Siemens Powers TH 188 Technical Instructions

Powers

Controls

Technical Instructions

TH 188 Unit Mounted Thermostat

With 20 Scim

Restrictor

With 40 Scim

Restrictor

Document No. 155-064P25

TH 188-2

April 1, 2005

TM

Description

Features

Application

Product Numbers

The TH 188 Unit Mounted Thermostat is a remote bulb, gradual acting pneumatic
instrument that maintains a pre-selected temperature by positioning pneumatic devices
that control a heating or cooling medium.

Replacement thermostats are available for Honeywell models LP916Bxxxx and Johnson
Controls model T-3300-2.

• Liquid filled sensing element

• Direct acting (DA), reverse acting (RA), and heating-cooling models (HC)

• Mounting bracket provided

The TH 188 thermostat is mounted in fan coil units and unit ventilators to control a space
temperature in applications where a wall thermostat cannot be used.

In unit ventilator applications, where the unit mounted thermostat pilots a Limitem, use a
thermostat having a 40 scim (11ml/s) restrictor. The Limitem has a built-in bleed to
prevent control air from being trapped.

To assure good control, the remote bulb sensing element must be located so that space
air passes over it continuously.

Table 1.

Thermostat Action

and

Model Number

Product Number

(5.5 ml/s)

(11 ml/s)

*Use 188-0024, but change restrictor.

HC (DA & RA) TH 188 HC --- 188-0030

DA TH 188D --- 188-0031
RA TH 188R 188-0024

Honeywell HC LP916Bxxxx --- 188-0033

Johnson HC T-3300-2 --- 188-0034

*

Siemens Industry, Inc.

Technical Instructions TH 188 Unit Mounted Thermostat

Control action Heating (DA) and Cooling (RA)

Type & Model

No.

Heating (DA)

Cooling (RA)

Changeover

Document Number 155-064P25
April 1, 2005

Specifications

Direct acting only

Operating

Physical

Reverse acting only
Operating range 60° to 85° F (16° to 29° C)
Operating pressure 30 psi (207 kPa) max.
Sensitivity range 1 to 5.25 psi per degree F
(12 to 65 kPa per degree C)
Factory sensitivity setting 2.25 psi per degree F
(28 kPa per degree C)
Temperature response 0.2° F (0.1° C)
Maximum ambient temperature
Case 135°F (57°C)
Bulb 231°F (111°C)
Scale graduations 1°F (0.55°C)
Normal air supply pressure See Table 2
Air usage
*HC, 40 scim restrictor 45 scim (12 ml/s)
DA, 40 scim restrictor 40 scim (11 ml/s)
RA, 20 scim restrictor 25 scim (7 ml/s)
Air capacity
*HC, 40 scim rest. 35 scim (9.5 ml/s)
*HC, 40 scim rest. cooling 120 scim (33 ml/s)
DA, 40 scim restrictor 35 scim (9.5 ml/s)
RA, 20 scim restrictor 120 scim (33 ml/s)
Thermal system Remote bulb, liquid filled
*HC air usage and air capacity is the same for Powers, Honeywell, and Johnson.
Weight 3 lbs. (1.36 kg)
Dimensions See Figure 9
Air connections Barbed fitting for 1/4-inch (6.4 mm)
OD plastic tubing
Finish Corrosion resistant zinc chromate

Page 2 Siemens Industry, Inc.

Table 2. Air Supply Pressure and Changeover.

Powers HC

25 Psi (172 kPa) 18 psi (124 kPa) 21 psi (145 kPa)

TH 188 HC

Powers DA

25 psi (172 kPa) -- --

TH 188D

Powers RA

-- 25 psi (172 kPa) --

TH 188R

Honeywell HC

18 psi (124 kPa) 13 psi (90 kPa) 15 psi (103 kPa)

Lp916Bxxxx

Johnson HC

20 psi (138 kPa) 15 psi (103 kPa) 17 psi (117 kPa)

T-3300-2

TH 188 Unit Mounted Thermostat Technical Instructions

Extension shaft 188-101

Document Number 155-064P25
April 1, 2005

Accessories

Operation

(See Figure 1).

Heating, Direct
Acting, 25 psi
(172 kPa) Supply

Cooling, Reverse
Acting, 18 psi
(124 kPa) Supply

Remote bulb mounting kit 808-517
Limit stop kit (25 sets/kit) 188-144
Restrictor replacement kit

five 20 scim (5.5 ml/s) and five 40 scim (11 ml/s) 188-159
Thermostat mounting bracket 188-077

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.

Heating-Cooling
Changeover

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.

Siemens Industry, Inc. Page 3

Technical Instructions TH 188 Unit Mounted Thermostat
Document Number 155-064P25
April 1, 2005

Operation,
continued

Figure 1. Thermostat Operation.

Calibration

General

The thermostats are factory calibrated at 72° F (22° C) and 7-1/2 psi (52 kPa) control
pressure.

1. Determine the temperature at the remote bulb.

2. Set pointer to the temperature found. Dial increments are approximately five degrees on
the major divisions with one degree subdivisions, beginning at 60° F clockwise to 85° F.

3. Loosen (do not remove) the return line test port screw. Slip the rubber line over test port
and connect to test gauge (Figure 2).

Figure 2. Test Port and Gauge used for Calibration.

4. With the knob set at the temperature of the remote bulb, the output (return line)
pressure should be between 7 and 8 psi (48 and 55 kPa) for all models. If the output
pressure is not 7 to 8 psi (48 and 55 kPa), remove knob after loosening its set screw.
Then rotate the set point adjustment post until pressure is 7 to 8 psi.

5. Replace knob and set pointer to temperature of the bulb, lock, and then turn to desired
setting.

The thermostat is now in calibration.

Page 4 Siemens Industry, Inc.

TH 188 Unit Mounted Thermostat Technical Instructions
Document Number 155-064P25
April 1, 2005

Calibration,
continued

Single Action

The direct acting or reverse acting thermostat may be calibrated at any pressure. It is
suggested that 7-1/2 psi (52 kPa) be used.

Dual Action

On the HC thermostat, calibration should be at 7-1/2 psi (52 kPa) for both DA and RA. This
is due to the use of one thermal system and a reversing relay to give the action desired. If
calibrated at 7-1/2 psi (52 kPa) for winter, the summer calibration will be 7-1/2 psi (52 kPa).
If 9 psi (62.1 kPa) is used for winter, 6 psi (41.4 kPa) will be summer’s calibrated pressure.
Figure 3 shows the relationship of the DA and RA calibration points. If the same pressure is
desired for both seasons, the RA section of the thermostat needs to be recalibrated (follow
the RA calibration procedures).

Figure 3. Dual Action Calibratio n Points.

Reverse Acting
Stage

See Table 1 for pressures and changeover pressures.
The reverse acting stage is factory adjusted and does not require field adjustment. If the

adjustment is disturbed, re-adjust as follows:

1. With the main air supplies set for 25 psi (172 kPa), connect a PRV with gauge to the

supply side of thermostat and set at 25 psi (172 kPa).

2. Remove knob and cover from thermostat, as shown in Figure 4.

3. Open bleed screw on test port and connect gauge.

4. Rotate adjustment post to give an output within 1/2 psi (3.4 kPa) of calibration pressure.

5. Adjust PRV for 18 psi (124 kPa) air supply; observe the control pressure. If it is not

within 1/2 psi (3.4 kPa) of calibration pressure, adjust the reverse acting adjustment until
it is. Apply thread locker to the screw setting.

6. Remove test port gauge; close bleed screw.

7. Replace cover and knob.

8. Lock knob in place with pointer at bulb temperature.

9. Set knob at desired temperature setting.

Siemens Industry, Inc. Page 5

Technical Instructions TH 188 Unit Mounted Thermostat
Document Number 155-064P25
April 1, 2005

Calibration,
continued

Heat-Cool Thermostat
Changeover

See Table 1 for air pressures and changeover pressures.
The changeover is a factory adjustment; it is sealed with thread locker and does not

require field adjustment. If for some reason this adjustment is disturbed, re-adjust as
follows: (See Figure 4)

1. Supply the thermostat with an air supply equal to the appropriate changeover

pressure.

2. With the exhaust adjustment (center screw) backed out several turns, turn the

switch spring adjustment (outer screw) down snug, and then back off, until air can
be heard bleeding out.

3. Turn the exhaust adjustment down snug and back off approximately 1/8 turn.

4. Seal both screws with thread locker to prevent further movement.

Installation

Figure 4. Heating/Cooling Changeover.

• A mounting bracket (188-077) is included for mounting the thermostat within the

various units.

• Figure 5 shows a typical installation where the mounting bracket is fastened to the

partition with two sheet metal screws (034-257).

• Mount the remote sensing bulb in the air stream of the return air (near the inlet of

the fan).

• Avoid metal-to-metal contact between the bulb and unit to prevent the soft copper

sensing bulb wearing through due to the rubbing action caused by the unit vibrating.

• Use the adhesive backed cable clip (141-311) supplied with the thermostat to fasten

the capillary and/or bulb in the return air stream.

• If there is not enough room to mount the thermostat in a convenient location, or if

the set point dial must be in the same compartment as the fan switch, order an
extension shaft kit (188-101). The thermostat may be mounted in any convenient
location within the eight-inch length of the shaft extension cable shown in Figure 6.

Page 6 Siemens Industry, Inc.

TH 188 Unit Mounted Thermostat Technical Instructions

Complaint

Check

Possible Cause

Corrective Action

Document Number 155-064P25
April 1, 2005

Installation,
contintued

Preventive
Maintenance

Return line

pressure

0 psi (0 kPa)

Return line

pressure 18

psi (124 kPa)

or greater

Figure 5. Unit Mounted.

The construction of the TH 188 Thermostat makes it virtually maintenance-free. But like
any pneumatic thermostat, it should be checked for calibration at least once each year
(usually just before the heating season).

Supply Air No Air As Required

*Heating 25

psi

(172 kPa)

*Cooling 18

psi (124

kPa)

Supply Air Pressure Too High As Required

*Heating 25

psi (172

kPa)

Calibration Out of calibration Recalibrate

Restrictor Clogged or dirty Clean or replace

Sensing element Loss of charge Replace sensing element

Direct acting stage Internal binding or leaking Replace thermostat

Calibration Out of calibration Recalibrate

Throttling pin Dirt built up around pin Clean or replace nozzle ass’y

Reverse acting stage Internal binding or leaking Replace thermostat

Calibration Out of calibration Recalibrate

Throttling pin Dirt built up around pin Clean or replace

Direct acting stage Internal binding Replace thermostat

Figure 6. Mounting with Extension Sha ft.

Table 3. Troubleshooting.

*See Table 1 for Honeywell and Johnson Controls pressures.

Siemens Industry, Inc. Page 7

Excessive

cycling

*Cooling 18

psi (124

kPa)

Calibration Out of calibration Recalibrate

Restrictor Clogged or dirty Clean or replace

Sensing element Loss of charge Replace sensing element
Reverse acting stage Improperly adjusted Re-adjust
Reverse acting stage Internal leaks or binding Replace thermostat

Lever assembly Sticking/binding of levers As Required

Technical Instructions TH 188 Unit Mounted Thermostat

Item

Part

Number

Description

Number

Req’d

Material

Document Number 155-064P25
April 1, 2005

Figure 7. See Tables 4 and 5.

Table 4. Parts List (See Figures 7 and 8).

1 -- Housing assembly 1 Zinc alloy
2 -- Cover plate 1 Aluminum
3 -- Upper lever 1 Aluminum
4 188-057 Sensing element 1 Copper
5 -- Adjustment post (HC & RA) 1 Steel
6 188-122 Knob 1 Zinc alloy
7 -- Adjustment knob (DA only) 1 Brass
8 034-370 8-32x1/4 sock. hd. set screw 1 --

9 -- Lower lever assembly 1 -­10 188-082 Nozzle assembly 1 -­11 188-080 Gasket 1 -­12 833-009 Seal Screw 1 Brass
13 -- Switch spring adjusting nut (HC only) 1 Brass
14 -- Exhaust adjusting screw 1 Brass
15 030-312K 6-32x5/16 pan hd. screw 1 Steel

15a -- Capillary clamp 1 Plastic

16 -- RA adjustment nut (HC & RA) 1 Brass
17 034-014K Restriction plate screw (HC & RA) 1 Brass

Page 8 Siemens Industry, Inc.

TH 188 Unit Mounted Thermostat Technical Instructions

Item

Part

Number

Description

Number

Req’d

Material

Document Number 155-064P25
April 1, 2005

Figure 8. See Tables 4 and 5.

Table 5. Parts List (See Figures 7 and 8).

18 -- Restriction cover 1 Steel
19 * Restrictor (2 Tabs 20 scim) 1 Brass

* (4 Tabs 40 scim) Brass
20 * Restrictor gasket, upper 1 Rubber
21 * Restrictor gasket, lower 1 Rubber
22 -- Restrictor plate screw (long) (HC & RA) 1 Steel

-- Restrictor plate screw (long) (DA only) 1 Steel
23 -- Cover screw 2 Steel
24 -- Filter 1 Felt
25 -- Polytube adapter 2 Brass
26 -- Mounting washer (HC & RA) 1 Steel
27 -- Mounting screw (HC & RA) 1 Steel

-- Mounting nut (DA only) 1 Brass
28 -- Pre-load spring 1 Spring Wire

*These parts are in Restrictor Replacement Kit (188-159) shown under Accessories.

Siemens Industry, Inc. Page 9

Technical Instructions TH 188 Unit Mounted Thermostat

+ 1 847-215-1000

Document Number 155-064P25
April 1, 2005

Dimensions

Dimensions in
Inches (Millimeters)

HC and RA Thermostats

DA Thermostats

Information in this publication is based on current spec i fications. The company reserves the right to make changes in specifications and model s as
design improvements are introduced. Powers is a registered tradem ark of Siemens Industry, Inc. Product or company names mentioned herein
may be the trademarks of their respective owners. © 2005 Siemens I ndustry, Inc.

Siemens Industry, Inc.

Building Technologies Divis i on
1000 Deerfield Parkway
Buffalo Grove, IL 60089

Figure 9. Dimensions.

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Document No. 155-064P25

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