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TH 188 Unit Mounted Thermostat
Accessories
Operation
(See Figure 1).
Heating, Direct
Acting, 25 psi
(172 kPa) Supply
Cooling, Reverse
Acting, 18 psi
(124 kPa) Supply
Heating-Cooling
Changeover
Siemens Industry, Inc.
Extension shaft
Remote bulb mounting kit
Limit stop kit (25 sets/kit)
Restrictor replacement kit
five 20 scim (5.5 ml/s) and five 40 scim (11 ml/s)
Thermostat mounting bracket
NOTE:
Pressures used refer to thermostats calibrated to Powers pressures. See Table 2
for Honeywell and Johnson Controls supply pressures.
When the remote bulb senses a falling temperature, the liquid within the thermal system
contracts. This allows the pre-load spring to lift the sensitivity arm and throttling pin, which is
in contact with the arm. The throttling pin lifts off the nozzle seat reducing the pressure
behind the nozzle. This is the direct acting signal of the thermostat. The reduced pressure
passes through the return line to a Normally Open (NO) valve. This allows more of the
heating medium to flow through the heating coil increasing the heat supplied to the space.
When there is a rise in temperature sensed by the remote bulb, the liquid within the thermal
system expands. This expansion moves the sensitivity arm and throttling pin downward,
increasing the load on the throttling pin and moving it closer to the seat in the nozzle.
Pressure builds up in the chamber below the nozzle until the force of the increased air
pressure against the bottom of the throttling pin exactly balances the downward force of the
sensitivity arm. This increased pressure reduces the flow of heating medium through the
heating coil by closing the normally open valve and reducing the heat supplied to the space.
With an 18 psi (124 kPa) supply pressure, the channel connecting the direct acting signal to
the return line port is closed, and the channel connecting the reverse acting signal to that
same return line port is opened.
When the remote bulb senses a falling temperature, the liquid within the thermal system
contracts, allowing the spring pressure to move the sensitivity arm away form the throttling
pin. This allows the built-up pressure to bleed off, lowering the pressure on the reverse acting
stage of the thermostat. Thus, the spring pressure opens the supply valve and closes the
exhaust valve, increasing the pressure in the return line.
With an 18 psi (124 kPa) air supply, the switch spring exerts enough force to overcome the
18 psi (124 kPa) air pressure acting on the switch diaphragm. The ball valve is therefore kept
closed, allowing no air to reach the changeover diaphragm. An air pressure increase to 25 psi
(172 kPa) will exert enough force on the switching diaphragm to overcome the force of the
switch spring and cause the switch diaphragm to move upward. This unseats the switch ball
valve and allows full supply pressure to act on the changeover diaphragm. The air pressure
force on top of the changeover diaphragm moves the diaphragm down to compress the
changeover rod spring. Attached to the changeover rod and changeover poppet. The motion
of the diaphragm, rod and poppet assembly closes the reverse acting port to the return line
and opens the direct acting port, making the thermostat direct acting. Lowering the air supply
back to 18 psi (124 kPa) will reverse the changeover procedure and the thermostat will again
be reverse acting.
Technical Instructions
Document Number 155-064P25
188-101
808-517
188-144
188-159
188-077
April 1, 2005
Page 3
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