Step 1 – Load DLL in Python using ctypes
import ctypes from ctypes import c_long, c_double import xlwings as xw
dll = ctypes.CDLL("C:/ASHRAE/ASHRAEDFDB.dll") dll.GetFittingData.argtypes = [c_long, c_double, c_double, c_double, ctypes.POINTER(c_double)]
Step 2 – Call from Excel via xlwings UDF
@xw.func
@xw.arg('fit_id', numbers=int)
@xw.arg('p1', numbers=float)
def ashrae_co(fit_id, p1, p2=0.0, p3=0.0):
co = c_double()
ret = dll.GetFittingData(fit_id, p1, p2, p3, ctypes.byref(co))
return co.value if ret == 0 else None
Now use =ashrae_co(401, 1.5) directly in Excel. ashrae duct fitting database excel link
The ASHRAE Duct Fitting Database (DFDB) is a digital repository of loss coefficients (C₀ or Co) for thousands of duct fittings (elbows, tees, transitions, etc.), based on ASHRAE Fundamentals Handbook, Chapter 34 (or 35 in some editions). The official version is distributed as a Windows DLL (Dynamic Link Library) or as part of commercial HVAC software.
Key file: ASHRAEDFDB.dll (32-bit or 64-bit)
⚠️ Note: ASHRAE does not provide a direct native Excel link. You must use programming or third-party tools to call the DLL from Excel.
The ASHRAE Duct Fitting Database is a collection of loss coefficients (C-factor, or technically Co for round and Co' for rectangular) for hundreds of HVAC fittings, including: Step 1 – Load DLL in Python using
Each fitting has a unique ID code (e.g., "E1-1" for a 90° smooth elbow) and its loss coefficient depends on geometry ratios (e.g., r/D, W/H, angle, area ratios). The official data is maintained by ASHRAE Technical Committee (TC) 5.2.
Important Note: The full database is copyrighted by ASHRAE. You will not find a legitimate, permanent "free Excel download link" on a public website. However, ASHRAE provides licensed access to the data, and several legitimate pathways allow you to bring it into Excel.
While you cannot legally redistribute the full DFDB, ASHRAE provides sample excerpts in the ASHRAE Handbook CD (2005–2013 editions). Many engineers have built their own trimmed Excel tables from these samples.
Some premium duct sizing add-ins (like ASHRAE’s own DFDB Toolkit for .NET) expose an ODBC interface. Step 2 – Call from Excel via xlwings UDF @xw
ASHRAE once offered a separate product called the ASHRAE Duct Fitting Database (DFDB) as a standalone Windows program or Excel add-in. As of recent years, this has evolved.
Where to find the official Excel download link:
This is the only safe, official "excel link" that exists. Save the download link from your ASHRAE account for future updates.