Unvented hot water systems come with a myriad of benefits for domestic use, but also some disadvantages. Plenty of industry jargon, as well as exhaustive information regarding unvented systems, exists, making the subject appear overwhelming.

Using our expert team of engineers’ profound understanding and hands-on experience of heating systems, we have compiled this guide to shed light on:

✅ what unvented hot water systems are

✅ the problems and issues they potentially solve in your home

✅ their advantages and disadvantages

Unvented hot water system explained

An unvented hot water system, also known as a ‘pressurised hot water system’ is a water cylinder vessel which takes its cold water supply directly from a supply pipe or dedicated storage cistern, such as the incoming water main. This keeps the water under constant extreme pressure.

The water entering the vessel is then heated directly or indirectly. The pressure is released when a hot water tap or appliance is turned on, providing an instant flow of hot water. As the system is completely sealed and pressurised, there is no need for ventilation, giving it the name ‘unvented hot water system’.

How does an unvented hot water system work?

There are two different methods for heating the water within an unvented hot water system:

  1. In a directly heated model where the cold mains water is warmed using an in-built immersion heater (an electric heating element that is positioned in the liquid intended to be heated).
  2. In an indirectly heated model where the water is heated using an external source, such as a gas boiler or solar panels.

When a hot water tap or appliance in the system is turned on, the high pressure from the incoming cold water enters from the bottom of the unvented hot water cylinder. The pressure forces the hot water along an outlet pipe leading from the cylinder, getting it to where it needs to be.

Unvented hot water systems are designed with several safeguards to prevent the risk of explosion, such as a pressure release valve or a temperature and a pressure release valve, and often an expansion vessel to allow for overheated water to be discharged.

Key differences between vented and unvented hot water systems

A vented hot water system, the more traditional option for domestic use, takes its cold water supply directly from a ‘header tank’, installed above the cylinder, usually in the attic or loft of your home. This system uses the gravitational force of the cold water entering the system from above to enable the flow of hot water to your taps and other appliances throughout your home.

A vented system is convenient to install and needs little maintenance, but requires sufficient space in your home to be installed near the header tank. This makes vented hot water systems unsuitable for homeowners with limited space.

Moreover, the vented hot water system’s dependency on the gravitational pull from the header tank can have a negative impact on the pressure flow of water if there is not enough height between the tank and the tap outlets. For instance, the water pressure of the downstairs taps will be greater than that of the upstairs taps.

Advantages of unvented hot water systems

Unvented hot water systems offer a range of advantages for your home:

Eliminates the need for extra storage

With cold water entering the system under mains pressure, unvented hot water systems operate without the necessity of a cold water tank. This feature makes them an ideal option for homeowners dealing with space constraints.

Reduced risk

By eliminating the need for a cold water tank, the system ensures your pipes won’t freeze or burst during winter months. Additionally, the absence of a water tank mitigates the potential risk of contamination, reducing the likelihood of Legionnaires’ disease.

Versatile installation

The robust pressure generated by the water entering the cylinder in unvented hot water systems allows unvented cylinders to be installed virtually anywhere. Both slimline and horizontal cylinder options are available, providing versatile installation possibilities in a home.

Reduced noise

In contrast to vented systems with a noisy cistern refill process, the absence of the cold water tank in unvented systems eliminates gurgling pipework sounds. The completely sealed system produces a quieter and more efficient operation.

Consistent and enhanced hot water supply

Unvented hot water systems benefit from a steady pressure supplied by the mains, providing a more reliable and constant flow of hot water compared to vented systems. This feature becomes especially crucial for homeowners with high-powered luxury bathroom fixtures, such as jetted baths, which demand a continuous and ample supply of hot water.

Rapid heat recovery efficiency

After use, unvented systems offer an exceptionally rapid heat recovery rate, requiring only 15-20 minutes to warm up. This efficiency ensures that hot water is readily available when needed, optimising both energy usage and convenience.

Learn more about how we assisted Bohunt School to construct a new three-storey Sixth Form Centre, requiring a range of heating and electrical support, including hot water services.

Disadvantages of unvented hot water systems

Mains dependency

The significant disadvantage of an unvented system is their direct connection to the mains water supply. In the event of a mains water supply interruption, whether due to maintenance or unforeseen circumstances, your home will be left without access to hot water. A minimum flow of 20 ml per minute from the mains is essential for the sustained operation of this type of hot water system.

Higher installation costs

Due to the heightened pressure they operate under, unvented systems are typically more expensive to install. Safety features, such as relief pipework, must be installed and maintained by specially qualified technicians with G3 recognised qualifications, (i.e. Building Regulations governing hot water supply and systems in England and Wales, or the Building Standards technical handbook in Scotland).

Call our expert team

If you’re experiencing any issues with your current hot water system, or are interested in learning more about our unvented hot water system installation and maintenance services, contact our expert team today!

Paine Manwaring is an established, professional and highly experienced supplier of plumbing, heating and electrical services to domestic and commercial customers throughout Sussex, Surrey and Hampshire.

For more information about the heating, plumbing and electrical services which we provide for both homes and businesses get in touch with a member of our expert and friendly team today on 01903 237522 or enquiries@painemanwaring.co.uk (Worthing) or 01243 784711 or info@painemanwaring.co.uk (Chichester).