Unvented cylinders — London
Unvented hot water cylinders & Megaflo installation London
G3 certified engineers installing unvented hot water cylinders — including Megaflo, Vaillant uniSTOR, OSO, Gledhill, and Telford — across all 33 London boroughs. Mains-pressure hot water for every tap and shower in your home. No header tanks, no pressure drop, full building regulations compliance.
What is it
What is an unvented hot water cylinder?
An unvented hot water cylinder is a sealed, pressurised vessel that stores hot water directly connected to the mains cold water supply. Because it operates at mains pressure — typically 1.5 to 3 bar in London — the hot water delivered to your taps and showers is at the same pressure as your cold water. The result is powerful, consistent flow from every outlet in the property simultaneously.
This is a fundamental difference from a traditional vented system, where a cold water storage cistern (the header tank) sits in the loft and feeds the cylinder and hot water outlets by gravity. Gravity-fed systems are limited to around 0.1 bar of pressure per metre of head — meaning a shower on the first floor of a London terrace might receive just 0.3–0.5 bar, which is barely enough for adequate flow and wholly inadequate for a modern thermostatic shower.
Unvented cylinders also differ from combination boilers. A combi heats water on demand as it flows through the boiler heat exchanger — there is no stored hot water at all. This works well in a one-bedroom flat, but the moment two outlets draw hot water simultaneously, the combi must split its heating output, and flow rate drops noticeably. An unvented cylinder stores 120–250 litres of water already heated to temperature, ready to deliver at full mains pressure to multiple outlets at once.
Because the system is sealed and pressurised, unvented cylinders require specific safety devices and must be installed by a G3 certified engineer under Building Regulations Part G. Every Prestige Engineers installation is carried out by a fully G3 qualified engineer and notified to building control.
Key differences at a glance
Vented system
- Cold water tank in loft
- Gravity-fed — low pressure
- Header tank takes up loft space
- Pump needed to boost shower pressure
- Open system — contaminants can enter tank
Unvented system
- Direct mains cold supply — no tank
- Mains pressure hot water everywhere
- Frees up loft space entirely
- No pump required for showers
- Sealed system — no contamination risk
Combi boiler
- Heats water on demand — no storage
- Pressure drops with multiple outlets
- No cylinder or tank
- Limited to one main outlet at a time
- Quick to heat but not suitable for large families
Why upgrade
Why London homeowners upgrade to unvented
London's Victorian and Edwardian housing stock was built with vented systems using loft tanks. Modern expectations — power showers, en-suites, simultaneous hot water demand across a family home — have outgrown those systems. Unvented cylinders are now the standard upgrade for any London property where hot water pressure or storage capacity has become a frustration.
Strong shower pressure
Hot water at mains pressure means every shower — ground floor, first floor, second floor — runs at the same pressure. No pump required, no pressure booster, no complex installation.
No loft tank required
Removing the cold water storage cistern and associated pipework from the loft frees up valuable storage space. This is a meaningful gain in London properties where loft conversion potential is high.
Space-efficient cylinder design
Modern unvented cylinders are compact and slimline. They fit in an airing cupboard, utility room, or under-stair cupboard — the same spaces used by a vented cylinder but with the header tank removed from the equation.
Compatible with all London boiler types
Unvented cylinders work with gas, LPG, oil, and heat pump boilers. They integrate seamlessly with existing central heating systems — you keep your current boiler and add the cylinder to serve the hot water circuit.
Sealed system hygiene
Unlike an open vented system where the header tank is exposed to the atmosphere — and potentially to insects, dust, and debris — an unvented system is completely sealed. This eliminates the risk of contamination in the hot water supply.
Improved energy efficiency
Unvented cylinders are insulated to current standards, with heat retention measured in hours rather than the poorly-insulated gravity-fed cylinders found in older London properties. Lower standing heat losses mean lower energy bills.
Brands we install
Unvented cylinder brands
We are brand-independent and install whichever cylinder best suits your property, budget, and hot water demand. Below are the brands we install most frequently across London, with notes on where each performs best.
Megaflo (Heatrae Sadia)
The market-leading unvented cylinder brand in the UK. Available in direct (electric immersion) and indirect (boiler-fed) models, from 80L to 300L. Known for reliability and wide parts availability across London.
Vaillant uniSTOR
High-quality indirect unvented cylinders designed to pair with Vaillant boilers. ErP-rated, available in 120L–270L, with a ten-year cylinder warranty when installed with a Vaillant system.
OSO Hotwater
Norwegian-engineered stainless steel cylinders built for durability. OSO models carry a 25-year cylinder guarantee and are well suited to London's hard water areas.
Gledhill Stainless
British manufacturer offering a wide range including the popular Stainless Lite Plus. Gledhill cylinders are a common choice for London housing associations and larger residential projects.
Telford Tempest
Competitively priced stainless steel cylinders with strong performance in both domestic and light commercial applications. A practical choice for landlords and HMO operators managing running costs.
Sizing guide
What size unvented cylinder do I need?
Cylinder sizing is determined by the number of occupants, the number of bathrooms, and peak demand patterns. Undersizing means running out of hot water during morning routines; oversizing wastes energy reheating water that is not drawn off. The guide below covers the most common London property types. For HMOs and properties with more than four bathrooms, we carry out a detailed demand calculation before recommending a size.
Suitable for a single bathroom and up to two regular users. Will supply a full bath or a power shower without running cold.
The most common size installed in London terraced and semi-detached homes. Supports two bathrooms running in sequence without recovery wait.
Recommended for families with three or more regular users, or properties with an ensuite and family bathroom. Provides enough reserve for simultaneous use.
Required for four-plus bedroom properties, houses of multiple occupation, or properties with more than two bathrooms. Building regulations may require a second cylinder or commercial-grade unit for large HMOs.
Building regulations
G3 building regulations: why they matter
Part G3 of the Building Regulations (England and Wales) governs the installation, commissioning, and maintenance of unvented hot water storage systems. It exists because an unvented cylinder stores a large volume of water under pressure and at high temperature. In the event of failure — whether from a faulty component, an incorrectly installed safety device, or an untrained installation — the stored energy can cause serious injury or structural damage.
The regulations are unambiguous: unvented hot water work must be carried out by an engineer who holds a current G3 qualification from a recognised body. The most common routes are the BPEC G3 qualification or an equivalent CIPHE-accredited pathway. Prestige Engineers hold current G3 certification and can provide documentation on request.
The installation must also be notified to your local authority building control. In practice this happens in one of two ways: the installing engineer self-certifies under a Competent Person scheme (the most common route), or you as the homeowner notify building control directly before work begins. We handle self-certification as part of every installation — you receive a compliance certificate to retain with your property documents.
This certificate matters beyond legal compliance. Mortgage lenders, home insurers, and solicitors acting on property sales will ask for evidence that notifiable building work was carried out lawfully. An unvented cylinder installation without a compliance certificate can delay or block a property sale and may invalidate home insurance relating to the hot water system.
What G3 compliance requires
- Installation by a G3 qualified engineer
- All mandatory safety devices fitted and tested
- Notification to local authority building control
- Compliance certificate issued to the homeowner
- Work carried out to manufacturer commissioning procedure
- Stored water temperature set to minimum 60°C (Legionella control)
What happens if G3 rules are ignored?
An installation carried out without G3 certification is illegal. The homeowner may face enforcement action from building control, and home insurance may be void for any claim arising from the hot water system. When selling the property, the lack of a compliance certificate will be identified in conveyancing searches and must be disclosed. Retrospective regularisation — obtaining a certificate after the fact — is possible but requires a G3 engineer to inspect and potentially correct the installation before a certificate can be issued.
Safety devices
Mandatory safety devices and why they exist
Every unvented cylinder installation must include a specific set of safety devices, each serving a distinct protective function. These are not optional extras — they are required by G3 building regulations, and their correct installation and operation is verified during commissioning. Understanding what each device does helps explain why G3 qualified installation is non-negotiable.
Expansion vessel
As water heats, it expands. In a sealed system there is nowhere for the expanded volume to go without an expansion vessel. The vessel absorbs this extra volume using a pressurised diaphragm, preventing dangerous pressure build-up inside the cylinder.
Temperature and pressure relief valve (T&P valve)
A mandatory last-resort safety device. If the cylinder overheats or pressure rises beyond safe limits — for any reason including expansion vessel failure — the T&P valve opens automatically and discharges hot water to a safe drain point. Without it, catastrophic failure is possible.
Motorised zone valve
Controls the flow of heated water from the boiler to the cylinder. The motorised valve is used in conjunction with a programmer and cylinder thermostat to stop heating the cylinder once the set temperature is reached, preventing continuous heating.
High-temperature cut-out thermostat (overheat thermostat)
A non-self-resetting safety thermostat that cuts power to the heating element or closes the heating circuit if the cylinder water temperature exceeds approximately 90°C. Unlike the main cylinder thermostat, this device requires manual reset to prevent automatic restart after an overheat event.
Line strainer
Protects all safety components from debris in the mains water supply. London's water supply contains particulate that can block the T&P valve seat, preventing it from seating correctly. A line strainer upstream of the cylinder is standard practice.
Installation process
How we install an unvented cylinder in London
A typical unvented cylinder installation in a London property takes one full day for a straightforward vented-to-unvented conversion. Properties requiring significant pipework modifications or conversion from a combi may require two days. We confirm the timeline at survey stage.
Survey and sizing
We assess your current system — whether vented cylinder with header tank, combi boiler, or direct electric — and recommend the correct cylinder size and model for your household.
Removal of existing system
The old vented cylinder, associated header and feed tanks in the loft (if present), and associated pipework are drained down and removed. For combi conversions we retain the boiler and extend the heating circuit.
Installation of cylinder and safety devices
The new unvented cylinder is positioned, connected to the mains cold supply and heating circuit, and all mandatory G3 safety devices are fitted and tested: expansion vessel, T&P valve, motorised valve, and overheat thermostat.
Commissioning and pressure testing
The system is filled, pressure tested, and the cylinder is heated through its first cycle. The engineer verifies correct operation of every safety device and sets thermostat temperatures to G3 requirements (typically 60–65°C stored temperature).
Building control notification
We self-certify the installation under the Competent Person scheme and submit notification to your local authority building control. A compliance certificate is issued to you, providing the legal record of the installation.
London installation costs
How much does unvented cylinder installation cost in London?
A full unvented cylinder supply and installation in London — including the cylinder unit, all G3 mandatory safety devices, labour, and building control notification — typically costs £900 to £2,000 for a mid-range 150–210 litre indirect cylinder. The range reflects differences in cylinder brand, property complexity, and pipework modifications required.
Entry-level cylinders such as a Gledhill Stainless Lite or Telford Tempest at 150 litres will sit at the lower end of this range. Premium cylinders such as the Megaflo HE or Vaillant uniSTOR with extended manufacturer warranties, or installations requiring full loft tank removal and pipework reconfiguration, will sit toward the upper end.
Additional costs to factor in for conversions from vented systems: loft tank disposal (typically £50–£150), any pipework modifications to accommodate the new mains-pressure circuit, and any remedial decorative work if pipework runs through walls or ceilings. We provide a fixed quote at survey stage with no hidden additions.
London rates. Prices include VAT. Fixed quotes provided at survey.
Ongoing maintenance
Servicing and maintaining your unvented cylinder
Unvented cylinders require an annual service by a G3 certified engineer. This is a mandatory requirement under building regulations and a condition of most manufacturer warranties. The annual service typically takes one to two hours and covers all safety-critical components of the system.
During an annual service we inspect and test the temperature and pressure relief valve (T&P valve), check the expansion vessel pre-charge pressure and replace the membrane if needed, verify the correct operation of the motorised zone valve and overheat thermostat, check all pipework connections for signs of weeping or corrosion, and confirm that the stored water temperature is achieving Legionella-control temperatures of at least 60°C.
The expansion vessel is the component most commonly requiring replacement outside of annual service. The rubber membrane inside the vessel degrades over time, particularly in London's hard water areas where calcium deposits can accelerate deterioration. A failed expansion vessel membrane causes the T&P valve to discharge regularly — a clear sign that servicing is overdue.
Stainless steel cylinders from brands such as OSO and Telford incorporate a sacrificial magnesium anode, which protects the cylinder wall from corrosion by preferentially corroding in place of the stainless steel. This anode should be inspected every three to five years and replaced when more than 50% depleted. Copper cylinders (less common in new installations) do not use an anode but should have their immersion heater element inspected periodically for lime scale build-up.
With annual servicing and timely replacement of wear components, an unvented cylinder will reliably last 20 to 25 years. Neglecting annual service is the primary cause of premature failure and the most common reason manufacturer warranties are voided.
Annual service checklist
Common questions
Unvented cylinders London: frequently asked
Can any plumber fit an unvented hot water cylinder?
No. Unvented hot water cylinders must be installed by an engineer who holds the G3 unvented hot water qualification under Part G of the Building Regulations. It is illegal for an unqualified person to install or commission an unvented system. The work must also be notified to your local authority building control — either by the engineer under a Competent Person scheme or by you directly. Prestige Engineers carry full G3 certification and handle all building control notification as standard.
What is the G3 qualification and why does it matter?
G3 refers to Part G3 of the Building Regulations (England and Wales), which governs the installation of unvented hot water storage systems. An engineer must hold a recognised G3 qualification — typically from CIPHE or a body such as BPEC — before carrying out any work on these systems. The qualification covers safe installation of all mandatory safety devices: the expansion vessel, temperature and pressure relief valve, motorised zone valve, and high-temperature cut-out thermostat. Without a G3 certified engineer, your insurance may be void and the installation will not comply with building regulations.
Is a Megaflo better than a combi boiler for a London home?
It depends on the property and hot water demand. A Megaflo (or any unvented cylinder) stores a large volume of hot water at mains pressure, meaning multiple outlets — showers, baths, taps — can run simultaneously without any drop in pressure. Combi boilers heat water on demand but struggle to supply two or more outlets at once, and performance can degrade if the incoming mains pressure is low, which is common in older London properties. For family homes with three or more occupants, or properties where multiple bathrooms need to run at the same time, an unvented cylinder almost always delivers a superior experience. For a one-bedroom flat where hot water demand is low, a combi remains cost-effective.
How long does an unvented hot water cylinder last?
A well-maintained unvented cylinder typically lasts 20–25 years. The cylinder body itself is usually the longest-lived component. The expansion vessel membrane may need replacing after 10–15 years, and the sacrificial anode (fitted on some stainless steel models) should be inspected every 3–5 years and replaced as needed. Annual servicing by a G3 certified engineer is the single most important factor in achieving maximum lifespan. Manufacturers such as Heatrae Sadia (Megaflo) and Vaillant offer extended warranty terms conditional on annual service records being maintained.
Do I need planning permission to install an unvented hot water cylinder?
You do not need planning permission. However, the installation must be notified to building control under Building Regulations Part G. A G3 registered engineer can self-certify the work under a Competent Person scheme, which means they notify building control on your behalf and issue you a compliance certificate. This certificate is important: it demonstrates the work is compliant to future buyers, insurers, and mortgage lenders. Prestige Engineers issue the compliance certificate as part of every unvented cylinder installation across London.
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