I have sized my system using this spreadsheet, installed it, accurately measured the CFM, and the SP reading is significantly higher than the sheet showed. Why?
There are many possible reasons for this. The most likely reason is that the fitting loss factors in the spreadsheet are different than the loss factors on the fittings you installed. Fittings are the single biggest component of losses in most cases. The factors used here represent the LOWEST LOSS FITTINGS AVAILABLE! The best industrial fittings are welded, smooth interior walled pipe. These fittings are generally best ordered from a local firm that provides dust collection parts as shipping can often be as expensive as the cost of the parts. These parts are called “PREFERRED”, meaning ‘expensive’. Using furnace (HVAC) fittings will increase the pressure drop by several inches (or decrease the volume until the pressure drop is the same). The next most likely reason is that the losses NOT included in the spreadsheet are different than the assumptions you used. Another possible reason is due to a bad blower installation. The blower should be installed with straight runs into and out of i
