When magnetic filters took the domestic heating industry by storm, manufacturers were quick to make larger more powerful versions for the commercial market. But how useful are they in commercial systems? Steve Crick, Managing Director of DosaFil, explains the science behind suspended solids and why magnets may not be the most effective solution for their removal.

It is no exaggeration to say that magnetic devices have completely changed the landscape of the domestic HVAC industry. When they were first introduced to the market in the early 2000s, residential boiler installers took to the product like a duck to water. And for good reason; for the first time, installers were able to easily and conveniently remove extremely problematic iron oxide (a consequence of corrosion) from their customers’ heating systems on an ongoing basis. What’s more, they were able to provide customers with proof of this protection by simply showing them the contents of the magnetic device at annual boiler services. A dream scenario for any gas engineer or plumber.

“Some may reason that powerful

magnetic devices provide an added layer of ‘security’ when paired with physical filters, but this an extremely weak (and expensive) argument.”

So, it really is no surprise then that manufactures wanted to repeat the domestic success of magnets in the commercial market. This initiative has proved a successful endeavour as many powerful magnetic devices can now be found sitting on closed heating systems in offices, retail outlets, supermarkets, healthcare facilities and hundreds, if not thousands, of other facilities. There’s just one problem with this scenario; magnet devices really aren’t all that helpful in commercial systems. Not only that, they can also be dangerous.

Before delving into the scientific explanations behind these bold statements, a bit of background information on system fouling is required in order to provide solid context.

In the absence of adequate water treatment, all commercial heating systems will experience fouling of the circulating water with suspended solids. This fouling typically comprises different types of corrosion products (which can form exceptionally quickly, especially where there is oxygen ingress as oxygen accelerates the rate of corrosion), limescale, and bio-fouling. All of these forms of suspended solids can lead to serious problems within commercial heating systems, including poor performance, reduced energy efficiency, component failure, premature repairs and compromised operating life. What’s more, detrimental levels of suspended solids can occur quickly, sometimes within a few months of a system being installed.


Conversely, a correctly applied and maintained water treatment programme will optimise system energy efficiency, operation, heat transfer and life expectancy. It is therefore essential for water quality to be carefully managed and controlled, a process which is best achieved via a combination of solid or liquid chemicals (inhibitors, online cleaning agents, dispersants and biocides, for example) and equipment.

When it comes to equipment, powerful magnetic devices have become popular as a means of removing ferromagnetic material from system water, something they do very well. Contention arises in the fact that much of what comprises suspended solid material in commercial heating systems is not magnetic. Even most types of corrosion products (typically found in heating systems) are not magnetic, such as iron hematite (red iron oxide), copper oxides, and aluminium oxides. The only species of corrosion product (found in heating systems) that is ferromagnetic is iron magnetite (black iron oxide). Furthermore, other types of suspended solids, such as scale, silts, organic materials, microbiological debris and biofilm particles, are not attracted to magnets. In other words, for all non-magnetic suspended solids, powerful magnetic devices are completely redundant.

Consequently, powerful magnetic devices are sometimes paired with a physical filter. This combination of equipment will remove all types of suspended solids, provided the micron size of the filter is appropriate for the system foulants.

What will also remove all types of suspended solids is a physical filter on its own (again, provided the micron size of the filter is appropriate for the system foulants). Put another way, a physical filter will remove iron hematite, iron magnetite, copper oxides, aluminium oxides, scale, silts, organic materials, microbiological debris, and biofilm particles – everything.

So why use a product which can only remove one type of suspended solid (a magnetic device), when another product can be used that will remove all suspended solids? It simply doesn’t make sense. In the presence of a suitable physical filter, magnetic devices are unnecessary. BSRIA guidance agrees: “(magnetic filters) are widely fitted when replacing boilers in domestic heating systems, mainly to protect the new boiler heat exchanger and pump seals from magnetite that has been previously formed in the old boiler and radiators. In principle, magnetic filtration should not be advantageous on a properly maintained system with low suspended solids”.

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