how to clean central heating system

How to Clean Central Heating System

Dirty central heating system? Find out how to clean a central heating system from a full-time heating engineer. I have been cleaning central heating systems for over ten years, and I can tell you it’s not easy or enjoyable.

Here, I will teach the processes I use that I find the most effective.

How to Flush Central Heating System

Knowing how to flush a central heating system could save you a lot of money, or make you a lot of money if you’re a heating engineer. Flushing a central heating system takes a lot of time and effort, therefore the high cost is justified.

Every time I install a new boiler, the central heating system is flushed and a magnetic system filter is fitted. A standard chemical flush is all that is needed most of the time.

Chemical flushing a central heating systemmy process

  • Drain central heating system
  • Add cleaner to the central heating system and refill
  • Turn the heating on at a high temperature and run it for at least a couple of hours.
  • The cleaner can be left in for days or weeks if needed. Read the instructions on the bottle of cleaner.
  • Drain central heating system
  • Leave drain off valve open and close all radiator bleed valves
  • Pump some fresh water through the system by opening the filling loop and check for it running clear from the hose outside in the drain
  • You can manipulate which way the fresh water goes by closing the valves on the flow or return or radiator valves. It depends on your system setup.
  • Refill the central heating system
  • Turn the heating on and wait for all the radiators to get hot
  • Drain central heating system
  • Make sure the water is clean when draining – Repeat if it’s not clean
  • Clean magnetic system filter, or fit one if there isn’t one
  • Put a bottle of inhibitor into the system and refill
  • Turn the heating on and make sure all the radiators get hot
  • Pat yourself on the back.

Magnacleanse flush

If it’s a really dirty system, then I will get my Magnacleanse (large magnets) out and use that. I connect them to the system and keep cleaning the sludge off the magnets.

It’s virtually the same process as above, except I will agitate (vibrate) the radiators when the heating is on to get the sludge moving around and out of the hose or onto the Magnacleanse magnets.

I agitate the radiators with a cordless SDS drill on hammer only mode with a special agitating drill bit connected. You can also use a rubber mallet and just keep lightly tapping along the bottom of the radiator.

Power flush

A power flush might be needed if the central heating system is really full of sludge and causing circulation problems. A power flush machine is a powerful pump that can be added to the system to force the thick sludge to move without the need to use the boiler.

You can also use a power flush machine with a Magnacleanse and or a heater to make it even better. The pump can be pumped in both directions, helping to reach all the thick sludge stuck in awkward places.

If your boiler is not working, then it might need to be repaired or replaced instead of flushing. Only flush it if you are sure it needs to be flushed, as you could be wasting a lot of time and money.

Draining Central Heating System

Draining the central heating system can be difficult sometimes. Like when there is no drain off valve, or sometimes you’ll have a drain off valve, but it won’t let the water out because it is over-tightened off someone else, and now the rubber seal is stuck or damaged.

To drain the system, you need to connect a long hose to a drain off valve and run the hose to a drain outside. You will then need to open the drain off valve and wait for the water to stop coming out of the hose, then vent all the radiators, starting with the highest first.

See how to drain the central heating system in more detail here.

Refilling Central Heating System

Before refilling the central heating system, make sure all the radiator bleed valves and any drain off valves are closed. If it’s a gravity open vented system, turn the water supply back on.

If it’s a pressurised system, open the filling loop and let the pressure get to 1.5 bar on the pressure gauge. Then start to bleed all the radiators with a radiator bleed key. Always start with the lowest radiators first and work your way up to the highest.

Learn how to refill a central heating system in more detail here.

How to Add Cleaner to Central Heating System

To add cleaner to a central heating system, it’s best to do it when the system is drained down and empty. Just make sure the drain off valve and radiator bleed valves are closed, then pour in into the top of a radiator or towel rail after removing the drain valve nut or blanking plug.

The easiest way to add cleaner to a central heating system is to pour it in the top of a towel radiator. If the system is empty, you can remove one nut with an adjustable spanner, wrap an old towel around it, and pour it straight in.

What if the central heating system is full?

If the central heating system is not drained, then you will either need to drain the radiator or drain the system first. To drain a single radiator, close both radiator valves, open the bleed valve with a radiator bleed key and catch the water in a bucket or towel.

When the water stops coming out, put some towels and a tub or bucket under one radiator valve and crack the nut in between the valve and radiator. Then open the bleed valve and the water should come out of the nut you have loosened.

Wait until there is more than a litre of water in the tub or slightly more than the bottle of cleaner, then close the radiator valve. Remove the blank nut or bleed valve nut from the radiator and pour the cleaner in. Leave the bleed valve open, so the cleaner will pour in nicely from the other end.

This will only be possible on most towel radiators and certain other radiators with big nuts at the top. If the nuts are on the side, you will need to screw in a funnel to pour in the cleaning chemical. For most towel radiators, you can pour straight into the top from the bottle.

How to Put Inhibitor in Central Heating System

How to put inhibitor in a central heating system is the same as adding a cleaner like above. If you don’t have any of those types of radiators, then you can put inhibitor (or cleaner) in one of these other ways –

  • Put inhibitor in a magnetic filter if you have one. Not all central heating systems do, and some filters are too small or not made for taking chemicals in.
  • Drain the system and remove a radiator, then pour inhibitor into a radiator pipe via a funnel.
  • Use a pressurised can of inhibitor and squirt in the filling loop.
  • Pour in the feed and expansion tank in the loft if you have an open vented system

How to Clean Central Heating System FAQs (Frequently Asked Questions)


How Often Should You Flush Your Central Heating System?

You should not need to flush your central heating system regularly. If it is flushed and cleaned properly once and fitted with a magnetic system filter, then it shouldn’t really need to be flushed again.

You can clean the magnetic filter as often as you like. If you are getting lots of sludge in the filter regularly after your central heating system was flushed, then you probably need to flush it again properly.

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