Tappys rough sketch.

Well that is another school day, it’s never entered my head nor has anyone pointed it out other than you Tappy.

my answer to that is, water finds its own level, as long as the system is sealed , the water in the expansion will move with the temperature both in the tank and the expansion pipe and the cold fill will move as well, things only go wrong when the boiler stat or the pump is faulty,

here a fag packet drawing of temperature movement I remember from college

If we’re talking Feed & Expansion tanks on CH systems, it’s simply expansion on the Vent Pipe(Expansion Pipes). It depends on where the Cold Feed & Expansion Pipes are connected in the system, that will impact on the Tank water level.  
With Low Head Vented Fully Pumped CH Systems in Flats for example; Close Coupled Tees were used for the Feed & Expansion Pipes.
This was illustrated in lots of Boiler MI’s at the time, particularly with Low Water Content boilers(SE). LWC boilers required a high flow rate through them, as their Heat Exchangers were a simple coil of copper pipes & the by-pass was critical as it aided this greatly.

With Expansion Pipes & Tanks on HW Vented cylinders, the Expansion Pipe Water simply heated up & the water level was higher than the Head the Tank provided, so you extended it higher before returning it back into the Tank. There was a formula/rule of thumb for this, which I can’t recall.

Thanks for the input Dick on the matter.
I think you are saying that the water in the expansion pipe does settle at a higher level than it does in the feed tank due to expansion of the water when heated?
However, with the 4% increase in volume of heated water expansion rate I would have thought that both would stay at the same level (each expanding by 4%).


What I'm saying is that the level will be slightly higher in the expansion pipe than it is in the expansion tank by natures gravity because of it being trapped in a column?
And the more head of water that the system has the higher it is projected up the expansion pipe column?
It's a tricky one and I could have got it wrong, however, the HEATAS tutor was definitely relating it to the more head the system had then the greater the hight the expansion pipe needed to be.
It's not really of any great importance, just something that I've always thought of as being very interesting right through my plumbing career and as a result of this staying in the back of my mind have always given my expansion pipes a good clearance above the tank.

Tappy, 

Any more thoughts anyone?

Ah, does he mean ‘Column Pressure’ where; at a depth of 10M the water pressure is Double that of atmospheric pressure & the Hydrostatic Pressure reduces the water to half it’s original volume. 
So, in CF’s illustration, the column on the right the water level will be higher & the higher the whole column, the lower the volume at the base.👍👍

Yeah, I had a similar situation when I did my ACS resit Thur/Fri/Sat last week. I did my Ted Rogers/Dusty Bin Gas Rating calculation & the Instructor was dumb struck!!......😂😂😂😂
Mind you, they just Train you Parrot Fashion from notes, most are Course Cowboys with no experience. I just keep my mouth shut & let them spout drivel, easiest way to get through it.  Which, as you can imagine, is hard for my good self😂😂 if you start correcting the daft cunts you’re there for days, wasting time!!