Unvented Cylinder Installation (G3)in Woolwich & Greenwich Mains-Pressure Hot Water, Installed Safely
Mains-pressure hot water to every tap in the house: Megaflo, Telford and Heatrae Sadia unvented cylinders supplied, installed, serviced and safety-inspected by a G3-qualified engineer with over 30 years on SE London homes.
G3 qualified engineer · Full annual service · Building Control notification handled
High-pressure hot water, done safely
An unvented cylinder is a pressure vessel. It has to be installed and maintained by someone qualified to do it: G3 exists for a reason.
- What it is
- Installation, servicing and safety inspection of unvented hot water cylinders, direct or indirect, coil-fed by a system boiler or immersion-heated. Includes discharge pipework, pressure and temperature relief, expansion vessel and full commissioning.
- Who it's for
- Homes and flats in SE London with two or more bathrooms, or one bathroom plus high hot-water demand, moving away from a gravity or combi setup. Also landlords running HMOs where multiple bathrooms need independent flow.
- When you need it
- When your current hot water arrangement can't keep up with demand, on a full central heating replacement, on a new bathroom or ensuite addition, or on the annual G3 safety inspection.
- Why a professional matters
- Unvented cylinders are Building Regulations G3 notifiable, installation, servicing and safety must be carried out by a qualified engineer. Poorly installed cylinders have caused fatalities from over-pressurisation.
Why unvented cylinders are the one appliance you don't cut corners on
The worst outcomes in domestic plumbing come from unvented cylinders installed or serviced by people who weren't qualified to.
Discharge pipe wrongly routed
Discharge from the T&P relief valve must run to a safe external termination, routing internally can steam-scald anyone nearby if the valve blows.
Expansion vessel loss
A tired expansion vessel means the T&P valve blows on every heat-up. Left unchecked, the safety valve itself will fail.
Overpressurisation risk
A cylinder without functional safety devices can become a pressure vessel with steam-explosion energy, historically responsible for UK domestic fatalities.
Failed Building Control
An unregistered installation is unlawful under Approved Doc G3, buyers' solicitors flag it, and it must be remedied before sale.
Common mistakes we fix
- Trusting a general plumber without G3 certification
- Skipping the annual service, mandatory for safety
- Not testing expansion vessel pressure annually
- Wrongly routed discharge pipework
- Missing Building Control notification
From your first call to the completion certificate.
- 1
Survey
Cold mains flow tested, cylinder location planned, discharge route confirmed, coil compatibility with boiler assessed.
- 2
Design & spec
Cylinder size (150L, 210L, 300L etc), direct or indirect, coil kW rating and controls specified and quoted in writing.
- 3
Installation
Cylinder positioned, all safety devices fitted (T&P, expansion, PRV), discharge pipework routed correctly, coil connected to boiler.
- 4
Commissioning
System filled, air purged, expansion vessel pre-charged to correct pressure, T&P tested, temperature control confirmed.
- 5
Certification
Building Control notified, G3 certificate issued, manufacturer warranty registered, annual service reminder set.
What you actually get for the money.
Full mains pressure at every tap
Multiple showers running simultaneously with no drop in flow.
No loft tank required
Frees loft space, eliminates freeze and legionella risks.
Compatible with power showers
Direct mains pressure supports power showers and body-jet systems.
Fast recovery
Large indirect coils re-heat 210L in 25–35 minutes on a modern system boiler.
Long service life
Correctly installed and annually serviced, 15–20 years is entirely normal.
Safe by design
G3-qualified installation and annual servicing keep all safety devices functional.
What a G3-compliant unvented installation involves
G3 is a rigorous qualification, and the standards attached to it are the reason unvented cylinders are safe.
Sizing the cylinder
150L for a two-person flat, 180–210L for a family home with 2 bathrooms, 250–300L for larger houses or high-demand HMOs. Undersized cylinders are the top cause of complaint six months post-install.
Direct vs indirect
Indirect (heated by boiler coil) is the norm for gas-heated homes; direct (electric immersion only) suits off-gas properties or as a backup. Twin-coil versions accept solar thermal or heat pump input.
Safety devices
Temperature and pressure relief valve, pressure relief valve, expansion vessel, tundish and discharge pipework, every one required, tested at commissioning and re-tested annually.
Discharge pipework routing
Discharge from the T&P valve runs to an external safe termination via correctly-sized tundish and D1/D2 pipework, complying with Approved Doc G3. This is where the majority of failed inspections happen.
Manufacturers we install
Megaflo (Heatrae Sadia), the SE London default; Telford Tempest and Hurricane, good performance and parts availability; Gledhill Stainless Lite for compact installations.
Annual service and G3 inspection
Expansion vessel pressure test, T&P valve test, safety device inspection, coil pressure check, discharge route re-inspection, and full written report, mandatory annually to keep safety and warranty valid.
Photos from real jobs, not stock imagery.
Questions we hear every week.
What is a G3 qualification?+
G3 is the specific Building Regulations qualification required to install, service and safety-inspect unvented hot water cylinders in the UK. Only G3-qualified engineers can legally do this work.
How much does an unvented cylinder installation cost?+
£2,200–£3,200 for a 210L indirect Megaflo in a typical SE London home, including safety pack, expansion vessel, tundish and discharge pipework. Larger or twin-coil cylinders £2,800–£4,000.
How long does the installation take?+
1–2 working days for a straightforward replacement. 2–3 days where relocating or converting from a vented system.
How often should an unvented cylinder be serviced?+
Annually, this is not optional. Expansion vessel loses pressure over time; safety devices need testing; discharge route must be re-inspected.
How much does the annual G3 service cost?+
£95 for a standard indirect cylinder, including full safety device testing, expansion vessel pressure check and written report.
What size cylinder do I need?+
180–210L for a typical 3-bed family home with 2 bathrooms; 250–300L for larger households or HMOs. We'll size on your usage pattern.
Do you handle Building Control notification?+
Yes, G3 notifiable installs are registered through our competent-person scheme at no extra cost. You receive the compliance certificate.
What happens if the safety valve blows?+
It means the expansion vessel or PRV has failed and needs immediate attention, call us and we'll come out as a priority. Do not ignore a valve that has discharged.
Ready to get unvented cylinders sorted properly?
Speak to Danny directly. Free quote in writing, honest advice, and a fair fixed price. No sales pressure, we’ve been at this for over 30 years for a reason.
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- Based in Greenwich · covers all SE London