The buffer (in any plumbing configuration- either in-series or at the point of hydraulic seperation in a primary/secondary) doesn't appreciably affect the average return temp or the overall duty cycle, but HUGELY affects the number of cycles and efficiency losses related to the fixed losses of ignition & flue purges, and the wear & tear on the boiler.
Cast iron or flat panel radiators add mass (and work linearly at lower temperatures than fin-tube), but you still have to do the math on the total mass & heat-emission at the temperature you're operating to know whether you'd need a buffer. But since you probabably still need a buffer on the other two zones to run at 120F withtout short-cycling, making it a commoned buffer in series with the return manifold and the boiler is probably the right thing to do ( and not a seperate buffer just for the basement zone.) Plumbed on the return manifold is slightly better than on the output side, since then it doesn't slow the response time of the system when under low- partial load- the radiation gets to it's peak temp sooner without having to raise the temp of the buffer first.
A quick refresher on the simplified math: At your low-load minimum of 120F your existing 40' zone with the 31lbs of water the fin tube would be delivering something on the order of 200BTU/foot x 40' =1600 BTU/hr into the rooms, but the boiler is putting out nearly 10x that at 15000 BTU/hr even at it's min-modulation, for a ~13,400 BTU/hr shortfall. Assuming a 10F hysteresis in the boiler you'd get 31lb x 10F/13,400= 0.023 hour= 1.4 minute (83 seconds) of burns- a seriously short cycle. If you raise the temp to 130F the output of the fin tube would rise to something like 250BTU/foot stretching it out a bit, but not enough to matter.
To deliver the full 15K in a 40' zone you need 375 BTU/ft, which would take 150F water, with a return temp over the condensing point. You'd need to buffer your existing zones to be able to operate in the condensing region without short-cycling the boiler into both lower efficiency & an early grave.
To stretch the burns to ~10minutes (.17 hours) with 13.4K of extra takes a system with 10F of hysteresis at the boiler takes 13400 x 0.17/10F= 228lbs of water, which is 228/8.34= 27 gallons. You already have 3.7 gallons in the zone, so a 20-25 gallon buffer will do. But if the hysteresis is closer half that (sometimes is) a 40gallon buffer would be better. (30-50 gallon electric HW tanks are CHEAP compared to a 30 gallon Boiler Buddy or a Ergomax BT26 or BT48 buffer, and won't add much pump head to a low-flow system like this.) It would take ridiculously large diameter & expensive distribution plumbing to achieve that much thermal mass.
Cast iron or flat panel radiators add mass (and work linearly at lower temperatures than fin-tube), but you still have to do the math on the total mass & heat-emission at the temperature you're operating to know whether you'd need a buffer. But since you probabably still need a buffer on the other two zones to run at 120F withtout short-cycling, making it a commoned buffer in series with the return manifold and the boiler is probably the right thing to do ( and not a seperate buffer just for the basement zone.) Plumbed on the return manifold is slightly better than on the output side, since then it doesn't slow the response time of the system when under low- partial load- the radiation gets to it's peak temp sooner without having to raise the temp of the buffer first.
A quick refresher on the simplified math: At your low-load minimum of 120F your existing 40' zone with the 31lbs of water the fin tube would be delivering something on the order of 200BTU/foot x 40' =1600 BTU/hr into the rooms, but the boiler is putting out nearly 10x that at 15000 BTU/hr even at it's min-modulation, for a ~13,400 BTU/hr shortfall. Assuming a 10F hysteresis in the boiler you'd get 31lb x 10F/13,400= 0.023 hour= 1.4 minute (83 seconds) of burns- a seriously short cycle. If you raise the temp to 130F the output of the fin tube would rise to something like 250BTU/foot stretching it out a bit, but not enough to matter.
To deliver the full 15K in a 40' zone you need 375 BTU/ft, which would take 150F water, with a return temp over the condensing point. You'd need to buffer your existing zones to be able to operate in the condensing region without short-cycling the boiler into both lower efficiency & an early grave.
To stretch the burns to ~10minutes (.17 hours) with 13.4K of extra takes a system with 10F of hysteresis at the boiler takes 13400 x 0.17/10F= 228lbs of water, which is 228/8.34= 27 gallons. You already have 3.7 gallons in the zone, so a 20-25 gallon buffer will do. But if the hysteresis is closer half that (sometimes is) a 40gallon buffer would be better. (30-50 gallon electric HW tanks are CHEAP compared to a 30 gallon Boiler Buddy or a Ergomax BT26 or BT48 buffer, and won't add much pump head to a low-flow system like this.) It would take ridiculously large diameter & expensive distribution plumbing to achieve that much thermal mass.