SANTA ROSA (CBS SF) — A malfunctioning rooftop HVAC heating unit filled a Santa Rosa school with carbon monoxide fumes Thursday morning, sickening students and forcing the evacuation of the building, authorities said.

According to the Santa Rosa Fire Department, crews were dispatched to the Bridge Church located at 301 Fulton Road at 10:26 a.m.

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Arriving firefighters found an orderly evacuation underway at the two-story building containing both the church and the Grace Christian Academy.

Students and teachers at the school told firefighters there was an odor inside the building. Numerous students and several of the staff were complaining of symptoms consistent with exposure to carbon monoxide.

Firefighters using a 4-gas monitor detected the presence of carbon monoxide in several classrooms and hallways in the building.

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In total 12 students and two staff members complained of symptoms consistent with carbon monoxide exposure, were treated and released at the scene.

Upon further investigation by firefighters and PG&E crews it was determined that the source of the carbon monoxide was an HVAC unit on the roof of the school that provides heat to several classrooms.

The HVAC unit was covered by a tarp due to a water leak around it during the recent rain. When the HVAC unit was turned on to provide heat, the tarp prevented proper circulation of clean air.

Once the tarp was removed the HVAC unit functioned properly. The rest of the HVAC units on the roof were tested and found to be functioning properly.

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Firefighters used fans to remove the carbon monoxide from the building.