Whenever a fire booster pump is designed into a fire sprinkler system supply, with the capacity (in itself) to exceed the system demand, it is considered by any hydraulics program to be an “over-sized” pump. This simply means that, based on the information given, the program is reporting that the pump could be reduced in pressure output and still be capable of accommodating the remote area being calculated.
This condition commonly occurs when a pump is required for a more demanding system area but a specific calculation is for a less demanding area. With an over-sized booster pump, the negative pressures being reported from the suction-side of the pump to the source are accounting for the friction losses within this piping; and, a negative required pressure at the source is quite simply saying that this is the amount of pressure that is not needed from the available pressure at the source.
When viewed with this in mind, there is really no better way to accurately report this circumstance. A few other hydraulic software programs use some “creative reporting methods” to avoid showing these negative values; we chose not to do so. We do have an option in the program that will report the Required Pressure of 0.0 PSI, at the Supply on the Hydraulic Summary report, which is also an accurate statement. The Hydraulic Analysis report, however, still reports the actual values.
A negative Required Pressure in no way implies any problem with the City/District water supply, quite to the contrary. The most obvious time that there would be a problem with the water supply is if the demand GPM of the system were to exceed the available GPM @ 0.0 PSI. (The City/District water provider may have their own restrictions for a minimum drawdown pressure.)