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Constant versus Variable flow

There are a number of ways to connect terminal units, like fancoil units and airhandling units, to

the installation. From the standpoint of hydronics there are however only two fundamentally

different ways to design the installation: Constant flow and Variable flow.

Constant flow

Constant flow installations are designed in such a

way that the amount of water flowing through the

installations doesnt change when the load changes.

This is usually achieved by using 3-way valves.

Fig 1: Constant flow system

When the load on the system is 100%, all the water will flow through the terminal unit. When there

is less cooling or heating needed, the 3-way valve will start to divide the water flow between the

bypass and the terminal unit. This way there will be less flow going through the terminal unit but

the amount of water going to the circuit will (theoretically) be the same.

Although from the angle of control this is a good way to design systems, it has one major drawback:

Energy consumption. Because the flow in the system is constant at all loads there is no opportunityto use a speed controlled pump to save energy. Additionally the differential temperature in the

system will be low because a lot of water is directed back unused. Low differential temperatures in

the system will have a negative effect on the efficiency of boilers and chillers.

Because the Constant flow system is not suitable for efficient, green, buildings the constant flow

system is falling out of favour. Most installations are now designed with Variable flow.

Variable flowIn Variable flow systems the water through the

terminal unit is controlled by using 2-way

valves. When the load is 100% the valve is fully

open and when less cooling or heating is

needed it closes to reduce the flow.

Fig 2: Variable flow

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Variable flow systems can be very energy efficient because the flow in the installation is reduced

when the full capacity of the installation is not needed. On average an installation will run on 40 to

60% of its capacity most of the time, so a very significant saving can be made on the pumping cost

when speed control on the pump is used. Furthermore, the differential temperature in the system is

maximised, which means that chillers and boilers are running on optimal efficiency.

Proper design and good commissioning (balancing) of a system with 2-way valves is however

critical to its operation. The 2-way valves must be sized properly, taking into account available

pump heads and authority. The system must be balanced properly to ensure the right flows during

full and partial load.

Pressure Independent Valves

Using Pressure Independent Balancing and Control Valves (PIBCV) in Variable flow systems will

reduce the complexity of design, simplify commissioning and will create the most efficient system.