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Cleaning a Central Heating System

Corrosion and sludge build up

Corrosion in central heating systems can lead to a build up of sludge which, over time, can cause problems and breakdowns. When a heating system is first installed, it should be cleaned through to minimise the possibility of corrosion. This process of cleaning central heating systems will remove things like flux used on soldered joints and small particles of copper from pipe cutting.

The oxygen contained in water can be released as it is heated and this leads to corrosion of steel components. This rusting process will also cause sludge to deposit in the system. Air getting into the system will have a similar effect. In addition, because we use a number of different metals for the components of a central heating system, the water can trigger a deterioration of some of these leading to further sludge build up.

Indications of Corrosion and Sludge

Boiler noises which gradually get worse over time
Pin holes developing in radiators
Radiators having a cold area at the bottom
Wearing of pumps and valves

Scale Build Up

In hard water areas, minerals in the water may gradually deposit on the metal surfaces and this can reduce efficiency as well as lead to early system failures. Some proprietary central heating systems also remove lime scale

Cleaning Central Heating and Flushing the System

Before using a proprietary system cleaner additive you should drain and replace the water in the system. This will allow some of the deposits suspended in the water to be removed first. Typically, when you drain a sludged up system you will find that the drained off water is quite discoloured.

Draining Down the Central Heating System

Before carrying any work, you must make sure that the boiler is switched off and you should isolate the electrical supply. You should also allow the system to cool if it has been running recently. With a vented system you will have a feed and expansion cistern or header tank and you will need to close off the water supply from here. With a sealed system water is added only by opening the filling loop. This is closed under normal conditions - so there is no feed to be turned off.

Locate the drain valve on your system. This should be at the lowest point in the pipe work and will have an outlet designed for the attachment of a hose. Run a hose from here to a suitable point outside and connect the hose to the drain off valve. Securing it with a jubilee clip will minimise the likelihood of leaks here but it’s sensible to place a collecting dish under the connection to catch any dribbles. Open the drain valve and allow the system to drain down fully. You should open the air bleed valves on upstairs radiators to ensure all water is allowed to drain off. Remember to close these once the system is drained down.

Refilling the Central Heating System

Turn off the drain valve and remove the hose used for draining down. With a vented system, switch the water supply back on at the feed and expansion tank in the attic to allow water to flow back into the system and begin to fill it up.

Filling loop used to top up after cleaning central heating systemWith a sealed or unvented system you will need to connect the filling loop – usually a braided hose - which is usually located just beneath the boiler, and open the valves at both ends. Open the filling loop and re pressurise the system to the manufacturer’s recommended pressure – normally around 1 bar.

Go round the radiators, one by one, opening the bleed valve and releasing any air. Do the downstairs radiators first and then the upstairs ones. This will minimise the chance of trapping air in the system. On a sealed system you should then re check the pressure and top up if necessary.

Adding the System Cleaner

Central Heating system cleaner bottleCheck manufacturer’s details for the required quantity of system cleaner and precise instructions before starting. Some come with a neutralising product which is added after the cleaning to neutralise the effects of the chemicals. Depending on whether you have a sealed or a vented system, adding the cleaning chemical will be done differently. For vented systems, the system cleaner comes in a plastic container and is added to the water in the feed and expansion tank. Make sure you add the chemical to the feed and expansion tank and NOT the cold water storage tank.

For sealed systems, the cleaner comes in a cartridge similar to those for bathroom sealants and mastic. Following the manufacturer’s instructions, connect the special attachment hose that is supplied to the air vent on one of the radiators and using a sealant gun, inject the cleaner into the system.

Run the Central Heating

Switch the electrics back on and run the central heating allowing it to run at its normal temperature for an hour. This will ensure that the cleaner is fully circulated around the system and properly distributed. Many boilers have an air release valve or auto air vent which releases air still in the system but you should also bleed the radiators to get rid of any trapped air. Leave the cleaner in the system for a week running the central heating system normally. This will allow the system cleaner to break down the sludge and scale into suspension in the water.

Drain and Flush the Central Heating System

Once the cleaner has had sufficient time to work, you will need to drain and flush the system at least a couple of times until the water drained off is clear. Once this is done, refill the system and add an suitable inhibitor to protect the system from further sludge build up. By adding inhibitors to central heating systems, you will reduce corrosion, prolong the life of pumps, valves and other components as well as minimising the likelihood of boiler breakdowns.

Boiler Service

Boilers should be serviced annually by a professionally qualified heating engineer who will carry out all servicing in accordance with the boiler manufacturers requirements. This is an extremely important thing to do in order to keep your system running efficiently. It is also vital to ensure that the boiler is safe and that no noxious fumes or gases escape into the home.

T&S Heating Limited