Small attic water tank question

Connacht

Folks,
I had water dripping out of my small attic water tank through the overflow pipe, so changed the whole ballcock, cost 11 euros. But when I replace it, I notice the ball sits very low in the tank allowing only about 3 inches of water, when the tank itself is around 12 inches deep. Is this normal?
In the larger tank, the ballcock sits almost at the top, allowing the tank to be virtually full at all times.
Excuse my ignorance ...

Wearb

If 3" when heating system is cold, that's fine. Have you added any new rads recently ?

Connacht

Thanks for that. No new rads. But is it ok that the ballcock is now at its high point, in other words if any more water enters the tank the ballcock will be submerged, which is how it was when I noticed the problem earlier because water was coming out the overflow pipe?

Wearb

That's fine, if as I said, the system is cold. Did you confirm ballcock was leaking? Any other work done recently, like stove or circ. pump?

I ask because a leaking ballcock need not be the only reason for overflow to occur.

Connacht

Thanks, no work done recently. System is 20 years old, main attic tank ballcock changed a few years ago, small one never changed til now. But how does the small tank operate normally anyway? If the ballcock is at its max height when cold/minimum water, then by definition it's going to be underwater when any extra water flows in. No comprendo.

Wearb

Because water expands when it heats it needs somewhere to do this. Sealed systems have an expansion vessel, open systems (like yours) have a feed and expansion tank. The water will continuously rise and fall in that tank as the water heats and cools.

If -when cold- the water is too high in the tank, it would rise up to the overflow pipe when hot. Also the more water in the system the bigger the expansion needed. That is why I asked about any additions.

Connacht

OK, thanks very much for your help. I'll monitor it.