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How to put inhibitor in central heating system

How to Add Inhibitor to Central Heating: Complete Guide

Last updated on June 10th, 2024

Not having inhibitor in your heating system can cause all sorts of problems.

Central heating inhibitor will protect your system against corrosion and limescale and essentially make your heating system last longer.

I have been a full-time central heating engineer for over a decade and have added many central heating chemicals to many systems.

Adding Inhibitor to Central Heating

Adding central heating inhibitor to a central heating system is the same process for cleaners, leak sealers, or any other heating system chemicals.

If the system has been drained, it’s a lot easier to add inhibitor, but you can do it without draining the system.

Dosing tool in radiator
Dosing tool in radiator

Common Ways

Here are 4 ways to add inhibitor to a central heating system:

  • Pour into a radiator
  • Pour into a central heating magnetic filter
  • Pour into a radiator valve pipe
  • Use a pressurised can of inhibitor


If you have a towel radiator, this is the best and easiest way to add central heating inhibitor to your system as you can pour it straight in the top.

On a standard modern radiator, which has a bleed valve with a large nut, you can remove the bleed valve from the side of the radiator and use a radiator dosing tool to pour in the inhibitor.

Replacing radiator bleed valve
Removing bleed valve

I have made my own dosing tool to keep it as small as possible, but you can buy a radiator dosing tool.

My radiator dosing tool
My radiator dosing tool

Video Guide

How to add inhibitor to a central heating system video

How to add inhibitor to a radiator:

  1. Lay an old towel under the radiator
  2. Close both radiator valves
  3. Open the bleed valve screw and let the water stream out into a bucket until it stops
  4. Place a tub under the radiator valve (a plumb tub is best by far)
  5. Undo the nut slightly in between the valve and radiator until water starts coming out
  6. Open the bleed valve screw so the water comes out of the bottom properly
  7. Catch the water in a tub until around a litre or so has come out (or however much inhibitor you’re adding)
  8. Tighten the nut back up
  9. Remove the radiator bleed valve nut with an adjustable spanner
  10. Fit dosing tool (unless it’s a towel radiator)
  11. Pour the inhibitor in the hole or dosing tool
  12. Refit the radiator bleed valve
  13. Open the radiator valves
  14. Bleed the radiator
  15. Turn the heating for an hour to get the sealer mixed in the system properly
Draining radiator with plumb tub
Draining radiator with plumb tub

Magnetic Filter

Adding central heating inhibitor to a magnetic filter is only possible with certain filters.

The Magnaclean Professional 2 is one of these filters and is one of the reasons it’s my filter of choice.

Magnaclean Pro 2 filter fitted
Magnaclean Pro 2 filter fitted

How to add inhibitor to a magnetic filter:

  1. Isolate the filter by closing the two valves on the filter
  2. Open the bleed valve on the filter to release the pressure then close it
  3. Open the top of the filter with the filter spanner and remove the magnet
  4. Empty the water from the filter by opening the nut on the bottom or by pulling the filter off the valves
  5. Add the inhibitor to the top of the filter
  6. Clean the magnet and refit it into the filter and screw the lid on (hand tight only on Magnaclean filters)
  7. Open the filter valves
  8. Bleed the filter
  9. Turn the heating for an hour to get the sealer mixed in the system properly

Radiator Valve Pipe

Sometimes you can’t add central heating inhibitor to a radiator or magnetic filter. If not, you can remove a radiator valve and pour it into the pipe using a small funnel.

How to add inhibitor to a radiator valve pipe:

  1. Drain the system (or partially drain)
  2. Loosen the two radiator valve nuts with an adjustable
  3. Remove the valve
  4. Fit the funnel in the pipe
  5. Pour in the inhibitor
  6. Refit valve
  7. Refill the system
Adding inhibitor to radiator pipe

Pressurised Can

You can also get pressurised cans of central heating inhibitor that can be squirted into the system via the filling loop or radiator.

No need to drain the system with these but it is difficult and they can make a mess.

Draining the System

Adding central heating inhibitor to a system is a lot easier when you drain (or partially drain) the system.

Drain heating system

This is sometimes necessary if the radiator valves aren’t isolating the radiator or you need to pour the sealer into a valve pipe.

How to partially drain the system:

If it's a vented gravity system, you will need to the turn water off at the stopcock first.
  1. Connect a drain-down hose to a drain-off valve and run the hose to a drain outside
  2. Open the drain-off valve to let the water out
  3. Wait for the water to stop coming out of the hose
  4. Vent the radiator on the highest floor by opening the bleed valve screws
  5. Wait for the water to stop coming out of the hose
  6. Close the drain off valve
  7. Add the inhibitor to a radiator on that same floor
Drain hose in drain

How much inhibitor do I need?

1 bottle of inhibitor is usually good for an average-sized heating system.

Different inhibitor products have different amounts whether it’s 300ml, 500ml, or a litre, they are all typically designed to cover a standard system of up to around 10 radiators.

Larger systems should use two bottles.

Can you put too much inhibitor in?

Not really. If you used 100% inhibitor with no water then maybe, but I can’t see how having too much central heating inhibitor would cause any problems.


Adding inhibitor to a central heating system can be a nightmare sometimes and if you haven’t done anything like this before, you might be better off paying a professional, especially if you have ancient radiators.

Having a towel radiator is the best case scenario for making it as easy as possible, just make sure you drain out at least a litre of water if you’re adding a litre of inhibitor.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you find it helpful.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
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