Steve, There's a great book published by the American Association of Industrial Hygenists called "Industrial Ventillation". It contains not only enough tables, charts, formulae,etc. to satisfy even a world class geek, but some very good "real world" examples of what type of configurations are "preferred" "good" "no-way". I've learned a lot about vents from this book, but in no way do I profess to be an expert. *CAUTION* I have noticed some self proclaimed experts that can calculate losses when "blowing" through a pipe haven't a clue what happens when you "suck" through the same pipe. Things get a little wierd there, and the "run of the mill" HVAC guys many times haven't a clue. The voice of experience speaks here. My suggestion (other than buying the book AND reading it) is to consider splitting up the systems. So what if there is one or five blowers on the roof... I'm sure the pidgeons won't complain a bit! What you will gain is a much better margin of error when the HVAC guy's theories work on paper, but not in your plant. Variables like building barometric pressure (changes seasonally with some heating/cooling systems) can be positive (the building blows) or negative (the building sucks). You can tell this when you open a door such as in a lobby. Most facilities types like the building to be positive so they can control the ingression of fresh air (remember, trust is good, control is better). In addition to this margin, you also have lower overall cost when you start getting up there in blower size as you are finding out. Another BIG plus is that you run very little risk of water/steam ladden air getting back down into your reflow or soldering system. That can get pretty ugly, especially if they don't forsee this and do not put a drip leg into the configuration that can handle the weight. I've seen the results of a poorly desighned vent of this size trap nearly 100 gallons of water in the vent, and suddenly the structure (originally intended to hold some sheet metal vent duct) decided that the weight of the water wins. Not pretty at all. So... to make this short (yeah, right) if you would like to discuss this r/t, please feel free to call me. Ed Popielarski QTA Machine 714-581-6601 ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################