Let's set the Wayback Machine to 2008. There was only one size of WSM, the 18.5" model. It came with a shallow water pan that held 1 gallon of water. The pan was a repurposed lid from the Smokey Joe. (Weber was famous for re-purposing existing parts from other grills in the design of the WSM.)
People complained, they wanted a higher capacity water pan. Many of us bought the
Brinkmann 812-0002-0 smoker charcoal pan as a replacement. It was used to hold charcoal in Brinkmann smokers, not water, but it was affordable and the perfect size and profile for a WSM water pan and held almost 2 gallons of water. Here's a pic, you'll notice is has steeper sides and a flat bottom.
Unbeknownst to WSM owners, Weber engineers like TVWBB member
Mike Durso was monitoring this forum and listening to our suggestions/complaints. In Fall 2008 for the 2009 model year, Weber introduced the brand new WSM 22.5" and a revised WSM 18.5". To increase water capacity in the 18.5" WSM, they repurposed the charcoal bowl from the Smokey Joe as the new water pan. Unlike the Brinkmann pan, it has a deep, round profile that hangs low over the charcoal chamber. While it does hold 2.5 gallons of water, it can sit right on top of lit charcoal and smoke wood if you really pile it up in the charcoal chamber, and some folks don't like that.
Brinkmann went out of business, their pans are getting harder to find, and the prices are going up, up, up. Thus some people look for alternatives elsewhere, pans with greater capacity but with a shallower profile.
For the 22.5" WSM, Weber repurposed the lid from the 18.5" Weber kettle as a water pan. Its shallow profile is similar to the pre-2008 WSM 18.5" water pan and it hold 3 gallons of water.
For the 14.5" WSM introduced in Fall 2013 for the 2014 model year, Weber made a small water pan using a part similar to the bowl from the discontinued Weber Torch.
So it's only the 2009 and newer WSM 18.5" that has what some people feel is a problematic water pan and perhaps in need of an alternative pan.
