Horizontal gas furnace installed in an attic between insulated rafters, with flexible supply ducts, a metal flue pipe and the gas valve compartment
Generic photo: Mark Doliner · CC BY-SA 2.0

Heat illness on the job: attic air handlers, crawlspaces, garages and rooftop units

Attics in summer can be far hotter than the outdoor temperature, and HVAC calls peak exactly when it is hottest. Heat exhaustion (heavy sweating, headache, nausea, dizziness, weakness) means stop, cool down and drink water; if it does not improve, get medical care. Heat stroke (confusion, slurred speech, passing out, seizures, very high body temperature, hot skin) is a 911 emergency: call, move the person somewhere cool and cool them fast while you wait.

Cross-checked ~5 min easyBrands: All

Before you start

  • Plan attic work for the cooler morning hours where possible
  • Do not work in a hot attic alone; tell someone where you are
  • Watch new workers closely; lack of acclimatization raises the risk

Diagnostic steps

Unlock all 4 steps with expected values

Full library + AI assistant + the OnCall Tech app for iPhone & Android. $34.90, one time.

Get lifetime access

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

More tech fundamentals guides

Guide · Fundamentals

Blower runs nonstop and no cooling on a new HVAC system: A2L refrigerant leak sensor

A2L systems carry a refrigerant leak sensor near the indoor coil. When it reports a leak, or when the sensor fails or loses communication, the control stops the compressor and runs the blower to dilute any refrigerant. This guide separates a sensor or wiring fault from a possible real leak; any real leak is referred to an EPA 608 technician.

8 steps ~40 minmoderate
Guide · Fundamentals

Confirming an inverter board's DC bus is discharged before you touch it

Inverter boards store high-voltage DC in large capacitors after the power is off, and a dark LED does not mean the board is safe. Remove power, wait the maker's time, then measure the DC bus at the marked test points until it reads 0 V. Only then unplug loads or handle the board.

7 steps ~20 minmoderate
Guide · Fundamentals

Load will not turn off: shorted triac or welded relay (board vs load)

When a load runs with no command, the board output that feeds it is usually shorted: triacs most often fail shorted and relay contacts can weld closed after inrush. Confirm the output is truly on with the load connected (not a ghost reading), then test the load itself, because a shorted or seized load is what usually kills the output.

8 steps ~40 minmoderate
Heat illness on the job: attic air handlers, crawlspaces, garages and rooftop units · OnCall Tech