AAON LL-540 - Manual (Page 25)

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and after the pump. There is also a needle
valve at each of these points to isolate the
pressure. To measure the pressure at any
given point, open the needle valve at that
point and close the other two needle valves.
One gauge is used so that the calibration of
the pressure gauge is irrelevant in the
calculation of the differential pressure.
Pipe Insulation
The water piping and components on units
with pumping packages are not insulated at
the factory. Insulation should be installed on
the water piping after the system has been
checked for leaks.