Currently have a 74 gallon hw heater. Noticed that our recently finished basement has been very hot and humid even w dehumidifier running (almost constantly). Walked into the mechanical room today and it was like a sauna. extremely hot and humid. the hw heater was running due to 2 people currently showering. putting my hand above vent hood i noticed a large amount of steamy air bypassing the vent all together. This, i assume, is why the mechanical room was so hot, and why the now insulated finished basement has been hotter and more humid. we had the hw heater installed several years ago, and it was upgraded from a 50 to 74, the plumber upgraded the exhaust vent pipe at the same time to make it up to code in nj. The town came out and inspected as well. currently the exhaust pipe inside the hw heater is 5" but the vent pipe is only 4". I assume thats correct (to minimal standards at least) considering the plumber upgraded it and the town inspected it. But would it vent better if it was a 5" or 6" vent pipe? Things definitely effecting backdraft and the negative pressure in the house currently are 2 portable AC units that vent outside, 2 100cfm+ bathroom fans, a range hood that's at least 350cfm. plus dryer. All of the above vent directly outside, and during the summer months many running at same time. Going to call a plumber this week to come check it out and hopefully fix the problem. I was looking for some advice from this forum for proper fixes to make sure we don't get jerked around and so its done right.