Flow in HVAC systems explained

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Flow in HVAC systems explained
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Flow in HVAC systems explained

– by Albrand Veldhuizen,  30/03/21

Flow is one of the most important parameters within an HVAC system. Without flow, nothing gets heated or cooled, supplied or discharged. As logical as this may seem, flow is a fragile parameter that can affect the capacity of both the heating and cooling systems. It can also lead to complaints such as drafts and noise and be the cause of many alarms and malfunctions. All in all then, good flow control is essential.

Transferring energy

The whole purpose of flow in a system is to transport energy. Examples include heat from the boiler to the onboard heaters, chilled water from the chiller to the cooling coils in the air handling units, and conditioned air to the various rooms.

When talking about cooling or heating, the capacity is expressed in kilo watt or watts. This common unit is actually a compound of Joules and seconds. A certain capacity (J) is delivered to a medium per second.

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Image 1: The different flows in an HVAC system

In the HVAC world, that medium is air, water or refrigerant – the energy carriers which ensure that the generated capacity ends up at the end users. Think about an air handling unit, fan coil unit or cabin.

Faces of Flow

Flow is a collective name that we give to a moving medium. In the field it is expressed in different units such as:

  • m³/h (air)
  • kg/s (water)

The essential difference between the two is that in one case we talk about a volume flow (m³/h) and the other about a mass flow (kg/s). If you know the density of the medium, you can easily convert them. Or you can use a converter on your phone.

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Image 2: Laminar and turbulent flow

Flow can be turbulent or laminar depending on the molecules of the medium and how they move in relation to each other. Turbulent flow is caused by resistance in the pipe network. A duct with many angled bends and constrictions provides a turbulent flow, for instance, and is often the cause of noise problems. With laminar flow, the molecules run neatly in layers past each other. Nice straight duct and pipework will give you a laminar flow.

Balancing the system

While diagrams of a chilled water system tend to look quite orderly, the reality onboard often resembles a complex artery system. The pipes in air systems also tend to meander through the ship like a large plate of spaghetti. It is up to the commissioning engineer to balance the system.

image 3 system before and after commissioning.png
Image 3: System before and after commissioning

Flow is generated by a pump or a fan. Naturally, the medium chooses the path of least resistance, leaving some users with far too much flow and others barely negligible. Control valves are used to create resistance in the pipe and evenly balance the flow across the system. Each end-user then receives exactly the amount of medium for which it has been engineered.

Natural flow

This is a phenomenon that we as a commissioner have no control over. Natural flow is nothing more than a draft that occurs on a cold window surface. Since the temperature of the window is colder than the indoor environment, the air cools down near the window. Cold air is heavier than warm air and falls down. This causes a cold air flow in the immediate vicinity of the window, which is perceived as a draft. Because it is a natural air flow and not related to any HVAC system pipe or outlet grill, there is little we can do about this.


Flow is one of the most important parameters in an HVAC system, without which nothing will be heated, cooled or ventilated. Flow is the expression we give to a moving medium like water or air and is given in volumes (m³/h) or mass flow (kg/s). To ensure every consumer has the right amount of flow the system needs to be balanced out. During this process the commissioning engineer also locates and tackles noise problems which are mainly due to turbulent flow.

If you have a particular issue regarding flow or commissioning please contact one of our engineers. For everything HVAC, stay tuned to heinenhopman.com.

Albrand Veldhuizen | Commissioning Engineer

Albrand has been working at Heinen & Hopman since 2006. He has worked himself up to the position of Commissioning Engineer and nowadays he is stationed at one of the largest yacht builders in Germany. During his many years working onboard numerous luxury yachts, he has developed a great expertise in HVAC systems for superyachts.