One of the main causes of sludge in the system is caused by the wrong placement of the pump to the vent pipe, without going into too much detail it (the pump) should be in a neutral position otherwise when the heating/hot water is called for by the timer/thermostats you get what is called pump over, water is sucked down the feed pipe and pushed out through the vent pipe hence pump over. This causes the water splashing into the expansion tank to pick up oxygen which in turn goes around the system causing the deterioration of radiators etc causing the sludge.

It also happens the other way round when the pressure pushes the water up the feed pipe raising the water level when the pump goes off, the water quickly tries to find its own level and can shoot up the vent pipe causing an overspill into the expansion tank. If the expansion tank is overfilled to begin with the water would flow out through the overflow pipe and then new water would top up the tank from the mains( even more oxygen) This you should have noticed by the water escaping via the overflow pipe outside.

You can check if this is happening by one person in the loft looking into the expansion tank and another person turning on the heating/hot water stats for 5mins and then switching off.

What's happened to my post answering you FE

Just back from picking up a G/child.

I answered you (FE) by saying :- That there should be no oxygen in a pressurised system unless there is an undiscovered leak underneath the floor or somewhere and the householder keeps topping up via the mains water link to the boiler and perhaps closed the automatic air valve thereby adding fresh water and not allowing the oxygen to escape.

Also some boiler manufacturers do not like you to put in additives as if you put too much in for the amount of water content it can damage the boiler seals in some cases.

Any oxygen in a new water fill should be vacated by the AAV and radiator bleeds in the first few days so therefore any deteriation of the system should be minimal.