I thought it was probably unvented but I'm not sure because it somehow is now missing. The tank has a return line. I found this online and emailed.
Part 080015AE is now obsolete and replace with part# 080078AE - Price 5.18 - please call our customer service line at 608-355-6558 to place an order for this.
Admin Assistant/Business Development
801 Lynn Ave
Baraboo, WI 53913
Phone: 608-355-6519 6519