Welcome! Here are some tips for using this forum.
Why do 5:1 or even 10:1 Turndown mod cons need a huge buffer tank?
A great thread awhile back had one pro-poster write that the even a 10:1 boiler had a hard time running a single 60ft zone of baseboard at 48F outdoor temp, and a 5:1 boiler trouble with outside temps over 35F. That 20k minimum firing rates are inadequate. Is call for heat too quickly satisfied due to low mass boiler, so it short cycles over and over, burner on and off circs run but no heat gets delivered? whereas it would get delivered if lowest firing rate was 8k or lower OR with a buffer tank burner wouldn't have to fire over and over, zones would just 'sip' from the buffer tank?
Would another solution be to use a reverse indirect as a buffer tank and therefore have one tank instead of two?
We are in the midst of deciding on a boiler and getting a great education from all the pros here on HH from the rich library of threads here.