Function of HVAC Accessories

TM 55-1905-223-24-17

1-17. Function of HVAC Accessor ies. Maintenance personnel should be familiar with the basic function of

the following accessories in the heating and cooling equipment in the subsystem.

Electrical Components Used with Duct Heaters.

(1)

Automatic Reset Thermal Cut-Out (ARTC). This is a thermally operated safety device that may

be either a bulb and capillary type or a disk type. This safety device is required to deenergize the

entire heater to prevent overheating. The automatic reset thermal cut-out may be capable of

handling small single phase loads or may require the use of a contactor for larger single phase

and three phase loads.

(2)

Heat Limiters. Sometimes called replaceable thermal cutoffs, these comprise one of the

methods of secondary thermally operated safety devices to protect the system against

overheating in the event of failure of the primary safety device, the automatic reset thermal cut-

out and its circuit. Heat limiters are set to operate at higher temperatures than the automatic

reset thermal cut-out. The heat limiters are normally installed in the power lines and open in

case of a double failure, that is, overheating of the system and failure of the primary safety

device. Wired this way the heat limiters are also independent of any contactor which may be

used in the operating circuit.

(3)

Manual Reset Thermal Cut-Out (MRTC). This is an alternate resettable secondary thermally

operated safety device set to operate at a higher temperature than the automatic reset thermal

cut-out. The manual reset thermal cut-out must either be in the power legs rated to handle the

resistive load and break a sufficient number of conductors to deenergize the heater. If any of the

manual resets is tripped, then it is necessary to correct the unsafe conditions and reenergize the

heater by pressing the reset button.

(4)

Contactors. A contactor is an electrical device which magnetically pulls in to close an electric

power circuit when the holding coil in the contactor is energized. Contactors are used when

either the control thermostat or the safety cut-outs or an air flow switch do not have sufficient

electrical rating to carry the resistive load.

(a)

Operating Contactors. These contactors are required to switch the electric load of each heater's

step and are operated by the control device, the automatic reset thermal cut-out, or the air flow

switch.

(b)

Back-Up Contactors. These are used for manual reset thermal cut-outs operating a second set of

contactors in series with the operating or safety contactors.

(5)

Room Thermostats. This is a temperature control device that is mounted on the wall in the area

which it serves. The voltage ratings of the thermostats must conform with the control voltage of

the heater.

(6)

Duct Thermostats. This is a contact device with a remote bulb mounted in the duct or plenum of

an air handling unit to measure air temperature. The bulb is mounted in such a way that it does

not see radiant heat directly from the heater.

1-20