The measurement of the draft is coupled with a visual inspection of the venting system to determine the probability that all of the combustion gases are getting out of the structure. The standard we use was developed from both technical analysis and field testing . The source of the combustion air for the appliance must be adequate and not come from a prohibited location. This test is to identify problems you might have if you change the building tightness. Having an open or not sufficiently connected return air system is a primary concern.
Open returns provide a significant depressurization source in the immediate vicinity of the unit.
Standards range from simply ensuring that the appliances work under the worst case conditions, to installing combustion air that meets current building code. Schematic of a combustion furnace. Air paths in a combustion furnace. Most appliances are equipped with safety devices to prohibit them from overheating and from allowing fuel into the device if there is no ignition source. You should check the limits if you are doing anything that might affect the heat transfer.