Can I add inhibitor to my heating system myself?

Inhibitor is a chemical that stops the steel radiators, copper pipe and aluminium heat exchanger in a mixed-metal system corroding against each other. Without it, the system makes its own sludge, and that sludge is what blocks radiators and wrecks pumps.

Safety

What you'll need

• An inhibitor test kit
• Inhibitor in a bottle designed for radiator dosing, or a filter-dosing pack
• A bleed key, cloths and a container
• Access to the filling loop

Step by step

1. Test before you dose.A dip-test strip kit costs very little. Take a sample from a radiator bleed point and check the level. Adding more when there is plenty is a waste; assuming it is fine when it is not is worse.
2. Look at the water you draw off.Clear or lightly coloured is a good sign. Black water means significant corrosion and sludge, and inhibitor alone will not clear that — the system needs flushing first.
3. Check whether a magnetic filter is fitted.If there is a filter on the return pipe near the boiler, it usually has a dosing point and is by far the easiest and cleanest route.
4. Let the system cool completely.Several hours with the heating off. You will be opening a point that holds system water.
5. Turn the heating off and isolate the radiator.For the radiator method, close both valves on a convenient radiator, counting the lockshield turns.
6. Release the pressure.On a sealed system, open a bleed valve on another radiator to drop the pressure before opening anything. Have towels and a container ready.
7. Dose through the bleed point.Inhibitor sold for this comes in a bottle with an adaptor that screws into the bleed valve boss, so the contents can be pushed into the radiator without spillage. Follow the product's instructions.
8. Dose through the filter instead if you have one.Isolate the filter with its valves, open the top, pour the inhibitor in, close and reopen the valves. Considerably tidier.
9. Reopen the valves and repressurise.Reopen the lockshield to exactly the turns you counted, open the TRV, then use the filling loop to bring the pressure back to about 1.2 bar cold. Close and disconnect the loop.
10. Run and re-bleed.Run the heating for an hour, then bleed any air introduced and check the pressure again once cold.
11. Note the date.Write it on a label by the boiler. Inhibitor should be checked annually and topped up after any work that drains part of the system.

Tips

• Test first. It costs a few pounds and tells you whether you need to do anything at all.
• Any work that drains a radiator removes inhibitor with the water. Top up after every such job.
• If the water drawn off is black, book a flush before dosing — inhibitor will not dissolve existing sludge.

Common mistakes

• Adding inhibitor to a sludged system and expecting improvement — Inhibitor prevents corrosion; it does not remove the magnetite already in the system. A sludged system needs flushing first, and then dosing to protect it afterwards.
• Forgetting to top up after removing a radiator — Draining a radiator for decorating takes a proportion of the inhibitor with it. Systems commonly end up unprotected through nothing more than repeated small jobs.

If it doesn't work

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