Back-to-back racks

Here we've included a link in the Part # column that provides a description of a rack and an image link that provides drawing of a rack.

Need help understanding the multiplex part number schema?

Back-to-Back Racks - 399 models
Description Part # Drawings
2 + 2-boilers 4" Header M04-B-4"
2 + 2-boilers 6" Header M04-B-6"
3 + 3-boilers 4" Header M06-B-4"
3 + 3-boilers 6" Header M06-B-6"
4+ 4-boilers 6" Header M08-B-6"