HVAC & Refrigeration

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Superheat's 28 and subcooling's 3, it's starved and low, bubble it before you even think about topping off.

  TranslationHigh superheat with low subcooling means the system is short of refrigerant, so find the leak with soap solution before adding any.

• Coil's iced up, static's at 1.1 and the filter's a MERV 13 in a one-inch rack, it's airflow, not charge.

  TranslationThe evaporator froze because airflow is restricted by an overly dense, undersized filter, so the problem is not the refrigerant charge.

• Sweat the drier in while flowing nitrogen, pull it down to 400 microns, then weigh in the charge.

  TranslationBraze the new filter-drier with nitrogen inside the pipe, evacuate the system to a deep vacuum, and charge it by weight.

• Contactor's pitted and the cap's reading 30 on a 45, no wonder it's pulling LRA and tripping.

  TranslationThe compressor contactor's contacts are burned and the run capacitor is weak, so the compressor cannot start and draws locked rotor current until its overload trips.

• Economizer's stuck open and the DCV sensor is reading outdoor air, the RTU's cooling the parking lot.

  TranslationThe outdoor air damper is jammed open and the CO2 sensor is mislocated, so the rooftop unit is conditioning far too much outdoor air.

• Zero superheat and the accumulator's frosted, it's flooding back, check the blower before you touch the TXV.

  TranslationLiquid refrigerant is returning to the compressor, most likely from low indoor airflow, so check the fan before blaming the expansion valve.

• Tower approach is 14 at a 72 wet bulb and the chiller barrel's fouled, schedule a tube brushing.

  TranslationThe cooling tower is leaving water far warmer than it should and the chiller heat exchanger is dirty, so the tubes need mechanical cleaning.

The components, pressures and temperatures of vapor compression refrigeration and heat pumps.

• Accumulatoralso suction accumulator, suction line accumulator
  A tank in the suction line that catches liquid refrigerant before it can reach and damage the compressor.
• Approachalso temperature approach, approach temperature
  The temperature gap between a fluid leaving a heat exchanger and the fluid or condition it is being brought toward.
• Compressoralso comp, pot
  The pump that raises refrigerant vapor from evaporator pressure to condenser pressure and keeps it circulating.
• Condenseralso condenser coil, outdoor coil
  The heat exchanger where hot, high-pressure refrigerant gives up heat and condenses back to a liquid.
• Defrostalso defrost cycle, defrost mode
  A cycle that melts frost off a heat pump's outdoor coil or a refrigerated evaporator so it can keep moving heat.
• Evaporatoralso evap, evaporator coil
  The coil or heat exchanger where low-pressure liquid refrigerant boils off, absorbing heat from air or water.
• Filter-drieralso drier, dryer
  A canister of desiccant in the refrigerant line that traps moisture, acid and debris.
• Flash gasalso flashing, flash
  Liquid refrigerant that turns to vapor because its pressure dropped, either at the metering device or where it shouldn't.
• Glidealso temperature glide, zeotropic glide
  The temperature change of a refrigerant blend as it boils or condenses at constant pressure.
• Head pressurealso head, discharge pressure
  The compressor discharge or condensing pressure of a refrigeration system.
• Heat pumpalso HP
  A refrigeration system that can reverse its flow to move heat indoors in winter as well as outdoors in summer.
• High sidealso discharge side
  The high-pressure half of a refrigeration system, from compressor discharge through condenser to the metering device.
• Metering devicealso expansion device, throttling device
  The restriction that drops refrigerant from high side to low side pressure and controls how much enters the evaporator.
• PT chartalso pressure-temperature chart, PT card
  Pressure-temperature chart: a table giving each refrigerant's saturation temperature at each pressure.
• Refrigerantalso freon, gas
  The working fluid that carries heat around a refrigeration or air conditioning system by boiling and condensing.
• Refrigeration cyclealso vapor compression cycle, refrigerant cycle
  The loop of compression, condensing, expansion and evaporation that moves heat from a cold space to a warmer one.
• Reversing valvealso 4-way valve, four-way valve
  The four-way valve in a heat pump that swaps which coil gets hot discharge gas, switching between heating and cooling.
• Saturationalso saturation temperature, sat temp
  The state where liquid and vapor refrigerant coexist, so its temperature is locked to its pressure.
• Subcoolingalso SC, liquid subcooling
  How many degrees liquid refrigerant is below its condensing temperature at the same pressure.
• Superheatalso SH, suction superheat
  How many degrees refrigerant vapor is above its boiling point at the pressure it is at.
• TXValso TEV, TX valve
  Thermostatic expansion valve: a metering device that adjusts refrigerant flow to hold constant evaporator superheat.

Moist air properties, sensible and latent heat, and how heating and cooling loads are calculated.

• Cooling load
  How much heat and moisture a cooling system has to pull out of a space each hour to keep it at design conditions.
• Design conditionsalso design temperatures, design temps
  The outdoor weather and indoor targets a heating or cooling system is sized to handle.
• Dew pointalso dewpoint, DP
  The temperature at which air becomes saturated and water starts condensing out when it is cooled.
• Dry bulbalso DB, dry-bulb temperature
  Plain air temperature as an ordinary thermometer reads it, kept dry and out of direct sun.
• Enthalpyalso h, total heat
  The total heat content of moist air, sensible plus latent, in Btu per pound of dry air.
• Humidity ratioalso grains, moisture content
  The weight of water vapor per weight of dry air, usually given in grains per pound.
• Latent heatalso latent
  Heat tied up in moisture, added or removed when water evaporates or condenses without changing air temperature.
• Psychrometricsalso psych
  The science of air and the water vapor mixed into it, used to design and diagnose heating, cooling and humidity control.
• Relative humidityalso RH
  The moisture in air as a percentage of what would saturate it at the same temperature, the number a humidistat shows.
• Sensible heatalso sensible
  Heat that changes the air's temperature, which you can read on a thermometer.
• SHRalso sensible heat ratio, S/T ratio
  Sensible heat ratio: the share of a cooling load or coil capacity that is sensible rather than latent.
• Sling psychalso sling psychrometer, psychrometer
  Shop name for a sling psychrometer, or any handheld instrument used to read wet bulb and dry bulb. Shop talk
• Tonalso TR, RT
  A unit of cooling capacity equal to 12,000 Btu per hour, the heat needed to melt a ton of ice in a day.
• Wet bulbalso WB, wet-bulb temperature
  The temperature a wet-wicked thermometer reads in moving air, lowered by evaporation in proportion to how dry the air is.

Airflow, static pressure, outlets, ducts, dampers and the people and tools that build and balance them.

• CFMalso cubic feet per minute
  Cubic feet per minute: the standard unit of airflow through equipment, ducts and outlets.
• Damper
  A blade or set of blades in a duct or opening that controls, balances or shuts off airflow.
• Diffuseralso supply diffuser
  A supply air outlet, usually in the ceiling, that spreads air into the room in a set pattern and mixes it with room air.
• Duct leakagealso duct leaks
  Air lost from supply ducts or pulled into return ducts through gaps at joints, seams and connections.
• Ductulatoralso duct calculator, duct wheel
  A slide-rule style duct sizing calculator that matches airflow and friction rate to round and rectangular duct sizes. Shop talk
• Economizeralso econ, airside economizer
  Dampers and controls that bring in cool outdoor air for free cooling instead of running compressors.
• Mudalso mastic, duct mastic
  Duct mastic, the thick paste brushed over duct joints and seams to seal them airtight. Shop talk
• Plenum
  A box on the supply or return side of an air handler that feeds or collects several ducts, or a ceiling cavity used as a return path.
• Registeralso vent
  A supply or return grille fitted with a damper so its airflow can be adjusted or shut off.
• Return airalso RA, return
  Air drawn back from the conditioned space to the air handler to be filtered and conditioned again.
• Static pressurealso static, SP
  The pressure air exerts outward on duct walls, the resistance a fan works against, measured in inches of water column.
• Test and balancealso TAB, T&B
  Measuring and adjusting a building's air and water flows so every outlet, coil and zone gets its design amount.
• Throwalso air throw
  How far a supply air jet reaches into a room before it slows to a chosen speed, such as 50 feet per minute.
• Tin knockeralso tinner, sheet metal worker
  Jobsite nickname for a sheet metal worker who fabricates and installs ductwork. Shop talk
• VAValso variable air volume, VAV system
  Variable air volume: a system that holds supply air temperature steady and varies the amount of air to each zone.

Air handlers, rooftop units, chillers, towers, splits and the parts techs replace most.

• AHUalso air handler, air handling unit
  Air handling unit: the cabinet of fans, coils, filters and dampers that conditions and moves air through a building's ducts.
• Barrelalso shell
  Shop name for a chiller's shell-and-tube evaporator or condenser vessel. Shop talk
• Capalso capacitor
  Short for capacitor, the part that gives compressor and fan motors the phase shift they need to start and run. Shop talk
• Chilleralso water chiller
  A refrigeration machine that cools water or glycol, which is then pumped to coils to cool a building or process.
• Contactoralso compressor contactor
  A heavy-duty relay that uses a 24 volt control signal to switch line voltage to a compressor and condenser fan.
• Cooling toweralso tower, evaporative cooling tower
  Equipment that cools condenser water by evaporating a small part of it into an airstream.
• Drain panalso condensate pan, condensate drain pan
  The pan under a cooling coil that catches condensate and routes it to a drain line.
• Fan coilalso FCU, fan coil unit
  A small terminal unit with a fan, a filter and a water coil that heats or cools one room or zone.
• Line setalso lineset, refrigerant lines
  The pair of copper refrigerant lines, one insulated suction line and one smaller liquid line, linking indoor and outdoor units. Shop talk
• Mini-splitalso ductless, ductless mini-split
  A ductless split system with an outdoor inverter unit connected to one or more indoor heads by refrigerant lines.
• RTUalso rooftop, rooftop unit
  Rooftop unit: a packaged air conditioner, often with gas heat, that sits on a roof curb and ducts straight down into the building.
• Schrader valvealso Schrader, Schrader core
  A spring-loaded valve core in a refrigerant service port, opened by the depressor in a gauge hose fitting.
• Service valve
  A manually operated valve on a condensing unit or receiver that isolates refrigerant and provides a gauge port.
• Swamp cooleralso evaporative cooler, evap cooler
  An evaporative cooler that blows outdoor air through wet pads, cooling it by evaporation and adding moisture. Shop talk

The ratios and seasonal ratings used to compare and specify HVAC equipment.

• COPalso coefficient of performance
  Coefficient of performance: heat moved or delivered for each unit of energy put in, with both measured in the same units.
• EERalso energy efficiency ratio
  Energy efficiency ratio: cooling output in Btu per hour divided by watts of input at one hot, full-load test point.
• HSPFalso heating seasonal performance factor
  Heating seasonal performance factor: a heat pump's seasonal heating output in Btu divided by its electricity use in watt-hours.
• IPLValso integrated part load value
  Integrated part load value: a single weighted chiller efficiency figure that reflects operation at 100, 75, 50 and 25 percent load.
• SEERalso seasonal energy efficiency ratio
  Seasonal energy efficiency ratio: an air conditioner's cooling output over a typical season divided by its electricity use.

Outdoor air, filtration, infiltration, the building envelope and building pressures.

• ACHalso air changes per hour, air change rate
  Air changes per hour: how many times a space's full air volume is replaced in one hour.
• Building envelopealso envelope, thermal envelope
  The walls, roof, floor, windows and doors that keep conditioned indoor space apart from the outdoors.
• Building pressurizationalso pressurization
  Controlling the air pressure inside a building relative to outdoors, usually by bringing in more outdoor air than is exhausted.
• DCValso demand-controlled ventilation, demand control ventilation
  Demand-controlled ventilation: adjusting outdoor air in real time to match how many people are actually in a space.
• ERValso energy recovery ventilator, enthalpy recovery ventilator
  Energy recovery ventilator: a unit that swaps heat and moisture between outgoing exhaust air and incoming fresh air.
• IAQalso indoor air quality
  Indoor air quality: how clean and healthy indoor air is, judged by contaminants, ventilation, humidity and comfort.
• Infiltrationalso air infiltration
  Outdoor air that sneaks into a building through gaps in its shell, pushed by wind, stack effect and fans rather than by design.
• MERValso MERV rating
  Minimum efficiency reporting value: a 1 to 16 rating of how well an air filter captures particles of different sizes.
• Stack effectalso chimney effect
  The buoyancy-driven airflow that pushes warm air out the top of a building and pulls outdoor air in at the bottom.
• Ventilation ratealso outdoor air rate, outside air rate
  The amount of outdoor air deliberately brought into a space to dilute indoor contaminants, usually in cfm per person and per square foot.

Field procedures, failure modes and the shop talk of diagnosing and repairing systems.

• Beer can coldalso beer-can cold
  A rough feel test: the suction line should be about as cold and sweaty as a can just out of the fridge. Shop talk
• Belly bandalso belly band mount
  An adjustable strap mount that clamps around the body of a replacement motor so it fits many different brackets. Shop talk
• Bubble italso bubble test, soap it
  To leak-check a joint or fitting by brushing on soap solution and watching for bubbles. Shop talk
• Burnoutalso compressor burnout
  A compressor motor failure that overheats the windings and contaminates the system with acid, sludge and carbon. Shop talk
• Dirty sock syndromealso DSS, dirty sock smell
  A musty, sweaty-sock odor blown out by an air conditioner or heat pump, caused by microbial growth on the indoor coil. Shop talk
• Floodbackalso flood back, flooding back
  Liquid refrigerant returning continuously to the compressor through the suction line while the system runs.
• Flowing nitrogenalso flow nitrogen, nitrogen purge
  Running a low flow of dry nitrogen through copper lines while brazing to keep oxide scale from forming inside. Shop talk
• Gaugesalso manifold, manifold gauges
  The manifold gauge set techs connect to service ports to read system pressures, saturation temperatures and charge. Shop talk
• Hard start kitalso hard start, start kit
  An add-on start capacitor and relay that gives a struggling compressor extra torque to start. Shop talk
• Huntingalso TXV hunting, valve hunting
  A TXV or control swinging back and forth, alternately overfeeding and starving the evaporator instead of settling. Shop talk
• Iced upalso iced, frozen coil
  Describes an evaporator coil or suction line coated in ice because it is running below freezing. Shop talk
• LRAalso locked rotor amps, locked rotor current
  Locked rotor amps: the current a motor draws at the instant of starting, before it turns, listed on the nameplate.
• Micron gaugealso electronic vacuum gauge
  An electronic vacuum gauge that reads very low absolute pressure in microns of mercury during evacuation.
• Non-condensablesalso noncondensables, non-condensable gases
  Air, nitrogen or other gases trapped in a refrigeration system that will not condense at operating conditions.
• Pigtailalso pig tail
  A short jumper wire spliced into a group of conductors so they can share a single terminal. Shop talk
• Pull a vacuumalso evacuate, pull it down
  To evacuate a refrigeration system with a vacuum pump, removing air and moisture before charging. Shop talk
• Pump downalso pumpdown, pump it down
  Moving a system's refrigerant into the condenser or receiver by shutting the liquid line and running the compressor.
• Recoveryalso refrigerant recovery
  Removing refrigerant from a system into a recovery cylinder instead of venting it to the atmosphere.
• Short cyclingalso short-cycling, rapid cycling
  A compressor or system turning on and off too often, with run times too short to reach steady operation.
• Sluggingalso liquid slugging, slug
  Liquid refrigerant or oil being drawn into a compressor's cylinders, where it can't be compressed and causes damage. Shop talk
• Splitalso temperature split, delta T
  The temperature difference between return and supply air across a coil, used as a quick health check. Shop talk
• Starvedalso starving, underfed
  Describes an evaporator not getting enough refrigerant, so part of the coil runs warm and superheat is high. Shop talk
• Sweatalso sweat a joint, solder
  To join copper pipe and fittings with a torch and filler metal drawn into the joint by capillary action. Shop talk
• Top offalso top it off, topping off
  To add refrigerant to a system that is low on charge without finding and fixing the leak. Shop talk
• Weigh inalso weigh in the charge, charge by weight
  To charge a system with a measured weight of refrigerant on a scale, set by nameplate charge and line set length. Shop talk
• Whipalso disconnect whip, AC whip
  The short length of flexible conduit and wire running from the disconnect to an outdoor unit or air handler. Shop talk

Frequently asked questions

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