None of the guides that I can find fully convey what you are trying to achieve.
Ere, let me have a go...
Consider the fact that different rooms in the house require different amounts of heat input to keep them at a stable, comfortable, temperature. Thus, heatloss calculations for different rooms will result in radiators of different sizes being specified (in simplistic terms, bigger rooms or rooms with bigger outside walls = bigger rads required).
Now, it stands to reason that a small radiator of only 250W output needs less heated water pumping through it than a massive 3000W one. If you were manning the pump you'd expect to have to push more water through the big one than the small.
However, all other things being equal, and with all valves fully open, with a single pump we'd be getting the same amount of water pushed through the 250W radiator as the 3000W. There's only so much heat the tiny 250W radiator can dissipate from the water hence the temperature of the water coming out (return) will only be a few degrees below that going in (flow). Conversely, the 3000W radiator can dissipate plenty such that its return temp is may well end up significantly lower than the flow.
There are numerous negative consequences of this situation:
1. The high return temperature of the 250W radiator may cause the boiler to cycle,
2. The 3000W radiator may be getting insufficient flow to realise its full heat output capability,
3. The rooms will heat up unevenly given that each radiator has a different average temperature (flow-return / 2),
4. Some rooms may not be getting their required heat output, some may be getting too much.
With only a certain amount of flow to go round if we throttle back the smaller radiators we are able to provide greater flow to the other, larger, radiators in the system. Furthermore, recall that we were assuming all other things being equal however in practice this is not the case - some radiators are further away from the pump/boiler hence their resistance to flow will vary. The higher the resistance the lesser the flow. Like electrical current, flowing water always takes the least path of resistance thus the furthest radiators can end up getting less than their fair share of flow even if they happen to require the most (i.e. because they're bigger).
Balancing is therefore required to ensure that each radiator gets the necessary flow - no more no less - in accordance with its size and position in the circuit. The aim is simply to get the same temperature drop across each radiator because then each one will perform the same relative to its design specification. Given that we have the same flow temperature to each radiator (more or less - the furthest may lose a degree or so due to losses in the pipes) then if each radiator has the same drop across it then each has the same average temperature across it. Radiator charts give outputs a given 'DeltaT' temperature e.g. 50C means an average radiator temperature of 70C in a 20C room thus if we achieve that then we achieve the specified output.
So, if you're still with me(!) you can hopefully see why we desire the same temperature drop across all radiators. But what temperature drop should that be? That is not determined by the radiators - remember they are designed for a given temperature relative to a room temperature of 20C. Thus, it doesn't matter if we have a flow of 75 and return of 65C or a flow of 80 and return of 60C - we still have an average temperature of 70C which is 50C higher than 20C as per the design specs.
Rather, the desired temperature drop depends entirely on the boiler and what it has been designed to provide. By convention, boilers were always usually designed to work with an 11C difference between flow and return. That is, the burner would heat the water flowing out of the boiler 11C higher than that coming in.
Condensing boilers, as you are aware, operate most efficiently with as low a return temperature as possible. Condensing mode starts around the 55C and so with an 11C difference than means our flow would have to be around 66C. That's not much good in the depths of winter - we need hotter rads to offset the extra heat losses. Hence, many condensing boilers have been designed to operate at a 20C drop hence we can have our 55C return for condensing benefits whilst still having a 75C flow to cope with winter heating demands.
Note however that not all condensing boilers are designed for a 20C drop - many are simply rehashes of old non-condensing boiler designs thus they inherit the 11C drop too. The installation manual will provide the specification drop.
So, we know our target differential - 11C or 20C as specified - but our balancing technique thus far can only make the drops the same across each radiator because we wouldn't want to throttle back every radiator given the strain on the pump and potential for noise. However, we are usually able to change one other variable which will have an effect on the whole system - pump speed. If we slow the pump down there will be less flow hence our radiators will dissipate more heat. End result = higher drops across the board. The converse is also true.
Changing the pump speed may not always be an option - many combis require the pump speed to be fixed at max in order for their instantaneous heating of hot water to occur (they have such a small loop of water in the primary circuit it has to be pumped round fast to transfer sufficient heat to the secondary side). Also, pumps only have a finite range and you may find it can't go low/high enough or that it becomes too low to overcome the resistance of the pipes etc (or to supply enough heat to the bigger radiators), or too high a flow resulting in excessive noise.
All in all, balancing is one thing, maximising condensing opportunity is another - it is not always possible to achieve both if the system has not been designed properly to do so.
I know that's a lot to take in and there are a whole range of different aspects to it but I hope this goes some way to addressing a few of your queries. Shout if you need further clarification and we go through it in more detail.
Mathew
