Why heating water is cleaned and protected, and the steps that keep a system healthy.
The water in a sealed heating system circulates for years, so keeping it clean and treated protects the boiler, the pump and the radiators. Use this as a revision checklist of why and how, then follow the product and manufacturer instructions.
- Understand the enemy. Inside an untreated system, oxygen and dissimilar metals drive corrosion, which produces a black iron-oxide sludge that collects in radiators and the boiler and harms efficiency.
- Flush before dosing. Clean the system out first, because adding inhibitor to a dirty system just protects the dirt. New systems are flushed of installation debris; older systems may need a more thorough cleanse.
- Add inhibitor. Dose the system with a corrosion inhibitor at the correct concentration so the protective chemistry can work throughout the system.
- Fit and maintain a filter. An in-line magnetic and debris filter captures circulating sludge and metal particles, protecting the boiler and making future maintenance easier; it needs periodic cleaning.
- Check the inhibitor over time. Inhibitor depletes, so it is checked and topped up during servicing and after any work that drained the system.
- Mind the make-up water. Repeated topping up introduces fresh oxygen, so persistent pressure loss should be investigated rather than just refilled.
Clean water, the right inhibitor and a filter keep a heating system efficient and long-lived, which also supports the efficiency expectations in current guidance. Confirm products, doses and procedures against the manufacturer instructions.
What the corrosion actually does
Corrosion inside a heating system does not only make sludge: it also produces hydrogen gas, which collects at radiator high points in the same way trapped air does. A radiator that keeps needing to be bled is often releasing this gas rather than air let in by a leak, and it is a sign the system's protection needs attention rather than just venting again.
The black magnetite that corrosion produces is heavier than water and sinks, settling in the bottom of a radiator, so a radiator that is warm at the top but stays cold and murky lower down points to sludge rather than trapped air. The harm it does is mechanical rather than chemical: gritty deposits wear pump bearings, stick valve mechanisms and choke narrow waterways, rather than eating into the metal the way an acid would.
A general-purpose heating inhibitor is formulated to guard against limescale as well as corrosion, which matters in hard-water areas where scale would otherwise build up in the boiler and pipework alongside any sludge.
On an open-vented system, a pump that is positioned or sized wrongly relative to the open vent and cold feed can pull air in or push water out at those connections instead of keeping the system under a small positive pressure. That is another route for fresh oxygen to reach the water, with the same corroding effect as frequent topping up.