Furnace flue

What is the purpose of an opening like this between the furnace unit and the flue pipe?


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This draft hood allows the draft over the fire to remain fairly constant by allowing house air into the flue system when winds are high and/or the stack effect is creating higher negative flue pressure than recommended by the manufacturer for good appliance operating characteristics. In an oil system, there would be an adjustable device in the flue pipe called a barometric damper to balance the flue pressures to what is recommended.

By adjusting/balancing the negative flue pressures, the fire flames are not distorted by the high draft (better, more complete combustion) and…the flue gases are not drawn through the heat exchangers at higher velocities that will reduce heat transfer time from the flue gases to house air/heating water resulting in lower system efficiencies.

Draft hood. . .

It’s necessary to aid in venting of the appliance. Standard practice for gravity-type vent systems. . .

Is that a Consolidated brand furnace?