Strange things happening in my header tank.....

[croydoncorgi]

Sorry - forgot to add advice.

Find and fix the air problem first. Then consider whether flushing or any other treatment is actually necessary. Note, however, that corrosion inhibitor is a must-have and WILL NOT WORK properly on a dirty system.

 

[Softus]

Air DOES leak into systems, past washers and especially O rings and dependent on the pressure difference between the water inside and the air outside. So if your (sealed) system pressure is too low, you will definitely get air leaking INTO the higher parts of the system.

Add the effect of the pump, and even more parts of the system will be at negative pressure with respect to atmospheric pressure.

Clearly this is the case for a sealed system, and I never suggested otherwise, but this topic is about an open vented system.

Obviously, the lowest pressure will be at the inlet port of the pump, so this is the first place to look for an air leak.

I agree that it is obvious. Even more obvious is the fact that it's rare - since ricarbo, who first mentioned this fault, has never seen it; only Slugbabydotcom has seen it. Since he has seen it, I accepted that it happens.

O rings such as used on radiator valve stems DO NOT WORK EFFECTIVELY if the pressure is low and especially leak air in the INWARD direction. The higher the pressure the tighter the seal.

I get the feeling this still applies only to sealed systems.

But if you have a sealed system DO NOT raise the pressure much above 2 Bar. More is not necessarily better.

Although, more sightings of the rare phenomenon would be better

 

[Softus]

They are called 'microleaks' and occur regulaly on negative pressure systems.

Tighten all glands on rad valves for starters.

Most common place the air can get drawn in.

Well, perhaps they are called "microleaks", but it isn't the regularly used term that you glibly imply. A quick search of Interwebbyland reveals that it's used occasionally on this forum, and never anywhere else except by members of this forum who post on other forums.

Anyway, all cynicism aside, and accepting that a pump can cause a net pressure of less than 1 bar on it's inlet side, it's going to be pretty easy to put this to rest.

If the OP's system is admitting air, which he has to keep venting, then, logically, just before he vents, the level of water in the F&E cistern will have risen above the level set by the float, by a volume of water equivalent to the (slightly compressed) volume of air admitted.

If the water level hasn't risen, then no air has been admitted, and it's simply good old electrolytic corrosion, with magnetite, hydrogen, and a clock that says it's time to get out the hosepipe. Or he could pay a ludicrous sum of money for a service of dubious value unless you have a low water content boiler.