The diffuser tank discussion continues and Peter Thomas, P.E. from Resource Compliance, Inc. was kind enough to offer us the following insight as to when a diffuser tank would be required. In most of my postings, I reference the International Fire Code (IFC) and International Mechanical Coden(IMC) quite often, but the Uniform Mechanical Code (UMC) is a code that I have very little experience with and as Peter points out, it is alive and well on the west coast. So it is worth noting that the Uniform Mechanical Code required Diffusion Tanks from 1994-2014 (the requirement removed in 2015 UMC)AND the UMC also required diffuser tanks be of steel construction. Most states utilize the International Mechanical Code, but select western states still use the UMC, or in the case of California the California Mechanical Code which is a derivative of the UMC. Here are some parts of the UMC that Peter provided: