Unvented Cylinder
Installation & Repair
in Watford
Mains-pressure hot water at every tap in the house. Full G3 unvented installation, annual servicing and repair by a Gas Safe engineer with 38+ years' experience.
What unvented cylinders actually covers
What it is
An unvented hot water cylinder is a sealed, pressurised cylinder fed directly off the mains. It replaces the old cold-water tank in the loft and the vented cylinder in the airing cupboard, giving you strong, balanced mains-pressure hot water at every outlet — including multiple showers at once.
Who it's for
Larger homes, families running two or more showers simultaneously, anyone converting a loft that needs the water tank gone, and homeowners on a system or regular boiler who want combi-like pressure without losing storage capacity.
When you need it
When you're planning a bathroom, a loft conversion, an extension, or upgrading a tired vented (gravity-fed) system. Also when your existing cylinder is leaking, scaled up, or the immersion has failed repeatedly.
Why professional help matters
Unvented systems are legally notifiable and can only be installed, serviced or altered by an engineer holding a G3 (Unvented Hot Water) qualification. Get it wrong and you're dealing with a pressurised vessel above 90°C — done properly, it's the best-performing hot water solution for most family homes.
The cost of leaving it
Unvented cylinders are safe, quiet and reliable — but only when installed and serviced correctly. The problems we're called to fix are almost always down to installers who cut corners on the safety controls.
What can go wrong
- A missing or seized expansion relief valve on a pressurised cylinder is a genuine safety hazard, not a paperwork issue.
- Skipping the annual G3 service voids the manufacturer warranty (typically 25 years on the cylinder) and lets scale destroy the immersion.
- Under-sized incoming mains, or shared cold feeds, cause weak performance at upstairs showers even with a brand-new cylinder.
- A cylinder installed without a tundish discharge to outside is non-compliant and can flood the property if the relief valve opens.
Common mistakes we see
- Choosing the biggest cylinder that fits the cupboard instead of sizing on real hot water demand.
- Fitting an unvented cylinder without first checking incoming mains flow rate and pressure.
- Using a non-G3 registered installer — Building Control notification cannot be issued and it invalidates your buildings insurance.
- Skipping the annual service to save £75, then paying £600+ for a new immersion, PRV set and thermostat two years later.
Our process
A clear, repeatable system — the same on every job, whether you're a homeowner or a landlord with a portfolio.
- 01
On-site survey
We measure incoming mains flow and pressure, check available space, count outlets and hot water demand — the numbers that decide cylinder size.
- 02
Fixed written quote
Cylinder brand and litre capacity, safety controls, pipework, discharge route, controls, commissioning and Building Control notification — all itemised.
- 03
Install
Old cylinder and (where required) loft tanks removed, new cylinder set on a solid base, secondary safety controls and tundish fitted, everything pressure-tested.
- 04
G3 commissioning
Full commissioning to G3 standards: pressure test, temperature and pressure relief valve function tested, discharge witnessed, benchmark logbook completed.
- 05
Registered & notified
Notified to Building Control under our G3 registration. Manufacturer warranty registered in your name (typically 25 years cylinder, 2 years parts).
What you get
Strong mains pressure at every tap
No more weak upstairs showers or waiting for the bath to fill. Two showers running at once with no drop-off.
No loft tanks
Frees loft space for insulation or conversion, removes the risk of a burst tank flooding the house.
Long manufacturer warranty
Up to 25 years on the cylinder itself when annually serviced by a G3 engineer — properly a fit-and-forget investment.
Quiet operation
No noisy filling of tanks in the loft, no water hammer at 3am when the ball valve slams shut.
Compatible with any heat source
Works with gas boilers, heat pumps, solar thermal and dual-fuel setups — future-proof for a heat pump upgrade later.
G3 compliant & notified
Full Building Control notification and Benchmark commissioning paperwork on completion.
Recent unvented cylinders work
Photos from real jobs completed by Matt and the Premier Heating team.
In detail
We install and service all the major unvented cylinder brands — Megaflo, Tempest, JAB, Vaillant uniSTOR, Telford, Range Tribune and more — in direct, indirect and twin-coil (solar-ready) configurations.
Sizing the cylinder correctly
Cylinder size isn't about house size — it's about how many baths and showers you run back-to-back. For a typical 3-bedroom family we'll usually spec 170–210 litres. Larger homes with multiple bathrooms need 250–300 litres. Undersize it and you run out of hot water; oversize it and you're paying to reheat water you don't use.
Incoming mains — the make-or-break number
An unvented cylinder is only as good as the cold feed pushing water into it. Before quoting we measure incoming static pressure and dynamic flow rate at the stopcock. If flow is under 20 L/min we'll flag it, and either recommend upsizing the mains or specify an accumulator so you don't end up with pressurised weak-flow showers.
Safety controls and the discharge route
Every unvented cylinder needs a working temperature and pressure relief valve (T&P), an expansion vessel or built-in air gap, a tundish, and a two-stage discharge pipe routed to outside. We fit and test all of these on every install — no exceptions.
Annual G3 servicing
An unvented cylinder needs an annual service by a G3-registered engineer to keep the warranty valid. We test the T&P and PRV, check the expansion vessel charge, descale the immersion where needed, check the anode where applicable, and log everything in the benchmark book.
Common repairs we handle
Weeping tundish (usually a failed expansion vessel or PRV), no hot water on gas (motorised valve or coil), no hot water on immersion (element or thermostat), poor flow at one tap only (thermostatic blender), or a leaking cylinder body (rare — but end-of-life). We'll always price the repair against replacement so you can make an informed call.
Frequently asked questions
Do I need an unvented cylinder or a combi?
If you run two showers at once, have three or more bathrooms, or you have a poor incoming mains that would strangle a combi, an unvented cylinder is almost always the right answer. We'll size on real demand at the survey and give you a straight recommendation.
How long does an unvented cylinder installation take?
A straight replacement in the same location is typically 1 day. A conversion from a vented (tank-fed) system to fully unvented, including removing loft tanks and re-piping, is usually 2–3 days.
How often does an unvented cylinder need servicing?
Once a year, by a G3-registered engineer. Skipping the annual service voids the manufacturer warranty on the cylinder.
What's the warranty on the cylinder?
Up to 25 years on the cylinder body itself (brand-dependent) provided it's annually serviced by a G3 engineer, plus 2 years on the ancillaries (PRV, T&P, expansion vessel). Our workmanship is guaranteed for 12 months.
Is an unvented cylinder safe?
Yes, when installed to G3 standards. That's why the qualification exists — every unvented cylinder in the UK has to be installed and serviced by a G3-registered engineer and notified to Building Control.
Do I still need a cold-water tank in the loft?
No — that's one of the biggest benefits. The unvented cylinder is fed directly off the mains, so loft tanks come out and the loft space is freed up.
Do you cover my area?
Yes — we cover Watford, Northwood, Rickmansworth, Pinner, Bushey, Hatch End, Stanmore, Middlesex and Greater London. If you're nearby but not listed, please call.
Related services
Areas we cover
Based in Watford, serving Hertfordshire and Greater London.
Need something not listed? Get in touch.
Mains-pressure hot water — installed and G3 certified.
Free on-site survey. Fixed written quote. Notified to Building Control. Backed by our 12-month workmanship guarantee.