Central heating flushing and inhibitor checklist

Heating water can circulate for years. Cleaning, corrosion protection and filtration have different jobs, so use them together according to the appliance, system and treatment instructions.

1. Assess the condition. Installation residues, corrosion debris, scale and poor flow need different checks. Black water can mean iron-oxide sludge, but it does not tell you the inhibitor level now or when the corrosion happened.
2. Clean appropriately. Remove installation debris from a new circuit and use a suitable method for deposits in an existing one. BS 7593:2019, as Sentinel summarises it, still recommends only three cleaning methods: power flushing, mains pressure cleaning and gravity cleaning. The method must suit the equipment and its condition. Inhibitor does not replace cleaning, even a combined product that also disperses particles: clean the system, then check the result.
3. Protect it. Add a compatible inhibitor to the specified concentration and check it once it has circulated. Some products also control scale; combined antifreeze and inhibitor products have their own concentration requirements. Not every inhibitor protects against freezing. Fernox's Alphi-11, a combined antifreeze and inhibitor, needs a minimum 25% concentration in use for corrosion protection; that gives frost protection to −11 °C. At 40%, its stated protection is −22 °C. Check the concentration once it has circulated, and use the treatment specified for the installation. These figures are for that product only, not ordinary inhibitor or other antifreeze.
4. Fit and maintain filtration. A magnetic filter catches magnetic debris that reaches it; combined filters also catch other particles. Service them as instructed. A clean filter, or one clean outlet, does not mean every branch is clean.
5. Recheck the treatment. Check the inhibitor level and general cleanliness every year. At five years, re-dose or send water for laboratory analysis as specified, and act on the results. After a partial drain or refill, test what is left and top it up as needed, rather than adding a full dose by default.
6. Find out why it needs fresh water. Every top-up brings in oxygen and can dilute the treatment. Find where water is being lost or let in; topping up is not a repair.

Gas and deposits

Corrosion can make hydrogen as well as sludge. Radiators that keep needing bleeding can also be down to air getting in, or filling problems. Having to bleed them does not tell you which gas it is, or why. Investigate the water condition, the pressure and where air could get in.

Sentinel's diagnosis guidance separates three gases. Hydrogen is a product of corrosion. It is common in new systems where too much flux was used, and in older systems that were not cleaned after new radiators or a new boiler were added. Air points to a mechanical fault, such as pumping over or joints under negative pressure. The Copper Board's guidance names micro-leaks on the suction side of the pump as one such path. A running pump lowers the pressure on its suction side, so a weep there can draw air in; with the pump stopped, the same joint sits under static pressure and lets water out. Hydrogen sulphide comes from bacteria and smells of rotten eggs. A water sample helps as well: a reddish tinge shows iron oxide from active corrosion, and the system may be drawing in air, while black particles are most likely magnetite, the final stage of corrosion. Hydrogen and corrosion debris are tackled with chemical cleaning followed by inhibitor. Powerflushing is not suitable where a system contains microbore or non-barrier plastic pipe.

Settled deposits can restrict lower radiator passages, but a cold lower area can also be caused by poor circulation. Inspect valves, flow and water condition before choosing a cleaning method. Magnetite particles can jam pumps and valves and narrow the waterways. They are not a measure of acidity: pH and chemical compatibility are separate checks.

Open-vented arrangements

In an open-vented system, the cistern provides static head and the pump creates pressure differences. The wrong pump setting, or the feed and vent in the wrong place, can cause pumping over or draw air in, adding oxygen to the water. Check the appliance’s required layout and that the circuit stays under positive pressure: being open to atmosphere does not make the pressure zero throughout.

Barrier pipe and draining down

Plastic pipe on a heating circuit with steel or iron parts should be barrier pipe. Plastic without a barrier lets a little oxygen pass through its wall; in the circuit that oxygen dissolves in the water and corrodes steel radiators, the pump and the boiler, adding to the sludge. A barrier pipe such as JG Speedfit’s has an oxygen barrier layer at the centre of its wall that keeps air out. The layer is not insulation and does not stiffen the pipe, so the pipe is still lagged and clipped as its instructions require.

Water drained from a system carries inhibitor, cleaner residues and corrosion debris. Fernox’s guidance is to run it only into the foul drain, or into a septic tank where the property has one. Road drains and rainwater gullies carry surface water straight to rivers, so system water never goes into them, and it is never emptied onto the garden or a lawn.