Provenance of Air in Heating and Cooling Systems
Provenance of Air in Heating and Cooling Systems
Where Does Trapped Air Come From in Heating and Cooling Systems?
Trapped air is one of the biggest problems faced in heating and cooling systems, causing issues such as ineffective heat distribution, gurgling and banging noises, insufficient flow rate and increased corrosion of parts.
This trapped air can come from various sources, such as:
- Air not expelled while the system is being filled
- Air dissolved in the cold water used to refill the system
- Air that gets in while the system is running
We take a look at where trapped air comes from and the systems you can use to help remove this air before it becomes a problem.
Air Not Expelled While the System is Being Filled
Before being put into operation, all hydronic systems are obviously full of air. Inaccurate design or installation of the system, which provides specific paths for the lines, can trap air during the filling phase. In particular, air tends to collect:
- In the top part of the heat emitters (fig. A)
- In pipe sections that must go around an obstacle (fig. B)
- In long horizontal pipe sections that then turn downwards
- In the top part of the risers
Air Dissolved in the Cold Water Used to Refill the System
This air is released only when the water in the system is heated. The quantity of air trapped in heating and cooling systems this way can be significant.
For example:
- A 1,000 L system (roughly 100,000 kcal/h) at a constant pressure of 2 bar and a water temperature of 20°C contains around 35 litres of dissolved air per m3 of water.
- When heating water from 20°C to 80°C, the volume of dissolved air decreases from 35 to 17 litres. This means that 18 litres of air are transformed into bubbles and micro-bubbles.
Air That Gets in While the System is Running
This trapped air can enter through the free area of an open vessel, or filter through the vent systems, gaskets, and fittings if the system operates under vacuum. This situation occurs when the sum of the system's static pressure and the pump's dynamic vacuum is negative. This situation occurs especially in the upper parts of the system where the static pressure is lower.
What Systems Are Used to Remove Air From a Heating and Cooling System?
Several different devices can remove air from a heating and cooling system, depending on whether you need to remove air bubbles, air pockets or micro-bubbles.
In general, three different devices can be used to remove air:
Manual devices to remove air bubbles and pockets
These are the simplest valves for removing air from the top of a radiator. When the valve is opened manually, the air comes out of the small opening at the side. They must always be installed in each terminal to vent the air that collects naturally in the top part.
Automatic devices to remove air bubbles and pockets
These are valves with a float that controls an automatic venting mechanism. As air collects inside the chamber containing the float, it displaces the water and lowers the float, causing the automatic valve to open and vent the air. After a certain volume of air has been expelled, water refills the chamber and pushes the float upward, thereby closing the air vent. The positioning of these devices must follow specific rules to ensure they work efficiently. They must be put at the top of the risers and at all points where air can stagnate. They can also be used on radiators to make it as easy as possible to fill the system.
Air separators to remove micro-bubbles
Correct installation of air separators prevents problems caused by micro-bubbles. By reducing the amount of air in the water, they enable it to absorb and remove the bubbles hiding in the critical zones of the system. These devices have a much larger flow section than an air vent, enabling a net reduction in the liquid's speed, which helps the air bubbles rise to the top.
How Are Micro-Bubbles Separated?
Air in the form of micro-bubbles is much harder to capture than larger air bubbles or pockets. This is why a mesh is combined with the air separators, which, by creating a swirling motion, help release the microbubbles and then merge them into larger bubbles that can be removed through the air vent. This process, called “coalescence,” is extremely important for minimising air in hydronic systems.
As the micro-bubbles unite, they form ever larger bubbles until they reach a sufficient volume for the float forces to be greater than the adhesion forces that hold them to the coalescence surface.
The bubbles then rise along the coalescence surface to the chamber above the main flow of the liquid, where they are collected and expelled using an automatic air vent with a float.
The surface along which the micro-bubbles merge is called the “coalescence media”. Some air separators use an internal media with metal mesh, while others use special polymers. In both cases, the coalescence media must have a large contact surface, making it easy for the bubbles to rise, and must produce low head losses. The concept of coalescence inside an air separator is shown in the image below.
Do Air Separators Work In Water-Glycol Systems?
Air separators can be used in systems with a water-glycol antifreeze mixture, for example, in refrigeration and solar panel systems, heat pumps, anti-freeze panels, and anti-snow ramps. Water-glycol mixtures are very viscous and therefore very effective at trapping air bubbles, including microbubbles, preventing their removal, which is why an air separator is necessary.
Where to Install Air Separators
Air separators must be installed in the part of the system where the solubility of gases in water is lowest. This is why, in heating systems, they should be fitted near the heat generator outlet, while in cooling systems, they should be placed before the chiller inlet.
This content was originally published in IDRONICS Issue 1 in December 2020.
FAQs
Where is the best place to install an air separator?
For maximum effectiveness:
- In heating systems – Install the air separator on the supply line, right after the heat source (e.g the boiler outlet), where the water is hottest and can hold the least dissolved air.
- In cooling systems – Install it on the return line, just before the chiller inlet, where the water is warmest from collecting heat from the building.
My system uses antifreeze (a water-glycol mix). Do I still need to worry about air?
Yes, even more so. Water-glycol mixtures are thicker (more viscous) than plain water, which makes it easier for air bubbles to get trapped and harder for them to rise naturally to a vent. Using an air separator is highly recommended in glycol systems to maintain efficiency and prevent corrosion.
If I have an automatic air vent, do I still need manual vents on my radiators?
Automatic vents are typically installed at the main high points of the system (like the top of risers or near the boiler). Manual vents on radiators are still necessary because air can become trapped in these localised high points that the main system vent cannot reach. They provide a targeted way to remove air from specific units.