While the induced draft heaters are more efficient in most cases, two things are important. One, even if installed in a shed a draft hood and vent must be installed to get combustion gasses out of the shed and enough ventilation has to be available for combustion air (see the instructions). Two, induced-draft heaters, in one way or the other, have metered air and gas in order to lower nitrous oxide emissions (Lo-NoX heaters). With the advent of low-profile heaters, this is especially necessary as the short distance between the flame and the heat exchanger does not allow for complete combustion of the gasses the way a taller heater does. The metering is required, partially because the amount of emissions can vary depending also upon the altitude of the installation. At least that is what a RayPak engineer explained to me many years ago.