This article is supplementary to the ultimate guide to balancing, please also check that out!
There seems to be a popular misunderstanding within the industry that a DT of 20°c is a specific commissioning target for gas boiler systems. A delta t of 20°c is not something to precisely aim for when commissioning, it is simply something to base the system design off.
It helps us accurately size emitters and also pipe sizes. We could have chosen 18c or 21c, it doesn't really matter. 20 is just a nice round number, Viessmann in fact have more recently started advising DT's of around 15°c even without hydraulic separation, however they also suggest lower design temperatures.
When it comes to commissioning the system, you can select a pump speed that gets you close to dt 20 but you don't need to go further than that. For example, If using a heat-only boiler with an external pump and setting 2 shows a DT of 14, and setting 1 a DT of 22, use setting one and recheck after balancing the system. There is no need to select setting 2 and close valves to strangle the flow from the boiler. Also, as your demand drops, your DT will naturally shrink.
More advanced boilers with internal pumps will have much more refinement to close into DT 20 but again this is certainly not important at this moment, especially as you haven't balanced the emitters yet.
What's more, your delta T of 20 will be less important and accurate as flow and return temperature are lowered by the modulating contoller.
As the controller causes the return temperature to drop and get ever closer to the room temperature, it becomes increasingly more difficult to shed the heat. This is because the 'heat transfer coefficient' of the radiators drop. For example, if you have a room of 20°c you'll never be able to get a DT of 20 if your flow temperature is for example 30°c. A more realistic target is a DT that's around 30% of the flow temperature.
For example; If we have a flow temperature of 70°c, (70 x 0.3) gives a DT of 21°c. If your flow temperature is 50°c this would give a DT of 15°c (50 X 0.3) and so on. This is not exact, it's just to get the flow rate in the right ballpark. There are more complex calculations for this is you need but really not worth it.
Its also worth mentioning, that even if your at DT is 15, this may increase your return temperature, but only by 2.5°c. As its the mean temperature of your radiator your modulating control is really adjusting, it just uses the flow temperature to help target that. Please comment if this needs more explanation!?
Regardless off this you still want to try and get the dt as WIDE as possible. It is after all the return temperature that dictates condense volume as discussed here.
Heat pumps however require a much more accurate DT in the 5 to 7 range, failure to not maintain this will result in high and low pressure issues within the unit.