DDC systems use microprocessor-based controllers that receive electronic signals from sensors, execute programmed logic, and send outputs directly to HVAC/R equipment. They are the backbone of commercial building automation systems (BAS) and can manage dozens of air handlers, chillers, rooftop units, and boilers from a central supervisory workstation.

• Typical inputs: temperature sensors (−40 °C to 80 °C), relative humidity sensors (0–100%), pressure transducers from vacuum to several thousand kPa
• Typical outputs: 24 VAC relay contacts (on/off), 0–10 VDC (modulating dampers and valves), 4–20 mA (VFD speed reference, control valves)
• Network communications: BACnet, Modbus, or LonWorks allow centralized monitoring, scheduling, trending, and alarm management across an entire facility
• Advanced capabilities: optimum start/stop, demand limiting, night setback, fault detection, and remote diagnostics

Electromechanical systems combine mechanical sensing elements with electrical switching. They are robust, well-understood, and still the dominant technology in residential and light commercial equipment.

• Common devices: bimetallic strip thermostats, diaphragm pressure controls, mechanical time clocks
• Example — refrigeration pressure cycling: a mechanical LP control cycles the compressor between 138 kPa and 276 kPa (20 and 40 psi) suction pressure
• Example — safety cutout: a mechanical HP control opens at 2 068 kPa (300 psi) discharge pressure to protect against dangerously high condensing pressure
• Limitation: less precision and flexibility than electronic or DDC controls; settings must be adjusted manually by a technician on site

Electronic controls use solid-state components — microcontrollers, sensors, and power electronics — to provide accurate, flexible control without mechanical moving parts. They bridge the gap between simple electromechanical devices and full DDC systems.

• EEV controllers: maintain refrigerant superheat within 2–6 K (4–11 °F) using a stepper-motor-driven electronic expansion valve
• VFD controls: adjust motor speed 0–100% based on temperature or pressure signals from sensors; 0–10 VDC or 4–20 mA signal input
• Multiple I/O: can handle multiple simultaneous inputs and apply advanced algorithms (PI, PID) from a single controller module
• Field note: all signals are low-voltage (0–10 VDC or 4–20 mA) — use a digital multimeter to verify signal presence and correct levels; never test with a line-voltage instrument on signal wiring

Pneumatic systems transmit control signals using compressed air at 83–138 kPa (12–20 psi). A pneumatic thermostat modulates its output air pressure in proportion to temperature, causing actuators to position dampers and valves.

• Still found in many large older institutional buildings (hospitals, universities); apprentices will encounter them during retrofits and service calls
• Mixed systems are common: pneumatic actuators driven by DDC controllers through electro-pneumatic (EP) transducers that convert a 0–10 VDC signal to 83–138 kPa air pressure
• Correct operation requires clean, dry compressed air; moisture causes corrosion of tubing, orifices, and diaphragms leading to erratic control or complete failure
• Regular maintenance: check air dryer, filter, and pressure regulator; verify main air pressure (typically 552–621 kPa / 80–90 psi supply) and branch line pressures

Control circuits coordinate system operation by energizing and de-energizing relays, contactors, and actuators in response to sensor inputs and user commands. In residential split-system air conditioning, the 24 VAC low-voltage control circuit is the most familiar example.

• Standard colour coding (split-system thermostat wiring): R = power (24 VAC from transformer), C = common, Y = compressor/cooling, W = heating, G = fan
• When the thermostat calls for cooling it completes R–Y (compressor contactor coil) and R–G (indoor fan relay) simultaneously
• Safety devices (high-pressure switch, low-pressure switch, condensate overflow switch) are wired in series with the Y circuit — any one device can interrupt cooling operation
• Trace control circuits on ladder diagrams; measure 24 VAC across each series device to locate where voltage is lost in a no-start fault
• Verify transformer sizing in volt-amperes (VA) — an undersized transformer will sag voltage when multiple loads energize simultaneously

Safety circuits protect people, equipment, and property by shutting down or preventing operation when abnormal conditions are detected. They are intentionally wired to fail-safe: opening any device in the series chain de-energizes the protected load.

• Typical series safety devices: high-pressure cutout, low-pressure cutout, oil differential pressure control, freeze-stat, motor overload relay, condenser water flow switch
• All safety devices are wired in series in the compressor or fuel valve control circuit — any single device can interrupt operation without affecting other circuits
• Many safety devices have manual reset to ensure a technician inspects the system before restart; look for a small button or lever on the device body
• Setting examples: HP cutout set just below the system MAWP in kPa and psi; freeze-stat set a few kelvins above 0 °C (32 °F) to prevent coil icing
• When diagnosing a tripped safety: identify which device opened, determine the underlying cause, correct it, reset the device, and document the event before returning to service

Operator circuits allow building occupants and technicians to start, stop, and adjust equipment under normal operating conditions — without bypassing safety functions.

• Common operator controls: on/off switches at air handlers, start/stop pushbutton stations, selector switches, room thermostats and humidity controllers
• Hand–Off–Auto (HOA) selector:
  • Auto — device responds to automatic control signals from sensors or BAS
  • Hand — device runs continuously regardless of automatic commands (subject to safety interlocks)
  • Off — device is de-energized regardless of any automatic or safety signal
• Operator controls must never defeat safety functions — in Hand mode, safety circuits must remain active to protect the equipment
• When commissioning: verify HOA positions do not override safety limits; confirm that the “Off” position truly isolates the device from all control signals

Humidity controls detect relative humidity (RH) and provide signals to activate humidifiers, dehumidifiers, or ventilation systems. Comfort applications maintain indoor RH between approximately 30–60%; critical environments (museums, archives, server rooms) may require tighter bands specified in both RH% and dew-point temperature.

• Capacitive sensor (most common): hygroscopic polymer film changes dielectric properties with RH; typical accuracy ±2–3% RH; output 0–10 VDC or 4–20 mA for DDC
• Mechanical (hair element): hygroscopic material (nylon or human hair) expands and contracts to actuate contacts; two-position output; less accurate but simple and low-cost
• Sensor placement: avoid locations near doors, windows, diffusers, or exterior walls — these produce false readings that cause hunting and equipment short-cycling
• Verify controller calibration using a psychrometric chart and a calibrated reference instrument; RH sensors drift over time and should be checked annually

Pressure controls monitor refrigerant, water, or air pressure to maintain system operation within safe and efficient limits. They appear in both safety and control circuit roles.

• High-pressure control (HPC): safety cutout — opens on high discharge pressure to protect compressor and piping; typically manual reset; set below system MAWP
• Low-pressure control (LPC): cycling control, loss-of-charge protection, or pump-down control; differential between cut-in and cut-out is the LPC differential setting in kPa / psi
• Pressure transducer: continuous analogue output (4–20 mA or 0–10 VDC); used by DDC for real-time monitoring, trending, and modulating control algorithms
• Differential pressure transducer: measures the pressure difference across a filter, coil, or pump; used for filter clog alarms, fan proving, and VAV box flow measurement
• Always confirm pressure units on specifications — kPa, MPa, psi, and in. w.g. (inches water gauge) are all used in HVAC/R; incorrect units during commissioning can cause unsafe setpoint entry

Temperature sensors are the most common sensing device in HVAC/R, measuring air, water, or refrigerant temperature to start, stop, or modulate equipment. Selecting the correct sensor type depends on the accuracy required, the temperature range, and the signal input of the controller being used.