Replying to Paul Daisy, you must be using a UL-listed CO detector, that won't alarm until you are practically toast. The specs on those are set to prevent false alarms, not to protect you from the lower level exposures that will still mess with your health and capacities.
There is no way you can operate any unvented heater in a tent and not have CO accumulating, not to mention all kinds of particulates, soot, water vapor, unburned hydrocarbons, etc. If you doubt me, get an instant readout CO meter like the kind gas appliance installers use, or get one from aeromedix.com and you'll see what you're really doing to yourself. CO takes WEEKS to get out of your blood. Please folks, don't use unvented heaters in tents or cabins if you want to maintain the ability to climb, carry loads, and think straight.
Some of my hunting buddies doubted me until one year I imposed a rule, no propane lanterns in the tent, and for the first time ever they could get up in the morning and not have an altitude headache.