By Bill Ivey

Principal Consultant

IEI is often hired to diagnose hot water delivery problems in high-rise residential buildings. Domestic hot water systems must be carefully designed and installed so that residents are supplied with hot water to faucets and shower valves in a reasonable amount of time. Waiting for water to get hot is not only inconvenient, but it also wastes water.

Accounting for Gravity

High-rise domestic water systems (both hot and cold) are more complicated than those for low-rise or even mid-rise buildings. They must be designed to account for gravity. This requires a booster pump, located in the basement or first floor, to deliver water to the upper floors. As the water moves through the pipes from the lowest floor to the highest floor, it loses approximately five pounds per square inch (PSI) of pressure per floor.

For a 21-story building, the pressure is approximately 100 psi lower on the 21^st floor than on the first floor. A minimum of 35 psi is necessary to properly operate most plumbing fixtures. If at least 35 psi is to be provided to the 21^st floor, the booster pump must boost the pressure to at least 135 psi at the first floor. The plumbing code requires that the pressure for a domestic water system not exceed 80 psi. Pressure reducing valves must be installed for the lower floors so that the 80-psi maximum pressure is not exceeded.

Maintaining the Temperature

High-rise buildings present another problem regarding delivery of hot water. Unless properly addressed, the hot water will cool in the pipes during periods of lower demand. To overcome this, plumbing designers add small pumps and sometimes small water heaters to constantly circulate hot water, thus maintaining the temperature in the distribution system.

Balancing Flow Rate

The circulation system must also be properly balanced, otherwise some circuits will not have enough flow to maintain temperature. This is accomplished by adding a balancing valve in each circuit. These valves are adjusted by the contractor to provide the specified flow rate. If the valves are not properly selected by the engineer or adjusted by the contractor, some circuits will have too much flow and some too little, resulting in long wait times for hot water. Pump selection is also critical. Excessive flow can damage the copper piping, sometimes resulting in leaks.