A system that has operated without problems for several seasons can start making noises in the pipes and leaving radiators cold after the summer shutdown. The boiler is running, the pump is operating, but the heat is not reaching where it should. In most such cases, the problem is not any of the equipment, but what has been accumulating in the pipes over the years.
How to tell whether the system needs flushing
The first sign can be seen in the radiator and identified simply by touching it. A radiator that is cold at the bottom but warm at the top indicates sediment — sludge settles by gravity and blocks the lower part of the channels. This is the opposite of classic airlock, in which the upper part is cold because air collects beneath the top plug. This single difference helps distinguish between two problems that require completely different approaches. If the symptom indicates air, start by bleeding the C.O. pump and the system, because flushing will not change anything here.
Symptoms of a sludge-filled system to watch out for:
- individual radiators do not heat up despite the thermostatic valves being fully open
- the circulation pump runs louder than in the previous season; you can hear humming and knocking in the pipes
- the water drained from the system is dark and cloudy, sometimes with noticeable sediment
- when bleeding the heat emitters, you can smell rot — this is hydrogen sulfide
- the system needs frequent topping up even though no leaks are visible
- the boiler reaches the set temperature quickly, but the rooms heat up slowly
In our experience, two or three of these symptoms most often occur at the same time. A single cold radiator is usually a local problem, whereas several heat emitters performing poorly at the same time already indicate a problem affecting the entire system.
Consequences of a contaminated system
Deposits in the system are not merely a matter of thermal comfort. Narrowed pipes and blocked radiator channels increase flow resistance, so the circulation pump has to overcome a greater pressure loss than the system was designed for. It works harder, consumes more energy, and magnetite particles go straight into the impeller. This is a common cause of pump seizure when the system is first started after the summer shutdown.
The second component at risk of damage is the heat exchanger. Deposits on its walls reduce heat transfer, so the boiler runs longer to reach the set temperature. With modern condensing boilers, where the exchanger channels are narrow, contaminated water from an unflushed system can lead to a failure during the first season after the heat source is replaced. That is why boiler manufacturers are increasingly making the warranty conditional on documented system flushing.
Sludge, magnetite, and hydrogen sulfide—where they come from
Contaminants in a central heating system are not uniform, and they cannot all be removed in the same way.
Magnetite and sludge are products of steel corrosion. Water in a closed system contains dissolved oxygen, which reacts with the surfaces of pipes and radiators. This produces a black, fine-grained sediment with magnetic properties—it circulates with the water, settles in areas with low flow velocity, and is drawn in by the pump. This is the predominant problem in steel systems, especially older ones and those frequently topped up with mains water.
Hydrogen sulfide and biofilm occur where water stagnates. Sulfate-reducing bacteria develop in systems shut down for the summer or operating at low temperatures. The result is a characteristic odor when bleeding the system, accelerated corrosion, and gas accumulating at the highest points of the system.
Limescale is a separate issue caused by water hardness, not corrosion. Limescale deposits require different treatment than flushing with an alkaline agent, and if their presence is suspected, it is worth consulting our technical department about the choice of method.
System flushing methods
Flushing with clean water
The simplest method is to fill and drain the water through drain points until the discharged water is no longer discolored. It removes loose sediment and particles suspended in the water, but will not remove deposits adhering to the walls. In a system that has not been flushed for a dozen or more years, water alone may not provide a lasting result, and the symptoms may return within just a few weeks.
Chemical flushing
The cleaning agent circulates through the system with all valves open, dissolving and breaking up the sediment so it can be flushed out. This method is effective against sludge and corrosion products, but requires following the procedure: using the specified concentration, allowing the minimum contact time, and thoroughly flushing the system afterward.
Dynamic flushing with a flushing pump
The highest effectiveness is achieved by forcing flow with external equipment. A flushing pump connected to the system pumps the solution at a much higher flow rate than the circulation pump and—more importantly—allows the flow direction to be reversed. Reversing the flow pushes sediment out of places that normal circulation merely bypasses, because in dead zones and return lines, water always flows along the same path. This is the only method that allows the system to be cleaned without dismantling it.
IBO CLEANER IC-1 — when and how to use it
IBO CLEANER IC-1 is an alkaline product designed to restore water circulation in new and old central heating systems. It removes sludge and accumulated hydrogen sulfide, which causes corrosion and the odor detected when bleeding radiators. It does not corrode steel and is safe for the installation.
A feature of major practical importance: IC-1 also works in cold systems with no access to hot water — that is, installations that have already stopped operating and cannot be heated before cleaning. Heating the system, if possible, speeds up the process but is not necessary for effective operation.
Usage instructions:
- Dosage 1:100 — one liter of product per one hundred liters of system capacity
- One liter is sufficient for a typical domestic installation with up to ten radiators; larger systems require another bottle
- The minimum circulation time is one hour with all valves open
- A higher concentration does not adversely affect the installation
- The product must not be left in the system — after cleaning, the system must be drained and thoroughly flushed
- Neutralization is not required
IC-1 is a product for professional use. It contains disodium metasilicate and ethylenediamine compounds, is irritating to the skin, and causes serious eye irritation. Protective gloves and eye and face protection are mandatory during use.
Flushing the installation step by step
The sequence below follows the cleaning procedure compliant with the BS 7593:2019 standard, referenced in the product documentation.
- Cool the system and release the pressure. Turn off the heat source and wait for it to cool. Flushing pumps operate at up to 35 °C and 2 bar — flushing a hot, pressurized installation is impossible.
- Remove or bypass sensitive components. Check that the circuit does not contain components whose manufacturer does not permit contact with cleaning agents.
- Add the product. In a closed system: if the installation is empty, add the product at any convenient point before filling; if it is filled, release the pressure and remove some water, then dose through any access point. In an open system: dose through the header tank, first draining enough water so that the entire product enters the circulating part of the system.
- Open all radiator valves. Any section isolated during flushing will remain uncleaned and, once the system is restarted, will release sediment back into the system.
- Ensure circulation for at least one hour. In a system that is not operating—with a seized circulation pump or blocked circuit—circulation is forced by an externally connected flushing pump. When the circulation pump is working, using a separate flushing device provides a higher flow rate than the heating circuit alone and allows its direction to be reversed.
- Use the reversing function. Changing the flow direction during flushing increases effectiveness in the most contaminated sections.
- Drain and flush the system. Water must pass through all consumers and drain points until it runs clear.
- Fill the system with fresh water and bleed it.
- Protect the system with a corrosion inhibitor. This is the final stage of the procedure, without which the entire process has limited durability.
Flushing pumps—selection based on system size
Our flushing pumps are vertically oriented devices equipped with their own tank, shut-off valves, and a reversing function. They remove deposits of calcium and silica, iron and manganese oxides, iron bacteria and sulfate-reducing bacteria, as well as organic substances. They are designed for use with non-acidic chemical agents.
| Parameter | FLUSH 20 | FLUSH 20 PRO | FLUSH 40 |
|---|---|---|---|
| Maximum flow rate | 40 l/min | 40 l/min | 70 l/min |
| Tank capacity | 17 l | 20 l | 40 l |
| Motor power | 150 W | 150 W | 370 W |
| Maximum operating pressure | 1.2 bar | 1.2 bar | 2 bar |
| Maximum liquid temperature | 35 (50) °C | 35 (50) °C | 35 (50) °C |
| Protection rating | IP54 | IP54 | IP54 |
FLUSH 20 and FLUSH 20 PRO are designed for descaling and cleaning heat exchangers and small water systems. FLUSH 40 is designed for cleaning entire non-commercial heating and water systems—it has a larger tank and significantly higher capacity. Detailed equipment for each version can be found on the product pages.
When choosing a unit, consider the capacity of the system. The pump tank should allow the solution to be prepared in a single cycle, while its capacity should provide a flow velocity higher than that of the circulation pump. In a typical single-family home with a radiator system, the FLUSH 40 is suitable, while version 20 is sufficient for servicing individual heat exchangers and boilers.
Flushing underfloor heating
Underfloor heating loops are not flushed in the same way as radiator systems. The loops are long and have a small cross-section, while the manifold allows you to control them individually—and that is exactly what you should use. Flushing is carried out loop by loop, closing the remaining circuits at the manifold so that the entire flow goes through one loop. Attempting to flush all circuits at once results in the flow velocity in each being too low to carry away sediment.
Underfloor heating operates at low supply temperatures, which encourages biofilm growth — a problem that occurs less often in radiator systems. It is also worth remembering that plastic pipes allow small amounts of oxygen to pass through if they do not have an oxygen-diffusion barrier, accelerating the formation of deposits throughout the system, including its steel components.
What to do after flushing the system? - Corrosion inhibitor
Flushing removes the effect, not the cause. Fresh water enters the clean system, bringing dissolved oxygen with it — and the corrosion process starts again. Without protection, the system becomes sludge-filled again, and the service work has limited durability. That is why adding an inhibitor is the recommended final step of the procedure.
IBO PROTECTOR IP-1 is a universal product that protects against corrosion and limescale deposits in all types of water-based central heating systems. Unlike cleaning agents, it remains permanently in the system.
- It protects all metals in the system — steel, copper, brass, and aluminum
- It has a balanced, buffered pH that prevents acid accumulation and corrosion
- It has an NSF CIAS certificate, often required by boiler manufacturers
- It limits gas accumulation in the system, allowing it to operate more quietly
- Dosage 1:100, one liter for a system with up to ten radiators
- Must be protected against frost
Mechanical protection complements chemical protection. A magnetic separator continuously traps magnetite before it reaches the pump rotor or heat exchanger, significantly extending the interval between successive flushes. You can find solutions of this type in the magnetic separators and iron removers category.
How often to flush the system
The frequency depends on the system materials, water quality, and operating conditions. Rather than following a calendar, it is worth remembering the factors that may shorten the interval between flushes:
- a steel system, especially an older one with sectional radiators
- hard water in the mains supply
- an open system in which the water is in constant contact with air
- frequent topping up of the system with mains water
- no corrosion inhibitor in the system
- summer shutdowns with completely cooled, stagnant water
Regardless of these conditions, two constant rules apply. Always flush the system when replacing the boiler — a new heat exchanger connected to a contaminated system collects all the sediment that has circulated through it for years. And always when replacing the circulation pump, because the same suspension that seized the previous one will also seize the new one.
Frequently asked questions
How can you tell that the central heating system needs flushing?
The most common symptom is a radiator that is cold at the bottom but warm at the top. Other symptoms include noise and knocking in the pipes, louder operation of the circulation pump, dark water when draining, and a rotten smell when bleeding the radiator. Several symptoms occurring simultaneously indicate the condition of the entire system, not a single heat emitter.
Why is the radiator cold at the bottom but warm at the top?
This is a typical symptom of sludge buildup. Sediment settles by gravity and blocks the lower part of the radiator channels, so water flows only through the upper part. If the upper part of the radiator is cold, the cause is air, not sediment—in that case, bleeding the radiator helps, not flushing.
Does system flushing have to be performed by an installer?
Chemical flushing is performed by an installer. Cleaning products are intended for professional use, require the correct concentration and contact time, and must be thoroughly rinsed out; effective cleaning also requires a flushing pump. Users can identify the symptoms themselves and check whether the problem is simply trapped air.
What is the difference between chemical flushing and water flushing?
Flushing with water removes only loose sediment and suspended particles. Chemical flushing dissolves and breaks up contaminants adhering to the walls of pipes and radiators, which water alone cannot remove. In systems that have not been cleaned for years, flushing with water alone produces an apparent and short-lived improvement.
How can you protect the system from becoming sludged again?
Fresh water containing dissolved oxygen enters the flushed system, increasing the likelihood of corrosion recurring. The recommended way to finish the procedure is to add a corrosion inhibitor, which remains permanently in the system and protects steel, copper, brass, and aluminum.
Is underfloor heating flushed in the same way as radiators?
No. Underfloor heating is flushed loop by loop, closing the other circuits at the manifold. Only then does the flushed loop achieve sufficient flow velocity to carry away the sediment. Flushing all circuits simultaneously is ineffective.
When is system flushing mandatory?
With every boiler replacement and every circulation pump replacement. A new heat exchanger connected to a contaminated system collects the sediment that has circulated through it for years, while the suspension that caused the previous pump to seize will also damage the new one. Some boiler manufacturers make warranty validity dependent on this.
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