Vented Cylinders
A traditional option, usually supplied by a loft storage tank.
SHOP VENTED →Hot Water Cylinders Explained: Types, Sizes & How to Choose the Right One
A complete guide to hot water cylinders, including vented and unvented systems, direct and indirect models, sizing advice and when to replace your cylinder.
SHOP HOT WATER CYLINDERS →
Understand the types
Vented, unvented, direct, indirect, solar and combination cylinders
Choose the right size
Find the right capacity for your household
Make a confident choice
Key factors, replacement advice and expert FAQs
The Essentials
What is a hot water cylinder?
A hot water cylinder is an insulated vessel that stores heated water for use throughout your home. Unlike a combi boiler, which heats water on demand, a cylinder keeps a supply of hot water ready for showers, baths and taps.
Water may be heated using a boiler, electric immersion heater, heat pump or compatible solar thermal system.
At a Glance
The different types of hot water cylinders
We stock a range of hot water cylinders to suit different heating systems and household needs.
Unvented Cylinders
Mains-fed for pressurised hot water, ideal where supply is suitable.
SHOP UNVENTED →Direct Cylinders
Heat water using an electric immersion heater.
SHOP DIRECT →Indirect Cylinders
Use heat from a boiler via an internal coil.
SHOP INDIRECT →Solar Cylinders
Designed to work with compatible solar thermal systems.
SHOP SOLAR →Combination Cylinders
Combine hot water storage and a cold water feed cistern.
SHOP COMBINATION →Vented vs Unvented: At a Glance
Vented System
- Supplied by a loft tank
- Gravity-fed water pressure
- Suitable for traditional systems
Unvented System
- Directly from the mains
- Can offer stronger pressure
- Suitable for multiple bathrooms where supply allows
Cylinder Size Guide
A practical guide to typical household hot water storage requirements.
| Household size | Indicative capacity |
|---|---|
| ●● 1–2 people | 90–150 litres |
| ●● 2–3 people | 120–180 litres |
| ●●● 3–4 people | 150–210 litres |
| ●●●● 4–5+ people | 180–250+ litres |
These are general guidelines. The right size depends
on your hot water usage, number of bathrooms,
simultaneous demand and heating system. Ask a
qualified heating professional to confirm the
appropriate capacity.
More Information
Everything to Consider Before Buying
Get a more detailed look at the key topics, with practical advice to help you choose the right cylinder.
Vented vs unvented hot water cylinders
Vented and unvented cylinders both store hot water, but they are supplied in different ways.
Vented cylinders are usually supplied from a cold water storage tank in the loft. They are common in traditional gravity-fed heating systems. Hot water pressure depends partly on the height of the storage tank and the installation.
Unvented cylinders receive cold water directly from the mains and store it under pressure. They can provide strong hot water performance and suit properties with several bathrooms, provided the mains supply is adequate.
Unvented systems require appropriate safety devices and installation by a suitably qualified installer. Always have the water supply and existing system assessed before changing cylinder type.
Direct vs indirect hot water cylinders
The difference between direct and indirect cylinders is how the stored water is heated.
Direct cylinders typically use one or more electric immersion heaters to heat water. They can be useful in properties without a suitable boiler connection.
Indirect cylinders use a heat exchanger, commonly a coil inside the cylinder, to transfer heat from a boiler or compatible heat source. Some also include an immersion heater for backup.
Solar thermal cylinders use suitable heat exchanger arrangements to receive energy from solar collectors, usually alongside another heat source. Combination cylinders incorporate hot water storage with a cold water feed cistern.
Choose the design that matches your existing heating equipment and installation requirements.
What size hot water cylinder do I need?
Hot water cylinder capacity is measured in litres. The right capacity depends on how much hot water your household uses and how quickly the cylinder can reheat.
Consider the number of occupants, bathrooms, showers, baths and whether several outlets are likely to be used at the same time.
A household of four might consider a cylinder around 150–210 litres as an initial guide, but actual requirements can be higher or lower.
Cylinder heat loss, usable stored water, recovery time and the available heat source also matter. An installer should calculate the required capacity rather than relying on household size alone.
When should you replace your hot water cylinder?
A well-maintained hot water cylinder can provide many years of service, although its lifespan depends on the material, water quality, maintenance and installation conditions.
Signs that a cylinder may need attention include:
- Leaks or corrosion around the cylinder
- Repeated loss of hot water
- Unusual noises or persistent heating problems
- Poor insulation and excessive standing heat loss
- Recurring faults with valves or controls
Not every fault requires a replacement. A qualified professional can determine whether repair or replacement is the better option.
If you are replacing an older cylinder, it may be worth considering a modern insulated model with suitable capacity and improved heat retention.
How to choose the right hot water cylinder
Start by identifying your existing heating system and whether it uses a vented or unvented cylinder. A like-for-like replacement may be simpler, but an upgrade could be appropriate when the whole heating system is being modernised.
- System compatibility: Match the cylinder to your boiler, immersion heater, heat pump or solar thermal system.
- Capacity: Choose a size based on actual hot water demand.
- Available space: Measure the cylinder cupboard, access route and installation clearances.
- Heat retention: Compare standing heat loss and insulation performance.
- Materials: Consider stainless steel, copper and other suitable constructions.
- Warranty: Check manufacturer terms and maintenance requirements.
Explore our hot water cylinder collection to compare the available types and capacities.
Installation: what you need to know
Hot water cylinder installation should be carried out by a competent professional familiar with the relevant system and current UK requirements.
Unvented hot water systems have specific safety requirements. In England and Wales, installers must follow the applicable Building Regulations, including G3 requirements where relevant.
Before installation, the professional should check the available space, pipework, water pressure, expansion arrangements, controls, discharge requirements and compatibility with the heat source.
Always follow the manufacturer's installation, commissioning and maintenance instructions.
Frequently Asked Questions
Is a hot water cylinder the same as a hot water tank?
In everyday UK usage, hot water tank and hot water cylinder often refer to the same appliance: an insulated vessel that stores heated water. However, a cold water storage tank is different because it stores unheated water for a water supply or heating arrangement.
How long does a hot water cylinder last?
Many hot water cylinders last around 10–20 years or longer, depending on their construction, water quality, maintenance and operating conditions. Manufacturer warranties vary, and regular servicing is particularly important for unvented systems.
Can I replace a vented cylinder with an unvented one?
It may be possible, but the change requires an assessment of your mains water pressure and flow, pipework, discharge arrangements, available space and safety requirements. A suitably qualified installer should assess the property before recommending a conversion.
Do I need a hot water cylinder with a combi boiler?
Most conventional combi boiler installations do not require a separate hot water cylinder because the boiler heats domestic hot water on demand. System and regular boiler arrangements commonly use a separate cylinder.
Is a larger hot water cylinder always better?
No. A larger cylinder provides more stored hot water but also takes up more space and may have greater total standing heat losses. The aim is to match capacity and recovery performance to your actual household demand.
Can a hot water cylinder work with a heat pump?
Yes. Heat pumps commonly use hot water cylinders, but the cylinder must be suitable for the heat pump's operating temperatures and output. Heat pump-compatible models often feature appropriately sized heat exchangers to support efficient heat transfer.
What's the difference between copper and stainless steel cylinders?
Copper cylinders are widely associated with traditional vented systems, while stainless steel is commonly used in modern unvented cylinders. The material alone does not determine performance. Construction quality, insulation, pressure rating, water compatibility and warranty should also be considered.
Find the right hot water cylinder at TradePlumbing
Explore our range of vented, unvented, direct, indirect, solar and combination cylinders, with options to suit a variety of heating systems and household requirements.
