Air can get trapped in your heating system for several reasons, affecting its efficiency and the warmth of your home. We’ll explore why this happens and how often you can resolve it by bleeding your radiators.
Reason 1 - Adding new water to your system
Whenever new water enters your heating system, whether through topping up the system or replacing parts, it can introduce air. Water naturally contains air, which can become trapped in the system.
Why does this happen?
All water naturally contains dissolved gases, including air. When this water enters the system, changes in temperature and pressure can release these gases, forming air pockets. If your boiler had recently been on say half hour ago, your water is still going to be hotter than the new colder water entering from the mains pipes. This causes the fluctuations and can result in additional air being released and trapped in the system.
Services. During maintenance, when parts of the system are opened, not only is new water introduced, but air can directly enter the system. For example, replacing a radiator or repairing pipework involves draining sections of the system and refilling them, which inevitably traps some air.
Reason 2 - Chemical Reactions
The chemical reactions that contribute to air in the system primarily involve corrosion of two kinds:
Oxygen Corrosion: Oxygen dissolved in water can react with metals in the system (like steel in radiators or iron in pipes), leading to corrosion. This reaction not only degrades the material but can also produce gases, including hydrogen, which can accumulate as air pockets.
Let’s get into it a bit more:
Oxygen corrosion occurs when oxygen dissolved in the water reacts with the metal of your radiators or pipes. Here’s a step-by-step breakdown:
- Oxygen Meets Metal: When oxygen dissolved in water comes into contact with the metal in your radiators or pipes, they don’t just ignore each other. Instead, they react. This is because metals like iron or steel in the system have a natural tendency to combine with oxygen - i.e. this is what causes rusting..
- Gas Formation: During this exchange between the oxygen in the water and the metals in your pipes or radiators, not only does rust form, but hydrogen ions in the water can also grab some of those free electrons. When hydrogen ions snag electrons, they turn into hydrogen gas. This doesn’t happen in huge, balloon-filling amounts, but over time, enough tiny bubbles of hydrogen gas can form and get trapped in high points of the system - your radiators!
- Air Pockets: These little bubbles of hydrogen gas join any other air that’s gotten into the system (because air isn’t just oxygen; it’s a mix of gases including nitrogen and, yes, hydrogen too). Together, they form those air pockets that can mess with the efficiency of your heating, leading to cold spots in radiators and the need to bleed the system.
So, in essence, oxygen corrosion in your heating system isn’t just about the metal slowly wearing away. It’s also about this sneaky side effect of creating hydrogen gas, contributing to those air pockets that need to be cleared out for your radiators to heat up properly.
Reason 3 - Heating and Cooling Cycles
The heating and cooling of water in the system affect solubility and pressure, which can lead to air entrapment. This can happen particularly if you turn your boiler on and off frequently (which is reasonable, as you may not need the radiators heating all day (or night)).
Solubility Changes: Gases are more soluble in cold water than hot. As water heats up, dissolved gases become less soluble and can form bubbles. When the system cools, not all of these gases redissolve, leading to accumulated air over time. Over a period of time, this air gets trapped and finds its way into the radiators, leaving cold spots and in-efficient heating.