Honeywell T874A F Thermostat User Manual

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TRADELINE

T874A-F Multistage

Thermostats/Q674A-E,G Subbases

Installation Instructions for the Trained Service Technician.

Application

Installation

The T874A-F Thermostats provide 24 to 30 Vac control

for heating and/or cooling systems as listed in Table 1.

WHEN INSTALLING THIS PRODUCT…

1. Read these instructions carefully. Failure to follow

them could damage the product or cause a hazardous condi-

tion.

2. Check the ratings given in the instructions and on the

product to make sure the product is suitable for your

application.

TABLE 1—THERMOSTAT HEATING AND/OR

COOLING STAGES.

T874

—

Heating Stages

Cooling Stages

3. Installer must be a trained, experienced service tech-

nician.

—

4. After installation is complete, check out product

operation as provided in these instructions.

The Q674A-E,G Subbases provide wiring terminals,

system and fan switching, and mounting bases for T874

Thermostats as listed in Table 2.

CAUTION

. Disconnect power supply to prevent electrical

shock or equipment damage.

TABLE 2—SUBBASE SWITCHING POSITIONS.

Q674

Switch Positions

System

2. To prevent interference with the thermostat

linkage, keep wire length to a minimum and

run wires close as possible to the subbase.

Model

Fan

Heat-Auto-Cool

Heat-Off-Cool

Off-Auto

Auto-On

None

. Do not overtighten thermostat captive mount-

ing screws because damage to subbase threads

can result.

. Do not short across coil terminals on heating

relay or gas control. This can burn out the

thermostat heat anticipator.

None

Off-Heat-Auto-Cool

Off-Auto

Auto-On

None

IMPORTANT: Thermostats are calibrated at the factory

by using subbases mounted at true level. Inaccurate

subbase leveling will cause thermostat control de-

viation.

Operation

On a 2-heat thermostat, the two stages of heat make

sequentially as the temperature drops. Make refers to the

mercury switch initiating a call for heat or cool.

There are about 2° F [1° C] between stages so that the

second stage makes only when the first stage cannot handle

the load. This is the interstage differential.

LOCATION

Locate the subbase about 5 ft [1.5m] above the floor in

an area with good air circulation at average temperature.

Do not mount the subbase where the thermostat may be

affected by:

—

drafts or dead spots behind doors and in corners.

hot or cold air from ducts.

radiant heat from sun, appliances or fireplace.

concealed pipes and chimneys.

unheated (uncooled) areas such as an outside wall

behind the thermostat.

Recycling Notice

M3375

This control contains mercury in a sealed tube. Do not

place control in the trash at the end of its useful life.

If this control is replacing a control that contains mer-

cury in a sealed tube, do not place your old control in the

trash.

Contact your local waste management authority for

instructions regarding recycling and the proper disposal of

this control, or of an old control containing mercury in a

sealed tube.

MOUNTING THE SUBBASE

The thermostat subbase can be mounted on a vertical

outlet box, horizontal outlet box or directly on the wall.

1. If you must mount the subbase on a vertical outlet

box, order Honeywell 193121A Adapter Assembly (Fig. 1).

The assembly includes an adapter ring, two screws and a

cover plate to cover marks on the wall. Install the ring and

cover plate on the vertical outlet box.

J.H. • Rev. 5-94 •1

• ©Honeywell Inc. 1994 • Form Number 66 00 - -11 11 66 55 —— 66

M3375

. Secure the cover plate (if used) and subbase with the

screws provided. Do not fully tighten the subbase screws.

. Level the subbase using a spirit level, as shown in

Fig. 1—Installation of subbase on outlet box.

Fig. 3, and firmly tighten subbase mounting screws. The

subbase mounting holes provide for minor out-of-level

adjustments.

IMPORTANT: An incorrectly leveled subbase will cause

the temperature control to deviate from setpoint.

Fig. 3—Subbase components and leveling

procedure.

For a wall installation, hold subbase in position and

mark holes for anchors. See Fig. 2. Obtain wall anchors

locally. Take care that the wires do not fall back into the

wall opening. Set aside subbase. Drill two 3/16 in.

[

4.8 mm] holes and gently tap anchors into the holes

WIRING THE SUBBASE

until flush with the wall.

All wiring must comply with local electrical codes and

ordinances. Follow equipment manufacturer wiring in-

structions when available. To wire subbase, proceed as

follows:

Fig. 2—Installation of subbase on wall.

1. Connect the system wires to the subbase as shown in

Figs. 8 through 16. A letter code is located near each

terminal for identification. The terminal barrier permits

straight or conventional wraparound wiring connections.

See Fig. 4.

Fig. 4—Wiring connections.

2. Your Q674 Subbase can require one or more jumpers

that may or may not be factory supplied. Refer to the wiring

diagrams for specific terminals to be jumpered. See Table 3

for proper application.

. Pull electrical wires through the cover plate (if

used) and subbase cable opening (Fig. 3). See Wiring the

Subbase section before pulling any wires.

CAUTION

Never install more than one wire per terminal

unless factory supplied jumper with spade termi-

nal is used. See Fig. 7.

IMPORTANT: Use 18 gauge, color-coded thermostat

cable for proper wiring.

0-1165—6

TABLE 3—JUMPER APPLICATIONS.

If your subbase has: Application Use Fig.

Fig. 6—Jumper adjacent terminals for special

system hookup. Strip wire 3/4 in. [19 mm].

RC, RH terminals

single transformer

system

adjacent terminals

special system

operation or LED

indication

nonadjacent

terminals

. Firmly tighten each terminal screw.

. Fit wires as close as possible to the subbase. Push

excess wire back into the hole.

5. Plug hole with nonflammable insulation to prevent

drafts from affecting the thermostat.

Fig. 5—Jumper RC and RH for single trans-

former system. Strip wire 3/4 in. [19 mm].

Fig. 7—For nonadjacent terminals, use jumper

supplied with subbase.

NOTE: Only the T874D Thermostat is shown with the

subbases. Thisthermostat/subbaseprovides2-stageheat-

ing and 2-stage cooling connections. If using another

model, see Fig. 16 and Table 4.

TABLE 4—THERMOSTAT BULB

CONFIGURATOIN.

Model No.

T874A

T874B

T874C

T874D

T874E

Heating Bulbs

Cooling Bulbs

C1,C2

H1, H2

C1, C2

T874F

H1, H2

60-1165—6

Fig. 8—T874A-D/Q674A in standard heating/cooling system.

Fig. 9—T874C,D/Q674B in standard heating/cooling system with separate transformers for heating and

cooling.

THERMOSTAT

SUBBASE

SYSTEM COMPONENTS

HOT)

(

HEAT RELAY 1

HEAT RELAY 2

FAN

SWITCH

FALL

AUTO

H1 ANTICIPATOR

HEAT CHANGEOVER

VALVE

FALL

H2 ANTICIPATOR

SYSTEM

SWITCH

COOL CHANGEOVER

VALVE

HEAT

OFF

RISE

FAN RELAY

COOL

COMPRESSOR

CONTACTOR 2

C1 ANTICIPATOR

C2 ANTICIPATOR

COMPRESSOR

CONTACTOR 1

RISE

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD

PROTECTION AS REQUIRED.

(HOT)

JUMPER RH, RC FOR SINGLE TRANSFORMER.

OPTIONAL LED ON SOME MODELS.

M933C

0-1165—6

Fig. 10—T874D/Q674B in standard heating/cooling system with single transformer for heating and

cooling. Exact replacement for York model no. 2TB04700324.

Fig. 11—T874D,E,F/Q674C in standard heating/cooling system.

60-1165—6

Fig. 12—T874A,D,F/Q674D in standard heating/cooling system.

Fig. 13—T874D/Q674E in standard heating/cooling system.

0-1165—6

Fig. 14—T874A-F/Q674G in standard heating/cooling system.

Fig. 15—T874C/Q674B in standard application.

60-1165—6

Fig. 16—Typical Q674 terminal designations. The thermostat and subbase used determines the number

of system components controlled.

TEMPERATURE LEVER STOPS

Some thermostats have factory-installed temperature

INSTALL AND ADJUST LOCKING LEVER

SCREWS

lever locking screws (Fig. 17) and/or stop brackets

The two screws with insulated heads should be used

only if the HEAT and COOL levers are to be locked in

place at a specific temperature control point. Do not use

standard screws that provide metal-to-metal contact with

the lever brackets.

(

Fig. 18). Use these only if the HEAT and COOL tempera-

ture set point lever ranges are to be restricted. If the compo-

nents are factory-installed, see the specific section below

for the adjustment procedure.

If a temperature range restriction is required and the

locking components are not factory-installed, it is neces-

sary to order a 4074ECK Envelope Assembly. This enve-

lope includes two stop brackets, one brass insert and one

bracket mounting screw for restricting the adjustable range

of the HEAT and COOL set point levers. Also, the enve-

lope includes two screws with plastic insulated heads for

locking the set point control levers in position. If the

components are to be used, see the specific section below

for the installation procedure.

To install:

1. Refer to Fig. 20 for screw hole locations.

2. Install the two screws with insulated heads in the

indicated holes. Do not overtighten screws.

3. Set the HEAT lever and the COOL lever at desired

temperature control points.

4. Firmly tighten both screws.

INSTALL AND ADJUST STOP BRACKETS

Install the stop brackets only if there is a need to restrict

the adjustable range of the heating and cooling temperature

set point levers.

Fig. 17—Location of lever locking screws.

To install:

LEVER LOCKING SCREWS

1. Turn to the back of the T874 Thermostat. Locate the

hole for the brass insert in the plastic base below the LED

window. See Fig. 19.

. Push the brass insert into the hole with your finger.

. Turn to the front of the T874 Thermostat.

. Place the two stop brackets in position with the tabs

in the slot between the HEAT and COOL levers. See

Fig. 20.

M18680

. Insert the mounting screw into the two slots in the

Fig. 18—Location of lever stop brackets.

stop brackets and attach to the brass insert. Tighten the

screw to pull the brass insert into the back of the thermostat.

LEVER STOP BRACKETS

. Loosen the mounting screw enough so the stop brack-

ets can slide for adjustment.

. Move the HEAT and COOL set point levers to the

maximum temperature that is desired.

. Slide the stop brackets until one rests against the

HEAT lever and the other against the COOL lever.

M18681

0-1165—6

. Firmly tighten the mounting screw.

MOUNTING THE THERMOSTAT

1. Remove the thermostat cover by pulling the bottom

edge of the cover upward until it snaps free of the cover clip.

10. If the HEAT and COOL levers are to be locked in

place at a specific temperature, use the two insulated head

screws supplied instead of the two adjustable lever stop

brackets.

NOTE: The cover is hinged at the top and must be removed

by pulling up at the bottom.

CAUTION

2. Carefully remove and discard the polystyrene packing

insert that protects the mercury switches during shipment.

Do not use standard screws that provide metal-to-

metal contact with the stop brackets. Short circuit

and potential equipment damage may result.

3. Turn over the thermostat base and note the spring

fingers that engage the subbase contacts. Make sure the

spring fingers are NOT bent flat, preventing proper electri-

cal contact with the subbase.

4. Set the adjustable heat anticipator indicators to the

Fig. 19—Installing brass insert.

respective current setting for each stage. See Setting the

Heat Anticipator section.

. Note the two tabs on the top inside edge of the

LED WINDOW

thermostat base. The tabs fit into corresponding slots on top

of the subbase. Mount the thermostat on the subbase.

6. Align the two captive mounting screws in the ther-

mostat base with the posts on the subbase. See Fig. 21.

Tighten both screws. Do not overtighten screws or damage

to subbase posts can result.

Fig. 21—Mounting thermostat on subbase.

BRASS INSERT

INSERT HOLE

BACK OF

THERMOSTAT

M955A

Fig. 20—Installing stop brackets.

HOLES FOR INSULATED

LOCKING LEVER SCREWS

COOL

LEVER

HOLE WITH

BRASS INSERT

HEAT

LEVER

BRACKET

TABS

Settings

SETTING THE HEAT ANTICIPATOR

Move the indicator to match the current rating of the

primary contro. See Fig. 22. When using the T874 Thermo-

stat with two stages of heating, set each heat anticipator to

match its respective primary control current draw. If you

cannot find the current rating on the primary control, or if

further adjustment is necessary, see NOTE and use the

following procedure to determine the current draw of each

stage.

ADJUSTABLE

LEVER STOP

BRACKETS

MOUNTING

SCREW

BRACKET

SLOTS

The current draw must be measured with the thermostat

removed from the subbase and the power on to the heating

system.

M18682

1. Connect an ac ammeter of appropriate range between

the heating terminals of the subbase:

Stage 1: between W1 and RH or R.

Stage 2: between W2 and RH or R.

60-1165—6

. Move the system switch to AUTO or HEAT (if

SUBBASE SETTING

used).

The subbase switching positions control the system

operation as described below (Fig. 23).

SYSTEM SWITCH (some subbases do not have all of

the following functions):

OFF: Both the heating and cooling systems are off. If

the fan switch is in AUTO position, fan is also off.

AUTO: Thermostat automatically changes between

heating and cooling system operation, depending

on the indoor temperature.

HEAT: Heating system is automatically controlled

by the thermostat. Cooling system is off.

COOL: Cooling system is automatically controlled

by the thermostat. Heating system is off.

FAN SWITCH

3. After one minute, read the ammeter and record the

reading:

Stage 1: __________ amperes.

Stage 2: __________ amperes.

NOTE: If equipment cycles too fast, set the indicator to a

higher current rating, not more than one-half division at

a time, and recheck cycle rate. Most conventional

-stage heating equipment is designed to operate at three

cycles per hour per stage, and 1-stage heating equipment

at six cycles per hour, at 50 percent load condition.

When using the T874 Thermostat in heat pump systems,

set the heat anticipator at 140 percent of the actual

primary control current draw to reduce the cycling rate.

ON: Fan operates continuously.

AUTO: Fan operates with cooling equipment as con-

trolled by the thermostat or with the heating equip-

ment as controlled by the plenum switch.

To move the subbase switches to the desired control

positions, use thumb and index finger to slide lever. Lever

must stop over desired function indicator position for proper

circuit operation.

Fig. 22—Adjustable heat anticipator scales.

Checkout

HEATING

Move the system switch on the subbase to AUTO or

HEAT (if used) and the fan switch to AUTO. Move the

heating set point lever on the thermostat about 10° F [6° C]

above room temperature. Heating should start and the fan

should start after a short delay. Fan starts immediately in

heat pump systems. Move the heat lever 10° F [6° C] below

room temperature. The heating should shut off; fan should

shut off after a short delay. Fan shuts off immediately in

heat pump systems.

Most heat pump systems should cycle 2-1/2 to 3 times

per hour.

. Hang the upper edge of the thermostat cover on top of

the thermostat base and swing the cover downward until it

engages with the cover clip.

TEMPERATURE SETTING

NOTE: In heat pump applications, time delays are involved

before the compressor is activated to prevent short cy-

cling. The delays are provided by a minimum-off timer

that prevents the compressor from starting for up to five

minutes from when the thermostat last turned off the

compressor, or from when the system first received

power.

Move the heating and cooling set point levers (Fig. 23)

to the desired comfort positions. One lever controls all

stages of heating, and another lever controls all stages of

cooling. The minimum differential between heating and

cooling set point is 3° F [1.7° C].

Fig. 23—Location of external components.

COOLING

Move the system switch on the subbase to AUTO or

COOL (if used) and the fan switch to AUTO. Move the

cooling set point lever on the thermostat about 10° F [6° C]

below room temperature. The cooling and fan should start

(

see NOTE above). Move the cool lever about 10° F [6° C]

above room temperature. The cooling and fan should stop.

FAN

Move the subbase system switch to OFF, and the fan

switchtoON. Thefanshouldruncontinuously. Movethefan

switch to AUTO. In the AUTO position, fan operation is

controlled by the heating or cooling system control circuit.

0-1165—6

NOTE: Hand heat will offset the thermometer reading.

After making each adjustment, wait ten minutes for the

thermometer to stabilize before comparing.

Calibration

THERMOSTAT

T874Thermostatsareaccuratelycalibratedatthefactory.

They do not have provision for field calibration.

Fig. 24—Thermometer calibration.

THERMOMETER

The thermometer in the thermostat has been accurately

calibrated at the factory. The thermometer should need ad-

justment only if it has been dropped or shifted due to mishan-

dling.

If the set point lever and the thermometer reading do not

agree, use the following procedure:

. Remove the thermostat cover by pulling up from the

bottom of the cover until it clears the mounting slots.

. Set the thermostat cover on a table near an accurate

thermometer.

3. Allowtenminutesforcoverthermometertosensearea

temperature; compare the readings. Be careful not to touch

the thermometer or breathe on it.

. If the readings are the same, replace cover and put the

system into operation.

. If the readings are different, insert a small screwdriver

in the thermometer slot (Fig. 24) and turn it until the ther-

M5070

mometers have the same reading.

6. Replace thermostat cover and put the system into

operation.

60-1165—6

Automation and Control Solutions

Honeywell International Inc. Honeywell Limited—Honeywell Limitée

985 Douglas Drive North

35 Dynamic Drive

Golden Valley, MN 55422

Scarborough, Ontario M1V 4Z9

www.honeywell.com/yourhome

0-1165—6

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