ASHRAE Terminology

thermostat designed for use in water.

changeover from one mode of operation to another without operator intervention (e.g., a thermostat that changes from heat to cool without need for manual operation of levers or setpoints).

the measured temperature at the location of the controlling device for a specific purpose (e.g., a room thermostat).

additional length of pipe or coil added to a direct-expansion evaporator in order to provide superheat at the thermostatic expansion valve sensing bulb without using evaporator tubing.

(1) pneumatic thermostat designed to maintain one temperature during the day and a lower temperature during the night. (2) changes thermostat output from direct acting (DA) to reverse acting (RA) for seasonal changeover. (3) two temperature controls in one enclosure.

change in superheat of a thermostatic expansion valve required to open the valve a predetermined amount.

in a thermostatic expansion valve, a connection from a selected point in the low-pressure part of the circuit to the system pressure-sensing side of the actuating element such that the selected point pressure is transmitted to the actuating element (e.g., diaphragm or bellows).

an estimate of the maximum volume of hot water that a storage-type water heater or an integrated heater can supply within an hour from the time that the water heater is fully heated (i.e., with all thermostats satisfied). This rating is a function of both the storage volume and the recovery rate.

see steam trap.

steam trap that relies on the density of water to raise a float-and-lever mechanism to operate a valve head. It discharges condensate as it forms and enters the trap body. It includes a thermostatic balance pressure or bimetallic air vent to allow free passage of air on start up and discharges incondensable gases reaching the trap during operation.

ability of a thermostat or control system to terminate the heat or cooling input at a temperature other than its setting and in advance of the time that the temperature at the thermostat or control system sensor normally would cause a control change.

a space or group of spaces, within a building with heating, cooling, and ventilating requirements, that are sufficiently similar so that desired conditions (e.g., temperature) can be maintained throughout using a single sensor (e.g., thermostat or temperature sensor).

(1) thermostat that has an element that is inserted directly into the airstream of a duct or pipe. (2) thermostat with the sensing member in the sensed variable but with the controlling element outside.

in a thermostatic expansion valve, an integral internal part or passage whereby the actuating element (e.g., diaphragm or bellows) is exposed to pressure leaving the valve.

device that acts to automatically control the temperature of a room or space in which it is installed by switching directly to the control device.

vapor thermostat charged with such an amount of liquid that a certain quantity of liquid remains in the bulb whatever the operating conditions may be.

manual or automatic reset of temperature control point of a thermostat, usually coupled with a start-up time for restoration of desired daytime temperature level.

a thermostat with the ability to preset different temperature/time settings for heating and cooling equipment.

device used in conjunction with a thermostatic expansion valve to ensure equable refrigerant distribution from the valve to individual parallel sections of an evaporator.

thermostat in which the sensing bulb can be located at a distance from the instrument proper while remaining flexibly connected to it.

(1) control setpoint at which temperature is to be maintained. (2) indicated temperature to which a thermostat is set.

selection of heating or cooling operation by means of a combination of manual and automatic switches, which usually are located at the room thermostat.

device for allowing the passage of condensate and preventing the passage of steam, or for allowing the passage of air as well as condensate. Common steam trap types include float and thermostatic (F&T) traps, inverted-bucket traps, and thermodynamic traps.

part containing terminals and switches, to which a thermostat is attached. It may also contain other auxiliary components.

(1) temperature-controlling device located on a surface to respond to surface temperatures and initiate control actions. (2) automatic control device having a sensing element responsive to the temperature of the surface being controlled.

an automatic control device used to maintain temperature at a fixed or adjustable setpoint.

device that adds heat within a thermostat in anticipation of overshoot.

in a gas charged thermostat, a device that compensates for fluctuations caused by temperatures, in certain parts of the power system, different from the temperature of the bulb.

difference between the cut in point and the scale setting.

trap installed on the discharge side of a heating unit and designed to pass air freely on start-up and condensate at a subcooled temperature, but to prevent steam vapor passing into the return. It can have a bellows or encapsulated metallic diaphragm containing a small quantity of volatile liquid. At the bottom of the diaphragm or bellows is attached a hardened, self centering valve head operating on the pressure side of the valve seat. At ordinary temperatures and atmospheric pressure, the valve is fully open to permit free passage of air and cold condensate. The trap discharges at a fixed temperature below that of steam saturation temperature and closely follows the steam pressure/temperature curve.

trap installed where low-temperature discharge is required. It incorporates a bimetallic element that, when heated, deflects and causes a downstream valve head to be drawn up, closing the orifice. It discharges air and cold condensate freely on start-up.

an automatic control device or system used to maintain temperature at a fixed or adjustable setpoint.

a device for controlling superheat by regulating the mass flow of refrigerant to a refrigeration load, actuated by changes in equalizer pressure and temperature sensing element temperature.

evaporator pressure regulator that is sensitive to temperature.

device within an electric controller for completing or interrupting an electrical circuit in response to a temperature change.

the state in which the system undergoes a normal change in operation, such as thermostat cycling or actuation of a defrost control.

(1) single temperature controller designed to control temperature at two distinct setpoints. (2) thermostat that handles two separate circuits in sequence.

an hour in which one or more zones is outside of the thermostat set point plus or minus one half of the temperature control throttling range. Any hour with one or more zones with an unmet cooling load or unmet heating load is defined as an unmet load hour.