Central heating inhibitor: what is it & how to add it

Central heating inhibitor or radiator inhibitor is a chemical added to a central heating system which prevents sludge from building up. It is an important element of maintaining a central heating system, ensuring its efficiency and protecting it from costly corrosion damage.

In an open vented system, central heating inhibitor can easily be added via the feed and expansion cistern in the loft. In households with a sealed heating system, such as those with a combi boiler, it is added via one of the radiators.

Central heating inhibitor overview

Over time in a typical, untreated central heating system, a chemical reaction will occur with the oxygen in the water and the steel of the radiators. The end result of this reaction is a type of iron oxide compound known as magnetite – or as it is simply called in the world of plumbing: sludge. It’s a murky, black, mud-like substance which can have devastating effects on a central heating system, such as:

  • Damage to the boiler
  • Damage to the radiators, such as leaks
  • Blocked pipes
  • Damage to the boiler’s heat exchanger, affecting the boiler’s ability to supply hot water for the taps
  • Radiators taking a long time to heat up, or don’t get hot in certain places.

While a power flush can remove central heating sludge, adding inhibitor will help to stop it from forming in the first place. As the saying goes, prevention is better than cure. Plus, it’s a lot cheaper: the cost of fixing corrosion and sludge problems can run into the thousands.

How to add central heating inhibitor

Inhibiting your radiators is not a difficult task. If you have an open vented heating system, you can simply pour the inhibitor into the feed and expansion tank in the loft. However, first of all, you will need to drain some of the water out.

Note that if you have a primatic hot water cylinder, you cannot add central heating inhibitor to your system. This is because the water for the radiators and the hot water are separated by only an air lock. The loss of the air lock would result in the contamination of the hot water for the taps with anti-corrosion chemicals.

How to add central heating inhibitor to the feed and expansion tank

To begin with, make sure that your central heating and hot water is switched off via the thermostat. Next, isolate the ball valve to the feed and expansion tank by turning off the mains or closing the isolation valve. Alternatively, you can tie up the ball valve with string or garden twine attached to a piece of wood laid across the top of the cistern.

Drain the system

You’ll now need to find the drain cock on your heating system. This is usually adjacent to a radiator on the ground floor. Using a jubilee clip, attach a hose pipe to it and run the hose pipe to a drain outside. Open the drain cock with a spanner. (Click here for more information on how to do this.)

Leave the system to drain so that enough water flows out. You don’t need to drain the system completely, but you should drain it enough so that the inhibitor actually circulates through the system and won’t simply sit in the feed and expansion tank. A good 2-3 minutes should do the trick. Close the drain cock when the time is up.

Add the inhibitor

An empty feed and expansion tank is a splendid opportunity to clean it. Use cloths and water to wipe out any of the muck inside. Don’t let it go down the supply pipe at the bottom.

Now it’s time to add your central heating inhibitor. Simply pour it into the tank, and then untie the ball valve or open the mains. The tank will fill up again.

Some air may have been ingested into the system, so before you turn the heating and hot water back on, you should bleed the radiators. Click here to learn how to do this.

How to add central heating inhibitor via a radiator

It is also possible to add central heating inhibitor via a radiator. You’ll need to do it this way if you have a combi boiler or a sealed system. However, this method works equally well if you have a feed and and expansion tank. Plus, this method will save you from having to go into the loft.

However, you will need a filling kit. These typically consist of a funnel with a half inch male connector, which screws into the top of the radiator in place of the bleed valve. Simply pour the central heating inhibitor into the funnel, and gravity does the rest. The Eezyfill filling kit is a bestseller on Amazon.

Turn off the heating and isolate the radiator

First of all, you’ll need to turn off the heating and the hot water. Then, isolate the radiator by closing the two valves at each end. Shut the thermostatic valve fully. Then, take the plastic cap off the lockshield. Before closing it with a pair of pliers, make a mental note of its position. You’ll want to return it to this position to ensure that the radiator stays balanced when you open it up again.

Open the bleed valve

Using a radiator bleed key, open the bleed valve. This will dissipate the pressure and expose the water inside the radiator to atmospheric pressure so that it flows out when you open the nut at the bottom.

Loosen the nut between the radiator and the TRV

In order to add central heating inhibitor, you’ll need to make room in the radiator by draining some of the water out. Place a plastic container, such as a piece of tupperware or an old ice cream tub underneath the nut between the thermostatic valve and the radiator. Use a spanner to loosen the nut, and catch the water as it runs out. Once enough water has flowed out, re-tighten the nut.

Attach the filling kit

Use a spanner to unscrew the radiator bleed valve. In its place, screw in your filling kit. It is also good practice to undo the bung at the other end of the radiator. This will ensure that the air inside can leave as the inhibitor is inserted, instead of coming back up through the funnel and splashing the inhibitor onto you or the carpet.

Insert the inhibitor

Gradually pour the central heating inhibitor into the filling kit. Take care not to overwhelm it, as the inhibitor may take a moment or two to flow in.

Congratulations – job done! Remove the filling kit and reattach the bleed valve and the bung at the other end. Open the thermostatic valve and restore the lockshield to its original position. If you have a combi boiler or a sealed system, you may need to top up the pressure via the filling loop.

 

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