How to Power Flush a Central Heating System Yourself
Black sludge (magnetite) reduces radiator output by up to 40%. A proper power flush restores full heat, extends boiler life and cuts gas bills. Here is the trade method — plus honest advice on when it is out of DIY scope.
Step-by-step method
1. Isolate the boiler and dose with cleaner
Add Fernox F3 or Sentinel X400 to the F&E tank or via a radiator. Circulate hot for 1–7 days to break down sludge before flushing.
2. Connect the flushing pump
Hire a Kamco CF90 (£75/day). Connect across the flow and return at the boiler tails. Run water at mains pressure with the pump reversing every 5 minutes.
3. Flush one radiator at a time
Close all TRVs and lockshields except one. Bang the rad gently with a rubber mallet to dislodge sludge. Repeat for every radiator — takes 5–8 minutes each.
4. Add inhibitor and fit a magnetic filter
After flushing runs clear, dose Fernox F1 or Sentinel X100 inhibitor (500ml per 10 rads). Fit a Magnaclean Professional 2 on the return — traps future magnetite.
Frequently asked questions
How much does a professional power flush cost?+
£450–£850 for a typical 3-bed UK home, £900+ for larger systems. DIY costs £150 in hire + chemicals + filter.
Will a power flush damage old pipework?+
Very rarely — but if you have pinhole-corroded copper it can accelerate failure. Chemical-only flush is safer on 40+ year systems.