What is a pre-plumbed unvented cylinder?

It is an unvented cylinder supplied with the heating side controls already fitted and wired at the factory, typically the zone valves and a wiring centre, and often the heating expansion vessel. The cylinder and its safety components are the same as a standard unit.

Is a pre-plumbed cylinder quicker to install?

Usually, when the controls are being redone anyway, because much of the pipework and wiring is done at the factory. On a straight cylinder swap where the existing controls stay, the saving mostly disappears.

Can I use a pre-plumbed cylinder with my existing boiler?

Often yes, if the boiler suits the control arrangement the cylinder is built for, usually two zone valves on a system boiler. Versions for regular boilers exist. An engineer should confirm the match before ordering.

Does a pre-plumbed cylinder need more space?

Generally yes. The zone valves, wiring centre and any expansion vessel sit on a frame beside or in front of the cylinder, so it needs a deeper or wider cupboard than a standard unit of the same capacity. Measure depth as well as height.

Does a pre-plumbed cylinder still need an annual service?

Yes. It is still an unvented cylinder, so the relief valves, expansion vessel, pressure reducing valve and discharge pipe need checking once a year, exactly as on a standard unit.

Is an unvented cylinder kit the same as pre-plumbed?

No. The kit is the set of safety parts every unvented cylinder must have, such as the inlet control group, expansion vessel and tundish. Pre-plumbed refers to the extra heating controls fitted on top of that.

Pre-Plumbed Unvented Cylinder: Is It Worth It?

A pre-plumbed unvented cylinder is a standard unvented cylinder with the heating side controls already fitted and wired at the factory, usually the zone valves, a wiring centre and sometimes the heating expansion vessel, all mounted on or beside the cylinder. It is worth paying for when a new system boiler or heat pump is going in and the controls are being moved into the cylinder cupboard anyway, because it saves site time and gives a tidier, tested layout. It is usually not worth it when you are only swapping an old cylinder and the valves and wiring elsewhere in the house are sound, or when the cupboard is too tight for the extra depth. The vessel and the safety components are the same either way. Here is how to tell which situation you are in.

What pre-plumbed actually means

Every unvented cylinder, pre-plumbed or not, comes with the same safety kit: a temperature and pressure relief valve, a thermostat and high limit cut-out, an inlet control group with a pressure reducing valve, an expansion vessel for the hot water and a tundish. That is the unvented cylinder kit the regulations require, and on a standard cylinder the engineer fits the loose parts on site.

Pre-plumbed goes further, onto the heating side. The maker adds the parts that control how the boiler or heat pump heats the cylinder and the radiators, pipes them together and wires them to a single terminal box. Exactly what is included varies by model, so the specification sheet matters more than the name.

Two-port zone valves for the heating and hot water circuits
A wiring centre that brings the boiler, programmer, thermostat and valves together
Often the heating system expansion vessel, mounted on a bracket
On some models an automatic bypass valve and a filling loop
On versions for open vented or regular boilers, sometimes a pump

When a pre-plumbed cylinder is worth it

The strongest case is a full system change. If a regular boiler with a loft tank is being replaced by a system boiler and an unvented cylinder, the controls have to be redesigned anyway. Putting them in the cylinder cupboard as one factory tested assembly is quicker on site, easier to fault find later, and keeps the valves where the next engineer will look for them. If you have not yet settled on a combi or system boiler with a cylinder, decide that first, because it changes which cylinder you need.

It also helps where time in the property is limited, for example a let flat between tenants or a home with a baby where the hot water cannot be off for long. Less pipework and wiring on site means fewer hours and less chance of a mistake in a wiring centre that someone has to trace later.

Heat pump installations are the other obvious fit. Heat pump cylinders are often supplied pre-plumbed because the controls are more particular about how the cylinder is heated, and the maker has already solved the layout. Our guide to the heat pump cylinder covers why an ordinary cylinder usually will not do on a heat pump.

When a standard cylinder is the better choice

If you are only replacing a failed cylinder and the boiler, zone valves and wiring elsewhere in the house are working, a pre-plumbed unit adds parts you already have. The engineer then either leaves your existing valves in place and ignores the new ones, which wastes money, or strips out working controls to use the new ones, which adds labour and risk for no gain.

Space is the other common reason. The valves and expansion vessel usually sit on a frame at the side or front of the cylinder, so a pre-plumbed unit needs a deeper or wider cupboard than a standard cylinder of the same capacity. London airing cupboards are often tight, and it is the depth that catches people out. If the door will not close on it, it is the wrong cylinder. Our page on horizontal and slimline hot water cylinders covers the options for awkward spaces.

Finally, a pre-plumbed cylinder is built around one way of controlling the system, usually two zone valves on a system boiler. If your heating uses a different arrangement, such as a three port mid-position valve, you are changing how the system works rather than just swapping a cylinder, and that should be a deliberate decision.

Does pre-plumbed change servicing, repairs or safety?

No. The vessel, the safety valves and the annual checks are identical. An unvented cylinder service still covers the relief valves, the expansion vessel charge, the pressure reducing valve and the discharge pipe, whether or not the zone valves are bolted to the side.

The pre-fitted parts are ordinary components. A zone valve head on a pre-plumbed cylinder fails the same way as one anywhere else and is replaced the same way. The only practical difference is that everything is in one place, which usually makes a repair quicker.

The installation rules are the same too. Fitting any unvented cylinder must be done by an engineer qualified for unvented systems, notified to building control, and given a discharge pipe to a safe point. Our guide on how to check a plumber is qualified for unvented work explains what to ask for.

Pre-plumbed does not mean plug in. The cylinder still needs connecting to the mains, the boiler, the heating circuits and a lawful discharge route, and the wiring centre still has to be matched to your boiler and controls. It saves time; it does not remove the need for a qualified installer.

Pre-plumbed or standard: a quick way to decide

Ask three questions. Is the boiler or heat source being changed at the same time? Are the existing zone valves and wiring old, unreliable or badly laid out? Is there room in the cupboard for the extra depth? Two or three yes answers point to pre-plumbed. If the only thing that has failed is the cylinder and the rest of the system is healthy, a standard unvented cylinder is normally the sensible choice.

Whichever you choose, capacity and incoming mains flow decide far more about how the hot water performs than the pre-fitted parts do. Check what size hot water cylinder do I need before choosing a model, and make sure the mains flow has been measured. If you are still comparing makes, our view on the best unvented cylinder brands covers the ones we see and fit most.

What to check on the quote

A good quote for either type should say which cylinder, what capacity, whether it is pre-plumbed and what that includes, where the discharge pipe will run, and whether the existing controls are being kept or removed. If a pre-plumbed unit is specified on a simple cylinder swap, ask why. Our checklist of what a plumbing quote should actually include covers the rest, and what a cylinder replacement actually costs explains which parts of the job drive the total.

Talk it through with an engineer

We supply and fit standard and pre-plumbed unvented cylinders and will tell you which suits your system rather than defaulting to one. If your existing controls are fine, we will say so and leave them in. For a full unvented cylinder installation, we measure the mains and look at the discharge route before quoting.

Call 020 4652 5111 to arrange a visit. Our rates are on the pricing page and are agreed before we attend.

Questions we get asked

What is a pre-plumbed unvented cylinder?: It is an unvented cylinder supplied with the heating side controls already fitted and wired at the factory, typically the zone valves and a wiring centre, and often the heating expansion vessel. The cylinder and its safety components are the same as a standard unit.
Is a pre-plumbed cylinder quicker to install?: Usually, when the controls are being redone anyway, because much of the pipework and wiring is done at the factory. On a straight cylinder swap where the existing controls stay, the saving mostly disappears.
Can I use a pre-plumbed cylinder with my existing boiler?: Often yes, if the boiler suits the control arrangement the cylinder is built for, usually two zone valves on a system boiler. Versions for regular boilers exist. An engineer should confirm the match before ordering.
Does a pre-plumbed cylinder need more space?: Generally yes. The zone valves, wiring centre and any expansion vessel sit on a frame beside or in front of the cylinder, so it needs a deeper or wider cupboard than a standard unit of the same capacity. Measure depth as well as height.
Does a pre-plumbed cylinder still need an annual service?: Yes. It is still an unvented cylinder, so the relief valves, expansion vessel, pressure reducing valve and discharge pipe need checking once a year, exactly as on a standard unit.
Is an unvented cylinder kit the same as pre-plumbed?: No. The kit is the set of safety parts every unvented cylinder must have, such as the inlet control group, expansion vessel and tundish. Pre-plumbed refers to the extra heating controls fitted on top of that.