Unvented Hot Water Cylinders Explained: Direct vs. Indirect & Key Benefits
- Sêan Reid
- 8 min read
For quite some time, an unvented cylinder has been a popular choice for installing a hot water system in homes and businesses. This is due to their ability to deliver high water pressure alongside a consistent performance.
This guide explores how unvented hot water cylinders work, and the differences between direct and indirect models. Whether you’re considering a direct unvented cylinder or an unvented indirect cylinder, this guide will help you make an informed decision.
What is an Unvented Cylinder?
An unvented hot water cylinder is a type of cylinder that stores hot water and delivers it directly from the mains water supply, without the need for a separate cold water storage tank. Because the system is sealed, it allows for higher water pressure at taps and showers, faster heating times, and more flexibility in installation.
Unvented cylinders are typically heated either by an immersion heater (direct) or via an external heat source such as a boiler, heat pump, or solar thermal system (indirect).
In summary:
- Delivers water from mains supply
- Does not require a separate cold water tank
- Allows for stronger water pressure and quicker heating times
- Can be heated using immersion or external heat sources
Unvented Hot Water System Diagram
The diagram below shows how water is delivered from the mains supply. An unvented system operates by connecting directly to the incoming cold mains water supply. It enters through the Pressure Reducing Valve (PRV). The PRV controls the system pressure to safe operating levels. The water then flows into the unvented hot water cylinder, where it’s heated by the internal coil. Once heated up, the hot water flows through the system to outlets such as showers and taps.
Key System Components
- Pressure Reducing Valve: Maintains safe water pressure throughout the system.
- Expansion Vessel: Accommodates water expansion as it heats up, preventing pressure buildup.
- Temperature & Pressure Relief Valve: A critical safety component that releases excess pressure or overheated water.
- Discharge Pipe: Safely channels any overflow water away from the property
Safety Warning (Installation Requirements)
- G3-qualified installer (certified for unvented systems)
- Building Control notification
- Minimum two independent safety devices
- Annual servicing by qualified engineer
Unvented hot water systems contain safety-critical components operating under high pressure and temperature. Untrained homeowners should never attempt maintenance, repairs, or modifications to these systems. .
Installing an unvented hot water cylinder is not a DIY job. UK Building Regulations (Part G3) require that unvented cylinders must be installed by a competent person who holds the appropriate G3 qualification (also known as the Unvented Hot Water Storage Systems certificate).
Benefits of Unvented Cylinders
There are several benefits as to why you would want an unvented cylinder in your home or business.
- High Water Pressure – As water is directly supplied from the mains, unvented cylinders can deliver strong flow rates to multiple outlets simultaneously.
- Faster Heating Times – Whether using a direct immersion heater or an indirect heat source like a boiler or heat pump, unvented cylinders heat water more quickly.
- No Cold Water Storage Required and Saves Space – Unlike vented systems, unvented cylinders don’t need a separate cold water tank. This makes installation less disruptive and ideal for properties with limited loft space.
- Compatible with a Range of Heat Sources – From gas and oil boilers to renewable options such as solar thermal systems and heat pumps, unvented cylinders are highly adaptable.
- Improved System Hygiene – Because of the pressurised and sealed design, the water provided will be cleaner and more efficient.
Unvented vs Vented Cylinders – What is the Difference?
| Feature | Unvented Cylinder | Vented Cylinders |
|---|---|---|
| Water Source | Mains water supply (pressurised) | Cold water storage tank (gravity fed) |
| Water Pressure | Higher, consistent pressure from mains | Dependent on tank height (lower pressure) |
| Space Requirements | No cold water tank needed; more compact | Requires elevated space for cold water tank |
| Installation Cost | Potentially higher due to specialist components | Lower cost due to simpler installation |
| Heating Method | Direct via immersion heater, or indirect from external source (heat pump, solar, boiler) | Indirect heating via external boiler |
| System Efficiency | More efficient; works well with renewables | Less efficient due to heat loss from tank |
| Suitability | Modern homes with high water demand | Older properties / Properties with loft space |
The main difference between vented and unvented cylinders is how cold water is supplied to the cylinder.
For vented cylinders, the system is reliant on being gravity-fed. This means the cold feed tank (header or feed & expansion) installed in an elevated position (like a loft) supplies cold water via a vent pipe into the cylinder to be heated up.
The height of the feed tank determines the water pressure. For example, the higher the tank, the greater the pressure.
Types of Unvented Cylinders – Direct and Indirect
Unvented cylinders can be manufactured in two main configurations: direct and indirect. The primary difference is where the heat is generated. Direct models use immersion heaters inside the cylinder. In contrast, indirect cylinders are heated by an external source, such as a boiler or a renewable source, via an internal coil. Understanding which type suits your setup depends on the needs of your existing heating system.
Direct Unvented Cylinder
An unvented direct cylinder implements heat sources located within the cylinder itself. This heat source is often an immersion heater.
Additionally, a second immersion heater can enable home and business owners to take advantage of off-peak energy tariffs.
How it works:
Water is heated directly by electric elements inside the cylinder, similar to a large electric kettle. A second immersion heater can be fitted to take advantage of off-peak energy tariffs (such as Economy 7), reducing running costs.
Best suited for:
- Properties without gas supply or boiler systems
- Electric-only heating setups
- Small flats or properties with minimal hot water demand
- Backup systems in properties with unreliable boilers
Considerations:
Running costs are higher due to electricity prices (typically 3-4x more expensive than gas per kWh). However, installation is simpler as there’s no dependency on external heat sources.
Indirect Unvented Cylinder
Unlike direct unvented cylinders, indirect tanks rely on an external energy source to heat up water. These can come in the form of primary or renewable and alternative heat sources.
Primary heat sources consist of gas, oil, and electric boilers. Whereas heat pumps, solar thermal systems, and biomass boilers are recognised as renewable and alternative sources for indirect unvented cylinders.
How it works:
Hot water from your boiler, heat pump, or solar thermal system circulates through a coil inside the cylinder, transferring heat to the stored water without the two water supplies mixing
Heat sources include:
- Primary sources: Gas boilers, oil boilers, electric boilers
- Renewable sources: Air source heat pumps, ground source heat pumps, solar thermal systems, biomass boilers
Best suited for:
- Homes with existing boiler systems
- Properties prioritizing energy efficiency and lower running costs
- Systems integrating renewable energy
- Larger homes with high hot water demand
- New builds designed for sustainability
Why indirect is the modern choice
Most contemporary installations choose indirect cylinders due to lower running costs, compatibility with renewable energy, and better long-term value. This is why Newark Cylinders specialises exclusively in manufacturing indirect, unvented cylinders – they align with modern building standards and sustainability goals.
Newark Cylinders only manufacture indirect unvented cylinders.
How do I know if I need a direct or indirect model?
If your system relies on a gas or oil boiler, an indirect cylinder is advisable. Alternatively, heat pumps, solar thermal systems, and biomass boilers can be used as heat sources in indirect systems.
If your property relies on an electric source for hot water and doesn’t depend on a boiler or external heating source, a direct cylinder is suitable. However, the running costs mean direct cylinders can be expensive to maintain.
In addition to the type of heating system you have (or want), it is worth considering energy-saving aims, the amount of hot water usage, and the size of your home.
Further details on unvented cylinder sizing can be found here.
Conclusion
Unvented cylinders have proven to be efficient, reliable, and suitable for modern homes and businesses. Furthermore, Indirect unvented models are capable of utilising primary and renewable heat sources.
Newark Cylinders specialise in manufacturing highly efficient, bespoke unvented (and vented) hot water cylinders tailored to your needs. Whether you require a custom size, multiple connections, or have unique performance requirements, our sales team can make it happen.
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