Water Distribution Systems

There are four main types of water distribution systems. They are defined by the number of pipes used in the system – 1-pipe, 2-pipe, 3-pipe, and 4-pipe. While this article will discuss primarily chilled water and condenser water system piping system design, it is important to understand the evolution from 1-pipe into the other three systems, all of which are used for heating as well as cooling.

1-Pipe Systems

A 1-pipe water distribution system is a system that has a one main pipe looping around the building and then returning.

1-Pipe Distribution System

Since 1-pipe systems are typically only used for heating, the supply and return are shown connecting to a boiler instead of a chiller.

The 1-pipe system has been used for many years in residential and smaller commercial buildings. It has been used as a hot water distribution system and seldom, if ever, for the distribution of chilled water

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This pipe is both the supply and return main. Its size is constant throughout, and all of the water in the system flows through it feeding one or more zone heating terminals.

A small amount of water is induced to leave the main at each riser by the use of a special flow fitting used on 1-pipe systems, sometimes referred to as a “monoflow” fitting. These fittings create a pressure drop in the main equal to or greater than the pressure drop through the riser, runout, zone terminal unit, and return piping.

Control of flow rate to the zone terminal units in a 1-pipe system is often difficult to achieve.

The pressure drop from the point where water leaves the main to where it returns is small and small changes in resistance in this line result in large changes in flow rate. As a result, many 1- pipe systems avoid flow rate control at the zone terminals and achieve capacity control by regulating airflow over the zone terminals instead.

Some advantages of the 1-pipe system include the simple design of the system that requires one pipe size. This simplicity of design leads to easy installation and low installed cost.

However, 1-pipe systems have several disadvantages. The pumping head is generally higher than that in other systems because of the resistances occurring in series. That means the pump and pump energy is larger than other distribution systems of comparable size.

The change in water temperature as the water moves through the system (the water gets colder after each successive terminal because of mixing) creates the possible need of larger units at the end of the main, which will complicate the selection of the zone terminal units and add cost due to oversized units near the end. Also, at part load, the end unit may be over or under capacity.

In order to keep the pressure loss through the unit coils low, the water velocity through the coils must be kept low. This results in coils with large tube diameter, a greater number of tubes in parallel, or larger coils than used with other distribution systems. Therefore, a physical space and terminal cost penalty exist when a 1-pipe system is used.

The 1-pipe system is poorly suited to chilled water distribution for several reasons. The water quantity used in chilled-water systems is usually considerably higher than that used for heating because the unit coils work on smaller temperature differentials in the cooling mode than in the heating mode. In order to economically accommodate higher flow rate, zone terminals used for chilled water would need to be redesigned so they are not prohibitively large, expensive, or space-consuming.

2-Pipe Systems

The 2-pipe water distribution system is used with both heating and cooling equipment containing water coils. It is equally useful for room fan coil units and medium or large central air handlers using combination hot water and chilled water coils.

The 2-pipe system can be used to distribute either hot or cold water, or alternate between the two. The same piping is used for both heating and cooling so there must be a definite outdoor temperature, which is called the “changeover temperature,” or some other indicator of building load, at which point the hot water in the piping is replaced by the chilled water and vice versa.

2-Pipe Reverse Return Distribution System

Some 2-pipe fan coil units are equipped with electric heat in addition to the heating capability of the hot water coil. This “touch up” electric heat can be used if heating is required for a fan coil but the system is still not changed over to the heating mode.

There are two forms of 2-pipe water distribution systems in common use:

  • 2-pipe direct return
  • 2-pipe reverse return

In a 1-pipe system, the supply and return main is the same pipe. The quantity of water flowing through the main is approximately constant and the main is built of one diameter pipe throughout its length. On the other hand, in the 2-pipe system, the supply and return mains are separate pipes and water leaving the supply main goes into the return main.

As water leaves the supply main and goes through the terminal units, the quantity of water flowing in the main is reduced, so the pipe diameter can be reduced. The opposite is true for the return main, which starts out small at the furthest terminal and has to be increased in size as water enters it.

Advantages of 2-pipe systems include the fact that a higher friction loss can be taken in both the piping and the zone terminal units and still have a total pumping head lower than that in the same size 1-pipe system because the zone terminals are in parallel water circuits, not series. Also, it is easier to balance the flow to each unit in this system than in the 1-pipe system, assuming branch balancing valves are installed in the piping as the system is installed.

Another advantage of 2-pipe systems is that the water temperature entering each zone terminal will be the same in temperature because the return water from each terminal unit does not mix with the supply water in the supply main.

However, the installed cost is greater than that for a 1-pipe system. In systems of the same size, even though the average pipe diameter in the 2-pipe system is smaller than that in the 1- pipe system, the extra pipe and greater number of fittings means that this system will have a greater first cost. Like the 1-pipe system, the 2-pipe system distributes only a common temperature fluid to the zone terminals.

Because the system cannot deliver hot water or chilled water simultaneously to the coils, it must be in either the heating or cooling mode. To change over from heating to cooling, the water in the mains must be completely circulated through the chiller and back to the unit before any cooling is available at the zones.

Changeover takes time. It is not practical to plan to change over frequently. Seasonal changeover is the most common method used. Two-pipe supplemental heating systems are also quite common, both for separate perimeter heating and zone reheat at the terminals.

Pumping head refers to the total pressure drop in ft wg that the water pump(s) must overcome to circulate the water through the system. Lower head results in lower pump energy consumption.

When a 2-pipe system is changed over from cooling to heating or vice versa, it is important that the water introduced is not too hot or too cold. This would cause thermal shock to the boiler or the chiller.

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3-Pipe Systems

The 3-pipe water distribution system has two supply mains feeding each zone terminal, one for chilled water and one for hot water, and a common return main. The chilled water supply and hot water supply lines are sized according to normal standards and the return is sized to handle the maximum flow rate (which is the cooling flow rate). As with 2-pipe systems, the return main can be either direct return or reverse return configuration.

3-Pipe Distribution System

Because of the two supply mains to each zone terminal, there is always hot and cold water present at the entrance to the zone coil ready to be used when needed. This gives any fan coil or air handler supplied by the 3-pipe water distribution system the ability to heat or cool at any time. No changeover from summer to winter cycle is needed in the 3-pipe system.

However, the operating cost of this system can become prohibitively high because of the mixing of hot and cold return water. It is important to be familiar with 3-pipe systems because they have been installed in existing buildings and are still in use.

ASHRAE 90.1 does not allow for the use of 3-pipe systems because of the mixing of hot and cold water in the common return pipe uses excess energy.


4-Pipe Systems

The 4-pipe water distribution system is actually two, 2-pipe systems in parallel; each system consisting of its own supply and return main. One system is always distributing chilled water to the units and returning it to the chiller. The other is distributing hot water to the units and returning the water to the boiler.

Unlike the 3-pipe system, there is no mixing of hot and cold water. By using two separate coils in each zone terminal unit, or one coil with a separate cooling and heating circuit, the heating and cooling systems are completely separated.

The chilled water flows through a cooling coil and the hot water flows through a separate heating coil. At no point are the two circuits connected. In a 4-pipe water distribution system, each terminal unit can become a separate zone of control, with its own thermostat. Both hot and cold water are available to all units at one time.

4-Pipe Distribution System

Four-pipe distribution systems are actually two 2-pipe systems in parallel. This system offers both hot and chilled water to all zones simultaneously, enabling the system to meet cooling and heating loads whenever and wherever they occur.

There is no need for seasonal or more frequent changeover. The hot and chilled water circuits are completely separate and the two water streams are never mixed. The design methods, valves, and controls are similar to 2-pipe and 3-pipe systems.

A 4-pipe system with a fossil fuel-fired boiler can deliver a competitive or lower operating cost than some 2-pipe systems with “touch-up” electric heat built into the unit. That is because the electric heaters in the 2-pipe unit must sometimes operate more often than is expected and electric resistance heat is expensive, and the heaters may require a larger building electric service. This operation occurs prior to the entire system changeover to heating. Fossil fuel rates typically offer an advantage to electric rates.

However, 4-pipe systems have a higher installed price than 2-pipe and most 3-pipe systems. The extra pipe and valves at the zone terminals tend to make the 4-pipe system the most costly in terms of installed cost. Four-pipe systems also require terminal units with dual coils or a 2-circuit coil, which costs more. Also, there are four pipes to run throughout the building, which takes more time and consumes more space for piping than the other systems.

For commercial buildings, the choice comes down to 2-pipe versus 4-pipe designs. The comfort and control advantage of 4-pipe over 2-pipe must be weighed against the higher installed cost of the 4-pipe system. Where the building configuration and layout of spaces may require long periods of both heating and cooling simultaneously, and occupant comfort is a requirement, 4- pipe makes the most sense. When the building lends itself to a seasonal changeover without large compromises in comfort, 2-pipe is suitable.