Jason,
I just had this issue. GBCI has not reviewed it yet, but my strategy was to average the two conditions. 180 with 130 degree return is a 50 degree delta-T. 150 with 130 degree is a 20 degree delta-T. I ran the simulation once with the boiler and plant at a 50 degree delta-T and ran it a second time with a 20 degree delta-T. Then I used the average values of the two runs. I left the system delta-T at 50 for both runs. The system delta-T did not seen to effect the simulation results.

A better approximation might be a weighted average using the ambient dry bulb in the TMY data to figure how many hours would be spent at each delta-T. Of course you'd want to exclude the hours when the system is not scheduled to operate.

When I emailed Carrier HAP support they said, "We cannot currently model Hot Water reset of boiler temperature in HAP 4.51. For purposes of LEED, you must write a narrative explaining that the program you are using for LEED does not have this capability.

We do have Supply Air Temp reset on a hot water coil in an air system.

Hot Water Boiler reset is on our list for a future version of HAP."

Good luck.

Reba Schaber Mechanical Engineer, P.E.
LEED Accredited Professional