What is wrong? › Commercial HVAC
A gas smell means everybody out, then the utility
Natural gas can ignite from a light switch or a phone, and a rooftop furnace section can leak into the duct that feeds your customers and staff.
What is probably happening
- A gas leak at the rooftop unit's heat section, the gas piping on the roof, or a kitchen appliance line.
- The rotten egg smell is an odorant added on purpose so you notice it. Take it seriously even if it is faint.
- A furnace section that just fired can give a brief whiff on start up, but you cannot tell the difference from inside. Leave anyway.
Do this in the next five minutes
- Tell staff and customers to leave now and take others with them.
- Do not light a match, smoke, turn anything on or off, use a flashlight, start a car, or use a phone inside.
- Once outside and away from the building, call 911 or your natural gas utility's emergency number.
- Keep people away from the building and the roof until the utility says it is clear.
- Only after the utility has made it safe, call a licensed commercial HVAC contractor to find and fix the fault.
Do not
- Do not go up on the roof to look. You cannot fix it and you could be at the source when it ignites.
- Do not turn the unit off from the thermostat or the disconnect. A switch can be the spark.
- Do not reopen for business because the smell faded. The utility clears the building, not your nose.
Who to call
Your gas utility first, then a licensed commercial HVAC contractor
The utility finds and isolates the leak for free and at once. The HVAC contractor repairs the heat section or gas train afterwards.
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What it should cost
$100 to $250 per hour
HVAC labor once the utility has cleared the building. The utility emergency response itself is not a contractor bill.
Source: Angi, HVAC repair cost, updated August 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the commercial hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- The utility has isolated the gas. Can you come today, and what is the diagnostic fee?
- Will you pressure test the gas train and the piping to the unit, and write down the result?
- Are you licensed for gas work on commercial equipment in this state, and what is the license number?
- If the heat exchanger is the problem, will you show me the evidence before quoting a replacement?
End the call if
- Anyone who tells you to go back inside before the utility has cleared the building.
- A replacement quote for the whole unit before anyone has found where the gas is coming from.