Central Heating Inhibitor
The protection that keeps sludge from coming back.
Corrosion inhibitor is the chemical that protects your heating system from the inside. It slows the reaction between water and steel that creates black sludge, the stuff that leaves radiators cold at the bottom and wears boilers out. Here is what central heating inhibitor does, how to tell when a system is short of it, and when a top-up is the right move.
Central heating inhibitor is a liquid added to the system water that slows corrosion and scale, so sludge stops forming and the water stays clean. Every wet heating system should be protected by it, and it needs topping up after any work that drains the system. What it cannot do is remove sludge that has already built up. A dirty system needs cleaning first, then inhibitor keeps it clean.
What central heating inhibitor does
Left untreated, the water in a heating system slowly reacts with the steel inside radiators. The result is iron oxide, the fine black magnetite that engineers call sludge. It darkens the water, settles along the bottom of radiators, collects in pipework and works its way into the pump and the boiler heat exchanger. Inhibitor conditions the water and protects the metal surfaces, slowing that reaction to a crawl so the system stays clean for years rather than months.
Much of London also has hard water, which adds limescale to the picture, and a properly treated system resists that too. The combination of corrosion and scale is what quietly drags heating performance down, so the small job of keeping the inhibitor topped up protects everything downstream of it.
Engineer noteInhibitor leaves with the water. Every drain-down, every radiator removed for decorating and every big top-up after a leak dilutes it. If your system has been opened up in the last year or two and nobody re-dosed it, assume the protection is lower than it should be.
Signs a system is short of inhibitor
You cannot see inhibitor, but you can see what its absence does. The common giveaways are grey or black water when you bleed a radiator, radiators that need bleeding again and again because corrosion is producing gas inside the system, bottoms of radiators slowly going cool as sludge starts to settle, and a system that was cleaned years ago and has never been checked since.
None of these proves the level on its own, which is why we test rather than guess. A small water sample tells us whether the system is protected, lightly corroding or already dirty enough to need cleaning before any dosing is worthwhile.
When a top-up is right, and when it is not
Inhibitor is protection, not a cure. It is the right move when the system is clean and needs keeping that way, and the wrong one when sludge has already taken hold.
A top-up is the right move when
- The water bleeds clear or only lightly tinted.
- The system has just been flushed or chemically cleaned.
- Water was drained for radiator, valve or boiler work.
- The level has simply never been checked.
It is not the fix when
- The water is already black, which means cleaning first.
- Radiators are already cold at the bottom from settled sludge.
- One radiator is cold, often a valve, air or balancing issue.
- The boiler is noisy on dirty water, which needs assessing first.
How we test and dose a system
Test the water
A sample shows corrosion levels and whether inhibitor is present.
Clean first if needed
Dosing dirty water protects the sludge, not the system.
Dose to the system size
The amount is matched to the radiators and water volume.
Record what was used
The product, the date and the readings where tested.
Getting the order right is most of the job. Inhibitor added to a sludged system is money down the drain, so if the water sample comes back dirty we will say so and explain the cleaning options, from a power flush for heavy sludge to a gentler clean for lighter contamination. Once the water is clean, the dose goes in and the protection starts working.
Inhibitor, BS 7593 and your paperwork
BS 7593 is the British Standard covering the treatment of water in domestic heating systems. It expects inhibitor to be added whenever a system is cleaned, filled or refilled, and checked from time to time afterwards. Boiler manufacturers generally expect treated system water too, so it is worth keeping a note of what went in and when. If you would like that in writing, we can provide a simple cleaning and inhibitor record with the work.
What it costs, and what to do next
An inhibitor top-up is a small job compared with the damage corrosion causes, and it is often added as the final step of a flush or clean rather than as a separate visit. The price depends on the size of the system and whether it is standalone or part of other work, and pairing it with a magnetic filter gives a cleaned system the best long-term protection. If a flush turns out to be needed first, you can see what affects the price. Either way, tell us what you have noticed and we will confirm the right move with a clear quote.
Related guides
Central heating inhibitor, answered
How often should central heating inhibitor be checked or topped up?
Can I add central heating inhibitor myself?
Does inhibitor remove sludge that is already there?
What happens if a system has no inhibitor?
Do I still need inhibitor if I have a magnetic filter?
Will inhibitor make my cold radiators warm again?
Not sure if your system is protected?
Send us your symptoms or a photo of the water when you bleed a radiator. We will test, tell you honestly what the system needs and confirm a clear quote for the agreed work.
