Powerflushing

Power flushing often seems to be suggested as a cure to all sorts of unexplainable issues. Heating systems don't normally need such radical treatment however if they do it is a good idea to locate what caused the problem in the first place.

Rust consists of hydrated iron(III) oxides Fe2O3·nH2O, iron(III) oxide-hydroxide FeO(OH), Fe(OH)3. For us this means that in order to get rust we need water, oxygen, and iron. For a heating system we must have water and radiators are made of steal which contains mostly iron. The only thing left to prevent your radiators rusting and falling apart is to remove the oxygen. Water in a tap often has extra dissolved oxygen in it so when a heating system is originally filled a small amount of rusting will take place. However once this occurs the water has no more oxygen (despite it being H2O it prefers to remain like that) so the rusting stops. In a heating system the same water that was original put into should remain in it.

The problem comes when new oxygen is introduced and this can happen in the following ways:-

If the water pumps over in the header tank this bubbles the water causing oxygen to be added. This is not helped as the water is hot and readily takes up this oxygen. For example this may be caused by the pump being incorrectly set, a blockage in an air separator or the vent and feed pipes fitted into the system in the wrong place.
A poor joint that may not let water out but lets air into the heating system.
Incorrectly fitted automatic air vents.
Draining and replacing the system water on a regular basis.
A leak that is not repaired but the system water is allowed to automatically refilled.

The above have to be fixed before a power flushing as the system will quickly return to rusting once the power flush has been done if they are not.

Most manufactures require that a heating system be cleaned when a new boiler is installed. This can be done using a powerflush or chemical cleaning of the system. Chemical cleaning is done by adding the chemical to the header tank prior to the work. On the day of installation the heating system is drained removing the debris. This is less aggressive than a full powerflush.

Powerflushing can still be a good solution and sometimes is the only solution. If the boiler being replaced is not working the chemical can't be added to the system and allowed to work the only answer is to power flush the system. Also if a traditional old system is being replaced with a combination boiler it is particularly important that the system is carefully cleaned as these boilers often contain a water to water heat exchanger that can act as a filter. This will over a short time remove the rust from the system at the expense of efficiency of heating the water and is often why old combination boilers don't get the hot water partially hot.

Do you need a power flush? Look out for the following: A cold zone at the bottom of most radiators when the heating is on, probably shaped like a hump (build up of sludge in the radiator). If the cold is at the top of radiators it is air, just bleed them. If you have a sealed system (common with most combination boilers) don't forget to top the pressure back up with the filling loop.