How to Power Flush a Central Heating System
Cold spots at the bottom of radiators, black water when bleeding, and a noisy boiler are all sludge. A power flush strips magnetite iron oxide from the system, restoring heat output by up to 40% and adding years of boiler life. Here is the trade procedure.
Step-by-step method
1. Kit list
Power-flush pump (Kamco CF90 or Fernox PowerFlow), 20L Fernox F3 cleaner, 500ml F1 inhibitor, magnetic filter (MagnaClean Pro2), plus 2m of 22mm braided hoses and a bucket for waste.
2. Dose the system with cleaner
Add Fernox F3 via a radiator or the filling loop. Run heating at full temp with all TRVs open for 1 hour to circulate the cleaner and soften sludge.
3. Connect the power-flush pump
Isolate the boiler. Connect the pump across the boiler flow and return (or across the pump if easier). Attach the dump hose to a foul drain — not a rainwater downpipe.
4. Flush radiator by radiator
Close all rad valves except one. Reverse-flush that rad for 3–5 minutes each direction while tapping the panel with a rubber mallet to dislodge sludge. Repeat for every radiator.
5. Whole-system rinse
Open all valves and dump-flush until water runs clear (usually 15–30 minutes). Refill fresh, dose with F1 inhibitor, and fit a MagnaClean on the return to catch future sludge.
Frequently asked questions
How much does a power flush cost in the UK?+
£450–£950 for a typical 6–10 radiator house depending on region. DIY hire is £80/day for the pump plus £50 in chemicals.
Is a power flush always necessary?+
No. If a MagnaClean has been fitted from day one and inhibitor topped up every 5 years, a chemical flush alone is usually enough.