What is considered large water volume, which would require the bypass pipe

PeteA

First time posting but long time viewer of the sites great content.

I am doing a ton of research and ready to revamp my heating system in my home. I currently have an old cast iron hot water boiler that feeds 5 radiators and 3 baseboards (14ft total), it's a 2 pipe (formerly gravity) system feeding the radiators and baseboards. regarding the radiators by the looks of some of the manuals I've seen circulated on this site and others I don't believe they are "large" compared to some of the dimensions I've seen radiators can be sized. One of the largest ones of the 5 is 20" tall 5 1/2" wide 5 thin tubes per section 38 sections. according to OCS each section holds a little more than .08 gallons of water per section.
what I am shooting for:
I am building my manifolds using 1 1/4 black pipe parts with full port 1/2 ball valves with 1/2 pex compression fitting where I will run 1/2 pex al pex home runs to every radiator and baseboard in the house. The new boiler is a crown Aruba 5 series that I selected based on both a heat loss calculation and a connected load calculation. My main question is regarding the boiler bypass requirement. I've looked through the Burnham website and the crown manual and I can't quite put my finger on exactly what defines "large" volume of water.
I am sure all of you will say that if I was to just connect the new boiler to the 3" pipe system then that's a large volume system totally understand that, but where does the "large" volume determination drop off? Is it driven by just the fact that I will still have radiators or once I eliminate all of the large pipes in the basement is this a more normal volume of water for a hot water system? If I still have to do the bypass piping would you folks suggest the simple one with the valves for throttling the water or the fancier calefi automatic 3 way valve. I am willing to spend the extra couple of dollars for the calefi since this will be a basically set it and forget it since it will regulate the temps automatically.

Thanks in advance, this site is awesome for all of the great help all of you offer everyone.

Pete

hot_rod

Calculation-wise you would determine the output of all the radiators and see how it matches the output of the boiler.  The radiators are in charge of the operating condition, temperature, of the boiler.

Sounds like you have both a high mass, cast iron radiators, and high water content with piping and radiators.  Your system sounds like the perfect candidate for return temperature protection.

The other option would be to pipe it up, run and see how long before return temperature rises to 130F. 

This idronics explains more about thermal equilibrium

PeteA

Thanks @hot_rod
I'll read through the idronics information to better understand the importance of getting the temps equalized. But in general based on the other main pieces of your response it would be safer for the life of the boiler and probably best for the system to plan on installing either the 3 way valve for the auto balancing to boost and maintain the 130 degrees or a bypass with throttling valves simply because of the radiator/basebord mix in the house. Better to be safe than sorry and have to cut it in later on.

hot_rod

yes, it would be easiest to install now instead of mid season