Shaun Dunn Whether you’re pricing up a replacement or trying to work out what’s behind the airing cupboard door, the first thing to settle is whether you’ve got a vented or unvented system. It affects your shower pressure, loft space, and who’s allowed to fit it.
Key takeaways
- A cold water storage tank feeding the cylinder usually means vented. Unvented cylinders take their supply straight from the mains, with safety controls including a tundish and relief valve.
- Unvented only gives strong pressure if the mains can back it up. Around 20 litres a minute and 1.5 bar dynamic is our benchmark when assessing a conversion.
- Unvented cylinders need annual servicing, commonly a manufacturer’s warranty condition.
- Moving from vented to unvented is a conversion, not a straight swap. A building warrant depends on the property and scope of work.
What Are Vented and Unvented Hot Water Cylinders?
The simplest way to put it: one takes its cold water from a tank, usually in the loft. The other’s plumbed straight into the mains. That difference is where everything else comes from, including how they behave and who’s allowed to fit them.
A vented cylinder is fed by a cold water tank in the loft, with gravity dropping water into the cylinder and a vent pipe running back over that tank to give expanding water somewhere to go. That height gives you your pressure, roughly 0.1 bar per metre between the tank’s water level and the outlet. Vented cylinders come as direct (an internal immersion) or indirect (a coil from your boiler or heat pump).
An unvented cylinder skips the loft tank and vent, taking cold water straight from the mains into a sealed, pressurised cylinder. An inlet control group reduces the incoming supply to a safe pressure, and because heated water expands, the sealed system needs somewhere for that extra volume to go, either an expansion vessel or a built-in air gap, with safety controls to act if anything moves outside its limits. The same direct/indirect split applies, and heat pump versions come as purpose-designed units. Sizing comes down to demand: roughly 150 litres for one bathroom, 210 for two, 250 or more for three.
How to Identify Which Type You Have
- Start in the loft. A smaller cold water tank feeding down into the cylinder is the giveaway for a vented system.
- Look around the cylinder. Vented systems have an open vent pipe running towards the tank. Unvented systems typically have an inlet control group, relief valve, and tundish.
- Check the label. Unvented units normally show a maximum operating pressure and model number; traditional vented copper cylinders usually don’t.
- Still not sure? Ask SD Plumbing & Heating next time we’re out and we’ll check in thirty seconds.
Vented vs Unvented: Which Is Better?
Neither is better in the abstract. It comes down to what your incoming mains can deliver, how much hot water you want at once, and whether you’ve got loft space going spare.
Pressure and Flow Rate
An unvented cylinder can give you strong pressure because it’s working from the mains, but only if the mains can keep up. A typical shower pulls around 8 to 12 litres a minute, so two running together can ask for roughly 20 litres a minute. Before quoting a conversion, we measure flow at a fully open kitchen cold tap, then check dynamic pressure at the stop tap: around 20 litres a minute and 1.5 bar dynamic is the benchmark we’re looking for.
Vented pressure is driven by the height between the storage tank and the outlet, which is why an upstairs shower just below the tank can feel weak while the kitchen tap downstairs runs fine. If the system’s otherwise doing its job, a shower pump can often fix that without replacing everything, typically £350 to £700 fitted.
Energy Efficiency and Running Costs
The cylinder itself doesn’t burn anything, so vented or unvented isn’t what decides your heating bill, what heats the water is. An indirect cylinder paired with a modern condensing boiler uses gas more efficiently than an older non-condensing one, while going electric means paying the electricity rate instead. Ofgem’s average Direct Debit rates for July to September 2026 are around 7.33p/kWh for gas and 26.11p/kWh for electricity, roughly 3.6 times the cost; from October, 7.97p/kWh and 26.32p/kWh brings that closer to 3.3 times.
Standing heat loss matters too: a modern factory-insulated cylinder typically declares around 1 to 2 kWh of heat loss every 24 hours, while an older copper tank in a loose jacket can lose considerably more. You’ll find the declared figure on the cylinder label.
Installation and Space Requirements
| Factor | Vented | Unvented |
|---|---|---|
| Water supply | Cold water storage cistern | Direct from the mains |
| Outlet pressure | Roughly 0.1 bar per metre of head | Controlled mains pressure, subject to incoming supply |
| Loft space | Needs space and access for the cistern | No cold water storage cistern required |
| Installation | Generally simpler for like-for-like | More involved, with safety controls and a compliant discharge route |
| Typical additional install cost | Baseline for comparison | Roughly £1,000 to £1,500 more |
| Annual service | Not normally required | Required annually |
| Typical lifespan | Around 15 to 25 years (copper) | Around 20 to 25 years (stainless steel) |
| Building warrant | Not normally required for like-for-like | Depends on the property and scope of work |
| Installer requirements | Ordinary plumbing; associated gas work needs Gas Safe registration | Needs the appropriate unvented hot water storage qualification |
If loft space is tight, losing the storage cistern is a genuine win, particularly in flats with no private loft. What unvented asks for instead is a properly designed discharge route for its relief valves.
Installation Requirements and Regulations
Fitting a vented cylinder is ordinary plumbing work: accessible loft space, proper structural support (a 227-litre cistern weighs around 230kg full, spread across several joists), enough height for workable pressure, and good insulation with a close-fitting lid. A like-for-like vented replacement generally isn’t notifiable work in Scotland, though accompanying gas work still needs a Gas Safe registered engineer.
Unvented is different because the cylinder holds heated water under pressure. Online guidance often points to Part G3, but that’s England and Wales, not Scotland. Here, unvented hot water storage falls under the Building (Scotland) Regulations and the Domestic Technical Handbook, including Standard 4.9, which requires a non-self-resetting thermal cut-out and a temperature and pressure relief valve, discharging through a tundish to a safe termination point. Whether you need a building warrant depends on the property and the work, and where there’s doubt, we’ll check with the verifier first.
For unvented work, the installer needs the Unvented Hot Water Storage Systems qualification, still commonly called the G3 ticket. If the boiler’s being changed too, that side of the job needs someone on the Gas Safe Register.
Maintenance and Lifespan
Unvented cylinders need servicing every year: we check the expansion provision, test the safety valves and inspect the tundish, and documented servicing is commonly a warranty condition. It typically costs £80 to £150. Vented systems need less routine attention, though the tank, ball valve and pipework should still be checked periodically.
Stainless steel unvented cylinders are commonly warranted for 20 to 25 years; copper vented cylinders typically last 15 to 25 years, depending on installation and water conditions.
Can You Upgrade From Vented to Unvented?
Yes, but it’s a proper conversion, not a straight swap. The storage tank and vent pipe come out, pipework may need altering to handle mains pressure, and the new cylinder needs its safety controls and a compliant discharge route.
The first question is whether your mains supply is good enough, which is why we check flow and dynamic pressure before quoting rather than assuming an unvented cylinder will automatically improve things. A typical conversion runs around £1,300 to £3,500. If you’re replacing the boiler or fitting a heat pump cylinder at the same time, doing both together often makes sense.
If you’re still not sure which system suits your house, or just want someone to have a proper look at what you’ve got, give us a call. We fit and service vented and unvented cylinders across Edinburgh and the Lothians, and we’ll talk you through the options before you decide whether converting makes sense.
