If you’ve ever seen the water drained during a power flush, you’ll know it can be startlingly black. Here’s what’s actually in it.
Magnetite (black iron oxide)
The main component of central heating sludge. Magnetite forms when steel and iron components corrode in water. The particles are very fine, black and magnetic. They settle in low-flow areas and build up into thick sludge. It’s also what a magnetic filter captures.
Red iron oxide (rust)
Forms when more oxygen is present — typically in open-vented systems, or systems topped up often. It turns water orange-brown and adds to sludge.
Limescale
London’s water is hard. Every time fresh water enters the system, minerals come in with it. Heated, they form scale on the hottest surfaces, especially inside the boiler’s heat exchanger, where they cause kettling and reduced efficiency.
Installation debris
Flux residue from soldering, swarf, fragments of pipe and PTFE tape — left behind during installation or alterations. Even new systems can contain it if they weren’t flushed at commissioning.
Microbial growth
In some systems, particularly open-vented ones, bacteria can grow, producing slime and gas. Cleaning chemicals and flushing help remove it; inhibitors with biocide may be recommended afterwards.
What a power flush doesn’t fix
- Pinholes or thinned metal in corroded radiators — see radiators and power flushing
- Worn pumps, valves or boiler components
- Poor system design or undersized radiators
- Balancing problems
assets/img/photos/power-flush-before-after-water.webpHow do you know it’s all been removed?
Looking at the water is a start — it should run clear. But clear-looking water can still contain dissolved contaminants and residual cleaner. That’s why engineers compare the system water with the incoming mains water using a conductivity (TDS) meter. When the two readings are close, the system has been flushed through thoroughly. Checking that every radiator heats evenly from top to bottom afterwards confirms the blocked areas have been cleared.
Where the debris goes
Everything removed is flushed to a foul drain with the dirty water. Heavy sludge can also collect in the machine’s tank or filter, and is disposed of responsibly. It shouldn’t be run into a surface water drain, which leads to rivers and streams.
Stopping it coming back
Once the debris is out, two things keep the system clean: the right concentration of inhibitor to slow new corrosion, and a magnetic filter to capture the magnetite that does form. Both should be checked at the annual boiler service.
See how debris is removed in how a power flush works, or book a power flush in London.