Three children and a man who was disoriented and incoherent were also found in the home near Vincent and School Streets just after 7 p.m. Thursday. The man and children have been hospitalized but are expected to recover.
RELATED: 1 dead, another in hospital from carbon monoxide poisoning in San Francisco
Police say a family member called police for a welfare check because they hadn't heard from their relatives in several days.
PG&E crews found a very high level of the colorless and odorless gas in the home.
The early investigation shows it came from a heater.
This latest incident stresses the importance of making sure carbon monoxide readers are working properly.
"Every single home, and every level on every single home should have a working and regularly tested carbon monoxide detector," said Steve Hill, with Contra Costa Fire Protection. "That is the silver bullet that can protect us from these tragedies."
VIDEO: How to protect your family from carbon monoxide poisoning
VIDEO: How to protect your family from carbon monoxide poisoning
Hill says most detectors have one button labeled "test" that can be pressed to show it works. If it is not working, a a trip to a hardware store can solve that problem.
"It is pretty simple," Hill said. "They are relatively not expensive and they make a big difference."
He says around 10,000 people die from carbon monoxide exposure each year. Most of those deaths happen in the winter, because more people are using heating sources to stay warm.
"Anything of those things that burn any kind of fuel are producing carbon monoxide," he said. "It is important they be well maintained and use properly according other manufacturers instructions."