Available draft Da
Enter a job
—in
where the appliance sits
prints on the output sheet — optional
Tag, make, model, and high-fire input. Several into a common: type each MBH here. The connector carries one. The common carries all.
Draft is a boiler, water heater, or furnace chimney. Losses is a generator, engine, or combustion-air job.
Appliance
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 |
Type what the appliance IOM or rating plate printed. We convert to the actual cfm the flue has to carry.
high-fire from the rating plate
350°F unless the appliance IOM says otherwise
Fuel, excess air, CO₂, O₂. 15% excess air is the default — confirm with the appliance or burner manufacturer. Change one number and the others follow.
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
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 — that is the warning you saw. A boot tee on the 8″ is the takeoff instead of a reducer; count the boot tee on the 8″, not both.
ASHRAE, lumped: Dt from total vertical rise. Δp = Σ k × Vh. Tap a ? for how you use that box. Field guide