Safety measures such as these are not a place to take shortcuts or be ignored!
Any combustion appliance open to the indoor air system (think furnace, unit heaters, wood stove, water heater, or gas-powered dryer) requires a supply of air to support the flame within the burner and a create draft up the chimney or out the vent. That air must come from somewhere. You may have one in your utility room, garage, crawl space, or furnace room. Right there, in your carefully insulated house is a great big vent hole leading to the great outdoors.. Those holes in your wall are there for a very important reason!
All current nationally-recognized model building codes now require some form of combustion appliance venting. Many local building codes have added additional requirements on top of the national building standards, requiring even more combustion air in a home with natural draft combustion appliances. The thinking for many years was that the air leakage in a home was adequate for combustion venting. This assumption was probably appropriate back in the day when windows were single pane and insulation was made up of newspaper and horse hair. In the case of older remodeled homes and newer well-sealed, more efficient homes, fuel burning appliances become starved for combustion air causing them to malfunction. Worse, the simultaneous operation of multiple venting appliances may create a negative pressure within the living space of a home that can overcome the stack draft pressures in a natural draft appliance. This dangerous situation can cause exhaust gas spillage and back-drafting of potentially deadly carbon monoxide into the living space.
As a result building codes now require the installation of combustion air vents whenever combustion appliances use living space air for venting.. end of story
#1 - If you dont have a combustion air vent installed inside your home GET ONE INSTALLED.. If you already have a combustion air vent in your home, do not block it under any conditions!
#2 - Always have a working Carbon Monoxide (CO) detector in your home if you have any combustion appliance present (oil, gas, wood, etc.).