I have several gas firepits that are in need of extra ventilation. They were installed with a single 3" vent to allow heat to escape from area under the firepit. They tend melt the electronic control unit because of the heat buildup.

What I am looking for is a formula that I can calculate the area, in sq.in., needed for proper ventilation. These units are 150k btu. I guess what I need is the amount of cfm of air needed for 150k btu.