What is a Terminal Unit in an HVAC system?

A small component that contains a heating coil, cooling coil, automatic damper, or some combination of the three. Used to control the temperature of a single room. Abbreviated TU.

In an HVAC system, the terminal unit is the point at which conditioned air is delivered to the space. The objective of the terminal unit is twofold:

●first, mix the correct proportion of hot and cold water to provide the desired supply of air temperature;

●second, to deliver the air at a constant volume.

Depending on the application, there are many different types of terminal units available on the market today.

How does a terminal unit work?

The terminal unit is responsible for mixing the correct proportion of hot and cold water to provide the desired supply of air temperature. It does this by using a heating coil and a cooling coil. The heating coil heats up the air as it passes through it, and the cooling coil cools down the air as it passes through it. The two coils are controlled by an automatic damper that opens and closes depending on the temperature of the air passing through it.

When the air reaches the desired temperature, the damper will close and the air will be delivered to the space at a constant volume.

What are some benefits of using a terminal unit?

There are many benefits to using a terminal unit in an HVAC system:

One benefit is that it allows for individual room temperature control. This is because each terminal unit is responsible for controlling the temperature of one specific room.

Another benefit is that it can help to improve indoor air quality by delivering fresh, conditioned air to the space.

Additionally, using a terminal unit can help to save energy and lower utility bills, as the system will only heat or cool the space as needed.

Is a terminal unit the same as a VAV?

No, a terminal unit is not the same as a VAV. A terminal unit is a small component that contains a heating coil, cooling coil, automatic damper, or some combination of the three. It is used to control the temperature of a single room. Abbreviated TU.

A VAV is a type of terminal unit that is commonly used in commercial applications. VAV units are similar to FAUs in that they consist of a heating coil, cooling coil, and blower assembly; however, VAV units also have an additional component called a VAV box. The VAV box is used to control the volume of air that is delivered to the space.

Products and reference designs of Terminal Unit 

As shown in the figure below, Terminal Unit consists of 18 parts, each part is Isolated AC/DC power supply、Non-isolated DC/DC power supply、Input power protection、Isolated DC/DC Power Supply、Signal input protection、Analog front end、Zero crossover、Signal isolation、Digital processing、Wireless interface、Clocking、Memory、Self-diagnostics/monitoring、Interface isolation、Wired interface、Serial interface、Output user interface、Signal output protection

Isolated AC/DC power supply

Isolated AC/DC power supply can generate a DC voltage for the other subsystems directly from the AC lines. Isolated AC/DC power supply includes PWM controllers、N-channel MOSFETs、Buck modules (integrated inductor)、Isolated DC/DC converters & modules、SR & load share controllers, the corresponding products are LM5039、CSD18531Q5A、TPSM82810、DCPA10505、UCC24630

Non-isolated DC/DC power supply

The backplane side of an analog input module usually has one or more digital processing blocks like MPU, MCU, FPGA or ASIC which all need to be powered from the backplane input voltage. Depending power levels we have DC/DC converters, LDOs and power management IC (PMIC) that can help to reduce power dissipation and size in the solution. Non-isolated DC/DC power supply includes Linear & low-dropout (LDO) regulators、Multi-channel ICs (PMICs)、Buck converters (integrated switch)、USB power switches & charging port controllers、Sequencers, the corresponding products are TPS7A05、TPS65266、TPS62933、TPS2553、LM3881

Input power protection

Over/under voltage, over current protection, miswire protection and inrush current protection might all be a part of the input power protection block. Traditionally this has been implemented with discrete components like diodes, fuses and PTC. Increasing protection requirements as well as higher efficiency requirements makes E-fuse, Hotswap and ideal diode solutions attractive. Input power protection includes eFuses & hot swap controllers, the corresponding products are TPS24701、TPS25921L

Isolated DC/DC Power Supply

The analog front end is usually galvanic ally isolated from the embedded processing block on the backplane. Depending on if the module has a field power connection or not an isolated power supply for analog front end from the backplane power input might be needed. TI has devices that can realize small and efficient fly-buck, fly-back and push-pull topologies. Isolated DC/DC Power Supply includes Linear & low-dropout (LDO) regulators、Isolated DC/DC converters & modules、USB power switches & charging port controllers、Transformer drivers, the corresponding products are TLV703、TPS55010、TPS2051B、SN6501

Signal input protection

Analog input modules interface with Current Transformers and Potential Transformers through connectors. The signal path is subject to electromagnetic disturbances (EMC) that can either damage sensitive electronics or create false measurements. An effective input protection scheme is necessary to comply with standards like the IEC61000. TVS diodes and common mode chokes can help clamp and reduce harmful interference. Signal input protection includes ESD protection diodes、TVS diodes, the corresponding products are TPD4S012、TVS3300

Analog front end

The analog front end is built using a multi channel ADC or Isolated Amplifier to convert current and voltage signals to the digital domain for further processing. High performance fully integrated ADCs from Texas Instruments can handle special needs like covering wide dynamic range, noisy environment, temperature variations, high common mode rejection. At times opamps are required to provide external gain and high input impedance for these sensors. Analog front end includes Precision op amps (Vos<1mV)、General-purpose op amps、Instrumentation amplifiers、Analog current-sense amplifiers、Precision ADCs、Series voltage references、Shunt voltage references、LVDS, M-LVDS & PECL ICs, the corresponding products are OPA387、TLV9301、INA381、ADS7042、LM4140、TLV431B、SN65MLVD206

Zero crossover

The analog front end is often built around one or multiple ADCs to convert current and voltage signals to the digital domain for further processing. For coherent sampling the ADC clocks can be adjusted to get the same number of samples per cycle irrespective of the input frequency. This can be done by detecting the zero cross-overs at the input using a MUX and a low cost comparator. Zero crossover includes Comparators、Analog switches & muxes, the corresponding products are LM2901、SN74CB3T3245

Signal isolation

Digital isolators are used if the output of the ADC needs to be isolated from the signal processing subsystem. Signal isolation includes Digital isolators, the corresponding products are ISO7721、ISO7742

Digital processing

Digital processing unit(s): Filtering, linearization and compensation of analog input data is easily realized in the digital domain on an MSP430 microcontroller. For safety modules, the Hercules microcontroller family can help achieve SIL qualification. The Sitara family of processors (DSP) can handle the majority of the common industrial Ethernet and fieldbus interfaces together with the microcontroller tasks. Optionally, level translators can be used if there is a mismatch between the various digital domains. Digital processing includes C2000 real-time microcontrollers、MSP430 microcontrollers、Arm-based processors、Digital signal processors (DSPs)、Fixed-direction voltage translators, the corresponding products are TMS320F28335、MSP430FR5969、AM6421、TMS320C6746、SN74LV1T04

Wireless interface

Wireless interface with the terminal unit enables communication with SCADA network and providing smart features by linking to cloud. Wireless interface includes Sub-1 GHz wireless MCUs、Low-power 2.4-GHz products、Wi-Fi products, the corresponding products are CC1312R、CC2651R3、CC3220MODA

Clocking

For highly accurate measurement of input signals synchronous operation between multiple Data converters is a must. The path delay between the embedded processor and the ADC results in variation in the clock arrival time and introduce additional measurement errors. For coherent sampling the clock frequency has to be tuned with greater resolution. A PLL synthesizer and a clock buffer minimize measurement errors with high accuracy clock and a strong drive. Clocking includes Clock generators、Clock buffers, the corresponding products are CDCE913、CDCLVC1102

Memory

Double Data Rate (DDR) memory has become more common in electronic systems as it provides faster data transfer and overall higher system performance. Conventional passive bus termination resistance is used for matching DDR termination line impedance to source impedance and thereby reduce the amplitude of reflection to 0. Active DDR Termination is used not only to reduce power dissipation (50% at 50% duty cycling) but also to improve data read/write accuracy & integrity. Memory includes DDR memory power ICs, the corresponding products are TPS7H3302-SP、TPS7H3302-SEP

Self-diagnostics/monitoring

A temperature sensor is required to monitor FETs as they are prone to self heating. It can be used as a simple thermal switch or to provide the information back to the MCU. A reset IC is used to monitor the power supply levels and provide a fixed delay during a power glitch or MCU forced reset. Self-diagnostics/monitoring includes Supervisor & reset ICs、Digital temperature sensors、Analog temperature sensors、Humidity sensors, the corresponding products are TPS3805H33、TMP175、TMP235、HDC3022

Interface isolation

Digital isolators are used to isolate the communication interface of the the data acquisition card as they interface with external monitoring systems. Interface isolation includes Digital isolators, the corresponding products are ISO6741、ISO7740

Wired interface

The data from the terminal unit can be sent out to the bus using any of the serial/parallel, high speed/slow speed industrial interfaces. Wired interface includes RS-485 & RS-422 transceivers、RS-232 transceivers、Ethernet PHYs、USB hubs & controllers, the corresponding products are THVD1410、SN65HVD82、MAX3232E、DP83822I、TUSB4020BI

Serial interface

The data from the terminal unit can be sent out to the bus using any RS-485, RS-232, CAN or UART. Serial interface includes RS-485 & RS-422 transceivers、RS-232 transceivers、Isolated RS-485 transceivers、Isolated CAN transceivers, the corresponding products are THVD1500、MAX3232E、ISO3088、ISO1050

Output user interface

The digital output of the MCU can be converted to user friendly outputs such as LEDs or analog outputs. Output user interface includes Low-side switches、RGB LED drivers、Precision DACs (≤10 MSPS)、Solenoid drivers, the corresponding products are ULN2004A、TLC5971、DAC8564、DRV110

Signal output protection

An effective output protection scheme is necessary to keep the data from getting corrupted due to noise. Transient suppressors and EMI/EMC filters are used to maintain the integrity of the data. Signal output protection includes ESD protection diodes、TVS diodes, the corresponding products are TPD4E1U06、TVS3300