“System flush” is a loose term, and people use it to mean different things. Most often, it refers to a mains-pressure flush — using water straight from the mains to push through the system, rather than a dedicated power flushing machine.
What is a mains (system) flush?
A cleaning chemical is usually circulated first using the system’s own pump. Then, using mains water pressure, each section or radiator is flushed through to drain until the water runs clear. It’s commonly used on sealed systems and is referenced in BS 7593 as one of the recognised cleaning methods.
How it compares to a power flush
| Mains / system flush | Power flush | |
|---|---|---|
| Water source | Mains supply | Machine recirculates, then dumps to waste |
| Flow | Limited by mains flow and pressure | High, controlled flow |
| Flow reversal | Limited | Yes, repeatedly |
| Heavy settled sludge | Less effective | More effective |
| Equipment | Minimal | Dedicated machine |
| Best for | Newer or lightly contaminated sealed systems | Moderate to heavy contamination |
When a mains flush is enough
If the system is relatively new, the water is only slightly discoloured and radiators heat evenly, a mains flush with a good chemical cleaner may be all that’s needed — for example, when adding radiators or replacing a boiler on a well-maintained system.
When a power flush is needed
When there’s settled sludge — radiators cold at the bottom, black water, noisy boiler — mains pressure and flow often aren’t enough to lift it. A power flush with radiator-by-radiator flow reversal is designed for that job.
Mains flushing on open-vented systems
Open-vented systems — the older type with a small feed and expansion tank in the loft — aren’t designed for mains pressure. Connecting them directly to the mains for flushing needs care to avoid over-pressurising the system, and the tank must be isolated. On these systems, a power flush (which runs at low pressure) is often the more suitable cleaning method.
What both methods have in common
Whichever method is used, a proper clean should include:
- A cleaning chemical suitable for the system’s metals
- Each radiator or section flushed, not just the main circuit
- Flushing until the water is clear and close to mains water quality
- All cleaner removed before refilling
- Inhibitor added at the correct concentration afterwards
If any of those steps are skipped, the result won’t be as good — whatever the method is called.
Which does your system need?
The honest answer comes from looking at the system and testing the water. If you’re getting a new boiler, the installer should explain which method they’ll use and why. If your system has clear signs of sludge, we can help.
Also compare power flush vs chemical flush, or see our power flushing service.