Garage heat

A properly operating indoor UNVENTED heater should NOT PRODUCE CO. A properly burning VENTED heater DOES NOT PRODUCE CO. What causes CO is an improperly adjusted burner (not enough O2) or an UNVENTED burner operated in a closed area which eventually DEPLETES enough O2 so as to cause "partial" combustion, and THAT leads to CO

Many modern "pilot" heaters have what is known as an "Oxygen depletion" pilot, which is supposed to go out and lock out the system if the O2 level drops. This is not only for the CO hazard, but in case someone is (illegally) using an unvented appliance in a poorly vented sleeping area

The problem with ANY fuel, unvented heater is EXPLOSION hazard. Also, depending on chemicals present, ANY fuel fired unvented heater can generate TERRIBLE obnoxious toxic vapors. One example "I bet" you guys have heard of is products such as "brakecleen" (CRC) that is ANY cleaning product containing trichloro based chemicals IE low pressure hydrofluro type products. THESE CAN PRODUCE phosgene gas, the stuff Hitler used. (Welding/ brazing refrigeration tubing on older refrigerants, R12, 22, etc does this as well)

There are LOTS of chemicals that become absolutely obnoxious when run through combustion. Photo / film development chemicals, hair bleach/ dye/ other hair (beauty) chemicals the list goes on and on

Some of these chemicals, if they aren't terribly toxic, become horribly corrosive when run through a burner. As a former HVAC service guy, equipment does not last long in places such as printing / publishing outfits, "back when" there were photo places, and laundry/ dry cleaners

And to repeat, they are an exposion hazard.........AKA "source of ignition"