Domakihub Add Inhibitor to a Central Heating System the Right Way
Stop rust sludge before it wrecks your boiler — a one-hour job that costs under €20.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
Corrosion inhibitor is the single cheapest thing you can do to keep a central heating system running cleanly — yet most homeowners, holiday rental managers, and hotel maintenance teams skip it entirely. The video above walks through the physical process on a real system; this article pairs those visuals with the exact steps, dosing figures, and Greece-specific tips so you can do it yourself without a callout fee.
Why Inhibitor Matters More Than Most People Realise
Steel radiators, copper pipework, and aluminium heat exchangers all sit together in the same water circuit. When oxygen gets into that mix, corrosion starts immediately, producing a dark sludge called magnetite. Left unchecked, the sludge settles at the bottom of radiators (creating cold spots), coats the boiler heat exchanger (cutting efficiency), and clogs the pump impeller until the pump seizes.
A bottle of liquid inhibitor — typically 1 litre for every 100 litres of system volume — neutralises the electrochemical reactions that cause this. Independent boiler manufacturer data consistently shows that a correctly dosed system loses less than 10 % of its heat-transfer efficiency over five years, compared with losses above 30 % in undosed systems of the same age.
Sealed Systems vs. Gravity-Fed Systems in Greek Homes
Most properties built or refurbished after the 1990s use a sealed, pressurised system with a filling loop and an expansion vessel. Older buildings — including many stone houses on the islands and apartments with exposed pipework — may still run an open-vented (gravity-fed) system with a small feed-and-expansion tank in the loft or roof space. The dosing method differs slightly for each, and the steps below call out which approach applies where.
In Greece, hard water in many regions (Athens, Thessaloniki, Crete) accelerates scale build-up on top of corrosion. Choosing a combined inhibitor-and-scale-preventer product addresses both problems in one dose and is worth the small extra cost for any property with a limestone-heavy water supply.
How to Add Inhibitor to Your Central Heating System
- 1
Check system pressure and bleed all radiators first
Before introducing any chemical, confirm the system is at its correct cold-fill pressure (usually 1.0–1.5 bar on sealed systems). Then bleed every radiator from the top floor down to the ground floor. Releasing trapped air now prevents air locks from redistributing the inhibitor unevenly after you dose. Top the pressure back up through the filling loop if it drops below 1 bar after bleeding.
- 2
Calculate your system volume and measure the dose
Rule of thumb: a typical Greek apartment or villa radiator holds roughly 8–12 litres; add 3–5 litres for the boiler and pipework. Count your radiators and multiply to get a rough total. Most inhibitor brands (Fernox F1, Sentinel X100, or local equivalents sold at plumbing merchants in Athens, Thessaloniki, and Heraklion) recommend 1 litre per 100 litres of water, with a minimum of 1 litre regardless of system size. Shake the bottle well before measuring.
- 3
Add inhibitor through the correct entry point
On a sealed system: locate the radiator closest to the boiler return pipe. Turn off both radiator valves, remove the bleed screw fully (keep a cloth ready), and use a dosing kit or a clean turkey baster to pour the inhibitor directly into the bleed hole before replacing the screw. On an open-vented system: simply pour the inhibitor slowly into the feed-and-expansion tank in the loft — gravity will carry it down into the circuit. Do not pour inhibitor into the boiler pressure-relief outlet or the condensate trap.
- 4
Restore pressure, run the system, and check for leaks
Reopen the radiator valves, re-pressurize to 1.2–1.5 bar via the filling loop, then fire the boiler on its highest heat setting. Let the system circulate for at least 30 minutes so the inhibitor disperses fully. Walk the property and check every radiator valve, bleed point, and pipe union for drips — inhibitor can reveal pre-existing weeps that higher pressure masked before. Re-bleed any radiator that develops a cold top during this run.
- 5
Test the inhibitor concentration and record the service date
Reputable inhibitor brands include test strips or sell a separate test kit. Draw a small sample from a bleed point into a white container and dip the strip; the colour change should confirm a full protective dose. Stick a dated label on the boiler or in your property maintenance log — inhibitor should be retested annually and topped up every 2–3 years, or immediately after any work that required draining the system. This record is also useful evidence for boiler warranty claims.
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FAQ
- Can I add inhibitor without draining the system?
- Yes — and that is exactly how it is normally done. You add the inhibitor into a live, water-filled system through a radiator bleed point (sealed system) or the feed tank (open-vented system). You only need to drain the system if you are adding inhibitor after a power flush, in which case you refill with fresh water plus a fresh inhibitor dose.
- How often does inhibitor need to be topped up?
- Test the concentration once a year using the manufacturer's test strips. In practice, a correctly sealed system loses very little inhibitor over time, so a top-up every 2–3 years is typical. However, any time the system is drained for repairs — even partially — you must add a fresh dose before refilling, because draining removes the chemical along with the water.
- Is one brand of inhibitor better than another for Greek water conditions?
- The major brands (Fernox F1, Sentinel X100, Adey MC1) are all effective when dosed correctly. For areas with very hard water — much of Attica, the Aegean islands, and parts of Crete — choose a product labelled as a combined inhibitor and scale preventer, or add a separate scale reducer to the dose. Check that the product is compatible with aluminium radiators if your system has them, as some older formulations can react with aluminium alloys.