Pressurised hot water cylinders – what are they?

Pressurised hot water cylinders, also known as unvented cylinders, are storage tanks that supply pressurised hot water to homes without relying on gravity. These systems are connected directly to the mains water supply; thus, they are a pressurised system, which allows them to provide hot water at high pressure for all taps, sinks, showers, and bath outlets in the house. A steady supply of mains pressure hot water for your property with a hot water storage cylinder.

They are the most popular hot water cylinders on the UK market, with stainless steel unvented hot water cylinders being the most prevalent due to the higher pressures that they can take over copper-manufactured cylinders.

Key Features of Unvented Hot Water Cylinders

Direct Mains Connection: Unlike traditional gravity-fed systems, pressurised cylinders are fed directly from the cold mains water supply.
High Pressure: By storing the water at a high pressure, typically between two and three bar, it enables strong water flow even to outlets at higher elevations.
Heating Methods: These cylinders can be heated either directly using electricity (immersion heaters) or indirectly using an external heat source like a central heating system boiler, heat pumps, or solar system.
Space-Saving: They eliminate the need for a separate cold water storage tank, reducing pipework and freeing up space in the home.

Advantages of an unvented cylinder

Delivers high-pressure hot water throughout the home.
Can be placed at lower levels in the water supply system.
Reduces overall pipework.
A pressurised unvented cylinder is ideal for properties with limited space.

How do unvented cylinders differ from traditional gravity-fed hot water cylinders?

Unvented cylinders, also known as pressurised hot water cylinders, differ significantly from traditional gravity-fed tanks in several key aspects:

Water Supply: Unvented cylinders are fed directly from the cold mains water supply, eliminating the need for a separate cold water storage tank in the loft. This direct connection allows for higher pressure and better flow rates throughout the home.

Pressure: Unlike gravity-fed systems, unvented cylinders operate at mains pressure, typically between two to three bar. This results in stronger water flow, especially to outlets at higher elevations.

Space Efficiency: By removing the need for a cold water tank in the loft, unvented hot water storage cylinders save space and reduce overall pipework in the home.

Installation Flexibility: Unvented cylinders don’t rely on gravity to deliver hot water, so they can be installed almost anywhere in the property, not just below the cold water tank.

Water Quality: The sealed nature of unvented systems reduces the risk of water contamination compared to open-vented hot water cylinders.

Expansion Management: Instead of using a vent pipe to handle water expansion during heating, unvented cylinders employ either an expansion vessel or an internal air gap within the hot water cylinders.

Performance: Unvented systems generally provide more consistent pressure and flow rates, especially beneficial in homes with multiple bathrooms or when several outlets are used simultaneously.

Noise: Unvented systems tend to operate more quietly as there’s no need for a cold water tank to refill after use of the hot water heater.

These differences make unvented cylinders a popular choice for modern homes, especially when space is limited or high-pressure/main- pressure hot water is desired throughout the property.

There are two main types of unvented hot water cylinders available:

Indirect Unvented Cylinders:

These cylinders are designed to work with external heat sources, such as gas or oil boilers (which are a system boiler) or heat pumps. They contain an internal coil through which hot water from the boiler or heat pumps circulates, indirectly heating the water stored in the cylinder. Indirect cylinders usually also include an immersion heater as a backup.

We go into full depth of this range here- indirect unvented cylinders

These indirect duplex stainless steel unvented hot water storage cylinders come with a 25-year warranty for domestic property use.

Direct Unvented Cylinders:

These direct cylinders have built-in electric immersion heaters that heat the water directly within the tank. They are suitable for homes without a boiler or as a standalone hot water solution. Direct cylinders typically have dual electric immersions for efficient heating.

We go into the full depth of this range here- direct unvented cylinders

Again these direct duplex stainless steel unvented hot water storage cylinders come with a 25-year warranty for domestic property use.

When choosing between direct and indirect unvented cylinders, the primary consideration is the existing heating system in the home. Homes with central heating systems with gas or oil boilers, heat pumps or solar thermal heat sources typically use indirect cylinders, while those relying solely on electricity for water heating would opt for direct cylinders.

What safety features are essential for unvented hot water cylinders?

Unvented cylinders, also known as pressurised hot water tanks, require several essential safety features to ensure their safe operation:

Temperature and pressure relief valve: This valve releases excess pressure if it builds up beyond safe levels or releases hot water if the temperature exceeds safe limits, preventing potential explosions.
Expansion Vessel: This component absorbs excess pressure within the system, helping to maintain safe operating conditions.
Thermostat: Controls the water temperature and prevents overheating.
Cut-out Switch: Acts as a safety feature to prevent overheating by shutting off the heating element if temperatures become too high.
Tundish: A visible device that allows for safe discharge of water from the relief valve into the environments, providing a clear indication of valve operation.

These safety devices work together to manage pressure, temperature, and expansion within the unvented cylinder system. It’s crucial to note that these safety features must be installed by a G3-qualified heating engineer and require annual inspection and maintenance to ensure they function correctly. Regular servicing is not only important for safety but also validates the manufacturer’s warranty and ensures compliance with “G3” UK building regulations.

What are the disadvantages of an unvented hot water system?

While unvented cylinders offer many advantages, they also come with several potential drawbacks:

Higher Installation Costs: Unvented systems are typically more expensive to install due to their complex nature and the need for additional safety features.

Specialised Installation: Only G3-qualified heating engineers can install a pressurised hot water tank system.

Regular Maintenance: Unvented cylinders require annual maintenance checks to ensure safety and compliance with regulations, adding to long-term costs.

Mains Dependency: You won’t have hot water if there’s a mains water supply interruption.

Compatibility Concerns: Some unvented systems may not be compatible with certain types of showers or mixer taps.

Space Requirements: While unvented hot water cylinders eliminate the need for a cold water tank, some unvented cylinders still require space for an expansion vessel, but units with internal expansion systems don’t.

Potential for System Failure: If safety features like pressure relief valves or expansion vessels malfunction, it could lead to more serious issues.

These drawbacks highlight the importance of careful consideration and professional advice when choosing between vented and unvented hot water systems for your property.

Pressurised domestic hot water systems

Pressurised hot water cylinders or unvented hot water cylinders are popular due to the energy-efficient benefits and exploitation of mains pressure water. They can be indirect or direct models to suit the customers’ requirements.

Pressurised Hot Water Cylinders