unvented

Unvented is an adjective meaning not provided with a vent. In building services, the term most often describes systems that do not rely on a vent pipe to atmosphere for their operation. The most common usage is in reference to hot water systems known as unvented cylinders or unvented hot water systems. These are closed, pressurized storage cylinders fed directly from the cold mains supply. They rely on a mechanical expansion vessel and a pressure/temperature relief valve to control pressure and temperature, rather than an open vent to atmosphere. Draw-off water from the cylinder is at mains pressure, allowing faster flow and higher demand performance, but installation and servicing require adherence to local plumbing codes and, in many places, certification by a qualified installer.

In space heating and combustion appliances, unvented or ventless appliances do not vent to the outside through a flue. They burn fuel in a sealed chamber and release combustion products into the living space; this setup is subject to strict safety standards because of the risk of carbon monoxide exposure and oxygen depletion if improperly installed or maintained. In many jurisdictions, unvented fireplaces, stoves, or heaters are banned or heavily regulated, often requiring direct-vent or sealed-combustion models with outside venting and detectors.

Unvented systems are contrasted with vented systems, which rely on a vent pipe and atmospheric vent to remove air, moisture, or combustion products. The use of unvented systems can offer advantages such as reduced pipework and higher water pressure, but safety risks require proper installation, regular maintenance, and compliance with regulatory standards and standards for appliances and installation.