The basic principles of system design are being lost due to combi's and sealed systems being more popular bit it should be important to know the basics.
There are dozens of ways to pipe a system all based on a few basic layouts.
Understanding how the position of the feed vent and pump effects the water circulation helps.
Here are a couple of drawings. (this could have been a far longer post but i have simplified a lot of things)
I'll start with this one. Basic common layout for a gravity HW, pumped heating on an old cast iron boiler.
The position of the feed connection to the system is called the neutral point. Everything from the pump to the feed is under positive pressure (from the pump) and everything after the feed is negative.
In this drawing the only part of the system under positive pressure is from the pump to the boiler (as this is effectively the point the feed connects). The rest of the circuit is under negative pressure (the water is being pulled).
Because the cast iron boiler holds so much water it also acts as a neutraliser and the water flow from the pumped side slows right down so has very little effect on the gravity side.
It used to be though having the pump on the return like above would help the gravity circuit. In reality it didn't have any real effect.
A system laid out like this will not draw air or pump over (unless a blockage is introduced).
Same system with the pump on the flow. The heating circuit is now under positive pressure (being pushed). This is the better pump position as pumps can push better than they pull.
Move on a few years and motorised valves are introduced and systems become fully pumped. Still using high water content cast boilers, so we get something like this.
Feed and expansion still connected to boiler so position of the pump does not cause it to pump over or draw air. Pump on the flow so the system is under positive pressure.
Same thing with pump on the return. System under negative pressure.
If you use a low water content boiler or the feed and expansion are taken from the pipework, where or how they are connected starts creating problems.
Pipe it as above and a suction effect is introduced to the expansion pipe and if the pump is set too high it will draw air. Fitted like this to a low water content boiler where the pump speed usually had to be set high this is why a lot of systems fitted in the 80's/90's are full of sludge.
Easy quick way to over come this was to combine the feed with the expansion like this
This stopped the problem as the pump had to pull water from the F&E tank rather than air. It was not ideal though.
So they developed this system
Provided the feed and expansion tees were kept within 150mm of each other, the pump would have very little effect on the expansion pipe. Go much beyond the 150mm spacing and it will pump over.
There are dozens of variations of these layouts. Just think how the position of the feed and expansion will effect the system or introduce blockages and think what would happen.
A bit quickly explained and i have missed out a lot but the basic principals are shown. Hope this is of some use to someone.