Unvented heating systems, also known as sealed heating systems or sealed central heating systems, are a type of heating system commonly used in residential and commercial buildings. Unlike traditional vented systems, unvented heating systems do not require a feed and expansion tank in the loft or attic space.

Here are some key characteristics and components of unvented heating systems:

1. Sealed System: Unvented heating systems operate under a closed-loop configuration, where the central heating circuit is completely sealed from the atmosphere. This means that the system does not rely on an open vent to release excess pressure or accommodate expansion.

2. Pressurized System: Unvented heating systems are pressurized, typically maintained at a higher pressure than the incoming water supply. The pressurization is achieved through the use of an expansion vessel, which accommodates the expansion of water as it heats up.

3. Components: The main components of an unvented heating system include a boiler, a hot water cylinder (often referred to as an unvented cylinder), an expansion vessel, pressure relief valves, circulating pumps, and various controls and sensors. The hot water cylinder is a key element, as it stores hot water for domestic use and can provide hot water at mains pressure throughout the property.

4. Benefits: Unvented heating systems offer several benefits. They provide high-pressure hot water, enabling strong showers and quick filling of baths. The absence of a feed and expansion tank eliminates the need for a loft or attic space, allowing for more flexible installation options and potential space savings. Additionally, unvented systems are generally considered more efficient, as they minimize heat loss associated with vented systems.

5. Safety Considerations: Due to the pressurized nature of unvented heating systems, safety is paramount. The system must be installed and maintained by competent professionals who adhere to relevant building regulations and safety standards. Properly sized and functioning pressure relief valves are critical to prevent excessive pressure build-up. Additionally, expansion vessels must be correctly sized and maintained to accommodate the expansion of water during heating.

It is important to note that regulations and requirements for unvented heating systems may vary by country or region. Local building codes and guidelines should always be followed, and professional advice should be sought for the design, installation, and maintenance of unvented heating systems to ensure compliance and safety.