Health experts are reminding people to check their HVAC systems after nine people were hospitalized with carbon monoxide poisoning this week.All nine have since been released. The county sheriff says it came from a malfunctionin g boiler and officials are now urging people to take proper precautions against the toxic gas.Jim Price has been operating on HVAC systems since he was 14-years-old.Now, he's the owner and operator of Air Tech Heating Inc. in Fond du Lac."Furnace, boiler, geo system, anything that heats your house should be checked annually," Price said.WATCH JACK PORTER'S FULL BROADCAST STORY HERE... HVAC experts remind homeowners to service heating systems ahead of winterIn addition to an annual check Price says there are little things you can do to stay safe."Most filters need to be changed at least every three months," Price said. "If you have a one inch filter you should be checking it monthly until you know how well your filter captures and how long it lasts."Each year, around 100,000 people nationwide visit the hospital for carbon monoxide poisoning. Around 400 annually are killed, according to the CDC.The Fond du Lac Fire Department says carbon monoxide detectors won't go off if they are reading anything below around 30 parts per million of carbon monoxide."A furnace will put out somewhere around zero five parts per million, all the way up to 100 parts per million," Price said.Price says just because an alarm isn't sounding, that doesn't mean you are in the clear.He says a slow leak could sneak below the detectors threshold. ...read more read less