GeoffF100 wrote:More accurately, the plumber said that a blockage in the fed pipe can prevent water getting in to the system. Less water = more "air". The radiators have all been bled. When radiators are bled, a lighted match shows that there is flammable gas, but that is normal.
There should never be a blockage. The header tank should have a well-fitting lid, as part of its insulation.
Marsh gas is a corrosion product.
Your rads are rusting. Perhaps due to air, if the pipe has blocked, more likely due to lack of inhibitor. The black sludge is sometimes called magnetite, another corrosion product. Dissolved steel. It used to be part of the your radiators, which are now on the way to pinholing due to rust.
Inhibitor can become diluted to the point of ineffectiveness. Perhaps by leaks, or partial draining without new inhibitor being added.
To prevent this one needs to check the header tank at least annually, and ensure there is sufficient inhibitor. In the same way that one checks the antifreeze/corrosion inhibitor in one's car.
Here is an example of a checker for the concentration.
https://www.sentinelprotects.com/uk/pro ... k-test-kitThe heating system that I designed and installed in our previous house gave no trouble in 29 years, other than a couple of valve motors. The gas boiler was never serviced nor did it need to be. The rads never needed routine bleeding, and when I had to bleed them after decorating, there was no gassy pong. The feed pipe did not block. The pump was the original. The (cast iron) rads did not clog up, there was no black in them.
All that was necessary to maintain this nirvana was an annual peek at the header tank, and a check on the inhibitor.
And the same will work for you.
Ivor