TradeCalculator.co.uk
Power Flush Calculator — Chemical Dose & Duration
Calculate chemical dose, flush duration, water usage and inhibitor re-dose for power flushing heating systems. Based on system volume and condition.
Total radiators to flush
Age of heating system
1L cleaner per X litres of system water. Sentinel X400 = 100 (1%); Fernox Cleaner F3 = 200 (0.5%). Always follow the product label.
1L inhibitor per X litres of system water. Sentinel X100 = 100 (1%).
Next up: related pages
How We Calculate This
This calculator estimates the chemical requirements, duration, and water usage for power flushing a central heating system. It helps plan the job and quote accurately for materials.
Calculation method
- System volume: Estimated from radiator count (rule of thumb ~10 L per radiator plus boiler and pipework)
- Chemical dose: System volume ÷ dose ratio. Default 1:100 (≈1% — Sentinel X400, one litre per ~100 L). Fernox Cleaner F3 is 0.5% (1:200). Always follow the product label
- Duration: Number of radiators × 30 minutes (a practitioner rule of thumb, not a standard-set time)
- Mains water usage: System volume × number of dump/refill cycles. The machine’s 80-150 L/min recirculation rate is not mains consumption
- Inhibitor: System volume ÷ 100 = litres of inhibitor (Sentinel X100 dosed at 1% of system volume)
The number of dump cycles and the 30-minutes-per-radiator timing are practical trade rules of thumb, not figures set by any standard. The estimated sludge mass is a rough indicative guide only and is not derived from any published standard. BS 7593:2019+A1:2024 itself does not prescribe a cycle count or per-radiator timing — it requires a chemical clean, a fresh-water flush until the run-off is clear, fitting a permanent in-line filter, and adding inhibitor at the final fill (re-dosed every five years, with an annual on-site water test).
Always follow BS 7593:2019+A1:2024 guidelines for system cleaning. Record all chemicals used and their concentrations on the system benchmark or handover documentation.
Frequently Asked Questions
Cleaner is dosed at roughly 1% of system volume — about 1 litre per 100 litres. A typical 10-radiator system holds about 100 litres, so 1 litre of cleaner is usually enough (Sentinel X400: one litre treats a system of up to 10 radiators; larger systems use twice as much). Fernox Cleaner F3 is dosed at 0.5% — 10 litres treats up to 2,000 litres, i.e. 1 litre per 200 litres. Heavily contaminated systems may need a higher concentration — always follow the product label. Use our Central Heating Volume Calculator to work out your system volume.
Allow approximately 30 minutes per radiator for thorough flushing. A 10-radiator system typically takes 5-6 hours. Heavily contaminated systems may take longer. The flush is complete when dump water runs clear from each radiator.
Most boiler manufacturers require the existing system to be cleaned and flushed before connecting a new boiler. This is typically a condition of the boiler warranty. BS 7593:2019+A1:2024 also requires system cleanliness for new boiler installations. At minimum, a thorough flush and inhibitor treatment is required.
A power flush machine recirculates the same water internally at about 80-150 litres per minute — that high flow rate is not mains consumption. Fresh mains water is only drawn when dirty water is dumped and the system is refilled. Each dump-and-refill cycle uses roughly the system volume, so mains usage is approximately system volume × number of dump cycles. A small, lightly-contaminated system can therefore be only a few hundred litres, while a large or heavily-fouled system needing extra dump cycles can run to a couple of thousand litres. This calculator estimates mains water as system volume × dump cycles.
After flushing, add a quality corrosion inhibitor at the correct dosing rate (typically 1 litre per 100 litres of system volume). Popular UK brands include Sentinel X100 and Fernox F1 Protector. Record the inhibitor type and concentration on the benchmark certificate. Use our Inhibitor Dosing Calculator for the exact quantity, and fit a magnetic filter to keep the system clean.
Was this calculator helpful?
Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.