What is wrong? › Commercial HVAC
No heat in the space: check power and thermostat, then call
Staff and customers will not stay in a cold shop, and OSHA's own guidance for indoor work is a 68 to 76 degree range, though it is a recommendation rather than a rule.
What is probably happening
- The rooftop unit's gas heat section is locked out after failed ignitions: a dirty flame sensor, a failed igniter, or a gas valve fault.
- A tripped high limit from restricted airflow, usually dirty filters, so the blower runs but the burners will not stay on.
- On a heat pump or electric heat unit, a failed heat strip, contactor or defrost control.
- Or simply the thermostat: schedule in unoccupied mode, or set to Cool.
Do this in the next five minutes
- Check the thermostat is set to Heat, above room temperature, and in occupied mode.
- Check the breaker for the unit and reset it once if tripped. If it trips again, leave it off.
- Change reachable filters if they are dirty.
- Turn the system Off, wait five minutes, turn it back to Heat. Many units reset their lockout this way. If it fails again, stop and call.
- If you smell gas at any point, leave and follow the gas leaf.
Do not
- Do not use portable propane or kerosene heaters indoors to bridge the gap. CO risk, and NFPA counts cooking and heating equipment among leading commercial fire causes.
- Do not keep cycling the power. Repeated lockouts mean a fault, and each try dumps unburned gas into the heat section.
- Do not go on the roof to look.
Who to call
A licensed commercial HVAC contractor
The heat section of a rooftop unit is gas or electric equipment inside the same cabinet as the cooling. It needs a mechanical contractor licensed for commercial gas work, not a plumber or an electrician.
BestHonestPro is a free comparison resource, not a contractor. There is nothing to sign up for and we do not sell your details. When we begin connecting callers to contractors, we will be paid a referral fee by the contractor, never by you, and this line will say so.
What it should cost
$100 to $3,000, averaging $350
HVAC repair, national range. A flame sensor or igniter is at the low end, a heat exchanger at the high end
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.
- Will you test the heat exchanger with a combustion analyzer and give me the readings?
- What caused the lockout: sensor, igniter, gas valve, limit or airflow?
- Is this repair a part I will be replacing every winter, or a one-off?
- If you say heat exchanger, will you show me the crack or the CO reading before quoting a unit replacement?
End the call if
- A heat exchanger condemned with no combustion reading and no photo.
- A replacement quote before the filters and the flame sensor have been checked.