ASHRAE Terminology

(1) in a hydronic system, a fitting or tank mounted in the piping system that separates entrained air from the water and discharges the air to an air vent. (2) in a steam system, a device that closes if either steam or water is present in the vent body and opens when air or noncondensables reach it.

the methodical proportioning of air and hydronic flows through the system mains, branches, and terminal devices using acceptable procedures to achieve the specified airflow or hydronic flow within testing, design, and installation limitations.

metering-type valve with locking positions to control hydronic flow.

steam, hydronic, or electric heating device located at or near the floor.

the hydronic circuit that determines the minimum differential pressure that the pump must produce to satisfy the zone loads (e.g., the circuit with the most-open valve). The critical circuit is the one with the highest pressure drop required to satisfy its load. At part-load conditions, the critical circuit can change based on zone loads.

(1) connection between supply and return line in a hydronic system. May be used to balance system pressure, maintain a minimum circulation flow rate or temperature, etc. (2) in a piping system, a connection in which a pipe carrying potable water is connected to a closed vessel (or system) that is above atmospheric pressure and that contains nonpotable fluid. This is typically the point where a backflow preventer is required.

heat exchanger assembly for transferring heat between two isolated fluid sources. The recovery system may be of air-to-air design or a closed loop hydronic system design. The system will include all necessary equipment such as fans and pumps, associated ducts or piping and all controls (operating and safety), and other custom-designed features.

(1) a perforated plate installed downstream of a fan in an air-handling unit designed to diffuse airflow equally across a downstream coil. Also called a diffuser plate. (2) in a multiple cooling tower installation, a pipe installed below and interconnecting the cold basin of multiple cooling towers to equalize the basin water levels and prevent the sump suction in one tower from drawing air or creating a vortex. (3) in refrigeration, a line or lines installed between compressors to equalize oil levels. (4) the bypass pipe in a primary/secondary flow system that hydronically decouples the primary from the secondary pumping system. See cross connection.

thermodynamic heating/refrigerating system to transfer heat. The condenser and evaporator may change roles to transfer heat in either direction. By receiving the flow of air or other fluid, a heat pump is used to cool or heat. Heat pumps may be the air source with heat transfer between the indoor air stream to outdoor air or water source with heat transfer between the indoor air stream and a hydronic source (ground loop, evaporative cooler, cooling tower, or domestic water).

hydronic system intended for operation at pressures exceeding 160 psig [1100 kPa (gage)] and/or temperatures exceeding 250°F (120°C). Compare to low-temperature hot-water system.

(also known as hydronic heating system or wet heating system) heating system for a building in which the heat-conveying medium is hot water and the heat emitters are radiators, convectors, or panel coils.

a thermal distribution system that uses water or a mixture of water and additives as the distribution medium in a building.

adjusting water flow rates through hydronic distribution system devices, such as pumps and coils, by manually adjusting the position valves or by using automatic control devices, such as automatic flow control valves.

science of heating and cooling water.

hydronic system intended for operation at temperatures below 200°F (93°C). Compare to high-temperature hot-water system.

piping system in which a single pipe loop provides the cooling or heating distribution to multiple devices or through secondary or tertiary loops.

(1) removal of air from a hydronic system. (2) removal of noncondensable gases from a refrigeration system. (3) removal of unburned gases from a combustion chamber. (4) to rid of whatever is impure or undesirable.

a systematic process or service applied to heating, ventilating, and air-conditioning (HVAC) systems and other environmental systems to achieve and document air and hydronic flow rates. The adjustment of fluid flow rates through distribution systems by manually adjusting the position of dampers, valves, etc., or by using automatic control devices to control the position of dampers, actuators, valves, etc.