HONEYWELL T8078C (01) PDF Document

The HONEYWELL T8078C controller has the following features:

• Modern styling ideal for occupied spaces like offices and hotels.

• Proportional + Integral (P+I) control for close temperature control under all operating conditions.

• Control modes and applications are configured by on-board switches.

• Choice of control Modes: 3-position modulating, thermal predictive modulating (TPM), and On/off.

• Choice of Applications: 2-pipe fan-coil cooling, 2-pipe fan-coil heating, 2-pipe fan-coil cooling + 2-stage on/off heating, 2-pipe fan-coil heating + 2-stage on/off cooling, 2-pipe fan-coil heat/cool changeover, and 4-pipe fan-coil heat + cool in sequence.

• Choice of control parameters: 1, 2, or 4 K for the Proportional Band and 2, 4 K for the Zero Energy Band.

• Automatic heat/cool changeover using a remote switch or a pipe thermostat.

• Control setpoint can be remotely adjusted by ±5 K.

• Energy Savings Input: A local contact closure or a central voltage input switches the controller to Energy Savings Mode, changing cooling and heating setpoints to pre-defined setup and setback temperatures.

• Sensor options: On-board or external (Wall mounted, or Remote/return air). The connection of an external sensor is detected automatically.

• Valve commissioning sequence for fast system check.

• Periodic valve exercise and synchronisation.

• Simple user diagnostics via a flashing LED inside the thermostat cover.

• Diagnostic output for communication of system information to a PC tool.

• Extra wide Input Voltage Specification: 18 – 30 Vac.

• Automatic calibration offset over full voltage range.

• Suitable for mounting on the terminal unit or on the wall (surface or wall-box mounting).

• Wiring access from the rear, and surface wiring knockouts at the top and sides.

• Locking front cover.

Power Supply:	24 Vac nominal 50/60Hz, operating range 18 – 30 Vac
Power Consumption:	1.2 VA nominal (electronics only)
Output Load Rating:	0.3 A (maximum) for all outputs at 24 Vac, with solid state switching
Setpoint Range:	15 to 30 °C
Configuration/System Selection:	Made via a bank of 6 on-board dip switches
Control Modes:	Modulating: 3-position modulating P+I control, based on adjustable proportional Band (Xp), and 150sec valve run-time On/Off: Primary stage control is P+I on/off output based on adjustable Proportional Band (Xp) and a fixed cycle rate (6 cycles/hour Cooling, 6 cycles/hour heating). Secondary stage control is P+I on/off output based on a fixed Proportional Band of 1K and a fixed cycle rate (6 cycles/hour Cooling, 12 cycles/hour heating) TPM: Special on/off control mode for thermo-electric actuators
Applications Systems:	• Heat / Cool Changeover • Cooling Only • Cooling + optional 2-stage On/off Heating • Heating Only • Heating + optional 2-stage On/off Cooling • Heating + Cooling Sequence Control
Proportional Band (Xp):	1K, 2K, or 4K (depending on system) selected using DIP switches. 1K for secondary 2-stage Heating or Cooling stage control.
Zero Energy Band (ZEB):	2K or 4K selected using DIP switches
Interstage Differential (ID):	Fixed at 1K, for Systems with secondary 2-stage Heating or Cooling
Control Point Stability:	±0.5K at 20°C
Ambient Temperature Range:	Operating range 0 to 40 °C
Storage Temperature Range:	-20 to 55 °C
Relative Humidity:	10 to 90%, non condensing
Remote Setpoint Adjustment:	±5K by remote unit, resistance input (Q979B1029, Q979C1036)
Energy Savings (Setup/Setback):	2 to 8K setup/setback possible, by means of external contact closure input. Value set by potentiometer on T8078C. Contact closure can be local or central, to control a group of T8078C (up to 50 max).
Heat/Cool Changeover:	Automatic changeover by means of external contact closure input. Input can be local (from aquastat) or central from switch/relay (controls up to 50 max).
Mounting:	Directly onto wall or wall-box (65x65mm junction box with 60mm screw pitch) or inside terminal unit or fan-coil. Also mounts on fan speed subbase Q6360A1025. Mounting accessory F42007789 available for other mounting configurations.
Wiring:	14 x screw terminals capable of accepting up to 1.5mm² stranded cable. Max length of wiring to actuators is 100m @ 1.0mm², 150m @ 1.5mm².
Enclosure:	Flame retardant plastic housing
Dimensions:	83.5 x 83.5 x 40 mm
Protection Class:	IP30 (IEC144)
Approvals:	CE mark, conforming to directives 73/23/EEC and 89/336/EEC. Standards applied include EN60730-1:2001, EN60730-2-9:1995, EN55014-1:1997, EN55014-2:1997.
Sensors:	On-board sensor, type NTC100K. Remote sensor T8109C1002 (1.5m cable) auto detected on power-up. Maximum sensor extension is 20m (using screened cable).

The following table outlines the various applications, controlled devices, and control modes for the HONEYWELL T8078C.

Basic Application	Controlled Device	Control Mode – details of application	Application No.
2-pipe fan-coil	Valve control	Modulating cooling	1
		Modulating heating	2
		On/off cooling	7
		On/off heating	8
		TPM cooling (thermo-electric actuator)	13
		TPM heating (thermo-electric actuator)	14
Terminal unit	Airside control (damper)	Modulating cooling	1
		Modulating heating	2
	Damper control	Modulating cooling	1
		Modulating heating	2
2-pipe fan-coil with auxiliary electric heat (1 or 2-stage electric heating elements)	Valve control	Modulating cooling + 1or 2-stage on/off heating	3
		On/off cooling + 1or 2-stage on/off heating	9
		TPM cooling + 1or 2-stage on/off heating	15
	Airside control (damper)	Modulating cooling + 1or 2-stage on/off heating	3
		Modulating heating + 1or 2-stage on/off cooling	4
2-pipe fan-coil with auxiliary cooling (1 or 2-stage chillers)	Valve control	On/off heating+ 1or 2-stage on/off cooling	10
		TPM heating + 1or 2-stage on/off cooling	16
	Airside control (damper)	Modulating heating + 1or 2-stage on/off cooling	4
		Modulating cooling	3
2-pipe fan-coil with heat/cool changeover	Valve control	Modulating heat/cool changeover	5
		On/off heat/cool changeover	11
		TPM (thermo-electric actuator) heat/cool changeover	17
4-pipe fan-coil	Valve control	Modulating cooling + heating in sequence	6
		On/off cooling + heating in sequence	12
		TPM cooling + heating in sequence	18
	Airside control (damper)	Modulating cooling + heating in sequence	1 or 2

The HONEYWELL T8078C offers three different output control modes to suit various actuators and system requirements:

Modulating Control

This is a 3-position control that precisely positions a control valve to meet cooling or heating demand. It uses two control outputs: one to drive the valve open and another to drive it closed. The controller sends pulses to move the valve to any position between fully closed and 100% open. The valve position is calculated using a P+I algorithm with an adjustable Proportional Band (Xp) and a valve actuator run-time of 150 seconds, providing optimal performance.

On/Off Control (with P+I input)

This control form cycles the output on and off over a fixed period. The on-time is adjusted by a P+I algorithm to meet the demand and control the space to the setpoint.

• The primary stage cycle rate is 6 cycles/hour for both cooling and heating. The Proportional Band (Xp) can be set to 1, 2, or 4K.

• Secondary stages have a fixed Proportional Band of 1K. The cycle rate is 6 cycles/hour for cooling and 12 cycles/hour for heating.

This method ensures closer control to the setpoint compared to conventional on/off control based on temperature differential.

TPM Control (for Thermo-Electric Actuators)

Thermo-electric actuators can be difficult to control due to time-lags in their response. The HONEYWELL T8078C uses a special Thermal Predictive Modulating (TPM) control to operate these actuators optimally. This type of on/off control provides closer control performance than conventional on/off or pulse-width modulating control when used with thermal actuators.

The HONEYWELL T8078C has two main operating modes, plus a special mode for initial setup.

Comfort Mode

This is the normal operating mode, where the T8078C controls the temperature to the setpoint selected by the user.

Energy Savings Mode

The T8078C features an Energy Management System. When an external input signal (a contact closure on terminals 10 and 11) is detected, the cooling and heating setpoints change to pre-defined setup and setback temperatures to maximize energy efficiency.

• The Setup/Setback values can be set between 2-8K using an on-board potentiometer.

• In cooling-only systems, the setpoint is increased (setup).

• In heating-only systems, the setpoint is decreased (setback).

• In heat/cool changeover systems, the heating setpoint is decreased in heating mode, and the cooling setpoint is increased in cooling mode.

• In cooling + heating sequence systems (any system with a Zero Energy Band), the cooling setpoint is increased and the heating setpoint is decreased by the same amount, effectively widening the Zero Energy Band.

Startup / Commissioning Mode

This mode is entered immediately on power-up to allow for system checks.

On power-up, the HONEYWELL T8078C will immediately start a test and synchronisation sequence to help installers and commissioning engineers verify that the system is wired correctly.

Test Sequence

For all control modes, the test sequence lasts for 60 seconds. It involves switching the 4 control outputs (connected to terminals 3, 4, 6, and 7) on and off in a specific sequence. If you need to repeat the test sequence, you can switch the power supply off and then on again.

Valve Synchronisation

The synchronisation process depends on the selected Control Mode:

• For modulating systems: After the 60-second test sequence, a 3-minute valve synchronisation begins. The valves are driven to the closed position to establish a baseline control reference.

• For on/off and TPM control systems: The valve closure sequence will last for 10 seconds.

The synchronisation sequence will be repeated 12 hours after the initial power-up and every 24 hours thereafter. This is scheduled to avoid disturbing temperature control during normal building occupancy hours. The Diagnostic LED will pulse during this sequence if no faults are detected.

The HONEYWELL T8078C provides user diagnostics and fault indication through an LED located on the bottom right side of the printed wiring board (the cover must be removed to see it). The LED uses predefined pulse sequences to indicate status or faults.

Fault Indication

Faults have the highest priority and are indicated by a repeating sequence of 1, 2, or 3 LED pulses, followed by a 6-second delay. Most faults are related to sensor or setpoint reading errors.

Fault	Suggested Action
Internal Sensor out of range	Replace controller
Remote Sensor out of range	Check sensor connections
Faulty setpoint measurement	Replace controller

Valve Position / Cooling or Heating Demand Indication

After an initial code sequence of pulses (1, 2, or 3 pulses to indicate which output is active), the LED will show the position or demand data. The duration of this data pulse provides the information:

• Valve position (Modulating Control): The LED stays on for a duration proportional to the valve position (e.g., 10 seconds ON = valve fully open).

• On/off or TPM demand: The LED stays on for a duration proportional to the on-period of the cycle (e.g., 10 seconds ON = 100% demand).

The following table shows how to determine the current operating mode and demand for all 18 control options based on the LED output.

	Modulating Control						On/off Control						TPM Control
	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Output Type 1 (1 Pulse)	Valve Position	Valve Position	Valve Position	Valve Position	Position of Heating Valve (Heating) / Position of Cooling Valve (Cooling)	Valve Position	On/off Cooling Demand	On/off Heating Demand	On/off Cooling Demand	On/off Heating Demand	Heating Demand (Heating) / Cooling Demand (Cooling)	On/off Cooling Demand	TPM Cooling Valve Demand	TPM Heating Valve Demand	TPM Cooling Valve Demand	TPM Heating Valve Demand	Heating Valve Demand (Heating) / Cooling Valve Demand (Cooling)	TPM Cooling Valve Demand
Output Type 2 (2 Pulses)	Valve closed	Valve closed	1st Stage On/off Heating demand	1st Stage On/off Cooling demand	Valve closed	Valve Position	Valve closed	Valve closed	1st Stage On/off Heating demand	1st Stage On/off Cooling demand	Valve closed	On/off Heating Demand	Valve closed	Valve closed	1st Stage On/off Heating demand	1st Stage On/off Cooling demand	Valve closed	TPM Heating Valve Demand
Output Type 3 (3 Pulses)	Valve closed	Valve closed	2nd Stage On/off Heating demand	2nd Stage On/off Cooling demand	Valve closed	Valves closed	Valve closed	Valve closed	2nd Stage On/off Heating demand	2nd Stage On/off Cooling demand	Valve closed	Valves closed	Valve closed	Valve closed	2nd Stage On/off Heating demand	2nd Stage On/off Cooling demand	Valve closed	Valves closed

The HONEYWELL T8078C uses a bank of 6 DIP switches to configure the Control Mode, Application System, and Control Parameters (Proportional Band and Zero Energy Band). These switches are located on the wiring board underneath the cover, on the bottom left side.

Follow these steps to configure the controller:

(1) Select Control Type, using switches S1 and S2

The choices are:

• TPM (optimised for thermal actuator control)

• On/off

• Modulating (3-position control) – also known as ‘Floating’ control.

(2) Select System Type, using switches S3 and S4

The choices are:

• 1-stage cooling (+ optional 2-stage on/off heating)

• 1-stage heating (+ optional 2-stage on/off cooling)

• 1-stage heat/cool changeover (activated by external input signal)

• heat + cool sequence control

(3) Select Control Parameters, using switches S5 and S6

• Proportional Band Xp = 1K, 2K or 4K (also dependant on system type)

• Zero Energy Band ZEB = 2K or 4K

Use the table below for specific switch settings:

CONTROL			SYSTEM			PROPORTIONAL BAND				ZERO ENERGY BAND
DEFINITION	S1	S2	DEFINITION	S3	S4	Xp (system dependent)	S3	S4	S5	ZEB	S6
Thermo-Electric Actuator (TPM)	OFF	ON	Heat / Cool Changeover	OFF	OFF	1K	ON	ON	ON	2K	ON
On/Off	ON	OFF	1-Stage Heating + 2-Stage On/Off Cooling	OFF	ON	2K	ON	ON	OFF	4K	OFF
Modulating	ON	ON				2K	OFF	OFF	ON
			1-Stage Cooling + 2-Stage On/Off Heating	ON	OFF	4K	OFF	ON	ON
							ON	OFF	ON
			Heat + Cool Sequence	ON	ON		OFF	OFF	OFF

IMPORTANT! The Installer must be a trained service engineer. Isolate the power supply before commencing installation.

Location

The T8078C can be located on the wall in the space to be controlled, or on the body of the terminal unit.

• With In-built Temperature Sensor: The controller must be located about 1.5m above the floor in a position with good air circulation at room temperature.

• With Remote Temperature Sensor: The position of the remote sensor is critical. For return air sensing, it must be in the return air stream. For room temperature sensing, it must be located as described for the in-built sensor. Note that the maximum cable run for a remote sensor is 20m using screened cable, and the T8078C must be power-cycled after connecting a remote sensor for it to be detected.

Mounting T8078C

The controller can be mounted directly on a wall or a suitable junction box (65 x 65mm maximum). Mounting screws are supplied.

Step 1. Unscrew the cover locking screw and remove the cover.

Step 2. Mount the T8078C to the surface using the provided mounting screws. If mounting on a terminal unit, use appropriate bolts or self-tapping screws.

Step 3. Make the required wiring connections. Terminals accept up to 1.5mm² stranded cable. Do not over-tighten terminal screws.

Step 4. Re-attach the cover and tighten the locking screw to complete the installation.

Wiring and Terminal Connections

Standard wiring access is through a hole at the top and back. There are also 4 wiring breakouts in the cover for surface wiring. The terminal connections are as follows:

1. 24 volt supply

2. 24 volt supply (0v)

3. Valve 1 open

4. Valve 1 close

5. Valve common

6. Valve 2 open / stage 1 on/off control (applications 3,4,9,10,15,16)

7. Valve 2 close / stage 2 on/off control (applications 3,4,9,10,15,16)

8. Heat / Cool changeover input

9. Heat / Cool changeover input

10. Energy savings input

11. Energy savings input

12. Remote sensor input

13. Remote common (for remote sensor / setpoint inputs)

14. Remote setpoint input

The HONEYWELL T8078C supports automatic or central Heat / Cool Changeover when configured for this mode. This feature is for 2-pipe fan-coil systems where changeover is done on a seasonal basis.

Configuration

To enable this mode, set System switches S3 to OFF and S4 to OFF.

Operation

• Automatic Changeover: This is initiated by a thermostat on the water flow pipe (such as the S4390A1004 pipe-thermostat). The thermostat detects whether heated or chilled water is present and sends the correct signal to the controller’s changeover input (terminals 8 and 9).

• Central Changeover: This is initiated by a central switch. This setup can be used to switch a group of up to 50 connected T8078C controllers from Heating to Cooling operation. When wiring multiple units, care must be taken to observe the polarity of connections.

While the HONEYWELL T8078C comes with an on-board temperature sensor, you can connect a remote sensor if required (e.g., for return air sensing).

Connection

Connect a suitable Honeywell Remote Sensor to terminals 12 (Remote sensor input) and 13 (Remote common).

Detection

The T8078C will automatically detect the presence of the remote sensor on power-up and will use its measurement instead of the internal sensor’s. Because detection happens at power-up, if you connect a remote sensor to a controller that is already powered, you must switch the power off and then on again for the sensor to be detected.

Wiring Requirements

• The maximum cable run between the sensor and the controller is 20m.

• The extension must use screened cable.

• Polarity of the sensor connection is not important, but the screen wire must be connected to either terminal 9 or 10 on the controller.

The HONEYWELL T8078C allows its setpoint to be adjusted remotely by up to ±5K. This is done by connecting a Remote Setpoint Adjustment module.

Suitable Modules

• Q979B1029 (Remote Setpoint Adjust)

• Q979C1036 (Sensor + Remote Setpoint Adjust)

Connection

Connect the module to terminals 13 (Remote common) and 14 (Remote setpoint input). The input is purely resistive, so the polarity of the connection is not important.

Wiring Requirements

• To ensure measurement stability, use screened cable for wiring these modules.

• Connect the screen wire to terminal 9 or 10 of the T8078C.

• The maximum cable run is 20m.

The HONEYWELL T8078C is compatible with a range of Honeywell valves and actuators as shown below.

Valve Actuators & Compatible Valves

Valve Actuators	Suitable Valves
M6410C M7410C Modulating 3-pos, 24V~ Max of 10 actuators (0.7VA each) to be connected to any T8078C output	Conical Sealing: V5822A (2-way), V5823A (3-way), V5823C (3-way+bypass) Flat Sealing: V5832A (2-way), V5833A (3-way), V5833C (3-way+bypass)
M8450A1000 Thermo-electric, 24V~, Stroke 8mm Max of 2 actuators (3VA each) to be connected to any T8078C output	Conical Sealing: V5822A (2-way), V5823A (3-way), V5823C (3-way+bypass) Flat Sealing: V5832A (2-way), V5833A (3-way), V5833C (3-way+bypass)
M100 (Thermo-electric, 24V~, Stroke 2.5mm) Z100 (Thermo-electric, 24V~, Stroke 4mm) Max of 2 actuators (3VA each) to be connected to any T8078C output	Conical Sealing: V5822A4xxx (2-way), V5823A4xxx (3-way), V5823C4xxx (3-way+bypass) Flat Sealing: V5832A4xxx (2-way), V5833A4xxx (3-way), V5833C4xxx (3-way+bypass)

Damper Actuators

Damper Actuators (Modulating 3-pos, 24V~)	Max no. to be connected to each T8078C output
4Nm – ML6161B2024	Max of 3 actuators (2VA each)
5Nm – ML6161E	Max of 2 actuators (4VA each)
10Nm – ML6174E	Max of 2 actuators (4VA each)
20Nm – N2024	Max of 1 actuator (6VA each)
34Nm – N3424	Max of 1 actuator (7VA each)

F42006646-001	Range stops (pack of 20)
F42007789-001	Wall-plate
Q6360A1025	Fan Speed Switching Subbase
Q979A1020	Wall mounted Remote Sensor
Q979B1029	Remote Setpoint Adjust Unit (±5K)
Q979C1036	Sensor + Remote Setpoint Unit (±5K)
T8109C1002	Return Air Sensor (1.5m cable)
S4390A1004	Pipe-mounted Changeover Thermostat