User Guide

ManualsBrandsBarber Colman ManualsHVAC Replacement PartsTwo Pipe Thermostat (55-85F)

thermostat is direct acting, the output pressure should rise.

If the thermostat is reverse acting, the output pressure will

drop.

Note:

The mount of the rise or drop in the output pressure

may vary depending on the ambient temperature in the test

area. If output is always 0 psig, the restriction may be

plugged. If output is always equal to supply or unable to

decrease below 3 psig, the nozzle may be plugged.

2. T check for active thermal element, adjust the setpoint

knob to obtain approximately 8 psig branch output.

Slightly warm the element with your hand or breath. A

direct acting thermostat will increase output pressure. A

reverse acting thermostat will decrease output pressure.

If the thermostat fails to function properly, refer to

REPAIR.

RUN/ADJUST

Theory of Operation

These thermostats are piloted relay devices incorporating an

internal pneumatic feedback principle which permits the use of

low mass bimetals and minimum leak from the

nozzle-restriction side of the circuit. This provides maximum

sensitivity with minimum air consumption. Operation may be

understood by referring to the schematic diagram in Figure-6.

The supply air pressure (1) is channeled from the main

chamber through the restriction (2) into the nozzle (3).

The nozzle-restriction combination controls the pressure to

the pilot diaphragm of the pneumatic relay. The bimetal

positions the nozzle lever over the nozzle to regulate the

pressure to the pilot chamber. The force exerted against the

pilot diaphragm (4) actuates the relay part of the system.

The relay is operated by the pressure on the pilot diaphragm

as follows: an increase in pressure on the pilot changer

diaphragm (4) overcomes the preload force on the pilot

diaphragm spring (5), the main valve plug spring (6) and the

air pressure on the plug (7), to move the supply main valve

plus off its seat. Supply air pressure then flows into the branch

chamber (8). The resulting pressure change is transmitted to

the feedback chamber (9), where it exerts a force on the

feedback plunger (10), moving it up. The force is transmitted

to the nozzle lever through the feedback levers (11 and 12)

and spring (13) to balance the force exerted by the bimetal.

This action provides linear relationship between temperature

at the bimetal and branch line pressure.

A reduction in pressure on the pilot diaphragm allows the

diaphragm to move away from contact with the bleed valve

seat (14). The bleed valve seat then moves off the main valve

plug and allows air from the branch chamber to bleed through

the bleed valve and out to atmosphere through exhaust port

(15). This reduces the force in the feedback chamber until it is

balanced by the reduced bimetal force.

Figure-6

At that point the pilot chamber pressure is just adequate to

cause the bleed valve to seat on the main valve plug and stop

flow of air out of the branch.

Changeover: In the thermostat, two bimetals are mounted on

the bimetal bracket. On TK-1201, one bimetal is direct acting

and the other is reverse acting. On the TK-1301 and TK-1601,

both are direct acting. The bimetal which is in control of the

thermostat is determined by the position of the switchover

lever (16). The position of the switchover lever is determined

by a spring-loaded diaphragm (17), which is actuated by the

supply main pressure. When the pressure is 15 psig, the

piston is at the bottom of its stroke. At this position, the bimetal

(18) rests on the nozzle lever and the bimetal (19) is raised off

the nozzle lever by the change-over lever. when the main

pressure is changed to 20 psig, the diaphragm overcomes the

spring force and moves the piston to its upper limit, moving the

over lever to allow the bimetal (19) to rest on the nozzle lever

and lift the bimetal (18) off the nozzle lever.