Unvented Hot Water Cylinder Installation London
Mains-pressure hot water, no loft tanks and strong flow at every tap and shower. We survey your property, specify the right cylinder, install it and commission it to Building Regulations G3, then hand over the paperwork.
What Is an Unvented Hot Water Cylinder?
An unvented hot water cylinder is a sealed storage cylinder fed directly from your mains water supply. Because it runs at mains pressure, it delivers strong, steady hot water to every tap and shower without a cold water tank or header tank in the loft. It's the modern replacement for the old tank-and-cylinder setup found in many London homes.
In simple terms, cold water arrives from the mains, passes through a pressure-reducing valve and a check valve, and is heated inside the cylinder by a boiler coil or an immersion heater. When water heats up it expands, so the cylinder uses an expansion vessel (or an internal air bubble) to absorb that extra volume. If anything goes wrong, safety valves release water through a visible tundish and a discharge pipe to a safe place.
The safety parts every unvented cylinder must have
- 1
Two independent temperature controls
A normal thermostat plus a separate high-limit cut-out, so stored water can't keep heating towards boiling point.
- 2
Temperature and pressure relief valve
Opens if the water gets too hot or the pressure gets too high.
- 3
Expansion relief valve and expansion vessel
Deal with the extra volume created as the water heats up.
- 4
Pressure-reducing valve and check valve
Keep the incoming mains pressure within the cylinder's limits and stop backflow.
- 5
Tundish and discharge pipe
Give any released water or steam a visible, safe route out of the building.
Benefits of an Unvented Cylinder
Strong, steady pressure
Hot water arrives at mains pressure, so showers and taps feel noticeably stronger than a gravity-fed system.
Several outlets at once
Two showers or a shower and a tap can run together without one dropping off, which is why it suits 2+ bathroom homes.
No loft tanks
The cold water storage tank is no longer needed, which frees up loft space and removes a freezing and leaking risk.
Flexible siting
Because it doesn't rely on gravity, the cylinder doesn't have to sit below a loft tank, so more locations become possible.
Quick bath filling
Higher flow rates mean a bath fills faster than from a low-pressure system.
Sealed system
The cold water is fed straight from the mains rather than sitting in an open tank, which reduces contamination risk.
The Honest Drawbacks
Most cylinder pages skip this part. Knowing it upfront helps you decide properly.
Works best when
- You have good mains pressure and flow
- You have two or more bathrooms
- You want loft space back
- You're happy to service it every year
Think twice if
- Your mains pressure or flow is weak
- There's no safe route for the discharge pipe
- You have one bathroom and a combi that works well
- You won't arrange yearly servicing
Two more points worth knowing. First, an unvented cylinder pushes hot water out using mains pressure, so if the mains supply is shut off you can't draw hot water until it's back on. Second, it's more complex than a vented cylinder, which is why it needs a qualified installer, a discharge pipe and regular servicing.
Is an Unvented Cylinder Right for Your Home?
| Your Situation | Our Honest View |
|---|---|
| Two or more bathrooms used at the same time | Strong fit. This is where unvented makes the biggest difference. |
| Good mains pressure and flow | Strong fit. We'll confirm with a pressure and flow test. |
| Loft conversion, or you want loft tanks removed | Strong fit. Removing the tanks frees up space. |
| Weak mains pressure or low flow | Test first. A booster, a different model or a vented system may suit better. |
| One bathroom, working combi boiler | Probably not needed. A combi already gives mains-pressure hot water. |
| Regular boiler and gravity system that works fine | A vented cylinder may be enough. See our vented cylinder page. |
| Landlord with several tenants | Strong fit, as long as the annual service is planned in. |
What Boilers and Heat Sources Work With an Unvented Cylinder?
An unvented cylinder needs something to heat it. It works with:
- ✓
System boilers
The most common pairing. A system boiler and unvented cylinder give strong simultaneous hot water. See system boiler installation and system boiler conversion.
- ✓
Regular (heat-only) boilers
Can heat an indirect unvented cylinder, though most people moving to unvented also change the boiler.
- ✓
Immersion heaters
A direct cylinder heats water electrically. Useful as the main source in a home without a boiler, or as a backup.
- ✓
Solar thermal and heat pumps
Suitable cylinders exist for these, but they need a specific design, so tell us if you're planning either.
It does not work with a combi boiler. A combi heats water on demand and has no cylinder, so the two are alternatives, not partners.
What Size Unvented Cylinder Do I Need?
Size depends on how many people live in the home, how many bathrooms you have and how you use hot water. Use this as a starting guide, and we'll size yours properly at the survey.
| Typical Home | Common Cylinder Size |
|---|---|
| 1-2 bedrooms, 1 bathroom | About 120 to 150 litres |
| 3 bedrooms, 1-2 bathrooms | About 150 to 210 litres |
| 3-4 bedrooms, 2 bathrooms | About 210 to 250 litres |
| 4+ bedrooms, 3+ bathrooms | About 250 to 300 litres or more |
A bigger cylinder is not automatically better. An oversized one takes up space, costs more to heat and loses more heat while it sits there. The aim is the smallest cylinder that comfortably covers your busiest morning.
Where Can an Unvented Cylinder Be Installed?
Common locations are an airing cupboard, utility room, garage or a suitably strengthened loft. Three things decide whether a spot works:
- 1
Structure and weight
The floor or shelf has to carry the cylinder and the water inside it.
- 2
A safe discharge route
The tundish must be visible and the discharge pipe must run to a safe termination with a continuous fall. This is often the hardest part in flats.
- 3
Access for servicing
The valves, expansion vessel and thermostats need to be reachable every year.
⚠ Weight is easy to underestimate
A full 150-litre cylinder holds around 150 kg of water before you add the weight of the cylinder itself. For a loft or an upper-floor position we check the structure first, rather than assume it will cope.
Will My Mains Water Pressure Work?
This is the question that decides whether unvented is a good idea at all, so we measure it rather than guess. We test both the pressure and the flow rate at your incoming supply. As a rule of thumb, many manufacturers ask for roughly 1.5 bar of pressure and around 20 litres per minute of flow, but requirements differ by model, so we check against the cylinder's installation manual.
If the supply is weak, there are still options: choose a cylinder designed for lower pressures, add a booster arrangement, or stay with a vented system, which doesn't depend on mains pressure.
How Does an Unvented Cylinder Installation Work?
Most installations take one to two days. Replacing a vented system takes longer, because the loft tanks come out and pipework is rerouted.
- 1
Survey and mains test
We check pressure, flow, siting, discharge route and your current system.
- 2
Specification and written quote
You get the cylinder size, type and a clear written quote before any work is booked.
- 3
Drain down and removal
The old cylinder and, where relevant, the loft tanks are drained and removed.
- 4
Fit the new cylinder
Positioned, connected to the mains, heating and hot water pipework, with the safety valves and expansion vessel.
- 5
Discharge pipework
Tundish and discharge pipe installed to the required route and fall.
- 6
Fill, test and commission
The system is filled, pressure tested and every control and valve checked.
- 7
Paperwork and handover
Commissioning checklist, Building Control paperwork and warranty registration, plus a walkthrough of how it works.
Is Unvented Cylinder Installation Regulated?
Yes. Unvented hot water storage systems fall under Building Regulations Part G (G3). The installation must be carried out by a competent person with the relevant unvented qualification, and it must be notified to Building Control or certified through a competent person scheme. This exists because a poorly fitted unvented cylinder, without the correct safety controls, can release steam or even rupture.
Our installers hold the unvented (G3) qualification, and every installation is commissioned and documented. At handover you should receive:
- ✓
A commissioning checklist
Such as the Benchmark checklist, showing the controls and safety devices were tested.
- ✓
Building Control notification or certificate
Proof the installation was notified or certified as required.
- ✓
Manufacturer warranty registration
Many warranties depend on correct installation and annual servicing.
If a company offers to fit an unvented cylinder and can't show an unvented qualification, treat that as a red flag. You can also ask us to explain exactly what we'll provide before you book.
Servicing and Maintenance
An annual service keeps an unvented cylinder safe and keeps most manufacturer warranties valid. A typical service covers:
- ✓
Expansion vessel
Checking the air charge and recharging if it has dropped.
- ✓
Relief valves and tundish
Testing the valves operate and the tundish and discharge pipe are clear.
- ✓
Thermostats and controls
Confirming both temperature controls work independently.
- ✓
Pressure-reducing valve
Confirming it holds the pressure the cylinder is designed for.
You can book it together with your boiler through our annual boiler service.
Warning signs to act on
- !
Water running from the tundish or discharge pipe
A safety valve is releasing water or steam.
- !
Little or no hot water
Can point to a failed thermostat, immersion element or valve.
- !
Banging, hissing or rumbling
Often scale, a failing expansion vessel or pressure issues.
- !
Rusty water or leaks around the cylinder
Can indicate corrosion or a failing connection.
⚠ If the discharge pipe is running
Switch off the heat source (boiler or immersion heater) and call a qualified engineer. Never plug, cap or restrict a discharge pipe, because it's the safety route for pressure and heat. If what's coming out is steam or very hot water, keep clear of it.
Unvented vs Vented Cylinders
| Feature | Unvented | Vented |
|---|---|---|
| Water pressure | Mains pressure | Gravity, set by loft tank height |
| Loft tanks | Not needed | Cold water tank needed |
| Shower pump | Usually not needed | Often needed for a good shower |
| Installer | Unvented (G3) qualified | Competent plumber, plus Gas Safe for the boiler |
| Discharge pipe | Required | Not required |
| Servicing | Annual service expected | Simpler, fewer safety parts |
| If the mains is off | No hot water until restored | Some hot water remains from the tank |
Not sure which suits you? Read our vented cylinder installation guide. If you also want a faster, simpler setup for a one-bathroom home, a combi boiler may be the better answer.
Direct or Indirect: Which Unvented Cylinder?
Indirect cylinder
Heated by a coil connected to your boiler. This is the usual choice for homes with a system or regular boiler, and often includes an immersion heater as backup.
Direct cylinder
Heated by one or more immersion heaters. Suits homes without a boiler, or as a standalone hot water solution.
Smart cylinders
Some modern cylinders heat only the top portion of water you need, which can reduce running costs. Ask us if that interests you.
Cylinder Installation Across London
We install and service cylinders across North, South, East, West and Central London.
Unvented Cylinder Installation: Common Questions
What is an unvented hot water cylinder?
A sealed storage cylinder fed directly from the mains. It delivers hot water at mains pressure, so you don't need a cold water tank or header tank in the loft.
How does an unvented cylinder work?
Cold mains water passes through a pressure-reducing valve and a check valve, then is heated by a boiler coil or immersion heater. Expansion is absorbed by an expansion vessel or an internal air bubble, and safety valves and a discharge pipe protect the system.
Do I need a G3-qualified engineer to install an unvented cylinder?
Yes. Unvented systems fall under Building Regulations Part G (G3) and must be installed and commissioned by a competent person with the relevant unvented qualification.
Does unvented cylinder installation need Building Control notification?
Yes. It must be notified to Building Control or certified through a competent person scheme. You should also receive a commissioning checklist and warranty registration.
How long does it take to install an unvented cylinder?
Most take one to two days. A like-for-like swap can be quicker. Replacing a vented system, with loft tank removal and pipework changes, usually takes longer.
Can an unvented cylinder go in the loft?
Sometimes. The loft floor must carry the weight and the discharge pipe needs a compliant route. We check both before recommending a loft position.
What size unvented cylinder do I need?
As a guide, 1-2 bedroom homes often suit 120 to 150 litres, 3 bedroom homes 150 to 210 litres, and larger homes with several bathrooms 210 to 300 litres or more. We size yours at the survey.
What mains water pressure do I need?
Many manufacturers ask for roughly 1.5 bar and around 20 litres per minute, but it varies by model. We test your actual pressure and flow before recommending anything.
Do I still need loft tanks with an unvented cylinder?
No. It runs from the mains, so the cold water storage tank isn't needed. With a sealed boiler system the feed and expansion tank can go too.
Can I use an unvented cylinder with a combi boiler?
No. A combi has no cylinder. Unvented cylinders work with system boilers, regular boilers, immersion heaters, solar thermal and suitable heat pumps.
Can I convert my vented cylinder to unvented?
No. A vented cylinder can't be converted. It's replaced with a purpose-built unvented cylinder and the loft tanks are decommissioned, often as part of a wider heating upgrade.
Does an unvented cylinder need servicing?
Yes. Annual servicing is strongly recommended and usually required for the warranty. It checks the expansion vessel, relief valves, tundish, thermostats and discharge pipe.
Why is my unvented cylinder dripping from the tundish?
A safety valve is releasing water or steam, often because of a failed expansion vessel, a faulty pressure-reducing valve or overheating. Switch off the heat source, never block the pipe, and call a qualified engineer.
What happens to hot water if the mains is turned off?
You can't draw hot water until the mains is back on, because the cylinder relies on mains pressure. A vented system can keep supplying limited hot water from its tank for a while.
How long does an unvented cylinder last?
Many modern stainless steel cylinders last 20 years or more with annual servicing. Water quality, servicing and the model all affect lifespan.
How much does unvented cylinder installation cost?
It depends on the cylinder size and model, direct or indirect, the siting, the pipework and discharge routing needed, and whether loft tanks are being removed. We survey your home and give you a written fixed quote before any work is booked.
Is an unvented cylinder worth it?
For homes with two or more bathrooms and good mains pressure, usually yes. If your mains pressure is low, or you have one bathroom and a combi that works well, it may not be necessary.
Can you install an unvented cylinder in a flat?
Often yes, subject to space and a compliant discharge route. Flats can be harder because the discharge pipe must terminate safely, and leasehold buildings may need consent from the freeholder.
Related Heating Services
Request Your Unvented Cylinder Quote
Tell us your property type, number of bathrooms and where the current cylinder sits. We'll arrange a survey, test your mains pressure and give you a written fixed quote.
Gas Safe Registered · G3-Qualified Installers · +44 7912 100949