Plumbing 7 August 2026

Vented or Unvented Hot Water Cylinder: Which One Does Your Home Need?

If your incoming mains can deliver a strong flow rate, an unvented cylinder will almost always give you the better shower — mains-pressure hot water at every tap, no loft tank, no pump. If it cannot, a vented cylinder fed from a cold tank in the loft is the honest answer, and no amount of new kit will change that.

That single measurement decides more cylinder jobs than anything else we look at. Everything below is how our engineers work through the rest of it when a customer asks which type they should be putting in.

What is the actual difference between vented and unvented?

A vented cylinder is the traditional British arrangement. A cold tank sits in the loft, gravity feeds the cylinder below it, and the pressure at your taps comes from the height difference between the two. It is open to the atmosphere through a vent pipe, which is where the name comes from. Simple, forgiving, and it will limp on for years with minor faults.

An unvented cylinder is sealed and fed directly from the incoming cold mains. There is no loft tank at all. Hot water arrives at your taps at roughly the same pressure as the cold, which is why showers feel completely different on the two systems. Because it is a sealed vessel holding hot water under pressure, it carries safety controls — an expansion vessel, a pressure relief valve and a temperature and pressure relief valve — and those controls are the reason it is a notifiable, licensed job rather than ordinary pipework.

There is a second split that people mix up with this one.

Direct and indirect cylinder — which do I have?

Indirect means the cylinder is heated by a coil inside it, fed by your boiler. Almost every cylinder we work on in Kent is indirect, because almost every home has a boiler doing the heating.

Direct means there is no coil — the water is heated by immersion heaters in the cylinder itself. You see these where there is no gas supply, or as a stop-gap. They are far more expensive to run on electricity alone.

Vented/unvented and direct/indirect are independent of each other. You can have an indirect unvented cylinder, a direct vented one, and every other combination. When you ring us about a cylinder, telling us both halves of that answer saves a lot of guesswork.

The question that decides it: what is your mains flow rate?

An unvented cylinder can only give you what the street main gives it. We measure flow rate at the kitchen tap, usually over a timed fill of a container, and check the standing pressure. If a property cannot supply enough flow, an unvented system will still work — but you will be disappointed with it, because two outlets running at once will fight each other.

This is where we sometimes talk people out of the upgrade they rang up wanting. Older properties on long shared supply pipes, and homes at the end of a run on a rural lane, are the ones most likely to fail the test. If the flow is not there, the sensible options are a vented cylinder, or a stored-water solution with a pump — not a sealed system that will underperform from day one.

Installing an unvented cylinder: G3 and why it matters

Unvented hot water storage is covered by Part G3 of the Building Regulations. The work is notifiable, and it must be carried out by someone holding a current unvented hot water systems qualification. Any gas work tied to it — moving the boiler, altering the gas supply, recommissioning — must be done by a Gas Safe registered engineer. We are Gas Safe registered under 951167, and we will not touch a sealed cylinder that has been fitted by someone who was not qualified to fit it without putting the safety devices right first.

That is not box-ticking. The discharge pipework from the relief valves has to be sized, routed and terminated so that if the vessel ever does dump boiling water, it goes somewhere safe and visible. Badly run discharge pipes are the single most common fault we find on unvented installations done on the cheap — and it is the one fault that turns an inconvenience into an injury.

The annual service is not optional

An unvented cylinder must be serviced every year. The expansion vessel loses its air charge over time; when it does, the pressure relief valve starts weeping and you get a slow, permanent trickle to the outside. Most people notice it as a damp patch on a wall, months after it started. The service also checks the tundish, the relief valves, the thermostat and the discharge route.

Skipping it will normally invalidate the manufacturer’s warranty on the cylinder, so the saving is false. Plumbing and servicing work is charged at £95+VAT for the first hour, which is always billable, and £47.50+VAT for each hour after that. A vented cylinder has no equivalent legal requirement, which is one genuine point in its favour.

Hard water changes the maintenance picture

Most of Kent sits in a hard water area, and it shows up in cylinders more than people expect. On indirect cylinders, scale builds on the outside of the boiler coil and slowly strangles the heat transfer, so the recovery time gets longer year on year until someone finally rings us about “slow” hot water. On direct cylinders, the immersion element scales up and burns out.

Two practical consequences. First, running the cylinder hotter than it needs to be accelerates scaling — although it must still be hot enough to keep the stored water safe, so this is not a case of turning it down as far as it will go. Second, if you are having an unvented cylinder fitted here, it is worth discussing scale protection on the incoming main at the same time, while the pipework is open. Retrofitting it later costs more in labour than doing it on the day.

So which should you fit?

Broadly, from what we see on jobs:

• Good mains flow, no loft tank wanted, more than one bathroom — unvented, nearly every time. The performance difference is obvious the first time you use the shower.
• Poor mains flow, or a loft tank already in place and working — vented. There is nothing wrong with a well-set-up vented system, and it is cheaper to live with.
• No gas supply — the cylinder type is the smaller question. Come at it from how the water is going to be heated first.
• Replacing like for like on a system that works — do not over-engineer it. Sometimes the right job is a straight swap, and we will say so.

Where a cylinder is being replaced alongside heating work, it is worth looking at the whole system together rather than in pieces — see our pages on plumbing and gas boiler installation, and the practical walk-through in how to change a hot water cylinder. If you already have an unvented cylinder that has never been looked at, book it in alongside your annual boiler service and get both done in one visit.

If you are not sure what you have got, or you want the flow rate measured before you commit to anything, ring us on 01732 792 454. We will tell you what your property can actually support, and any installation work comes with a free fixed written quote before we start.