Gases In The System

Dissolved Gases

What they are:
Dissolved gases are gases (mainly oxygen and nitrogen) that are mixed into the water in molecular form. This happens when the water in the heating system is initially filled or topped up from the mains supply, where gases are already dissolved due to water treatment processes and atmospheric pressure.

How they form and enter the system:

• Mains water naturally contains dissolved gases from exposure to air.

• When the system is filled or refilled, these gases enter with the water.

• Chemical reactions (such as corrosion) within the system also release more gases into the water.

Why they’re a problem:
When the heating system is turned on and water is heated, these dissolved gases come out of solution (just like bubbles forming when a fizzy drink is warmed). They then become entrained or free gases, contributing to corrosion, insulating the metal surfaces, and reducing heat transfer efficiency.

Entrained Gases

What they are:
Entrained gases are microscopic gas bubbles suspended or trapped within the circulating water. Unlike free gas pockets, they remain dispersed in the fluid rather than accumulating in one place.

How they form:

• Dissolved gases coming out of solution due to heating form entrained bubbles.

• Water turbulence, flow disruptions, or poor system design can mix air into the system.

• Ongoing corrosion reactions generate gas which becomes entrained in the circulating water.

Why they matter:
Although small, entrained bubbles have a massive impact over time:

• They act as thermal insulators, reducing how efficiently heat transfers from water to radiators.

• They circulate continuously, feeding corrosion throughout the system.

• Over time, they coalesce into larger gas pockets (free gases).

Free Gases (Gas Pockets)

What they are:
Free gases refer to large, visible gas pockets that accumulate in high points of the system – often in radiators, pipes, or heat exchangers. These are the most obvious and disruptive form of trapped gas.

How they form:

• Entrained gases naturally rise to the top of pipework or radiators due to buoyancy.

• Over time, they collect in poorly vented or uneven parts of the system.

• Mechanical failures (like leaking seals or vacuum pulls during cooling) can also introduce air.

Why they stay trapped:

• Wet heating systems are closed loops, so once gases enter, they have no easy way to escape.

• Traditional bleed valves only remove visible gas pockets, not the dissolved or entrained gas.

• Poor circulation, lack of degassing technology, and absence of automated removal systems allow gases to persist.

Why they’re dangerous:

• Free gases block water flow and reduce radiator effectiveness.

• They cause cold spots, gurgling noises, and uneven heating.

• Worse, they foster internal corrosion and sludge formation (magnetite), leading to blockages and premature system failure.