Background:

Located in the Dallas TX area, in a 130+ year old home (completely renovated/insulated/etc) that has had a totally hydronic geothermal system (radiant floor and water coils) in place and functioning wonderfully for 10 years. Due to reasons to complicated to explain in a post we have lost our water source for the system. While exploring options for restoring the water source, I am considering adding a gas boiler to the system which would provide heat for this winter giving us more time to deal with the water source issue and if restored options (gas or electric geothemal) for future heating.

Pertinent facts :

approx 130K BTUs required
past water temp for heating 120 degrees
buffer tank 120 gal
all pumps and tekmar dual point controls already in place
if installed the boiler will also replace gas hot water (will add indirect)

Question:

Because the buffer tank is already in place, won't it negate the efficiency difference between a modulating boiler and a "Bang - Bang" ? Won't the buffer tank and proper control set points actually perform the "modulating"? In this scenario is there any value in spending more in initial cost and on going maintenance without the efficiency payback?