Why install a filter on top of using a chemical cleaner and inhibitor?
The best way to explain it is to compare it to car care. You would still put an oil filter on a brand new car; you wouldn’t wait for the engine to get a bit grubby before deciding to fit a filter onto it. That’s the same theory behind installing a filter on a hydronic heating system, even if you’re using chemicals.
In a heating system that is old and has been installed for quite some time, even if you clean it and use an inhibitor, you can never guarantee it would be 100-percent-cleaned. Modern components in heating systems are actually very susceptible to debris. Modern condensing technology is marked by incredibly fine flows, and they can be blocked with very small amounts of debris. So best practice is always to use a filter, even if you have used a cleaner and an inhibitor.
It’s also important to keep in mind that an inhibitor is not a preventer. It slows down the rate of corrosion, but does not necessarily stop all of it, and the inhibitor can get used up in that process. This is why, in the United Kingdom, commercial systems are very strongly regulated to make sure you always have the right amount of chemical in the system. These systems must be checked four times a year from multiple system checkpoints, because inhibitor degrades.
On a residential system, it’s completely impractical to expect to get into a household more than once annually to check on what’s happening inside that system. So we recommend instead is to:
1) Check the heating system water annually
2) Every fifth year top off the inhibitor in the system, compensating for any degradation