heating and hot taps from one source.
The clever bit is the tank in the loft. It sits above every radiator, so gravity keeps the system full. As water heats, it expands. Instead of building up dangerous pressure, the extra water simply rises up a pipe and into the tank. When it cools, it drops back down.
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The Main Parts
If you’ve got one of these systems, you’ll spot most of the following.
- Feed and expansion tank. A small plastic tank, often in the loft, with a ball valve like the one in a toilet cistern. It tops the system up automatically.
- Open vent pipe. A pipe that rises from the system and ends over the tank. It lets air and steam escape, so the system can never over-pressurise.
- Cold feed pipe. This runs from the tank down into the system and supplies the water.
- Hot water cylinder. A large, usually copper, tank in an airing cupboard. It stores hot water for your taps and shower.
- Boiler and pump. The boiler heats the water and the pump moves it around.
- Motorised valve. Most systems have one, which directs hot water to either the radiators or the cylinder.
Why Is It Called “Open Vented”?
Because it’s open to the atmosphere. The vent pipe means the water is never fully sealed in. Air can get in and out through the tank.
That’s the main difference from a sealed system, where the water is locked in a closed loop under mains pressure. We’ll compare the two below.
Open Vented Vs Sealed Systems
| Feature | Open vented system | Sealed system |
|---|---|---|
| Header tank in the loft | Yes | No |
| Pressure source | Gravity (height of the tank) | Mains water, set at the boiler |
| Typical operating pressure | Low, roughly 0.1 to 0.3 bar at the boiler for a typical loft tank | Around 1 to 1.5 bar when cold |
| Expansion handled by | Feed and expansion tank | Expansion vessel |
| Hot water storage | Vented cylinder (low pressure) | Often unvented cylinder or combi boiler |
| Common in | Older homes | Modern homes |
Table Source: Water Regulations Advisory Scheme (WRAS); Worcester Bosch; Which?
One useful bit of physics sits behind those pressure figures. Every metre of vertical water height creates about 0.098 bar of pressure. So a tank two or three metres above the boiler gives you a fraction of a bar, far lower than a sealed system.
What Are The Upsides?
Open vented systems have stuck around for decades for good reasons.
- Simple and proven. There’s little to go wrong.
- Forgiving of old pipework. The lower pressure is kinder to older joints and fittings.
- Safe by design. The open vent means pressure can’t build up.
- Easy to repair. Most heating engineers know these systems inside out.
- Works well with a tank-fed shower. The hot and cold supplies are matched in pressure.
What Are The Downsides?
They’re not perfect, and this is why many people upgrade.
- Low pressure. Showers can feel weak unless you fit a pump.
- Loft space. You need room for the tank, and it must be insulated so it doesn’t freeze.
- Corrosion risk. Because air can enter the system, oxygen gets in and rust can form. Over time this creates black sludge that clogs radiators and boilers.
- Heat loss. A tank in a cold loft and long pipe runs can waste energy.
- Not ideal for modern boilers. Many efficient combi boilers are designed for sealed systems.
How To Spot One In Your Home
Have a quick look in your loft. If you find a small plastic tank with pipes running into it, you almost certainly have an open vented system. There’s often a larger, separate cold water tank next to it too. Don’t mix the two up.
Another clue is a large hot water cylinder in your airing cupboard alongside a conventional (non-combi) boiler.
Looking After It
A little care goes a long way.
- Check the tank now and then. Make sure the water level looks right and the ball valve isn’t sticking.
- Insulate the tank and pipes. This helps in winter, but leave the space beneath the tank uninsulated, so warmth from the house stops it freezing. Follow the guidance from a qualified installer to insulate the tank and pipes.
- Use a corrosion inhibitor. It slows rust and sludge.
- Get an annual service. For gas boilers, use a Gas Safe registered engineer. You can check registration on the Gas Safe Register.
- Consider a powerflush if radiators have cold spots. This is often a sign of sludge.
Should You Switch To A Sealed System?
Not always. If your system works well and your bills are reasonable, there’s no rush.
But switching can make sense if:
- You want better shower pressure.
- You’re fitting a new boiler and want a modern efficient one.
- You’d like to free up loft space.
- Your system is constantly sludging up.
Conversion means removing the tanks, adding an expansion vessel and a filling loop, and often changing the cylinder. It’s a job for a qualified heating engineer, and quotes vary a lot, so get a few.
Final Thoughts
An open vented central heating system is simple, reliable and a little old-fashioned. Understanding how the loft tank, vent pipe and cylinder fit together makes it far easier to spot problems and make sensible upgrade decisions.
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FAQs
Q1. Is An Open Vented System The Same As A Gravity-Fed System?
Not quite, but they’re closely linked. “Gravity-fed” usually describes the low-pressure water supply from the loft tanks. An open vented heating system relies on gravity to keep the system filled.
Q2. Can I Have A Combi Boiler With An Open Vented System?
Generally, no. Combi boilers work with sealed systems and don’t need a tank or a cylinder. Fitting one usually means converting the system.
Q3. Why Is My Loft Tank Overflowing?
Often the ball valve has failed, or the system is over filling. Sometimes it’s caused by a leaking coil in the cylinder. Get an engineer to check it, as it can waste water and damage your ceiling. Check whether your home insurance covers escape-of-water damage.
Q4. Do Open Vented Systems Need A Pressure Gauge?
No. Unlike a sealed system, you don’t top up pressure yourself. The tank does that job automatically.
Q5. Are Open Vented Systems Being Phased Out?
They’re no longer typical in new homes, but they aren’t banned, and millions are still in use. Repairs and replacement parts remain widely available.
Sources & References
- HM Government. (2026). Approved Document L: Conservation of fuel and power and Approved Document G: Sanitation, hot water safety and water efficiency. GOV.UK.
- Gas Safe Register. (2026). Gas safety advice and guidance for homeowners and heating installations.
- Energy Saving Trust. (2026). Heating and hot water: Improving energy efficiency in UK homes.
- Water Regulations Advisory Scheme. (2026). WRAS approvals and water fittings regulations guidance.
- Which?. (2025). Central heating systems explained.
- Worcester Bosch. (2025). Advice and guides: Home heating, boilers, and hot water systems.
Disclaimer: This article is provided strictly for general informational purposes concerning home central heating systems and is not intended for promotional purposes or commercial endorsement. The information contained herein does not constitute professional plumbing, heating, or engineering advice. Readers should independently verify all facts and seek professional advice from a qualified, registered heating engineer before undertaking any repairs or modifications.
