Available draft Da
Enter a job
—in
Outputwhere the appliance sits
Draft is a boiler, water heater, or furnace chimney. Losses is a generator, engine, or combustion-air job.
Listing
Atmospheric water heater or boiler with a draft hood / draft diverter. Type B vent. Flue gas 350°F unless the appliance IOM says otherwise. The chimney has to pull — pass if available draft Da ≥ 0. No draft range. Two or more on one vent: ASHRAE multiplies total k by 1.5.
Categories I – IV
Two questions: does the vent run under negative pressure (the chimney pulls) or positive pressure (the appliance pushes)? And is the appliance non-condensing or condensing?
| Cat | Pressure | Condensing | Pick |
|---|---|---|---|
| I | Negative — the chimney pulls | Non-condensing — gases stay above dew point | Draft hood, or UL 103 if there is no hood |
| II | Negative — the chimney pulls | Condensing — gases drop below dew point | UL 1738 |
| III | Positive — the appliance pushes | Non-condensing — gases stay above dew point | UL 103 |
| IV | Positive — the appliance pushes | Condensing — gases drop below dew point | UL 1738 |
Tag, make, and model. Input is under Firing. Add a row if more than one unit shares the vent.
Type what the appliance IOM or rating plate printed. Fuel and excess air sit with the input. We convert to the acfm the flue has to carry.
high-fire from the rating plate
350°F unless the appliance IOM says otherwise
5.3% in the vent after dilution
from the analyzer, if you have it
M = 1.60 lb products / 1,000 Btu · hood dilution is already in M. Confirm burner excess air with the appliance manufacturer — do not type it here.
default 60°F
auto 29.92
0.40 ≤18″ · 0.30 above
Height, centerline length, diameter, and fittings for this run.
ups only · appliance outlet to cap
this size: include that rise + horizontals
H is the ups, outlet to cap. L is this diameter’s full centerline — that rise plus every horizontal and offset. Do not type only the horizontal.
Fittings
k on this VhCount what’s in this flue. Elbows are k, not extra length.
10″ down to 8″: leave this flue at 10″, tap that button, type 8″ on the new one. The reducer k shows up between them. Do not put it under Other k. A boot tee on the 8″ is the takeoff instead of a reducer; count the boot tee on the 8″, not both.
prints on the output sheet — optional
ASHRAE, lumped: Dt from total vertical rise. Δp = Σ k × Vh. Tap a ? for how you use that box. Field guide