The fire started just after midnight on Ella Boulevard, near Rankin, just off the North Freeway. Smoke could be seen billowing from the building.
Residents said repairs were being made on Tuesday after being without hot water for the last five days.
They say maintenance on the boiler room had recently wrapped up before this happened.
"Yeah, I was asleep. My son came knocking on the door, we heard the fire trucks and stuff, and we all ran over here and saw it was the boiler," resident Diane Garcia said.
Everyone safely escaped, but the building could be seen without power.
Red Cross was handing out blankets to residents who were unable to go back into their apartments.
Investigators are trying to determine if the fire started in the boiler room.
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