Unvented Water Heater

water heater
a heater and storage tank to supply heated water
Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating. In industry, both hot water and water heated to steam have many uses.
An appliance for supplying hot water for purposes other than space heating or pool heating.

unvented
not provided with vents
(of strong, usually negative feelings) Not expressed
Not provided with ventilation
Unvented roofs are those which do not have an active ventilation system. More and more modern construction use unvented roof designs. In cold climates, this requires that the interior air barrier (or other condensing surfaces) stay warmer than the dew point so as to avoid condensation.