3 Tanks In A Line: Venting

The situation is there are 3 natural gas water tanks in a line. A 40 gallon tank and 2 -30 gallon tanks (in that order from building exit point to the rear). A friend raised the question that they were not vented properly. He was worried about the available combustion air in the basement and stated that if there was not enough combustion air when the 40 gallon tank turned on it could create a pull on the rear tanks vents and pull CO into the basement. These tanks are all located in a 750 sq. ft unfinished basement. What is code on venting 3 tanks in sequence like this?

Thanks for any replies

venting

The same as for a single tank. There has to be adequate provision for combustion air, (at least on square inch of free area for each 1,000 btu's), and the flue has to be adequate for the total btu's of all the burners.