Parallel Chiller System

To provide some redundancy in the HVAC design, most designers will require two or more chillers. Multiple chillers also offer the opportunity to improve on overall system part load performance and
reduce energy consumption. Parallel chiller plants are straightforward to design and are easily modified for variable primary flow.

Rooftop cooling unit with chiller units and compressors.

Basic Operation

Figure below shows a parallel water-cooled chiller plant. Chilled water is circulated by the chilled water or primary pump through both chillers to the load and back to the chillers. The chilled water loop can
be either constant flow or variable flow. Variable flow systems increase the complexity but offer significant pump work savings. They also resolve the issue about chiller sequencing that occurs with parallel chillers, constant flow.

Basic Parallel Chiller System Operation

Variable flow systems are covered in Primary/Secondary Systems and Variable Primary Flow Design. A condenser loop is required for water-cooled chillers. This includes a condenser pump, piping and a cooling tower or closed circuit cooler. The condenser loop operates whenever the chillers operate.

For constant flow systems, the chilled water temperature range varies directly with the load. Depending on load diversity, the chiller design temperature range will be less than the temperature range seen at each load. In this case, the chiller temperature range is 8°F while the cooling coil range is 10°F . The overall result is increased chilled water pump and pipe capital cost plus higher annual pumping cost.

Basic Components

Parallel chillers experience the same percent load. For example, consider a chiller plant with a 100-ton and a 1000-ton chiller operating at 50% capacity. With both chillers operating, both chillers will operate at 50% capacity. The 100-ton chiller will be at 50 tons and the 1000-ton chiller will be at 500 tons. This occurs as long as the flows don’t change (i.e,. variable primary flow) and both chillers see the same return water temperature.


In most cases, the sum of the chiller capacities meets the design for the building or process. Additional capacity can be added, if required, by oversizing the chillers. It is common for parallel chillers to be the same size and type although this not a requirement. Water-cooled, air-cooled or evaporatively-cooled chillers can be used. Air and evaporatively-cooled chillers do not require a condenser loop including piping, cooling tower and pump.


Pumps can be constant or variable flow. The chilled water pump is sized for the design flowrate. Figure below shows one main chilled water pump providing flow to both chillers. An alternative method is to have two smaller pumps serving dedicated chillers. Figure below also shows dedicated condenser pumps and cooling towers for each chiller. The pumps and piping are sized for the design condenser flow for each chiller. Whenever the chiller operates, the condenser pump operates.

Typical Parallel Chiller System

Cooling Towers

Water-cooled chillers will require cooling towers. Figure above shows dedicated cooling towers for each chiller. A common cooling tower is also possible but not common for parallel chillers.

Parallel Chiller Sequence of Operation

Parallel chiller plants create a unique situation when used in a constant flow system. Consider the system operating at 50%. From a chiller performance aspect, turning off one chiller and operating the other at full capacity is desirable. However, this will not happen. At 50% capacity, the return water will be 49°F. The chiller that is turned off will let the water pass through it unchanged. The operating chiller will only see a 50% load (49°F return water), and will cool the water down to the set point of 44°F. The two chilled water streams will then mix to 46.5°F supply temperature.

If the system is operated in this manner, the warmer chilled water will cause the control valves to open (increase flow) to meet the space requirements. An iterative process will occur and the system may stabilize. The issue is whether the cooling coils can meet the local loads with the higher chilled water temperature. Depending on the actual design conditions, the building sensible load could be met but high chilled water temperature will make it difficult to meet the latent load. Since this scenario is likely to occur during intermediate weather, dehumidification may not be an issue. In areas where humidity is an issue, this arrangement can result in high humidity within the space.

One solution is to operate both chillers all the time. This works and is a simple solution, however, it is not energy efficient and causes unnecessary equipment wear.

Another possibility is to lower the operating chiller’s set point to offset the mixed water temperature. This also works but has some difficulties. Lowering the chilled water setpoint requires the chiller to work harder, lowering its efficiency. In extreme conditions, it can cause chiller stability issues.

Adding isolation valves to stop flow through a chiller when it is not operating is not recommended for a constant flow system. It is unlikely that the pump will be able to provide design flow if all the chilled water is directed through just one chiller. The pump will ride its curve and a loss of flow will occur. Without design flow, it is unlikely that all the individual loads will receive their required flows. In the event the pump could actually provide the flow through one chiller, the maximum allowable flow rate for the chiller may be exceeded resulting in serious damage to the chiller.

The safe answer is to operate both chillers all the time chilled water is required, however, this is as expensive as operating a single chiller plant. Staging on the pumps and cooling towers is similar to that outlined for single chillers.

Parallel Chiller Plant Example

Typical Parallel Chiller System

Consider the same model building used in the Single Chiller example. The design load performance is identical to the single chiller plant. There are small changes in real applications when two chillers are used instead of one. For instance, pump and chiller selections are not likely to offer identical performance, other than being half the size.

Parallel Chiller Plant Example – energy and performace

What is more interesting is the annual energy usage is the same for both single and parallel chillers. This occurs because both chillers were operated to provide 44°F supply chilled water at any plant load. With both chillers operating, all the pumps and towers had to operate as well. There was no opportunity to use only one chiller at light loads, shut down one tower and condenser pump and shift the single chiller further up its performance curve.

This could be accomplished by switching to variable primary flow, which would allow a chiller to be isolated at light loads, as well as to reduce the chilled water pump size and to lower its operating cost.


What are the benefits of using multiple chillers in a parallel chiller system?
Using multiple chillers in a parallel chiller system provides redundancy in the HVAC design, ensuring that the system can still operate even if one chiller fails or is taken offline for maintenance. Additionally, multiple chillers can improve overall system part-load performance and reduce energy consumption. This is because each chiller can operate at a more efficient part-load condition, reducing the overall system’s energy usage.
How do variable flow systems in parallel chiller plants improve pump work savings?

Variable flow systems in parallel chiller plants improve pump work savings by allowing the chilled water pump to operate at a lower speed or capacity during part-load conditions. This reduces the energy consumed by the pump, resulting in significant pump work savings. Variable flow systems also resolve the issue of chiller sequencing that occurs with parallel chillers in constant flow systems.

What is the difference between constant flow and variable flow systems in parallel chiller plants?

In a constant flow system, the chilled water pump operates at a fixed speed, circulating a constant flow rate of chilled water through the chillers and the load. In a variable flow system, the chilled water pump speed is varied to match the changing cooling demand of the load. Variable flow systems are more complex but offer significant pump work savings and improved system efficiency.

How do parallel chiller plants improve overall system part-load performance?

Parallel chiller plants improve overall system part-load performance by allowing each chiller to operate at a more efficient part-load condition. This is because each chiller can be sized to meet a specific portion of the total cooling load, reducing the need for oversized chillers that operate inefficiently at part-load conditions. By operating multiple chillers in parallel, the system can take advantage of the most efficient operating points of each chiller, resulting in improved overall system efficiency.

What are the design considerations for a parallel chiller plant with variable primary flow?

When designing a parallel chiller plant with variable primary flow, consideration must be given to the chiller selection, piping layout, and control strategy. The chillers must be selected to operate efficiently at variable flow rates, and the piping layout must be designed to accommodate the varying flow rates. The control strategy must also be designed to optimize the operation of the chillers and pumps to achieve maximum system efficiency.

How does chiller sequencing affect the operation of a parallel chiller plant?

Chiller sequencing refers to the order in which the chillers are started and stopped to meet the changing cooling demand of the load. In a parallel chiller plant, chiller sequencing can affect the overall system efficiency and reliability. Improper chiller sequencing can lead to inefficient operation, increased energy consumption, and reduced system reliability. Variable flow systems can help resolve the issue of chiller sequencing by allowing the chillers to operate in a more flexible and efficient manner.

What are the maintenance considerations for a parallel chiller plant?

Maintenance is critical for ensuring the reliability and efficiency of a parallel chiller plant. Regular maintenance tasks include cleaning the condenser coils, checking and optimizing the refrigerant charge, and performing routine inspections of the chillers and pumps. Additionally, the control strategy must be regularly reviewed and updated to ensure that the system is operating at maximum efficiency. By performing regular maintenance, the system can operate reliably and efficiently over its entire lifespan.