garage heating

CO emissions are a function of burning fuel, not unit efficiency. A more efficient unit will only produce less emissions by virtue of its lower fuel consumption. I do have a "ventless" torpedo-heater and there is no way you are going to run that in an enclosed area.

If you are determined to use one of the ventless types do yourself a favor and ask the local fire department which ones the would or more likely would not recommend (Fire Chief will probably recommend you not use any of these, possibly equating their safety to Russian Roulette). Next call your insurance agent and see if use of such a device will void your policy (although UL approved, many policies exclude portable heating devices). Something to think about.