Can someone point me in the right direction to a spreadsheet, formula, or other related information that will show me to approximate/calculate total duct leakage based upon pressure pan tests during a blower door. I’ve been told by a few that it can be done but have not found the resources. Any help would be greatly appreciated.
You can get an approximate number but nothing legitimate / allowed by code
Easiest method is run blower door with ducts & unit sealed off & then open them up - difference is your supposed leakage
There is a method called Modified Blower Door Subtraction that can be used to approximate duct leakage to the outside, but not total duct leakage. That method also requires measuring the pressure in the duct and modifying your readings based on that number. Here is a link to the TEC Blower Door Manual: https://energyconservatory.com/support/minneapolis-blower-door-manu... It is explained on Page 44 of that manual.
The DeltaQ duct leakage test uses only a blower door fan to measure duct leakage to outside at actual HVAC system pressures, as they exist/occur during system operation. So, for example, a leak at the return plenum is not treated the same as a leak near a duct termination at a register. The test consists of multiple pressurization and depressurization ramps up and down with the blower door; these are done first with the AHU off and then with the AHU on. The differences in flow and pressure at various ramping levels allow extraction of actual leakage flows to outside for supply and return independently. Duct leakage to outside is accurate to roughly 1% of the envelope's Q50 flow rate (i.e., you can accurately measure 15 cfm of duct leakage in a 1500 cfm50 home). The test is sensitive to wind (more than simple envelope pressurization), which is a downside. The benefit is you get actual leakage at operational pressures and flows, and you get to set up the blower door fan once, do not mask off registers or set up a duct blaster fan. A pretty good test, IMHO.
Duct leaks are very common; in many homes, duct leaks are responsible for significant energy losses.
For ducts located in an unconditioned attic, any leaks in the supply system tend to depressurize a house, while return-system leaks tend to pressurize a house. Either condition can cause problems.
Duct leaks outside of a home’s thermal envelope waste more energy than duct leaks inside a home’s thermal envelope.
Even if ducts are located inside of a home’s thermal envelope, duct leaks can still connect to the outdoors. For example, supply system leaks in a ceiling between the first and second floors of a two-story home can pressurize the joist bay, forcing conditioned air outdoors through cracks in the rim joist area.
It’s much easier to seal duct seams during new construction than in an existing house.
Testing for duct leakage:
Energy auditors have developed several methods for testing duct tightness. These methods vary from fast and dirty to time-consuming and accurate. Builders interested in tight duct systems should familiarize themselves with the range of available duct testing options:
Using only a blower door;
Using a blower door and a pressure pan;
Using a Duct Blaster;
Using a Duct Blaster and a blower door; and
Using a theatrical fog machine.