HVAC BMS

Sensible heat

Heat that changes the dry-bulb temperature of air without changing its moisture content. It is what a thermometer measures.

Latent heat

Heat absorbed or released when water changes phase (e.g., vapor condensing on a cooling coil) with no temperature change. Latent load drives dehumidification.

Conduction

Heat transfer through direct contact within or between solids, e.g., through a wall or a coil's tube and fin surfaces.

Convection

Heat transfer carried by a moving fluid (air or water). Forced convection across coils is how AHUs move heat into or out of air.

Radiation

Heat transfer by electromagnetic waves between surfaces, with no medium required. Why a sunny window or radiant panel affects comfort even when air temp is fine.

Dry-bulb temperature

Ordinary air temperature measured by a standard sensor, unaffected by moisture.

Wet-bulb temperature

Temperature read by a sensor cooled by evaporation. It reflects both heat and moisture and sets the lower limit for evaporative cooling (e.g., cooling towers).

Dew point

The temperature at which air becomes saturated and moisture begins to condense. A coil must cool air below dew point to dehumidify.

Relative humidity (RH)

The amount of moisture in air compared to the maximum it can hold at that temperature, expressed as a percent. Changes with temperature even if moisture content does not.

Humidity ratio

The actual mass of water vapor per mass of dry air (lb/lb or grains/lb). Unlike RH, it does not change when air is only heated or cooled sensibly.

Enthalpy

Total heat content of air (sensible + latent) per pound of dry air, in BTU/lb. Used for enthalpy economizer decisions.

Psychrometric chart

A chart relating dry-bulb, wet-bulb, dew point, RH, humidity ratio and enthalpy. Used to plot air processes such as mixing, cooling and dehumidifying.

Mixed air temperature (MAT)

Temperature of outdoor air and return air after blending in the AHU mixing box, before the coils.

BTU (British Thermal Unit)

The heat required to raise one pound of water by 1°F.

Ton of refrigeration

A unit of cooling capacity equal to 12,000 BTU/h.

CFM

Cubic feet per minute — the unit of airflow.

GPM

Gallons per minute — the unit of water flow.

Sensible heat equation (air)

Q (BTU/h) ≈ 1.08 × CFM × ΔT (°F).

Water-side heat equation

Q (BTU/h) ≈ 500 × GPM × ΔT (°F).

Delta-T (ΔT)

The temperature difference between supply and return (air or water). Higher water ΔT means less flow is needed for the same heat transfer.

Low delta-T syndrome

When chilled water return temperature comes back too close to supply. It forces extra flow and extra chillers/pumps to run. Common causes: 3-way valves, oversized or leaking valves, dirty coils, low air-side setpoints.

Static pressure

The pressure air exerts outward on duct walls, independent of velocity. Typically measured in inches of water column (in. w.c.).

Duct static pressure

Static pressure measured in the supply duct, usually about 2/3 of the way down the main run. The AHU supply fan VFD is modulated to hold its setpoint.

Differential pressure (DP)

The pressure difference between two points, e.g., across a filter, across a coil, or between supply and return water mains.

Building static pressure

Pressure of the building relative to outdoors. Usually held slightly positive to limit infiltration, often via relief/return fans or relief dampers.

Fan/pump affinity laws

Flow ∝ speed; pressure ∝ speed²; power ∝ speed³.

System curve

A curve showing the pressure a duct or pipe system requires at each flow rate. Resistance rises roughly with the square of flow.

Operating point

Where the fan (or pump) curve crosses the system curve — the actual flow and pressure the system runs at.

Static pressure reset (trim & respond)

A sequence that lowers the duct static setpoint until VAV dampers start requesting more air, then nudges it up. Saves fan energy. Defined in ASHRAE Guideline 36.

VFD speed control vs damper throttling

A VFD reduces fan speed, cutting power roughly with the cube of speed. A throttling damper adds resistance while the fan runs at full speed, saving much less energy.

Central plant

Equipment that produces heating or cooling energy for the building: chillers, boilers, cooling towers and primary pumps.

Distribution

The pumps, piping and ductwork that move water or air from where it is produced to where it is used.

AHU (Air Handling Unit)

A central unit that mixes, filters, heats or cools and moves air for many zones. Contains dampers, filters, coils and fans.

RTU (Rooftop Unit)

A packaged, self-contained air handler on the roof, typically with its own DX cooling and gas or electric heat.

DOAS (Dedicated Outdoor Air System)

A unit that conditions 100% outdoor air for ventilation, often decoupled from the zone heating/cooling equipment.

VAV box (Variable Air Volume)

A terminal unit with a damper that varies the airflow delivered to one zone to meet its load. Often has an airflow sensor and a reheat coil.

Fan coil unit (FCU)

A small terminal unit with its own fan and a hot/chilled water coil serving one room or zone.

Terminal unit

A per-zone device (VAV box, fan coil, radiant panel) that controls how much conditioning one space receives.

Chiller

A machine that uses a refrigeration cycle to remove heat from water, producing chilled water for coils.

Boiler

Equipment that heats water (or makes steam) for heating coils, reheat and radiation.

Cooling tower

A heat-rejection device that uses evaporation to reject a water-cooled chiller's condenser heat to outdoor air. It serves a chiller; it does not replace one.

Heat exchanger

A device that transfers heat between two fluids without mixing them, e.g., a plate-and-frame HX for water-side economizing.

Heat pump

A refrigeration machine that moves heat rather than generating it and can run in heating or cooling mode by reversing the cycle.

Reversing valve

The valve in a heat pump that switches refrigerant flow direction to change between heating and cooling mode.

DX (Direct Expansion)

Cooling where refrigerant evaporates directly in the air-side coil instead of using chilled water.

VRF (Variable Refrigerant Flow)

A multi-zone DX system in which one outdoor unit serves many indoor units, varying refrigerant flow with inverter compressors.

Refrigeration cycle — four components

Evaporator (absorbs heat), compressor (raises pressure/temperature), condenser (rejects heat), expansion valve (drops pressure).

Evaporator

The heat exchanger where refrigerant boils and absorbs heat, e.g., from chilled water in a chiller.

Condenser

The heat exchanger where hot refrigerant gas rejects heat and condenses to liquid, to condenser water or outdoor air.

Chilled water loop

The loop between the chiller evaporator and the AHU cooling coils; nominally around 44°F supply and mid-50s °F return.

Condenser water loop

The loop between a water-cooled chiller's condenser and the cooling tower; nominally around 85°F to the chiller and 95°F back to the tower.

Cooling tower range

Temperature difference between water entering and leaving the tower.

Cooling tower approach

How close the tower's leaving water gets to the outdoor wet-bulb temperature.

Blowdown

Draining some tower water to remove concentrated dissolved solids left behind by evaporation.

Makeup water

Fresh water added to a cooling tower to replace evaporation, drift and blowdown.

Water-side economizer

Using the cooling tower and a heat exchanger to make chilled water without running the chiller when outdoor wet-bulb is low enough.

Air-side economizer

Using cool outdoor air for free cooling by opening outdoor air dampers beyond minimum when conditions allow.

Dry-bulb vs enthalpy economizer changeover

Dry-bulb changeover enables economizing based on outdoor temperature alone; enthalpy changeover also accounts for humidity, avoiding bringing in cool but very humid air.

Primary/secondary pumping

A plant arrangement with constant-flow primary pumps through the chillers and variable-flow secondary pumps to the building, hydraulically separated by a decoupler (common pipe).

Variable primary flow

A plant arrangement with a single set of variable-speed pumps serving both chillers and building load, with a minimum-flow bypass.

Head pressure control

Controlling condenser fans or flow to keep refrigerant condensing pressure high enough for proper operation, especially in cold weather.

Low-ambient lockout

Logic that prevents equipment (e.g., an air-cooled chiller or DX compressor) from running below a set outdoor temperature.

kW/ton

A chiller efficiency metric — lower is better. Water-cooled chillers are roughly 0.55–0.65; air-cooled roughly 0.95–1.2.

COP (Coefficient of Performance)

Heat moved divided by energy input. Heat pumps exceed 1 because they move heat rather than create it.

Defrost cycle

Heat pump logic that periodically reverses to melt frost from the outdoor coil. Can be timer-based or demand-based.

Auxiliary / supplemental heat

Electric resistance heat that supplements a heat pump when it cannot keep up, often enabled below an outdoor air temperature.

Water-loop heat pump system

Many heat pumps on a shared water loop, letting heat rejected by zones in cooling be used by zones in heating.

Outdoor air, return air, exhaust/relief dampers

The three AHU mixing-box dampers. OA brings in fresh air, RA recirculates building air, EA/relief removes excess air.

Minimum outdoor air

The least OA the AHU must bring in while occupied to meet ventilation requirements (ASHRAE 62.1).

MERV rating

Minimum Efficiency Reporting Value — a 1–16 scale of filter efficiency. Higher means finer filtration and usually more pressure drop.

Filter DP switch / sensor

Measures pressure drop across a filter to indicate when it is loaded and needs replacing.

Freezestat (low limit)

A safety that trips (typically near 38°F) when air leaving a coil gets too cold, shutting down the fan and protecting the coil from freezing.

Duct smoke detector

A life-safety device that shuts down the AHU when smoke is sensed in the duct, usually hardwired.

High static safety

A pressure switch that shuts down the fan if duct pressure gets dangerously high, e.g., from dampers closing unexpectedly.

Sensor

A device that measures a physical condition (temperature, pressure, humidity, flow, CO2) and sends a signal to the controller.

Thermistor

A temperature sensor whose resistance changes non-linearly with temperature. Commonly 10kΩ NTC (resistance drops as temperature rises).

RTD (Resistance Temperature Detector)

A temperature sensor (e.g., Pt100/Pt1000) whose resistance rises almost linearly with temperature. More accurate and stable than a thermistor.

Current switch / current transducer

Senses motor current to prove a fan or pump is actually running (status/proof).

Airflow measuring station

A device that measures airflow (CFM) in a duct, e.g., for minimum OA verification.

CO2 sensor

Measures carbon dioxide as a proxy for occupancy, used for demand-controlled ventilation.

AI (Analog Input)

A controller input reading a continuous value, e.g., a temperature or pressure sensor.

AO (Analog Output)

A controller output sending a continuous command, e.g., valve position or VFD speed.

BI / DI (Binary / Digital Input)

A controller input reading an on/off state, e.g., fan status or a filter alarm.

BO / DO (Binary / Digital Output)

A controller output sending an on/off command, e.g., start a fan or enable a pump.

0–10 VDC signal

A common voltage analog signal for sensors and actuators. Susceptible to voltage drop on long wire runs.

4–20 mA signal

A current-loop analog signal. Unaffected by wire resistance, and 0 mA indicates a broken wire because 4 mA is 'live zero'.

Actuator

The motor or device that physically moves a damper or valve in response to a control signal.

Final control element

The device (valve, damper, VFD) that actually changes the manipulated variable.

Spring-return actuator

An actuator with a spring that drives it to a fail-safe position on power loss, e.g., OA damper closes.

Two-position control

On/off or open/closed control with no intermediate positions.

Floating (tri-state) control

Actuator control using two binary outputs: drive open or drive closed. Position is held when both are off.

Modulating control

Continuous control that can position a device anywhere between 0–100%.

2-way vs 3-way valve

A 2-way valve throttles flow (variable flow system). A 3-way valve diverts or mixes so total flow stays roughly constant.

Equal-percentage valve characteristic

A valve whose flow changes by an equal percentage per equal increment of stroke. Used on water coils to offset the coil's non-linear heat output so the combined response is more linear.

Linear valve characteristic

A valve whose flow is proportional to stroke. Often used on 3-way mixing or bypass applications.

Cv (flow coefficient)

The GPM of water a valve passes at 1 psi pressure drop when fully open. Used to size valves.

Valve authority

The valve's pressure drop divided by the total pressure drop of the branch it controls. Too low authority means poor control at partial openings.

PICV (Pressure-Independent Control Valve)

A control valve with a built-in regulator that holds flow constant for a given position regardless of system pressure changes.

VFD (Variable Frequency Drive)

A drive that converts incoming AC to DC and back to variable-frequency AC to change motor speed. The final control element for fan/pump speed loops.

VFD minimum speed

A floor on drive speed to protect the motor and equipment. Can cause integral windup if the loop keeps demanding lower speed.