Coefficient 2 of the Part Load Performance Curve input range

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The EnergyPlus.idd file does not specify an input range for the pump coefficients:

N7, \field Coefficient 1 of the Part Load Performance Curve
     \default 0.0
N8, \field Coefficient 2 of the Part Load Performance Curve
     \default 1.0
N9, \field Coefficient 3 of the Part Load Performance Curve
     \default 0.0
N10, \field Coefficient 4 of the Part Load Performance Curve
     \default 0.0

However, the simulation will throw an error if pump power is negative, so you should probably stay away from negative coefficient values. You can see where the program is using the coefficients in the pump power calculation; there may be some other implicit limits based on what's physically realistic for pump operation.

From the docs, the fraction of full load power is defined according to this equation

$$FractionFullLoadPower=C1 + C2 \times PLR + C3 \times PLR^2 + C4 \times PLR^3$$

c1, c2, c3, c4 = (0, 1, 0, 0) is the EnergyPlus default as highlighted in the EnergyPlus example files.