Open Vented vs Unvented Hot Water Systems: A Dutyholder's Guide

1 day ago

9 min read

An open vented system stores hot water in a cylinder fed by a cold water tank in the roof, with a vent pipe open to the atmosphere. An unvented system is a sealed cylinder fed directly from the mains, with no loft tank and no open vent. Neither is automatically safer for Legionella. What matters on your site is how each one is installed, controlled, monitored and maintained.

This guide is written for facilities managers, estate managers and compliance managers in England who inherit a hot water system, or have to decide whether to change one. It covers how the two designs differ, what the rules require, where the Legionella risks sit, and what to do before you commit to a swap.

TL;DR

A vented cylinder is gravity-fed from a cold water storage tank and has an open vent pipe. An unvented cylinder is sealed and fed by mains pressure.
Legionella control is the same for both: store hot water at 60°C or higher and deliver it to outlets at 50°C or higher, keep cold water below 20°C where possible, and avoid stagnation.
Unvented cylinders over 15 litres are notifiable work under Part G3 of the Building Regulations and need a competent installer. Their safety valves must be tested, not forgotten.
Swapping vented for unvented is not like-for-like. Check mains pressure and flow, pipework, the old tank and your Legionella risk assessment first.

How the two systems work
What the rules require in England
Vented or unvented: where the Legionella risk actually sits
Which one suits which site?
Switching from vented to unvented: a checklist
Our perspective
How we can help
FAQ
Sources

How the two systems work

Open vented (gravity-fed)

A vented cylinder is fed by a cold water storage cistern, usually in the loft. Gravity supplies the pressure, so outlets on upper floors can feel weak without a pump. A vent pipe rises from the top of the cylinder and discharges over the cistern. It lets expanding water and air escape safely. HSE describes gravity-fed cold water systems with storage tanks and larger water heaters as a distinct category from smaller mains-fed systems with point-of-use heaters, and the guidance treats them accordingly.

The tank is the feature that matters for Legionella. It is a large volume of cold water that can sit still, warm up in a plant room or loft, and collect debris if the lid is not secure.

Unvented (mains-fed, sealed)

An unvented cylinder connects directly to the mains supply. Because the water is heated in a sealed vessel, it expands with nowhere to go, so the system needs an expansion vessel or internal air bubble, plus safety devices. Approved Document G defines the expansion vessel and pressure relief valve as part of the unvented installation.

You get mains-pressure hot water at every outlet, no loft tank and no open vent. A building that has a mains-fed cold supply but still uses a break tank, for example because of low mains flow, keeps a storage vessel that needs the same attention as any cistern.

Side by side

Feature	Open vented	Unvented
Cold water feed	Storage cistern (often in the loft)	Direct from the mains, or a booster set
Pressure at outlets	Gravity only, a pump may be needed	Mains pressure
Open vent pipe	Yes	No
Cold storage tank	Yes, needs inspection and temperature checks	Not for the cylinder, but break tanks may exist elsewhere
Safety depends on	Open vent staying clear	Temperature and pressure relief valves, thermal cut-outs, expansion vessel
Installer requirement (over 15 litres)	Notifiable in England under Building Regulations	Notifiable, competent person required
Typical Legionella focus	Tank condition, stagnation, temperature	Valve testing, stagnation in break tanks, scale, temperature

What the rules require in England

Notifying building control

In England, both types of hot water cylinder installation need building control involvement, either through the local authority or a registered competent person. If a contractor offers to swap a cylinder “off the books”, treat that as a warning sign for your insurance and your audit trail.

Unvented systems over 15 litres

Part G3 deals with unvented hot water storage because, unless installed correctly, the cylinder could explode. Practical consequences for a site manager:

The installer must be a competent person who can self-certify, or the work must be inspected by building control.
At least two independent safety devices are expected, such as a non-self-resetting thermal cut-out and a temperature relief valve. Scotland’s technical handbook, which is a different regime from England’s, sets relief valves to open at no more than 90°C. Check the equivalent requirements in Approved Document G for England.
Keep the commissioning certificate, such as a Benchmark checklist, in your compliance file. Without it, building insurance, property sales and local authority checks can become awkward.

Legionella duties do not change

Your duty under health and safety law is to assess and control the risk, whatever the plumbing looks like. HSE’s technical guidance, HSG274, is the benchmark. The HSE healthcare Legionella guidance says hot water cylinders (calorifiers) should store at 60°C or higher, with distribution at 50°C or higher, using thermostatic mixing valves, within one minute of running the outlet. In healthcare, the stricter 55°C figure applies, and HTM 04-01 is the document your estates team will work from. We cover the numbers in more detail in our note on HSG274 temperature limits.

Vented or unvented: where the Legionella risk actually sits

The question we hear most often is whether an unvented cylinder is safer. The honest answer is that the risk profile of a building is shaped more by the complexity of the system, how it is used and how well it is maintained than by the type of cylinder. A large, oversized and under-used store is a problem whether it is vented or not.

Risks on a vented system

The cold water tank. It is open to the atmosphere through its overflow and vent arrangements. Lids, screens and insect mesh need checking, and the water temperature should be monitored against the 20°C target.
Stagnation. If a tank is oversized for the building’s current use, water ages and warms. Our guide to why stagnant water causes Legionella explains the mechanism.
Pumps and long pipe runs added to compensate for low gravity pressure, which can create dead legs.

Risks on an unvented system

Safety valves. With no visible vent, internal safety rests on mechanical devices. These are not fit-and-forget: scale can make a valve seize, which is a known failure point when an over-heat event happens. Test and service them on a schedule, using a competent engineer.
Break tanks. Mains-fed pressurised systems often hide a break tank. These get overlooked in routine monitoring and can become warm and stagnant if they are undersized for the flow or if building use changes.
Greater reliance on monitoring. Closed systems cut out the atmospheric contamination route, which is why estate managers in healthcare often favour them. The trade-off, as HTM 04-01 design guidance implies, is more dependence on safety devices and a more specialised maintenance schedule.

What both need

A current written risk assessment that names the cylinder, any tanks and the distribution loop.
Regular temperature checks at the cylinder flow and return, and at representative outlets.
Flushing of infrequently used outlets, and removal of dead legs.
Thermostatic mixing valves serviced to a schedule where they protect vulnerable users. Remember that a TMV can create a warm, low-flow section of pipe if it is poorly placed.
Training for the people who do the checks, and a logbook that shows they were done.

Which one suits which site?

There is no universal winner. Use the criteria below against your own building.

If your site has…	Leaning towards
Low mains pressure or poor flow	Vented, or unvented with a booster set after a flow survey
Several floors needing consistent pressure	Unvented, subject to mains capacity
An ageing loft tank with poor access	Unvented, or a refurbished tank with a good inspection regime
Limited plant space	Unvented, with no need for a loft tank
Vulnerable occupants (care, clinical)	Whichever design lets you hold temperatures and monitor reliably, usually with HTM 04-01 guidance
Low occupancy or seasonal use	A smaller store, either type, to avoid stagnation

Healthcare, schools and hospitality each bring their own risk factors. If that is you, our pages on healthcare premises, schools and colleges and hospitality and leisure set out what we look for.

Switching from vented to unvented: a checklist

Experts treat the move from a legacy open vented system to unvented as a significant engineering change, not a like-for-like swap. Before you ask for quotes:

Measure the mains. Ask for a pressure and flow test at peak demand. If the mains cannot support the load, you may need a booster set or pipework upgrades.
Review the existing pipework. Old pipes and joints designed for low pressure may not suit mains pressure.
Decide what happens to the tank. If the loft tank is no longer needed, it should be removed or fully disconnected, not left to stagnate. If the cold water system still needs storage, it needs to stay in your monitoring plan.
Choose a competent installer registered with a suitable scheme, and confirm they will notify building control and issue the commissioning certificate.
Plan the work as a project. General facilities-upgrade planning steps, such as scoping, sequencing and coordinating trades, apply here too, although the Legionella controls are specific to water.
Disinfect and commission. New or altered systems should be flushed, disinfected and sampled before going into use. Heating systems that were drained and refilled deserve the same care.
Update the risk assessment and logbook. The old schematic is now wrong. The new system needs new monitoring points, new valve test dates and a new responsible person.

Keep the paperwork. If something goes wrong later, a dated certificate and a revised risk assessment are your evidence of good management.

Our perspective

We see both designs on the sites we assess, and our view is simple: stop asking which cylinder is better and start asking which one your team can control. A well-run vented system with a sound, clean, monitored tank is a safer place than a neglected unvented one with seized valves and a forgotten break tank.

The mistakes we most often find are not about type. They are an unvented cylinder with no valve test records, a cold water tank nobody has opened in years, a loop that returns below temperature because a pump was never rebalanced, and an outlet in a disused wing that has not run since the last term. In each case the system was not the problem. The gaps were in the schedule and the records.

Our approach is to make compliance simple. We write the risk assessment around your actual system, set out what needs to be measured and by whom, and give you a logbook you will use. For larger sites, we look at why large hot water systems pose risk because volume is often the hidden issue. If you are planning a change, bring us in before the engineers arrive, not after.

— Sammi

How we can help

Whether you run a vented system, an unvented one or a mixture across several buildings, the starting point is a risk assessment that reflects what is actually installed. Our Legionella risk assessments start from £185 one-off. If you need evidence of what is happening in the water, Legionella water testing and analysis starts from £55 per sample. For tank cleaning, disinfection and replacement, see our system disinfection and flushing service, and for valve servicing and remedial works, our compliance services page. Where you want continuous evidence of temperatures, automated water temperature monitoring reduces the manual rounds. Your staff can also learn the basics through Legionella awareness training, on-site or online.

FAQ

Is a vented or unvented cylinder better for Legionella control?

Neither is inherently better. HSE’s guidance focuses on temperature control, avoiding stagnation and good maintenance for all systems. An unvented cylinder removes the loft tank, but it relies on safety valves and may still have break tanks that need checking.

How can I tell whether a cylinder is vented or unvented?

A vented cylinder has a vent pipe running from the top, usually over a cold water tank in the loft. An unvented cylinder is fed directly by the mains, usually with a visible expansion vessel, a tundish and a discharge pipe from the relief valves. If you are unsure, ask a competent engineer and record the answer in your risk assessment.

Do I need a qualified installer for an unvented cylinder?

Yes. Unvented systems over 15 litres fall under Part G3 of the Building Regulations and need a competent person, or building control inspection. Ask for the commissioning certificate and keep it in your compliance file.

Can I keep my open vented boiler if I fit an unvented cylinder?

Often yes, but it depends on the heating circuit and what the installer finds on survey. The mains supply must also support the new cylinder. Get the installer to confirm both in writing before work starts.