Hydrogen sulfide, the rotten-egg sewer gas that deadens the sense of smell as it reaches deadly concentrationsIn: Confined space entry field guide
HAC
Hot-aisle containment, which encloses the hot exhaust aisle and ducts it to the return, keeping the room coolIn: Aisle containment QA
HACCP cold-holding temperature
The food-safety control point: cold TCS food held at or below the food-code limit, commonly 41°F, monitored and logged, with the danger zone running 41°F to 135°FIn: Commercial refrigeration field guide
Hail vs wind damage
Hail leaves random bruises and granule loss; wind leaves directional creased, torn, lifted, or missing shinglesIn: Storm and hail damage restoration
Listed accessory joining two single-pole breaker handles so they switch togetherIn: Multiwire branch circuits
Handrail
The graspable rail on stairs and ramps, set at 34 to 38 inches, sized for the hand and for the catch of a stumbleIn: Metal railing and guardrail fabrication
Stock acclimated to outdoor conditions before planting, so it transitions into the bed instead of shocking or burningIn: Seasonal color and annual rotation
Hardiness zone
A USDA map band based on average annual coldest winter temperature, used to tell whether a perennial survives local wintersIn: Landscape design and plant selection
Hardness (gpg / ppm)
Dissolved calcium and magnesium; 1 grain per gallon is about 17.1 ppm as calcium carbonateIn: Water treatment field guide
Hardscape drainage
Moving water off the surface and out from behind and under a patio, walkway, or wall, so it is never trappedIn: Hardscape drainage
Harmonic order
Whole-number multiple of the fundamental; the 5th is 300 Hz on a 60 Hz systemIn: Harmonics and power quality
The walk of a task to find the hazard to each body part and select PPE against it, required and, in general industry, documentedIn: PPE hazard assessment and selection
Hazard classification
NFPA 13 grouping of a space by contents (Light, Ordinary 1-2, Extra 1-2) that sets the design densityIn: Fire sprinkler system design guide
Hazardous (classified) location
An area where flammable gas, combustible dust, or ignitable fibers may be present in enough quantity to make electrical equipment an ignition riskIn: Hazardous classified locations
Hazardous energy
Any energy that can injure on release: electrical, mechanical, hydraulic, pneumatic, thermal, chemical, or gravityIn: Lockout/tagout field guide
The energy the pump adds, expressed in feet of water; about 2.31 feet equals 1 psi for cold waterIn: Hydronic pump install field guide
Head box
A reservoir built above the gap on the pour side that creates hydraulic head to drive grout acrossIn: Grout types and baseplate grouting
Head entrapment / 3.5 to 9 inch rule
A bounded opening within reach must be 3.5 in or smaller, or 9 in or larger; the range between traps a head and is a strangulation hazardIn: Playground safety surfacing
Head lap / side lap
The horizontal and vertical overlaps between underlayment courses, printed as lines on most sheetsIn: Roofing underlayment types
Head-to-head coverage
Spacing heads so each throws to the next, giving close to 100 percent overlapIn: Sprinkler system design
Headlap
The overlap of a slate by the course two above it, giving three thicknesses at every line; 3 in standardIn: Slate roof field guide
Headlap and exposure
Headlap is how far a course laps the one below; exposure is the part of the tile left to the weather; more lap means less exposureIn: Tile roof installation
Ventilation and air conditioning that performs infection control in a hospital, treating the room's air as a clinical deviceIn: Healthcare HVAC ventilation field guide
Apparent temperature from air temperature plus humidity, in °F, available from weather apps and the OSHA-NIOSH Heat Safety ToolIn: Heat illness prevention
An electric tank that moves heat from the air into the water at two to four times resistance efficiencyIn: Water heater types
Heat pump / COP
Equipment that raises the temperature of recovered heat using electricity; COP is heat delivered divided by electricity consumedIn: Data center waste heat reuse
Heat recovery
A VRF that can heat some zones and cool others at once, moving heat between them off one outdoor unitIn: VRF commissioning field guide
Heat stroke
Medical emergency: confusion, slurred speech, collapse, high core temperature; call 911 and cool immediatelyIn: Heat illness prevention
Effective embedment depth, how far the anchor reaches into the concrete, in inches or mmIn: Concrete anchor installation
Helical pier / screw pile
A steel shaft with one or more helical plates, screwed into soil until the plates bear in firm strata, carrying load in compression or tension.In: Helical piers and screw piles
Helical plate (helix)
The steel plate welded to the shaft at a true pitch that bears on the soil; its diameter and number set the bearing area and the capacity.In: Helical piers and screw piles
High-efficiency filters; HEPA 99.97% at 0.3 micron, ULPA 99.999% or better near 0.12 micronIn: Cleanroom HVAC field guide
HEPA air scrubber / AFD
Air filtration device with a HEPA filter that lowers airborne spores and, ducted outside, creates negative pressureIn: Mold remediation field guide
Hermetic / semi-hermetic / open
Motor sealed in a welded shell, bolted serviceable housing, or external with a shaft sealIn: HVAC compressor types
Herringbone
A 45 or 90 degree zigzag pattern that resists vehicular creep; recommended for driveways and streetsIn: Paver layout and field border
HET
High-efficiency toilet, flushing at 1.28 gpf or less, the WaterSense threshold and about 20 percent below the federal maximumIn: Fixtures and water efficiency
HFC / HFO
Hydrofluorocarbon (high GWP, A1) and hydrofluoroolefin (low GWP, often A2L)In: Refrigerant types and A2L
HGI / grease trap
Hydromechanical grease interceptor, the small flow-rated unit sized in gallons per minuteIn: Grease interceptor field guide
The ranking of systems from most to least effective: eliminate, then passive guardrails, then travel restraint, then fall arrest lastIn: Rooftop permanent fall protection
High leg (wild leg)
On a four-wire delta, the line that reads about 208 volts to neutral; must be identified and kept off 120-volt loadsIn: Three-phase wye vs delta
High side / low side
The high-pressure half from compressor discharge to the metering device, and the low-pressure half from the metering device to the compressor suctionIn: Refrigeration cycle field guide
High-build immersion lining / DFT
A thick, dense, product-matched film; DFT is its cured dry film thickness in milsIn: Tank coating and lining
High-heat procedures
Extra controls above an upper trigger temperature: more water and rest, closer watching, pre-shift warningIn: Heat illness prevention
High-intensity (luminous)
Gas heater with a glowing ceramic emitter at a high surface temperature, commonly near 1800°F, for spot heatIn: Infrared radiant heater field guide
The audible, visual, and remote alert that trips above the lag-pump level when the pumps are not keeping up, before the wet well overflowsIn: Sewage lift station field guide
High-limit
The safety control above the operating control that cuts the burner on excess temperature or pressure, often manual-resetIn: Boiler startup commissioning
High-temp underlayment
A tile-rated membrane that holds up to the heat tile traps against the deck, where a standard ice-and-water shield would flowIn: Tile roof installation
High-voltage spark test
A charged brush swept over a dry, exposed membrane that sparks to ground at a pinhole; for bare membranes onlyIn: Electronic leak detection
Hot mix asphalt, the conventional mix produced commonly around 300 to 350°FIn: Warm mix asphalt (WMA)
HMA / hot mix asphalt
Asphalt produced and placed hot, the standard product of an asphalt plantIn: Asphalt plant production
HMA / WMA
Hot-mix asphalt and warm-mix asphalt; WMA compacts at lower temperatures and extends the window in the coldIn: Cold-weather asphalt paving
HOA
Hand-off-auto switch; in hand the device runs regardless of the BMS, bypassing the automation, the classic commissioning findIn: BMS and DDC controls commissioning
How long the test pressure is maintained; a code minimum around 10 minutes for examination, often 2 hours or more for spec acceptanceIn: Chilled water hydro test package
Holding strength
The tested load an attachment resists in slide and pull-out, used with a safety factor in the layoutIn: Snow guards and retention
Hole / floor opening
A gap of 2 inches or more in its least dimension in a walking or working surfaceIn: Jobsite housekeeping
Hole balance
The ratio of the weakest sampling hole's flow to the strongest, kept above a floor so no hole is starvedIn: Aspirating smoke detection
Holiday / pinhole
A pinhole, void, or bare spot in the film where the substrate is exposed, found by spark or sponge detectionIn: Tank coating and lining
Holiday detection
Electrical testing, wet sponge or high-voltage spark, that finds pinholes and bare spots in the filmIn: Protective coatings and blasting
Hollowcore
A flat floor or roof plank with voids run through it to cut weight, joined by grouted keywaysIn: Precast erection and connections
Home run / slack / rack dress
The three parts of one drop's installed length: the routed run, the service loop, and the run up the cabinet to the portIn: Cable tray fill and copper takeoff
Honeycomb
Voided, stone-exposed concrete where the mortar never filled between the coarse aggregateIn: Placement and consolidation
Hook-and-loop
Reusable fabric strap, VELCRO ONE-WRAP the common brand, that binds a bundle without crushing itIn: Data center rack cable management
A cable or rail between anchors with an energy absorber that lets a worker stay tied off while moving along the roofIn: Rooftop permanent fall protection
The sloped horizontal pipe that collects conductors and carries the combined flow toward the building storm sewerIn: Interior storm drainage piping sizing
Horizontal vs vertical carrier
Horizontal carriers tie the waste into a horizontal branch; vertical carriers turn it down into a stack on stacked floorsIn: Fixture carriers and supports guide
The glow bar that heats to incandescence and lights the burners directlyIn: Gas furnace field guide
Hot weather concreting
Placing and protecting concrete when high air temperature, low humidity, wind, and sun speed up evaporation and hydration, per ACI 305In: Hot weather concreting
Hot work
Any operation producing sparks, flame, or heat that can ignite combustibles, such as welding, cutting, grinding, brazing, or torch workIn: Hot work permit field guide
Hot work permit
The authorization and checklist that controls welding, cutting, and other spark- or flame-producing workIn: Fire watch and impairment
Hot-air weld
A seam fused by hot air and roller pressure, melting the two thermoplastic sheets into oneIn: Single-ply seam QA
Hot-gas bypass
A valve that routes hot discharge gas back to the low side to fake a load and hold flow off the surge line at very low load, at an energy penaltyIn: Centrifugal chiller surge control
Hot-mix asphalt (HMA)
Asphalt and aggregate mixed and laid hot, the standard driveway mat; also called blacktop or bituminous concreteIn: Asphalt driveway installation
Hour meter / engine hours
The running-time counter on a machine that triggers the service interval, the equivalent of the odometer on a truckIn: Fleet maintenance program
Hourglass / 2 to 1 ratio
A sealant bead about twice as wide as deep, thinned in the middle so it stretches without peelingIn: Joint sealant replacement
An arrangement where the robot runs the structured task and a person sets it up, supervises it, checks quality, and handles every exceptionIn: Construction robotics
Humidity ratio
The actual mass of water vapor per pound of dry air, in grains or pounds; 7,000 grains equal one poundIn: Psychrometric chart
Distributing direct current at higher voltage, here 400 V or 800 V class, to a rack busbar rather than AC to per-server suppliesIn: Rack DC power distribution (HVDC)
Hybrid cable
A single jacketed line bundling optical fiber for signal and copper conductors for DC power up to the RRUsIn: Cell tower and antenna install
High-pressure water removal of deteriorated concrete that leaves the rebar and sound concrete intact without micro-crackingIn: Bridge deck construction
Hydrogen sulfide (H2S)
The rotten-egg gas in sewer gas, toxic at high concentration and deadens the sense of smell as it climbsIn: Sewer gas odor field guide
The push of standing water against a wall, growing with depth, which can double or triple the wall's loadIn: Hardscape drainage
Hydrostatic relief valve / uplift
Hydrostatic uplift is groundwater pressure pushing up under an empty shell; the relief valve, in the main-drain sump over gravel, lets that water into the pool to equalize so the shell does not float.In: Commercial pool construction
The inverted-U reflection a round target makes on a GPR profile; the peak sits over the target and gives its depthIn: Concrete scanning before you drill
Hyperscale
A very large, standardized, self-built facility run by a cloud or platform operatorIn: Data center types field guide