There are several types of HVAC water systems that are designed with diversity. It could be a chilled water system, heat pump loop water system, heating water system, or (most commonly) a reheat water system. When diversity exists in an HVAC water system, the TAB of this system requires a system review and careful consideration of the approach to the TAB process.

The HVAC water system diversity is defined as the percentage difference in the pump capacity versus the connected load. For example, if the pump is sized for 300 gpm and the total connected load to all elements is 450 gpm, then the diversity of the HVAC water system is as follows:


Diversity = [(450-300)/450] x 100 = 33.33%


Keep in mind that diversity is not a calculated value in the design process. Diversity is a byproduct of the calculated block load of the building and the calculated individual loads within the building (individual space load requirements). In addition, the water coil selection process can affect the amount of diversity in a system as the manufacturers’ selections do not always match the design water flow quantity (gpm) requirements.

To properly address the TAB process in a system with diversity, an understanding of a pressure independent system and pressure dependent system is required.

Pressure dependent devices have their water flow (airflow) vary with the system pressure. As the system pressure increases the flow increases at the device and as the system pressure decreases the flow decreases at the device.

Pressure independent devices have their water flow (airflow) stay at the required flow regardless of the system pressure (as long as the system is operating within its capacity limitations). The pressure independent device monitors the flow (pressure) to the device and adjusts the devices’ control valve (damper) to maintain the required flow.

A comparison of an airside VAV system to a waterside variable water volume system can help clarify the difference between pressure dependent systems and pressure independent systems.



  • In a variable volume air system, the fan modulates speed to maintain the static pressure set point as the VAV boxes modulate damper position to maintain the required airflow to the space. The VAV box is controlling an airflow setpoint/requirement to maintain space conditions. The VAV box reports the airflow quantity to the BAS/DDC system.

  • When one VAV box or multiple VAV boxes change damper position, airflow does not change at the other VAV boxes (as long as the system static pressure setpoint is maintained) since they are pressure independent.


  • The pump is controlled to maintain a system differential water pressure. As the control valves open and close, the pump speed varies to maintain the system differential pressure setpoint.

  • Since the control valves are controlling to maintain space temperature or a coil/element discharge air temperature, the water flow through the coil/element can change as the system pressure and demand changes. The actual water flow (gpm) through the coil/element is not being reported to the BAS/DDC system. The control valve is NOT controlling to a required water flow (similar to a VAV box), but to space temperature or a coil/element discharge air temperature.

  • The change in water flow at a given coil/element is dependent upon what elements in the system are opening and closing, the control speed (PID loop) of the VFD controlling the pump and control valve, and the location of the system differential pressure senor. The system is DYNAMIC and therefore pressure dependent.


Figure 1 is a water system profile diagram displaying how the water flow can vary at a coil/element based on the system coil/elements location with respect to the system differential pressure sensor location.

There has been an increase in use of pressure independent control valves (PICVs) to try and address the challenges of a pressure dependent HVAC water system. An understanding of PICVs is required to allow for the correct valve selection, application, and operation.

PICVs do NOT maintain a consistent inlet water pressure, nor outlet water pressure to the control valve. As the inlet pressure varies (internal pressure across the PICV), the pressure drop across the rolling diaphragm and spring changes which in turn effects the outlet orifice. The regulating element (be it ball- or globe-style) would remain unchanged — this varying pressure simply changes the outlet orifice downstream of the regulating element — thus maintaining a constant flow rate (at the same control valve position). The accuracy of the flow rate at the PICV can vary from 5% to 17%, depending on the manufacturer and application.

This is where the selection, application, and use of the PICV become important. In most applications, the PICV does not control to a water flow quantity (gpm). The control valve typically controls to space temperature or a discharge air temperature. In most HVAC water system applications, the PICV is not reporting the water flow quantity (gpm) back to the BAS/DDC system for control valve adjustment and regulation of water flow to a water flow setpoint. Certain PICVs have the option to report water flow quantity back to the BAS/DDC system by use of an ultrasonic flowmeter or a pressure differential measurement with a correlation to valve position. However, this option is typically not utilized for control in an HVAC water system.

In addition, a PICV is not the same as an automatic flow-limiting device (AFLD) coupled with a control valve. The automatic flow-limiting device limits the amount of water flow to the coil/element as long the automatic flow-limiting device is operating within its design pressure range. There are different types of automatic flow-limiting devices; some utilize pressure regulating orifices to limit maximum flow to the coil/element, while others use variable orifices to limit maximum water flow to the coil/element. The automatic flow-limiting device assists in the proportioning of water flows throughout the water system by limiting the maximum water flow to the coils/elements. The PICV does not help in the proportioning of the water flow in a HVAC water system due to either stroke limiting, accuracy, inherent valve authority, or the combination of all of these elements. In some cases, the PICV requires more system energy (pumping capacity) to obtain the maximum design water flow for the coil/element.

Figure 2 shows a typical office building and a hot water reheat system. There is a connected load of 450 gpm coils/elements with a 300 gpm pump serving this system.

The pump cannot satisfy the TOTAL system water flow requirement, but specifications require that the TAB agency measure and set the design water flow quantity at each coil/element.

How is this to be accomplished? Can diversity be simulated? Does one open all control valves 100% and proportion the available water flow? Or do you proportion the water flow at each coil/element to the percentage of diversity?

Following are testing scenarios showing the challenges of testing an HVAC water system with diversity. Please keep in mind that these scenarios are not recommended procedures, but are scenarios that display system testing shortcomings. The above building example is utilized in all scenarios and assumes that manual balancing valves are installed at all coils/elements.


Testing Scenario 1

Open the connected load equal to pump capacity and proportion the water flow — Diversity Simulation.

  • Open the proper amount of coils/elements equal to the pump capacity of 350 gpm (Open all control valves on floors two though floors eight and close the all control valves on floor one and floor nine).

  • Proportionally balance the water flow to the coils/elements with the control valves open.

  • Next, close all control valves on floor two and floor three (100 gpm of control valves). Open all the control valves on floor one and floor nine (350 gpm of control valves).

  • Proportionally balance the coils/elements on floor one and floor nine.

  • Keep performing the various iterations in simulating diversity by establishing a connected load equal to the pump capacity.

It should be noted after a couple iterations, each time a control valve or a set of control valves are opened or closed, the system changes and therefore the water flow to the coils/elements change. With manual/settable balancing valves, the balancing valve position would have to be changed in each testing scenario. The attempt of simulating diversity can create an infinite number of testing scenarios and balancing valve positions.

It is very difficult and in most situations impossible to simulate diversity. The weather load, solar load, people load, equipment load, etc., for the building or space is an everchanging scenario. Manually manipulating valve positions to try to simulate diversity is only a means to establish a connected load for testing purposes. Depending on the test scenario selected, the setting of the manual balancing valves could add additional unrequired resistance to the HVAC water system.


Testing Scenario 2

100% Load Simulation (Figure 3).

  • Open all coils/elements 100%.

  • Test the pump for total water flow. Note: The pump will probably provide more than scheduled water flow.

  • Start proportioning coils/elements closest to the pump. There will not be enough water flow delivered from the pump to set each device to design water flow. In this example, it is assumed that the pump is able to deliver 330 gpm in this system.


Testing Scenario 3

100% Load Simulation set to x% Diversity.

  • Calculate the % System Diversity. In this case the system diversity is as follows:

Diversity = [(450-300)/450] x 100 = 33.33%.

  • Open all control valves 100%.

  • Proportion the water flow at the coils/elements to 33% of the coil/element required water flow.

  • There is added resistance to the system since manual balancing valves have been set (closed) to get 67% flow with a 100% open building. This can create a higher required differential pressure set point for the system.

  • A coil/element might not be able to obtain required water flow since the balancing valve was closed to limit a design flow condition.

  • The 100% open building “should never” occur. As the building load changes, the water flow requirement varies through the building as required.

  • The pump total water flow could be reduced due to the added resistance on the system and reduced capacity at certain coils/elements.


More testing thoughts

There are several considerations when testing an HVAC water system with diversity. A review and analysis of the system is required to determine the best approach/method. When testing an HVAC system with diversity, common sense must be employed. The project team should not get caught up in obtaining numbers or design values. Following are some thoughts and goals to consider when testing and/or designing an HVAC water system with diversity.

  • What controls the control valves? Discharge air temperature? Space temperature?

  • How much diversity is there in the system?

  • What type of system is being tested? Hot water reheat, heat pump loop, chilled water, etc.?

  • What is the design intent of the system? Are there special system design considerations?

  • Project scheduling could dictate testing procedures. System availability for testing and occupancy can create challenges with the testing process.

  • Is repeatability of the measured water flows required? Can the system be setup in the same testing mode to repeat the measured water flows?

  • Verify pump performance.

  • Set up system to operate as efficiently as possible.

— Establish the differential pressure operating setpoint.
— Obtain water flow to the coil/element with the greatest resistance (possibly largest gpm loads) without creating excessive resistance in the system.

  • Make sure that all coils/elements can provide required capacity.

  • How much is 0.5 gpm, 0.7 gpm, 1.2 gpm?

— To obtain these lower water flows, sometimes manual balancing valves are required to be closed too far. This balancing valve position can create an opening smaller than a strainer opening and become restricted with debris.
— Is the balanced gpm in a heating system really important to be +/-10% for the “smaller gpm” elements? According to studies, 90% of the heat transfer of a coil will be obtained at 50% water flow at 180°F water temperature.

  • Utilize discharge air temperature sensors on VAVs, coils, etc., to assist in function testing and troubleshooting. This provides the ability to trend data on the BAS/DDC system.

  • Utilize branch balancing devices to each floor or building and any other strategic locations. Where is it going to be difficult to get water flow? Where is it easy to get water flow?

  • Use automatic flow-limiting devices for systems with diversity, remodels, phased projects, etc.

  • Make sure the control valves are sized properly.

  • Make sure the balancing valves are sized properly, size the balancing valve for water flow and NOT pipe line size.

  • Utilize a reverse return design to assist in proportioning water flows.

  • Know and understand the system design intent and the procedures for testing.

  • Focus on what is important to provide an efficient system at optimum operating conditions.


The TESTING of an HVAC water system with diversity can become more of an art than a science! Each system is different and could require a different process or approach.

It can’t be overemphasized that an HVAC water system is dynamic. The building load is constantly shifting. The HVAC water system is not a true pressure independent system. Let the control valve do the work to limit the amount of resistance in the system. After testing is complete, system performance can be trended (if available on the BAS/DDC system) to determine if the system differential pressure setpoint can be adjusted to minimize pump operation and energy usage. ES


A special thank you to Mr. Kevin Poniatowski, national sales manager of Bray Commercial Division and Mr. Jeff Jones, president of PRO Hydronic Specialties, for their valuable input and technical assistance with the writing of this article.