First of all, I want to thank those that helped me out deciding between the B3 and the B4. My local heating/cleaning tech who also sells and services Biasi confirmed that the B4 was the size for our house. My set-up includes the outdoor reset with the Taco PC702 along with the Taco SR 504 EXP 4 zone switching relay. I now have 4 adjustable Grundfos circulators, 3 for heatings zones and 1 for our Super Stor,water tank. While we are a good week from putting even the slightest bit of heat on, I do have a circulator question. Each one is adjustable with a Low/Med/High flow setting or anywhere in between. I forgot to ask the guy...but what are the advantages or disadvantage of any of the settings? Currently,they are all set to medium. Forgot to mention, I have the Riello 40 F3 Burner and boy...is the whole set-up quiet! Thanks, Bill **Actually, the burner and boiler is ultra quiet, BUT the exhaust pipe (Roar) is on the loud side. I can't really describe the sound other than the roar of hot air. Any other sound is minimal. Its loud enough that I want to wrap the exhaust pipe in something.