How to fill this calculator

Chimney work is two different problems that happen to share fittings and a diameter. Either a hot stack is supposed to pull — buoyancy is on your side, and the question is whether it is enough — or a machine is fighting friction, and buoyancy does not get to help. That is the first box on the calculator, and it is not a fuel or a listing.

Draft is a boiler, water heater, or furnace chimney. The stack is hot, outdoor air is cold, and that density difference is theoretical draft, which we always credit, including on UL 103 and UL 1738. A draft-hood appliance has to produce available draft at or above zero or the hood spills; a UL 103 or UL 1738 appliance has to sit inside the outlet-pressure window the manufacturer published. Either way the chimney is doing work for you.

Losses is a generator, engine exhaust, or combustion-air intake, where there is no chimney pulling for you. You size friction against the pressure the machine will tolerate. Theoretical draft is shown on a generator so you can see it, but it is not subtracted. Pass if backpressure is at or under the data-sheet limit.

Heating appliances are Draft. Generators, turbines, and outdoor-air ducts are Losses.

Gas appliances in the United States are classified Category I, II, III, or IV under NFPA 54 / ANSI Z21. That classification is a statement about how the appliance is allowed to be vented, not a brand and not a listing, and it comes down to two questions. Does the vent run under negative pressure (the chimney pulls) or positive pressure (the appliance pushes)? And do the gases stay above dew point, or do they drop below it and make condensate in the vent?

Negative or positive is the static pressure in the vent, not whether a fan sits on the burner — a fan-assist appliance can still be Category I if the manufacturer says the vent runs under suction. Non-condensing or condensing is dew point in the vent, not whether the brochure says “condensing heat exchanger.” A boiler that condenses in the heat exchanger but is vented so the stack stays hot can still be Category I. Use the category the manufacturer printed.

A hooded water heater or an atmospheric boiler is Category I. Most modern condensing boilers are Category IV: they push, and they make condensate. Category III is a non-condensing appliance that pressurizes the vent — power-vent, and some fan-assist jobs the manufacturer listed as positive. Category II is the rare one, where the chimney still pulls but the gases condense, so you still need a condensate-rated special gas vent and a drain.

You will usually find the category on the rating plate, on the manufacturer’s specification sheet, and in the appliance installation and operating manual, and sometimes in a separate venting guide. Confirm the category with the manufacturer if the plate is not readable.

A fan on the appliance does not automatically make it Category III. The manufacturer says whether that fan still leaves the vent under suction (Category I) or pressurizes it (Category III).

The three cards on the calculator are product listings for the vent. They are not the fuel, and they are not the category: a draft hood is a feature of the appliance, natural gas is what is burning, and Category IV is how that appliance is allowed to be vented. Pick the listing that matches what the manufacturer said the vent has to be, then pick the fuel under Burner.

• Draft hood

  Category I · flue 350°F

  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.

• UL 103 heating appliance

  Category I / III · flue 450°F

  Boiler or furnace, no draft hood. Category I when the chimney still pulls; Category III when the appliance IOM says the vent is under positive pressure. Factory-built chimney listed for that pressure — not special gas vent. Flue gas 450°F unless the appliance IOM says otherwise. Pass if outlet pressure P sits in the appliance IOM draft range.

• UL 1738 special gas vent

  Category II / IV · flue 200°F

  Condensing appliance. Special gas vent — not Type B, not a UL 103 chimney. Category II if the chimney still pulls, Category IV if the appliance pushes. Flue gas 200°F unless the appliance IOM says otherwise. Pass if outlet pressure P sits in the appliance IOM draft range.

Hooded water heaters and gravity boilers take Type B (the draft-hood card). Commercial boilers and furnaces without a hood take a UL 103 chimney, or masonry sized as one. Category III is the same listing, listed for positive pressure — the gases are still hot, they just do not get a gravity chimney. Category II and IV take UL 1738 special gas vent, because those jobs make condensate. The manufacturer will name the listing in the spec sheet or the appliance installation and operating manual; they often will not name a chimney manufacturer.

This is the temperature of the gases leaving the appliance, at the outlet, before they travel the chimney. Density, velocity, and theoretical draft all move with it: cooler gas is denser and slower and makes less draft, hotter gas is the opposite. The temperature has to stay under the rating of the vent material.

These are defaults. Confirm flue-gas temperature with the appliance manufacturer. Draft hood is 350°F, because dilution air at the hood drops roughly 500°F combustion gases into the 300–350°F band. UL 103 is 450°F. UL 1738 / condensing is 200°F. A common condensing check is entering-water temperature plus about 30°F, so a 140°F EWT boiler is around 170°F at the outlet. No. 2 oil is around 560°F. Wood is around 500°F. Generator or engine exhaust comes from the data sheet, commonly 800–1,100°F at the turbo outlet.

A better number comes from a stack thermometer or the manufacturer’s published outlet temperature, then the specification sheet, then the appliance installation and operating manual. On a condensing boiler the venting section often gives a maximum vent temperature rather than a typical one; when you are checking draft, use the temperature actually leaving the appliance. A hydronic appliance leaving at 210°F is 210°F even if the UL 103 card default is 450°F.

The vent has to survive the temperature and the pressure of the gases you put in it. The appliance manufacturer tells you which listing the vent has to carry (Type B, UL 103, UL 1738) and roughly how hot the outlet is. The installation manual for the flue material tells you the maximum continuous temperature of that product, the pressure it is listed for, and whether it can take condensate. Those two have to agree. If the flue-material IOM says 149°F, do not put 210°F gas in it.

Read the ladder from cool to hot — PVC, then CPVC, then polypropylene, then stainless special gas vent. UL 1738 is the listing those products sit under, 550°F and below. UL 103 is a different listing, 1,000°F and below, for heating-appliance chimneys. Generator exhaust is hotter still, and it is not a gas-appliance vent listing.

Materials

• PVC

  149°F

  Category IV, only when leaving temperature stays under this.

• CPVC

  194°F

  Category IV, hotter condensing than PVC will take.

• Polypropylene

  230°F

  Category II and IV condensing vent.

• Stainless special gas vent

  550°F and below

  AL29-4C / 316L under UL 1738. Category II and IV.

• Type B double-wall

  400°F typical

  Category I with a draft hood. Negative pressure only.

Listings

• UL 1738 special gas vent

  550°F and below

  Category II and IV. PVC, CPVC, polypropylene, and stainless all sit under this listing. 550°F is the listing ceiling, not the rating of the PVC.

• UL 103 factory-built chimney

  1,000°F and below

  Category I without a hood, and Category III when the chimney is listed for positive pressure. Continuous rating is 1,000°F.

• Generator / engine exhaust

  1,400°F continuous · 1,800°F intermittent

  Diesel and gas engines. Not a gas-appliance vent listing. Exhaust is far hotter than a boiler.

UL 1738 is 550°F and below, which is the listing ceiling rather than the rating of every product sold under it. PVC, CPVC, polypropylene, and stainless special gas vent (AL29-4C, 316L) are all sold as UL 1738, but PVC is still typically 149°F continuous, CPVC 194°F, polypropylene 230°F, and stainless is what uses the top of the range. A Category IV boiler leaving at 130°F can be PVC if the manufacturer of the appliance and the manufacturer of the pipe both allow it; the same boiler on a high-temperature bypass leaving at 210°F is not PVC.

UL 103 is 1,000°F and below. That is a factory-built heating-appliance chimney for Category I without a hood, and for Category III when the product is listed for positive pressure. It is not special gas vent. Type B is cooler still, typically in the 400–480°F band, negative pressure, non-condensing only. Masonry has to be lined for the temperature and the condensate, and the liner manufacturer’s book is the flue book on that job.

Generator and engine exhaust is typically listed 1,400°F continuous and 1,800°F intermittent. Diesel and gas engines leave the turbo in the 800–1,100°F band, sometimes hotter, which is why you do not hang UL 1738 on an engine and why a generator is a Losses job rather than Category I–IV. The engine data sheet is the appliance book; the exhaust-system manufacturer’s installation manual is the flue book.

Pressure matters as well as temperature. Type B and masonry are negative-pressure products. UL 1738 stainless is listed for positive pressure, but the ceiling is the product you bought, not the listing. Special gas vent is typically listed to 20 in. WC. UL 103 and generator exhaust are typically 60 in. WC. Some manufacturers exceed those levels; that has to be confirmed in the flue-material IOM. PVC and polypropylene often sit lower, around 6 in. WC. The Tools tab filters as you type temperature, pressure, and condensate; the product you bought still governs.

IOM means installation and operating manual. Two of them show up on a vent job and they answer different questions, and they are also not the only paper on the desk.

The appliance manufacturer is who knows the fire and the outlet, and that knowledge is scattered on purpose. The rating plate on the unit has input, fuel, sometimes the category, and sometimes a sea-level versus altitude rating. The specification sheet and the submittal usually repeat the input and add efficiency, excess air or CO₂, outlet temperature, the draft or backpressure window, and the allowed vent listing — often easier than the book. The appliance installation and operating manual has category, hood or no hood, venting rules, draft range, the high-altitude table, and combustion setup, and you go there when the spec sheet is silent. Some brands publish draft range only in a venting addendum or a bulletin, and a phone call to a manufacturer tech is a legitimate source. On a generator the engine data sheet is the appliance book: exhaust flow (and whether it is scfm or acfm), exhaust temperature, and maximum backpressure. Those numbers go into this calculator. Tag the appliances B-1, B-2, B-3 on the sheet so the output matches the drawings.

The flue-material manufacturer is who knows the pipe. Their installation manual does not know your boiler’s draft range; it tells you maximum continuous temperature, whether intermittent spikes are allowed, joint method, supports, clearances, pitch, condensate drains, and which terminations the listing allows. An open top, a velocity cone, a rain cap, and a screen are not interchangeable, and some of them will fail a tight gravity stack — that is a fittings problem, but the flue book is what allows the fitting in the first place.

The job itself is height, length, the fittings that are actually in the run, and, if you have one, a combustion analyzer. Drawings first, tape measure when the drawings lie. You size from the appliance manufacturer and you install from the flue book. If the appliance is leaving at 230°F and you used PVC rated 149°F, the flue-material temperature rating is exceeded.

SCFM is standard cubic feet per minute — volume at 70°F. ACFM is actual cubic feet per minute — volume at the temperature in the flue. This calculator sizes on ACFM.

Hot gas takes more space than the same gas at 70°F, so ACFM is the larger number. 1,200 scfm at 900°F is about 3,080 acfm. Size on 3,080. If you type 1,200 as acfm, the flue is undersized.

ACFM = SCFM × (460 + T) / 530

T is flue-gas temperature in Fahrenheit. On the Calculator, switch Flow to scfm and type the catalog number; we convert it at the flue-gas temperature you typed. The Tools tab does the same conversion.

On a boiler or water heater, type the rating-plate input. We compute ACFM from that. On a generator, type ACFM at exhaust temperature. If the data sheet listed SCFM, convert it. Do not type catalog SCFM into the acfm box.

Engine sheet “1,200 acfm at 900°F”: type 1,200 acfm and 900°F. Engine sheet “1,200 scfm at 900°F”: switch Flow to scfm and type 1,200. That is about 3,080 acfm.

Input is how hard the appliance fires at high fire — the heat you are dumping into the products of combustion — and the chimney has to carry that, not the turndown rate. More input is more mass, more acfm, more velocity, and more pressure drop. Read it off the rating plate on the appliance; the specification sheet and the appliance IOM repeat the same number. Type it in the units on the plate: MBH is thousands of Btu/h (200 MBH = 200,000 Btu/h); Btu/h is the same energy not divided by 1,000; GPH is oil nozzles; boiler horsepower is treated as 42,000 Btu/h input per HP; CFH is cubic feet of gas per hour.

Elevation is where the appliance sits. It sets the barometer and the density of the flue gas. Sea level is 0 ft and 29.92 in. Hg; Denver on grade is 5,280 ft, barometer about 24.9. Look up the city if the appliance is on grade. Type the input as it reads on the plate. If the manufacturer derated the burner for altitude — that usually starts around 3,000 to 5,000 ft, and the appliance IOM will say so — type the derated number. We do not invent a second elevation correction on top of that.

15% excess air is the default for gas. Confirm that number with the appliance or burner manufacturer. If they printed a different excess air, CO₂, or O₂, type that. Carbon dioxide, oxygen, and excess air are three ways of looking at the same fire, so you only have to type one of them and the others follow. The mass factor M moves with them.

EA% = 100 × (CO₂max / CO₂ − 1) · O₂ ≈ 20.9 × EA / (100 + EA)

On a draft-hood appliance the hood adds room air after the burner. Do not type the burner’s 15% there — dilution is already in M. Condensing gas defaults to 10% CO₂. If nobody published a number and you do not have an analyzer, leave the default.

On a UL 103 or UL 1738 appliance the manufacturer publishes a window of pressure they will tolerate at the outlet, in inches of water column. Negative is suction, the chimney pulling on the appliance; positive is backpressure, the appliance pushing into the vent. The calculator’s P is that outlet pressure — losses minus theoretical draft — and you size so P sits inside the window. Min is the most suction allowed (the most negative number). Below that is excess draft: specify a barometric damper at the appliance and set it in the field with a manometer, and do not add the barometric as a fitting k, because it relieves whatever is over the setpoint. Max is the most backpressure allowed; above that the flue is too small or too long, so you enlarge it or drop a fitting.

If you leave the box alone we fill −0.25 to +0.25 in. WC, a wide commercial-boiler window used so a missing book does not fail a job that would have passed a typical manufacturer. It is not a substitute for the published number. Real windows are all over the map: some commercial boilers are −0.10 to +0.10, some fan-assist Category I furnaces are a few hundredths of an inch of suction, and many Category IV appliances are a positive window (they push) such as 0 to +0.50 or 0 to +1.0. Draft-hood appliances do not use a range at all — they simply have to draft (Da ≥ 0). Generators do not use a range either; they use a maximum exhaust backpressure from the engine data sheet, commonly 0.25 in. WC, sometimes 0.50, on the Losses tab.

The window is printed on the specification sheet, in a venting addendum, or in the appliance installation and operating manual. Copy both numbers. If the manufacturer never published one, the default ±0.25 is the typical commercial assumption — say so in the notes on the sheet.

P = Δp − Dt · size so min ≤ P ≤ max

Two boxes, two jobs, and the vertical lives in both of them. Vertical rise is the ups only — appliance outlet to the cap — and it is the only number that makes theoretical draft. Horizontal does not go there. Centerline length is the full pipe of that diameter: that same rise, plus every horizontal and offset. Elbows stay on the fittings row; do not add equivalent length.

Twenty-two feet up the stack and six feet of offset is 22 ft of vertical rise and 28 ft of centerline length. Ten feet up in the room, twenty feet across the roof, fifteen feet up the chase is 25 ft of rise and 45 ft of length. A 400-ft building with 25 ft of stack still has 25 ft of rise. If you have two diameters, each flue gets only its own centerline. Take both numbers off the drawings, then confirm with a tape when the building is real. A boot-tee projection used as the small run is often under a foot, so type the stub, not the main. Combustion-air jobs have no rise, because there is no hot stack.

Dt = 0.2554 B H (1/To − 1/Tm) · k_friction = F L / D

A reducer or increaser is not a fitting you count. The run changing size is two flues (or two ducts). Tap Add size change and type the next inside diameter. The calculator then has a large run and a small run, and it needs to know how you actually got from one to the other, because two drawings that both say “10 down to 8” are not the same loss.

If there is room for an elbow, then a reducer, then a flangeless outlet adapter, the 90 lives on the large diameter and the diameter drop is the reducer. Loss is contraction, k = 0.5 [1 − (D2/D1)²] on the small velocity head — 10″ to 8″ is 0.18, not 0.59. The 0.59 some sheets book is the full velocity-head change, 1 − (D2/D1)⁴, which is not an irreversible loss. The adapter on the small end is not extra k if the ID stays that size.

If there is no room for a 90 then a reducer then the adapter, a boot tee, tee, or wye whose small projection off the main is the appliance connection is the other way to step down. The calculator sizes that stub on the takeoff fitting (a boot tee is k = 0.65 on the small velocity head). Count the boot tee on the small run. Length is the projection, often under a foot. Do not also add a concentric reducer; the takeoff already is the size change.

Stepping up is the same button the other way: a 10″ exhaust into a 12″ stack is two flues, an increaser, k = (1 − A1/A2)² on the smaller velocity head. Type 12″ on the second flue. Most appliance connections step down at the unit, either with a reducer in the breeching or with a takeoff off a common, and most chimneys step up, or stay the same, from the breeching into the stack. The fitting on the small run is usually the whole job; another ten feet of large pipe is cheap compared with swapping a boot tee for a 90° tee on the small end.

Reducer k = 0.5 [1 − (D2/D1)²] · Increaser k = (1 − A1/A2)² · boot / tee / wye on the small run → no concentric k

A 12-inch Category I chimney, no draft hood, about 500 acfm leaving at 210°F. The gases are already cool, so this stack does not have much buoyancy to spend. The manufacturer’s literature says Category I, no hood, so this is Draft, UL 103, and you copy the draft range. Flue-gas temperature is 210°F because that is what is leaving, not the 450°F default on the card.

Site at grade, nearly sea level. Vertical rise is the ups only: 8 ft in the room plus 13 ft of chase is 21 ft, not the 12 ft of roof offset. Centerline length is 33 ft — that same 21 ft of rise plus the offset — because horizontal is friction and does not make draft. Fittings on the 12″ are four 45s, one 90, a boot tee, and a drain, and then you pick the top. The flue-material manufacturer is what allows the termination; the calc is what tells you whether this chimney can afford it.

An open top is k = 0 when you size on the 12″ outlet, plus a light screen (Other k 0.25). Theoretical draft about 0.066 in, losses about 0.060 in, available draft just positive — it drafts. A bird screen at the listed k = 0.5 is extra resistance on that open top, so losses go up and available draft goes down; never a credit. Swap in a velocity cone, k = 1.25 instead of the open top, and losses jump to about 0.086 in and available draft goes negative. The cone is there to throw the plume off the roof, but this chimney does not have the draft to spend, so you leave the open top or you enlarge, and you do not also count a rain cap with the cone.

Cool gases and a short stack make a tight gravity chimney. Termination k often decides the result. Also check summer design temperature: less temperature difference means less theoretical draft.

Same room, a duct bringing outdoor air to the appliance, about 450 acfm through the wall. The fan or the appliance has to pull it — there is no chimney helping — so this is Losses, combustion air, not Draft. Theoretical draft is zero because the air is not hotter than outdoors. Air temperature 60°F, allowable from the manufacturer, often 0.25 in. Type acfm; at 60°F, scfm and acfm are almost the same number, but you still pick the unit the schedule printed.

Ten inches across the room, 8″ at the appliance: add a size change. Elbows live on the 10″. The 8″ is the boot-tee projection, often under a foot, so boot tee = 1 on the 8″ and no concentric reducer, because the boot tee is the takeoff. Loss comes out about 0.16 in against 0.25 allowable, so it passes. Swap the boot tee for a 90° tee and it may not, which is why the fitting on the small run matters more than another 10 ft of 10″. Do not also type 0.59 as Other k — that is the old velocity-head-change reducer, and this job is a takeoff.