Mark Boccetti takes a detailed look at deaeration and the importance of removing air from new and retrofit heating systems
Unlike dirt removal, deaeration is a practice that has not taken hold as widely in the UK domestic heating industry.
The installation of dirt separators on all new boiler installations is pretty much common practice, in order to negate the dangers of dirt damaging heating circuits.
However, the continual removal in air from a heating system is in reality more important than dirt removal, as air will begin to cause damage to a system almost immediately if not removed, whereas dirt is a longer burn and will only start to cause damage once corrosion has taken hold.
How air affects a heating system
One of the most common complaints from homeowners surrounds radiator performance, specifically cold spots and irregular heating patterns, which can continually leave some rooms colder than others. Another common source of irritation is knocking and banging noises, emitted from radiators or pipework.
Manual venting often provides a temporary fix but it will only ever be a short-term remedy for problems that are, in most cases, being caused by air circulating and congregating within a system. The expansion and contraction of air bubbles generates many of the annoying noises householders’ experience, while the collection of air bubbles in a radiator corner or pipe bend leads to the formation of air pockets that will block system flow, reducing radiator performance.
The science behind deaeration
The science behind deaeration is based on Henry’s Law (or Henry’s coefficient), which states that the partial pressure of the gas within a liquid will come to equilibrium to the partial pressure of that same gas external of the liquid; which explains why a fizzy drink will eventually go flat.
If we take one litre of water as an example: at 20oc under atmospheric pressure, one litre of water will contain 35ml of dissolved gases. Through high temperature and low pressure these gases come out of solution, so if you increase the pressure on water, not only do you increase its boiling point but you also increase the amount of gases that it retains or absorbs. When gases are dissolved in water, they hold no volume, but when they come out of solution they do.
As water will always revert back to its natural state, even when deaeration does take place, the system will eventually reabsorb the gases that have been removed.
The gases (Oxygen and Nitrogen) re-enter the central heating circuit via various means: through micro leaks, through connection points (diffusion) and via expansion vessels. Oxygen and Nitrogen can both cause problems within a system, but as Oxygen is naturally contained within water it is that which causes the corrosion. Nitrogen is not detrimental to the circuit from a corrosion perspective, but from a noise perspective it is the chief culprit and is the root of irritating radiator and system noise. Because of the combined problems of the harmful gases, is it important therefore to remove all.
By installing a deaerator, gases can be expelled when they come out of solution, thus dramatically slowing the process of corrosion. In fact, on new systems, with new pipework, new radiators and a new boiler, the corrosion is pretty much halted; although it’s always recommend in this instance to add an inhibitor during commissioning to add further safeguards.
Where a deaerator is fitted onto an existing system, homeowners will still enjoy the same benefits of air removal, which includes quieter systems, less friction and better transference of heat in the main heat exchanger; the latter being key to avoid long-term system damage. When a boiler is fired up and heat is introduced, microbubbles will immediately start to form on the wall of the heat exchanger and can therefore hinder the transfer of heat between the heat exchanger and the system fluid.
The quality of system fluid is arguably the most important component in any heating system. If it is allowed to deteriorate then most of the components that are wet on the circuit will become less efficient and will break down, so it is essential to maintain the quality of the fluid via dirt, and importantly, air removal. As little as 5cm of air within a radiator can reduce the output of that radiator by up to 75%, while independent tests have also proven that as much as 6% can be saved on household energy bills by installing a deaerator, such as Spirotech’sSpiroVentRV2.
Mark Boccetti is technical advisor at Spirotech UK