For definitions and system sizing refer to the system design section.

The plumbing system designer must consider many "moving targets" in his design such as:

1. Suction pressure - present and future and the possibility of significant variances due to losses in the meter and back flow preventer at high capacities.

2. Estimating the demand curve [load vs. time].

3. The final as built compared to the original design of the piping system.

4. Special requirements such as air conditioning equipment, process equipment, boiler makeup etc.

5. The building dynamics - i.e. present use and occupancy vs. future use - as an example an office complex that converts to high-density executive rental suites or a condo that converts to a hotel. Densely populated buildings use more water even though the fixture unit count can be the same as less populated buildings.

All of the above can result in a system significantly different than anticipated, and on this basis standard practice is to over design.

It is generally accepted that Hunter's curve used for calculating peak capacity in GPM is very conservative, however this continues to be the basis of most designs.

The above issues usually result in an oversized system and corresponding oversized pumps and motors thereby wasting energy for the life of the system (usually 20 years).

A variable speed system designed to the same system capacity and pump differential pressure as a constant speed system will adjust to the variable parameters and operate at the most efficient point and speed, which can provide very impressive savings.

For systems where variable speed can be considered adjustable drives can be:
(1) fluid (2) mechanical (3) electro magnetic (4) wound rotor (5) variable voltage (6) variable frequency

For reasons of:
(1) equipment first cost (2) drive efficiency (3) use of standard motors (4) less equipment to maintain and install etc., variable frequency is the best choice.

In addition to energy savings variable speed VFD drives provide the following:
1. Soft no-load starting - reduces shock load on driven equipment.

2. Smooth controlled speed changes - reduces pressure and flow surges in the system.

3. Infinite stepless speed control.

4. Reduced wear on the equipment.

5. Critical Speed (noisy operation) avoidance