The Impact of Trapped Air in Heating and Cooling Systems
The Impact of Trapped Air in Heating and Cooling Systems
The Problem With Trapped Air in Heating and Cooling Systems
Trapped air is one of the most common and disruptive problems in hydronic systems. It can lead to noise, inefficiency and even long-term damage to your heating and cooling system. In this guide, we explain why air gets trapped, the problems it causes and how you can fix it.
What Causes Trapped Air in a Hydronic System?
Air can enter a system during initial filling, through leaks in pumps or valves, or from hydrogen gas released by corrosion inside the pipes. This air naturally rises and collects at the highest points, such as the tops of radiators or within pipe loops, creating airlocks.
Key Problems Caused by Trapped Air
Trapped air in heating and cooling systems can cause several problems, including:
Gurgling Noises and Banging Pipes
- Cause – Air bubbles moving through pipes and components create a gurgling sound, especially at startup. Sudden pressure changes can also cause cavitation, a phenomenon in which vapour bubbles form and implode violently, producing a hammering or rattling noise.
- Impact – Annoying sounds that disrupt comfort in homes and buildings.
Cold Radiators and Insufficient Heating/Cooling
- Cause – Air gathers at the top of radiators or fan coil units. Since air is a poor conductor of heat compared to water, it impedes heat transfer.
- Impact – You get cold spots on radiators, uneven room temperatures and a system that struggles to reach the set temperature, leading to discomfort.
Reduced Flow and System Blockages
- Cause – Large pockets of air can completely block the flow of water in a pipe, a situation known as an airlock. This is a severe problem in systems like underfloor heating, where pipes are long and laid in loops.
- Impact – Some parts of the system receive no hot or cold water, leaving entire rooms without heating or cooling.
Increased Energy Bills and System Strain
- Cause – To compensate for the reduced efficiency caused by air, the boiler or chiller and the circulation pumps must work harder and run for longer periods.
- Impact – You will see a noticeable increase in your energy consumption and utility costs.
Internal Corrosion and Damage
- Cause – The oxygen in trapped air reacts with ferrous metals (like steel) in the system, causing two types of corrosion:
- General Corrosion – Forms magnetite sludge that can clog narrow passages in valves and pumps.
- Localised Pitting – Creates small, deep holes in pipes and components, leading to leaks and premature failure.
Pump and Valve Cavitation Damage
- Cause – Air and vapour bubbles can cause cavitation. When these bubbles implode near metal surfaces, like a pump's impeller, they create tiny, high-pressure jets that erode the metal over time.
- Impact – Reduced pump performance, loud noise and costly pump or valve replacement.
How to Fix and Prevent Trapped Air
Trapped air in heating and cooling systems can be removed using a few different methods and devices, depending on the size of the air bubbles dissolved in the water.
Manual Venting
- What it is – Using a radiator key to open a small manual valve (the bleed valve) at the top of a radiator to release trapped air.
- When to do it – When you hear gurgling sounds or a radiator has cold spots at the top.
Automatic Air Vents
- What it is – Mechanical valves installed at high points in the system that automatically open to release air as it accumulates.
- Why it's important – This is the most effective long-term solution for preventing air-related problems in the entire piping network.
Air Separators
- What it is – An air separator is a mechanical device installed directly into the main piping of a hydronic heating or cooling system. Its primary function is to efficiently capture and remove micro-bubbles in the circulating water. Unlike a simple air vent at a high point, it uses a mesh screen to actively separate air from the water stream. Once separated, the air is vented from the system through a built-in automatic air vent.
- Why it's important – While manual and automatic air vents address air that has already accumulated at high points, an air separator is a proactive and systemic solution that tackles hard to remove micro-bubbles.
FAQs
What are the main problems caused by trapped air in my heating or cooling system?
Trapped air can cause several serious issues, including:
- Noisy pipes – Gurgling or banging sounds from air bubbles and cavitation.
- Reduced efficiency – Insufficient heat exchange in radiators or fan coils, leading to cold spots and higher energy bills.
- Flow problems – Blocked or reduced water flow, which is especially damaging to systems like underfloor heating.
- Corrosion – Oxygen in the air bubbles reacts with metal components, causing general corrosion (sludge) and localised pitting, which can damage the system.
- Pump and valve damage – Cavitation can occur, where vapour bubbles implode, damaging pump impellers and valves.
What is the difference between manual and automatic air vents?
- Manual air vents – These are simple valves (often found on radiators) that must be opened by hand to release large air pockets. They are essential for venting individual terminals.
- Automatic air vents – These devices use a float mechanism to release accumulated air without manual intervention. They are typically installed at the highest points of the system and in areas where air tends to collect.
What is cavitation and why is it a problem?
Cavitation is the formation and implosion of vapour bubbles inside a liquid. It happens when water pressure drops suddenly (e.g in a fast-spinning pump impeller), causing the water to vaporise and form bubbles. When these bubbles move to a higher-pressure area, they collapse implosively, creating noise and eroding metal surfaces, which can lead to pump and valve failure.
Are air separators necessary in systems that use antifreeze (water-glycol)?
Yes, they are highly recommended. Water-glycol mixtures are thicker than plain water, which makes it even harder for air bubbles to escape. An air separator is crucial in these systems to prevent air from being trapped and causing issues.