Design and Installation Data:
Pressure System Sizing

Designing a copper tube water supply system is a matter of determining the minimum tube size for each part of the total system by balancing the interrelationships of six primary design considerations:

  1. Available main pressure;
  2. Pressure required at individual fixtures;
  3. Static pressure losses due to height;
  4. Water demand (gallons per minute) in the total system and in each of its parts;
  5. Pressure losses due to the friction of water flow in the system;
  6. Velocity limitations based on noise and erosion.

Design and sizing must always conform to applicable codes. In the final analysis, design must also reflect judgment and results of engineering calculations. Many codes, especially the model codes, include design data and guidelines for sizing water distribution systems and also include examples showing how the data and guidelines are applied.

Small Systems

Distribution systems for single-family houses can usually be sized easily on the basis of experience and applicable code requirements, as can other similar small installations. Detailed study of the six design considerations above is not necessary in such cases.

In general, the mains that serve fixture branches can be sized as follows:

  • Up to three 3/8-inch branches can be served by a 1/2-inch main.
  • Up to three 1/2-inch branches can be served by a 3/4-inch main.
  • Up to three 3/4-inch branches can be served by a 1-inch main.

The sizing of more complex distribution systems requires detailed analysis of each of the sizing design considerations listed above.

Pressure Considerations

At each fixture in the distribution system, a minimum pressure of 8 psi should be available for it to function properly - except that some fixtures require a higher minimum pressure for proper function, for example:

  • Flush valve for blow-out and syphon-jet closets - 25 psi
  • Flush valves for water closets and urinals - 15 psi
  • Sill cocks, hose bibbs and wall hydrants - 10 psi

Local codes and practices may be somewhat different from the above and should always be consulted for minimum pressure requirements.

The maximum water pressure available to supply each fixture depends on the water service pressure at the point where the building distribution system (or a segment or zone of it) begins. This pressure depends either on local main pressure, limits set by local codes, pressure desired by the system designer, or on a combination of these. In any case, it should not be higher than about 80 psi (pounds per square inch).

However, the entire water service pressure is not available at each fixture due to pressure losses inherent to the system. The pressure losses include losses in flow through the water meter, static losses in lifting water to higher elevations in the system, and friction losses encountered in flow through piping, fittings, valves and equipment.

Some of the service pressure is lost immediately in flow through the water meter, if there is one. The amount of loss depends on the relationship between flow rate and tube size. Design curves and a table showing these relationships appear in most model codes and are available from meter manufacturers.

Some of the main pressure will also be lost in lifting the water to the highest fixture in the system. The height difference is measured, starting at the meter, or at whatever other point represents the start of the system (or the segment or zone) being considered. To account for this, multiply the elevation of the highest fixture, in feet, by the factor 0.434, the pressure exerted by a 1-foot column of water. This will give the pressure in psi needed to raise the water to that level. For example, a difference in height of 30 feet reduces the available pressure by 13 psi (30 x 0.434 = 13.02).

Friction losses in the system, like losses through the water meter, are mainly dependent on the flow rate of the water through the system and the size of the piping. To determine these losses, water demand (and thus, flow rate) of the system must first be determined.

Water Demand

Each fixture in the system represents a certain demand for water. Some examples of approximate water demand in gallons per minute (gpm) of flow, are:

  • Drinking fountain - 0.75
  • Lavatory faucet - 2.0
  • Lavatory faucet, self closing - 2.5
  • Sink faucet, WC tank ball cock - 3.0
  • Bathtub faucet, shower head, laundry tub faucet - 4.0
  • Sill cock, hose bibb, wall hydrant - 5.0
  • Flush valve (depending on design) - 3.5
  • Shower head - 2.2

Adding up numbers like these to cover all the fixtures in an entire building distribution system would give the total demand for water usage in gpm, if all of the fixtures were operating and flowing at the same time-which of course does not happen. A reasonable estimate of demand is one based on the extent to which various fixtures in the building might actually be used simultaneously. Researchers at the National Institute of Standards and Technology studied this question some years ago. They applied probability theory and field observations to the real-life problem of simultaneous usage of plumbing fixtures.

The result was a system for estimating total water demand is based on reasonable assumptions about the likelihood of simultaneous usage of fixtures. Out of this study came the concept of fixture units.

Each type of fixture is assigned a fixture unit value which reflects:

  1. Its demand for water, that is, the flow rate into the fixture when it is used;
  2. The average time duration of flow when the fixture is used;
  3. The frequency with which the fixture is likely to be used.

Assigned fixture unit values vary by jurisdiction. Consult local plumbing codes for values used in your area.

Totaling the fixture unit values for all the fixtures in a system, or for any part of the distribution system, gives a measure of the load combined fixtures impose on the plumbing distribution and supply system. This fixture unit total may be translated into expected maximum water demand following the procedure prescribed by your local code.

Keep in mind the demand calculations just described apply to fixtures that are used intermittently. To this must be added the actual demand in gpm for any fixtures which are designed to run continuously when they are in use; for example, air-conditioning systems, lawn sprinkler systems and hose bibbs.

Pressure Losses Due to Friction

The pressure available to move the water through the distribution system (or a part of it) is the main pressure minus:

  1. The pressure loss in the meter;
  2. The pressure needed to lift water to the highest fixture (static pressure loss);
  3. The pressure needed at the fixtures themselves.

The remaining available pressure must be adequate to overcome the pressure losses due to friction encountered by the flow of the total demand (intermittent plus continuous fixtures) through the distribution system and its various parts. The final operation is to select tube sizes in accordance with the pressure losses due to friction.

In actual practice, the design operation may involve repeating the steps in the design process to re-adjust pressure, velocity and size to achieve the best balance of main pressure, tube size, velocity and available pressure at the fixtures for the design flow required in the various parts of the system.

Table 6 shows the relationship among flow, pressure drop due to friction, velocity and tube size for Types K, L and M copper water tube. These are the data required to complete the sizing calculation.

NOTE: Values are not given for flow rates that exceed the maximum recommendation for copper tube.

For the tube sizes above about 1-1/4 inch, there is virtually no difference among the three types of tube in terms of pressure loss. This is because the differences in cross sectional area of these types become insignificant as tube size increases. In fact, for this reason, the value for Type M tube given in Table 6 can be used for DWV tube as well.

Pressure loss values in Table 6 are given per linear foot of tube. In measuring the length of a system or of any of its parts, the total length of tube must be measured, and for close estimates, an additional amount must be added on as an allowance for the extra friction losses that occur as a result of valves and fittings in the line. Table 7 shows these allowances for various sizes and types of valves and fittings.

Water Velocity Limitations

To avoid excessive system noise and the possibility of erosion-corrosion, the designer should not exceed flow velocities of 8 feet per second for cold water and 5 feet per second in hot water up to approximately 140°F. In systems where water temperatures routinely exceed 140°F, lower flow velocities such as 2 to 3 feet per second should not be exceeded. In addition, where 1/2-inch and smaller tube sizes are used, to guard against localized high velocity turbulence due to possibly faulty workmanship (e.g. burrs at tube ends which were not properly reamed/deburred) or unusually numerous, abrupt changes in flow direction, lower velocities should be considered. Locally aggressive water conditions can combine with these two considerations to cause erosion-corrosion if system velocities are too high.

Due to constant circulation and elevated water temperatures, particular attention should be paid to water velocities in circulating hot water systems. Both the supply and return piping should be sized so that the maximum velocity does not exceed the above recommendations. Care should be taken to ensure that the circulating pump is not oversized, and that the return piping is not undersized; both are common occurrences in installed piping systems.

Table 6 applies to copper tube only, and should not be used for other plumbing materials. Other materials require additional allowances for corrosion, scaling and caking which are not necessary for copper. This is because copper normally maintains its smooth bore throughout its service life.