The principles behind a hydronic system

The principles behind a hydronic system

In this module, you will learn about the basic physical theories behind hydronic heating and cooling, and how the high energy density of water makes it very efficient for energy transport and distribution. You will then get an example of a hydronic system in practice that illustrates the many benefits.

Welcome to this module about the basic principles of hydronic systems such as radiant cooling and heating.

The principles behind radiant cooling and heating are the same, but in this module we´ll predominantly discuss radiant cooling.

In any building, there is an ever on-going exchange of heat radiation between the surfaces of all bodies. Heat radiation is emitted from persons, equipment, furniture, floors, ceilings and walls.

The amount of radiation from a body depends on its emissivity, size and the body’s temperature. The higher the emissivity, size and temperature, the higher the radiation.
A large body or surface with a lower temperature than the other surfaces in a room will receive more radiation than it emits. That is what we exploit when a chilled ceiling or floor is introduced into a room.

Here chilled water is circulated in thin pipes and is used to chill the ceiling or floor. The water chilled surface – a ceiling or a floor – absorbs so much of the heat radiation exchanged in the room that the room temperature can be kept under control.

Furthermore, a chilled ceiling also absorbs a large amount of heat by means of convection.
In a chilled ceiling or hydronic cooling system, chilled water is used as the cooling medium instead of air. But why is that an advantage?

First of all, the density of water is 800 times higher than the density of air. Moreover, water’s heat capacity is 4.2 times higher than the heat capacity of air. And that means water can contain significantly more energy than air. In other words, we need much less water than air to transport the same amount of energy.

Consider this comparison: A hot air balloon holds 3500 litres of air. The amount of energy contained in the balloon is the same as the amount of energy that can be contained in one litre of water. It is the combination of water’s density and its unmatched heat capacity that is utilised in hydronic systems.
And because water carries energy in a much denser form, hydronic systems are designed in ways that are much more space-efficient than forced air systems. Take a look at the difference in pipe sizes for hydronic versus forced air systems.

For example, circular air ducts installed in a forced air system need to be approximately 400 to 500 mm to carry 10 kW of cooling capacity. To provide the exact same cooling capacity, the hydronic system would need only 40 mm pipes.

That means that the duct work of a forced air system takes up a lot more space than the pipe work of a hydronic system.
Now, we will look at a practical example of how the principles of hydronic cooling are applied in a tall building and the benefits that follow.

This building is designed with a forced air system and has 100 floors of offices and residential areas. If this building had a hydronic system instead, the building would have had 105 floors in the same space! That is 5 extra floors of rental income for the building owner and much more efficient building overall.

So hydronic systems can reduce the required floor height of a building which allow for more floors in the same building, which again enables increased utilised floor space and rental income.

Hydronic systems do not just save space; they also dramatically improve the efficiency of a building. Let’s look at our skyscraper example from a cost comparison perspective.
First of all: A forced air system uses much more power for the distribution of air than a hydronic system does. As shown here, the needed power for distribution is about 10 times higher with the forced air system compared to the water based hydronic system.
This shows how hydronic systems not only increase the utilised floor space; they also significantly improve the efficiency of a buildings HVAC system.

To recap, hydronic systems have the following characteristics and advantages:
- They utilise the high energy density of water for very efficient energy transport and distribution.
- That results in smaller pipe distribution networks in the building and improved space performance.
- Because of the superior energy density of water, hydronic systems also consume much less energy for energy transportation than forced air systems.
- Hydronic systems with chilled ceilings or floors can remove the building’s sensitive heat loads and this allows for significantly reduced air volumes.
- And last but not least, hydronic systems guarantee an optimal indoor environment for the occupants of the building.