Unvented Hot Water Cylinder Costs UK — Supply and Fit Prices for Megaflo-Style Systems in 2026

What Is an Unvented Cylinder?

An unvented hot water cylinder stores hot water at mains pressure. Cold mains feeds directly into a sealed, insulated steel cylinder, the water is heated (by a boiler coil, an immersion heater, or both), and it leaves the cylinder at close to mains pressure. There is no cold water storage tank in the loft and no open vent pipe — hence "unvented". Megaflo is the best-known brand, to the point that many customers say "Megaflo" when they mean any unvented cylinder, much like "Hoover" for vacuum cleaners.

Because the water is delivered at mains pressure, you get strong, even flow at every tap and shower in the house — and crucially, that pressure does not drop when more than one outlet is in use. That single benefit is why most customers want one.

Unvented vs Vented vs Combi

To quote confidently and explain the upgrade to a customer, you need to be clear on how the three common setups differ.

Vented (gravity) cylinder

The traditional setup: a copper cylinder in an airing cupboard fed by a cold water storage tank in the loft. Hot water is delivered by gravity, so pressure depends on the height of the loft tank above the outlet. The result is weak flow upstairs, decent flow downstairs, and showers that often need a pump. Vented cylinders are cheaper and simpler, but the low-pressure performance is exactly what customers are trying to escape.

Unvented cylinder

Mains-pressure hot water stored in a sealed cylinder, as described above. Excellent flow to multiple outlets at once, no loft tank required (which frees up loft space and removes a freezing/leak risk), but it must be installed and serviced by a qualified person and carries a few more failure points.

Combi boiler

A combination boiler heats water on demand with no stored cylinder at all. Great for smaller properties and flats, but flow rate is limited by the boiler's output and drops noticeably when two outlets run together. For a family home with two or more bathrooms, a combi often can't keep up — which is the main reason people move to an unvented cylinder with a system boiler instead.

Why People Switch to Unvented

Understanding the motivation helps you size the system correctly and close the job. The main reasons customers ask for an unvented cylinder are:

• Mains-pressure hot water to multiple outlets: A shower and a tap can run at the same time without the flow collapsing. This is the headline benefit.
• Adding bathrooms: A loft conversion or extension with an en-suite usually pushes an old gravity system beyond what it can deliver.
• Removing the loft tank: No cold water storage tank means no loft leak risk, no freezing pipes and reclaimed loft space.
• Better showers without pumps: Strong shower flow without bolt-on pumps that are noisy and fail over time.
• Future-proofing for heat pumps: Unvented cylinders pair well with air source heat pumps and are often specified during a low-carbon heating upgrade.

Sizing: How Big a Cylinder Do You Need?

Sizing is where quotes most often go wrong. Too small and the customer runs out of hot water; too large and you've oversold and the cylinder takes longer to reheat. The right size depends on the number of bathrooms and occupants, not the size of the house.

• 150L: Suitable for a 1–2 bathroom home with up to 3 occupants. The common starting size for smaller properties.
• 210L: The most popular size in UK homes — comfortable for a 2-bathroom family home with 3–4 occupants.
• 250L+: For 3+ bathrooms, larger households, or homes with high-demand fittings like body-jet or rainfall showers. Cylinders run up to 300L and beyond for large properties.

Always factor in recovery time — how quickly the cylinder reheats. A property with back-to-back showers in the morning needs either more stored volume or a higher boiler output feeding a fast-recovery coil. When in doubt, size up one step; the cost difference on the cylinder is modest compared with the cost of a callback for "no hot water".

The Safety Components

An unvented cylinder stores hot water under pressure, so it must have a set of safety devices to prevent it becoming dangerous if a thermostat fails. These components are not optional — they are a legal requirement and the reason the job needs a qualified installer.

• Expansion vessel: Water expands as it heats. The expansion vessel absorbs that increase in volume so pressure stays within safe limits. Some cylinders use an internal air gap (air bubble) instead of an external vessel.
• Temperature & pressure (T&P) relief valve: A fail-safe that releases water if temperature or pressure exceeds safe limits — the last line of defence against the cylinder overheating.
• Expansion (pressure) relief valve: A secondary valve that discharges if pressure alone climbs too high.
• Tundish: A visible air break in the discharge pipework that lets you see when a relief valve has operated and prevents back-siphonage.
• Discharge pipe (D1/D2): Routes any released hot water safely to a point where it cannot scald anyone — this pipework has specific Building Regs requirements on size, fall and termination.

Getting the discharge pipe wrong — too small, wrong fall, or terminating somewhere a person could be scalded — is one of the most common reasons an unvented install fails inspection. Price the time to run it correctly.

Why It Must Be Fitted by a G3-Qualified Installer

Installing or working on an unvented hot water system is "notifiable" work under Building Regulations, and it must be carried out by someone holding the G3 unvented hot water qualification. This is a specific competency certificate, separate from a general plumbing or gas qualification, that covers the safe installation, commissioning and servicing of unvented systems.

If a customer is comparing quotes, a meaningfully cheaper price from someone without a G3 ticket is a red flag — the install will not be compliant, may not be insurable, and could be dangerous. As a qualified installer, this is a genuine selling point: make your G3 certification visible in your quote.

Building Regulations G3 and Notification

Unvented hot water installations fall under Part G of the Building Regulations (specifically G3). The work is notifiable, which means Building Control must be informed. There are two routes:

• Self-certification: If you are registered with a competent person scheme (such as those covering unvented hot water), you can self-certify the work and notify the local authority on the customer's behalf. This is the quicker, cheaper route.
• Building Control notice: If you are not on a relevant scheme, the homeowner must submit a Building Notice to the local authority and pay for an inspection — this adds cost and delay.

The customer should receive a compliance certificate on completion. Being able to self-certify is both faster and a reason customers should choose a registered installer over a cheaper unregistered one.

The Install Process

A straightforward swap of a vented cylinder for an unvented one in the same location is the simplest version of the job. A full first-time install — especially where the loft tank is being removed and pipework reconfigured — takes longer. A typical install involves:

• Draining down the existing system and removing the old cylinder (and loft tank, if applicable)
• Checking incoming mains flow rate and pressure — an unvented system needs adequate mains flow to perform, and a poor mains supply may rule it out or need an accumulator
• Positioning and securing the new cylinder, often on a reinforced base given the filled weight
• Connecting cold mains, hot distribution, the boiler heating coil and immersion heater
• Fitting and setting the expansion vessel, relief valves, tundish and discharge pipework
• Filling, venting, commissioning, checking relief valve operation and setting the thermostat
• Completing the Building Regs notification and handing over the compliance paperwork

A like-for-like swap might be a day's work; a full reconfiguration with pipework alterations and loft tank removal can run to two days.

Typical UK Prices 2026

Here are realistic 2026 figures for supplying and fitting an unvented cylinder. As always, these vary by region, brand and complexity — but they give you a defensible starting point.

Cylinder supply (parts)

• 150L unvented cylinder: £500–£750
• 210L unvented cylinder: £650–£950
• 250L+ unvented cylinder: £850–£1,200+

Premium brands (Megaflo, Telford, Heatrae Sadia) sit at the upper end; budget and own-brand merchant cylinders come in lower. Stainless steel cylinders generally cost more than enamel-lined steel but resist corrosion better in hard or aggressive water areas.

Fitting / labour

• Like-for-like cylinder swap (1 day): £400–£700
• Full first-time install with pipework alterations (1.5–2 days): £700–£1,400

Full supply-and-fit

Pulling it together, a complete supply-and-fit job commonly lands at £1,500–£3,500, including the cylinder, safety components, sundries, labour and Building Regs notification. Simpler swaps sit toward the bottom; larger cylinders, loft tank removal, mains upgrades and extra pipework push toward — and occasionally past — the top.

What Drives the Cost Up

When two quotes for the "same" job differ by £1,000 or more, it's almost always one of these factors:

• Cylinder size and brand: A 250L Megaflo costs hundreds more than a 150L own-brand unit.
• Removing the loft tank and reconfiguring pipework: Far more labour than a straight swap in the same spot.
• Poor mains supply: Low flow or pressure may need an accumulator or pump, adding several hundred pounds.
• Discharge pipe routing: A cylinder sited far from an external wall or safe termination point means more pipework and time.
• Electrical work: A new immersion circuit or wiring for controls may need a separate electrician.
• Making good: Boxing in, base reinforcement, tiling and decoration after the work.
• Region: London and the South East typically carry the highest day rates.

Quick Reference: Unvented Cylinder Prices UK 2026

FAQ

Can any plumber install an unvented cylinder?

No. The work is notifiable under Building Regulations and must be carried out by someone holding the G3 unvented hot water qualification. A general plumbing or gas ticket on its own is not enough.

How long does an unvented cylinder last?

A quality cylinder typically lasts 15–25 years, but it must be serviced annually — checking the expansion vessel charge and relief valves — to stay safe and keep the warranty valid.

Do I need good mains pressure?

Yes. An unvented system delivers mains-pressure water, so it needs an adequate incoming flow rate and pressure to perform. A weak mains supply may require an accumulator, a pump, or rule out an unvented system altogether — which is why a flow and pressure check is part of any honest quote.

Is an unvented cylinder better than a combi?

For homes with two or more bathrooms, usually yes — it supplies strong hot water to several outlets at once, which a combi struggles to do. For a small flat or one-bathroom home, a combi is often the simpler and cheaper choice.