Not a problem, no rush.
Just interested in what you all think & how you go about the issue of minimum flow rates through boilers & to ensure boiler controlled pumps are not damaged by pumping against a closed head.
I will start by saying that the best way in my opinion is to install a low loss header or similar. I realise that in a typical domestic situation that might be viewed as overkill or unnecessary and I am sure that the extra costs involved for time and materials would be too much in most cases.
I have found in the past that built in bypass valves can be inadequate. In some circumstances the flow rate through the bypass has not been enough to prevent overheating particularly when they are the only bypass on an S plan system with spring return valves.
I have also found their lack of bore can raise noise levels beyond the acceptable. That is purely down to the pump (fixed speed) pumping against too high a resistance as you are no doubt aware. I realise that the new pumps are 'clever', however, from experience with variable speed pumps they can be a nuisance. Sometimes they need to be set on 100%.
In order to avoid possible problems and allow the Boiler and pump to work comfortably, (in the absence of a Low loss header or plate etc), I would install a larger automatic bypass externally with a decent pipe run, as that is probably the next best thing.
Until the new Boiler is up and running with its new 'clever' pump, you cannot always predict the performance. It is easy and cheap to install an external bypass that gives as much control as you will need, whilst you're piping things up.
Setting an external auto bypass where there is also an inbuilt bypass is a matter for the Engineer on site to observe what is required. They are both sprung valves and will work together to some degree.
I am talking here of my own thoughts and experiences over the years. No doubt others will think differently.
I hope I answered your question well enough?
If not, let me know.
Do you teach at a college or were you talking about an apprentice who worked for you?