Cleaner goes in first, gets circulated, then gets flushed out. Inhibitor goes into clean water afterwards and stays there. Doing it the other way round wastes the inhibitor, because the cleaner you add later strips it back out again. For a typical 10 radiator house you need one 500ml bottle of cleaner, a full flush, then one 500ml bottle of concentrated inhibitor. The whole job takes an afternoon, and it is the single cheapest thing you can do to keep a boiler alive.
This guide covers both halves properly: the clean, the flush, and the dose, with the numbers you actually need at each stage.
In this article
- Why does the order matter?
- What is the difference between a cleaner and an inhibitor?
- How much do I need for my system?
- The 7 step guide
- How do you add inhibitor without draining the system?
- FAQ
Why does the order matter?
An inhibitor is a corrosion and scale preventer. It works by coating clean metal surfaces. If your system is already carrying black magnetite sludge, that sludge sits between the inhibitor and the metal it is supposed to protect, so you get a fraction of the benefit for the full price.
Worse, if you dose inhibitor and then decide to clean the system a month later, the cleaner does its job and removes the inhibitor along with the sludge. You have paid twice.
The correct sequence is always the same:
- Clean. Break down the sludge and scale that is already in there.
- Flush. Get the loosened debris and the spent cleaner out of the system.
- Inhibit. Dose clean water so it stays clean.
BS 7593:2019, the code of practice for domestic central heating in the UK, is built on the same logic. It calls for the system to be cleaned, dosed with a corrosion inhibitor, and the inhibitor concentration checked every year. That annual check is the part almost everyone skips.
What is the difference between a cleaner and an inhibitor?
They are opposites, and mixing them up is the most common mistake we see at the trade counter.
| Cleaner | Inhibitor | |
|---|---|---|
| Job | Removes existing sludge, magnetite and scale | Stops new corrosion and scale forming |
| How long it stays in | 1 hour to 7 days, then flushed out | Permanently, topped up after any drain down |
| When to use | Cold spots, noisy boiler, black water, before a new boiler | Every system, always, checked annually |
| Typical products | Fernox F8, Sentinel X800 | Fernox F1, Sentinel X100, LavaTherm Super Concentrate |
If your radiators are cold at the bottom but hot at the top, that is sludge and you need the cleaner. If everything works fine and you just want it to stay that way, you only need the inhibitor.
How much cleaner and inhibitor do I need?
Work off system volume, not guesswork. A rough but reliable rule for a domestic system is about 10 litres per radiator, plus the boiler and pipework. So a 10 radiator house is roughly 100 to 120 litres.
One 500ml bottle of concentrated inhibitor treats around 100 litres, which is broadly 8 to 10 single panel radiators. Sentinel X100 Super Concentrate states 100 litres for 500ml on the bottle. Always read your specific label, because standard and super concentrate strengths differ and the ratio is not the same across brands.
Two practical points. First, a bigger house is not automatically two bottles: measure by radiator count, not floor area. Second, if you are even slightly over the volume, round up. Under dosing gives you partial protection, which is the same as no protection in the places it did not reach.
Super strength cleaner that breaks down sludge, limescale and corrosion debris. One bottle treats a typical domestic system.
Starts breaking down sludge and magnetite within an hour of circulating, so you can clean and flush in a single visit.
The 7 step guide: cleaner then inhibitor
You will need: your cleaner, your inhibitor, a funnel or dosing adaptor, a radiator bleed key, an adjustable spanner, PTFE tape, towels and a shallow tray or bucket.
Step 1: Turn everything off and let it cool
Switch the heating off at the programmer and leave it until the radiators are cold to the touch. Working on a hot system is how people scald themselves. Note your boiler pressure while it is cold, which should read roughly 1.0 to 1.5 bar.
Step 2: Add the cleaner
On a sealed system the easiest route is through a towel rail or an accessible radiator. Close both valves, put a tray under the bleed valve, open the bleed valve to release pressure, then drain off slightly more than the volume of your cleaner bottle. Remove the top blanking plug with a spanner, insert the funnel, and pour the cleaner in slowly. Refit the plug with fresh PTFE on the thread, then reopen both valves.
If you have a magnetic filter fitted, you can often dose straight into the filter canister instead, which is faster and less messy.
Step 3: Circulate it
Repressurise if the gauge has dropped, bleed any air out of the radiator you used, and run the heating at normal temperature. How long depends on the product: Sentinel X800 begins working within an hour, while some cleaners are designed to be left in for up to a week for heavily sludged systems. Check your bottle. For a badly blocked system, longer is better.
Step 4: Flush the system out
This is the step people skip, and skipping it is why their inhibitor underperforms. Turn the heating off, let it cool, then drain the system down completely from the lowest drain cock. Refill with clean water and drain again. Repeat until the water running out is clear rather than grey or black.
If you have a magnetic filter, clean the canister out at this point. It will have caught a lot.
Step 5: Refill with clean water
Fill the system back up, working from the lowest radiator upwards, and bleed each radiator as you go. Bring the pressure back to 1.0 to 1.5 bar cold.
Step 6: Dose the inhibitor
Same method as the cleaner. Isolate a radiator or towel rail, drain off enough to make room, pour the inhibitor in through the funnel, reseal and reopen the valves. Or dose through the magnetic filter if you have one.
Do this while the water is still fresh. Every day you leave a newly filled system undosed is a day it is corroding.
Prevents scale and corrosion in mixed metal systems. Same job as the branded concentrates at a fraction of the price.
The inhibitor most UK installers specify by name. Protects sealed and open vented systems against rust and limescale.
Step 7: Repressurise, bleed and write it down
Top up to 1.0 to 1.5 bar cold, bleed every radiator, and run the heating for an hour so the inhibitor distributes. Then write the date and the product on a label stuck to the boiler or inside the cupboard door. In three years when you cannot remember whether you dosed it, that label is worth more than the bottle was.
One honest caveat: draining and refilling a sealed system is within reach of a competent DIYer, but anything involving the boiler itself belongs to a Gas Safe registered engineer. If you are not confident with the filling loop or the pressure vessel, get someone in.
How do you add inhibitor without draining the system?
You do not need to drain the whole system to top up inhibitor. There are three routes, in order of how easy they are:
- Through a magnetic filter. Isolate the filter, open the canister, pour the inhibitor straight in, reseal and reopen. Five minutes. This is why a filter earns its money twice.
- Through a towel rail or radiator. The method in Step 6 above. You lose maybe a litre of water rather than the whole system.
- Through the feed and expansion tank. Open vented systems only. Pour it straight into the small tank in the loft and let it find its own way down.
What you cannot do is add inhibitor to a sludged system and expect it to fix cold radiators. That needs the clean first.
Frequently asked questions
How often should I add inhibitor to central heating?
Check the concentration annually and top up whenever the system has been drained or partially drained, for example after a radiator swap or a leak repair. Assume you need to redose after any work that lost water. Test strips or a lab test kit will tell you where you stand rather than guessing.
Can you put too much inhibitor in central heating?
Slightly over dosing is not harmful and is far safer than under dosing. Grossly over dosing is wasteful and can leave residue, so stick to roughly one 500ml concentrate per 100 litres. If you have accidentally added two bottles to a small system, it is not an emergency, but do not add more.
Is it worth putting inhibitor in a heating system?
Yes, and the maths is not close. A bottle of inhibitor costs under £20. A replacement heat exchanger runs into the hundreds and a new boiler into the thousands. Most manufacturer warranties also require correct water treatment, so skipping it can invalidate the cover you have already paid for.
How much does it cost to put inhibitor in a central heating system?
DIY, the chemical alone is roughly £8 to £25 depending on brand and bottle size, plus a funnel. A full clean and inhibit with a power flush by a professional typically runs into the hundreds because of the labour and equipment. Doing the dose yourself between professional services is the cheap middle ground.
Do I still need inhibitor if I have a magnetic filter?
Yes. A filter catches magnetite that has already formed. An inhibitor stops it forming in the first place. They are complementary, not alternatives, and a system with both stays cleanest. If you are fitting a filter anyway, dose the inhibitor at the same time.
Can I use cleaner and inhibitor together at the same time?
No. The cleaner will strip out the inhibitor you just paid for. Clean, flush thoroughly, then inhibit. The only exception is a combined product specifically sold to do both, and even then a proper two stage clean gives a better result on a system that is already dirty.
Everything for the job, in stock
Cleaners, inhibitors, biocides and magnetic filters from Fernox, Sentinel and LavaTherm. Trade prices, no account needed.
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Related reading: Sentinel X100 Inhibitor 1 Litre if you prefer a ready mixed dose, magnetic heating filters such as the Fernox TF1 Complete for catching magnetite before it settles, and Fernox F7 Biocide if you are treating a system with bacterial contamination. Product guidance from Fernox and Sentinel.
