Sludge in your heating system: symptoms, cleaning and dirt separators

In short: heating sludge is mostly magnetite, a black iron oxide produced by corrosion of the steel in radiators and pipes. It settles wherever the water moves slowly: the bottom of radiators, underfloor heating loops, the heat exchanger of the boiler or heat pump, the pump rotor. The signs: radiator hot at the top but lukewarm at the bottom, brown or black water when bleeding, a noisy pump, a boiler going into lockout. The fix comes in three stages: clean the circuit, fit a magnetic dirt separator on the return, then fill with compliant water so the corrosion stops.

Where does sludge come from?

A heating circuit is sealed: in theory, the same water circulates for years. In practice, oxygen gets in regularly: with every top-up, through a failing expansion vessel that forces you to add water, through automatic air vents when the system drops into negative pressure, or through older underfloor heating pipes that are not oxygen-tight. That oxygen attacks the steel: it forms magnetite (a very fine black powder) and rust. Add limescale if the system was filled with mains water, and installation debris (swarf, jointing paste, soldering flux) on a new system.

These particles travel with the water and settle wherever the flow is low. Magnetite, as its name suggests, is also attracted by magnets: it clings to the rotor of permanent-magnet pumps, which means every modern high-efficiency circulation pump, and eventually seizes them.

The tell-tale signs

The first case is the most telling. Put your hand on it: a sludged radiator is hot over the top two-thirds and cold at the bottom, even once bled. A radiator with air in it is exactly the opposite: see Bleeding a radiator: when and how to do it.

Step 1: look at the water

A clear glass is all you need for a first diagnosis. With the system cold, draw a little water from the boiler drain valve or a radiator bleed screw:

• Clear, slightly yellow: the water is healthy.
• Cloudy, brown: corrosion under way, a clean is needed.
• Black, with sediment at the bottom of the glass: heavily sludged system. Hold a magnet against the side of the glass: if the sediment follows it, it is magnetite.

To go further, measure pH and conductivity with a conductivity meter or a 3-in-1 heating water meter. In Switzerland, guideline SICC BT 102-01 requires demineralised heating water with a conductivity below 100 µS/cm and a pH between 8.2 and 10 (8.5 at most if the system contains aluminium). High conductivity or a low pH point to corrosive water. For a full picture, the Fernox System Health Check laboratory analysis kit tests a sample and returns a report. If the system has already been treated with an inhibitor, the Fernox Express Inhibitor test strips check that it is still correctly dosed.

Step 2: clean the circuit

Fitting a dirt separator to an already loaded circuit without cleaning it first means filling its collection chamber within days and leaving the deposits stuck to the bottom of the radiators where they are. So you start with the clean, at one of two levels depending on the state of the system.

Chemical cleaning in circulation

This is the approach within reach in a single-family home. You add the cleaner to the circuit (through a radiator, the filling valve or a dosing adapter on the separator), run the heating at normal temperature for anything from a few hours to a few weeks depending on the product and the level of fouling, then drain, flush with clean water until it runs clear, and refill. Suitable cleaners: Fernox Cleaner F3 for routine maintenance, Fernox Power Cleaner F8 for a heavily fouled system, BCG-R 13 or BCG-HR in drums for larger volumes. If the problem is limescale rather than sludge, the Fernox DS-40 descaler does the job, but it is acidic: the circuit must then be neutralised with Fernox Neutralizer.

Precautions: follow the manufacturer's dosage and contact time, check the product is compatible with aluminium if the system contains aluminium radiators or a heat exchanger, and always flush until the water runs clear: a cleaner left in the circuit keeps attacking the metals.

Power flushing

For a heavily sludged system, or underfloor heating with blocked loops, chemical cleaning alone is no longer enough. A flushing pump is connected to circulate water and cleaner at high flow, radiator by radiator or loop by loop from the manifold, reversing the direction regularly to dislodge the deposits. This is a job for a heating engineer equipped with a Fernox Powerflow flushing pump, and usually takes half a day for a house.

Step 3: fit a dirt separator

A cleaned circuit fouls up again if nothing catches the particles. The dirt separator is fitted on the return, just before the boiler or heat pump: that way it protects the heat exchanger and the pump, the two most expensive components. It combines two principles: a widened chamber that slows the water down and lets the heavier particles drop into a collection chamber at the bottom, and a powerful magnet that captures magnetite, which is too fine to settle. A drain valve at the bottom lets you discharge what has been caught in a few seconds, with the heating running.

How to choose one

• The size: that of the return pipe. DN 20 (3/4") for a small wall-hung installation, DN 25 (1") for most single-family homes, DN 32 (1 1/4") and above for an apartment building or a high-output heat pump. An undersized separator creates a pressure drop and holds back the whole system.
• Magnetic, as soon as the system contains steel, which is almost always. A magnet that can be removed from outside is cleaned without opening the circuit.
• With isolation valves, to take out the strainer or magnet without draining. Models sold without them need one on each side.
• Rotating connection: some models swivel to fit a horizontal or vertical pipe, useful in a cramped plant room.
• Combined air and dirt: if the system has no deaerator, a unit that does both solves repeated bleeding and sludge at the same time.
• Insulation shell: limits heat loss and condensation. Many models come without one; it is added separately.

A few models for each situation

• Single-family home, DN 25 return: CALEFFI DirtmagPlus with ball valve, Fernox TF1, Spirotech SpiroTrap MB3, Evenes Magnus or Dirtstop XL with isolation valves.
• Wall-hung boiler, little space: Dirstop Mini DN 20, Evenes Mini DN 20, Giacomini R145 XS, or the DirtmagSlim units designed to sit directly beneath a Vaillant, Junkers or Brötje boiler.
• Air and dirt in a single unit: CALEFFI DiscalDirtmag, SpiroCombi MB3, Evenes combined deaerator and dirt separator, IMI Zeparo ZUKM, or the Spirotech SpiroVent RV2 + SpiroTrap MB3 set, which pairs the two separate units.
• Heat pump, high flow rates: Easyflow DN 40, Easyflow DN 50, XF magnetic dirt separator with bypass or flanged SpiroTrap.
• Tight budget: XS type 5459 magnetite separator or Evenes dirt separator, to be completed with two isolation valves.

Maintenance

In the first weeks after fitting, drain the collection chamber every two to four weeks: it fills up fast. After that, twice per heating season is enough, plus a clean of the magnet and strainer during the summer shutdown. What comes out is a good indicator: if the drained water is still black after several months, corrosion is continuing and it is the water that needs treating.

Step 4: stop the sludge coming back

A separator catches the particles; it does not stop the corrosion that produces them. For that, you need to act on the water and on the oxygen.

• Fill and top up with demineralised water, as guideline SICC BT 102-01 requires: a Purotap single-use cartridge for a top-up, a Purotap micro DS filling station or a Purotap micro Multiflex filling unit for a complete fill. The step-by-step method is in our guide Topping up your heating system with water.
• Remove the oxygen entry points: expansion vessel at the right pre-charge (see How to choose an expansion vessel), filling hose disconnected after use, automatic air vents closed once the system has been bled, and no leak tolerated: a circuit that needs regular top-ups is losing water somewhere.
• Corrosion inhibitor: a product such as Fernox Protector F1 or BCG-K32 forms a protective film on the metals. It is most useful on a system filled with mains water or one that mixes steel, copper and aluminium. First check what the boiler or heat pump manufacturer specifies: with demineralised water to SICC, some do not want one. Check the dosage once a year with test strips.
• One water check a year: pH and conductivity, at the start of the season. Any drift is corrected with a demineralised top-up or a treatment, long before the radiators go cold.

Frequently asked questions

My radiator stays cold at the bottom even though I've bled it. Why?

Bleeding releases air, which sits at the top. A radiator cold at the bottom has a layer of sludge stopping the water from circulating through the lower part. Bleeding changes nothing: the radiator needs cleaning (take it off and hose it through, or clean the whole circuit) and a separator fitted so it doesn't come back.

Where should the dirt separator go: on the flow or the return?

On the return, just before the boiler or heat pump: the water arrives there carrying everything it has picked up around the system, and the unit protects the heat exchanger and pump. The air separator, on the other hand, goes on the flow, at the hottest point, where the air is released.

Is a separator enough on its own, without cleaning first?

On a new or lightly loaded system, yes. On a system already giving black water, no: the collection chamber would be full within days, and the deposits stuck to the bottom of the radiators will not shift without cleaning.

Do I need a separator with a heat pump?

Even more than with a boiler. The plate heat exchanger of a heat pump has very narrow channels that block quickly, and most manufacturers require a magnetic separator on the return for the warranty to apply. Choose a model matching the return pipe size, with no reduction: a heat pump needs a high flow rate.

How often should the separator be drained?

Often in the first weeks after fitting, then twice per season. Open the bottom valve for a few seconds with the heating running, until the water runs clear.

Doesn't demineralised water corrode more?

No, quite the opposite. Without dissolved salts, water is a poor conductor of electricity and electrochemical corrosion slows right down. That is why the Swiss guideline prescribes it. What corrodes is oxygen: hence the importance of a sealed circuit and a healthy expansion vessel.

Everything for a clean circuit

Browse our air and dirt separators, our heating water treatment range (cleaners, inhibitors, Purotap filling stations and test kits), our automatic air vents and our radiator plugs, drain valves and bleed valves.