5.2 Natural Draft Venting

5.2 Natural Draft Venting

Quick Answer: Natural-draft and other Category I appliances vent because hot flue gas is buoyant, not because a fan is pressurizing the pipe. Use Type B gas vent (or a properly lined chimney) for listed draft-hood and Category I equipment. The connector runs from the draft hood or flue collar to the vent. Give the connector real vertical rise, pitch any horizontal run up toward the vent, keep listed clearances, and fasten joints the Massachusetts way (three corrosion-resistant fasteners on single-wall metal). Two Category I appliances may share a common vent; a Category III or IV appliance may not. If the path includes a damper or power exhaust, 248 CMR 5.12(1) requires a non-bypassable solenoid with a CO interlock or draft-proving switch.

This section is PSI Natural Draft. It is still NFPA 54-2012 Chapter 12 as amended by 248 CMR 5.12, not 248 CMR 10.16. You will size the common vent in Chapter 6. Here you learn what may share a vent, how the connector must be built, and which Massachusetts damper and joint rules sit on every Category I job.

Type B, Type B-W, and what they are not

A Type B gas vent is a listed double-wall vent for listed gas appliances with draft hoods and other Category I appliances. The inner wall carries flue gas; the air space cuts clearance to combustibles (the listing, commonly one inch, is on the pipe — do not invent a smaller number). Type B is a non-positive product. It is the everyday chase vent for an 80% furnace and a draft-hood water heater.

Type B-W is the listed vent for listed vented wall furnaces. It is not a generic substitute for Type B in a basement common-vent, and it is not a Category IV concentric kit.

Type L vent appears in the national type-of-system table for certain listed combination gas-and-oil appliances. It is a different listing than Type B. If the appliance can be converted to solid fuel, or is an unlisted combination gas-and-solid-fuel unit, NFPA 54 sends it to a chimney, not to Type B.

Single-wall metal pipe is still in Chapter 12 as a Category I option in limited locations. It needs more clearance to combustibles than Type B (six inches is the figure most national texts print for ordinary single-wall). Massachusetts did not delete single-wall, but 248 CMR 5.12(9) is strict about how you fasten those joints. In occupied houses, Type B is the usual right answer because of clearance and heat loss.

Type B passing through an unused masonry flue is a listed lining method for Category I. NFPA 54 treats that Type B (or a listed lining system) as not exposed to the outdoors for sizing purposes — a fact you will use again on exterior-chimney questions in section 5.3 and in Chapter 6. It is still a gas-only path once you label it as a gas liner.

Draft hoods and draft controls

The draft hood is part of the appliance listing. Leave it in the same room and the same atmospheric pressure as the burner. Do not shrink it, extend a “homemade collector” above it, or enclose it so dilution air cannot enter. The hood needs non-positive pressure at its relief opening. Spillage at the hood is a failed draft test: the vent is restricted, too long horizontally, too cold, too large, or fighting a competing exhaust.

A barometric draft control is a different device, common on some boilers and conversion burners. It is listed with the appliance. It is not a license to put a manual disc damper in the connector and “tune draft” by guess. NFPA 54 Chapter 12 treats manually operated dampers and automatically operated vent dampers as their own subsections. Automatic dampers must be listed and installed so the burner cannot fire against a closed damper. Massachusetts goes further when the vent path is a hood or power exhaust — that is the solenoid rule in 5.12(1), not a sticker on a barometric gate.

Vent connectors versus vents

Say the words the way the code does:

248 CMR 5.12(9) replaces NFPA 54 12.11.6, Joints. Fasten joints between connector sections and connections to flue collars or draft hood outlets by one of these methods:

1. Single-wall metal piping: no fewer than three corrosion-resistant fastening devices at each joint.
2. Listed vent material assembled and connected in accordance with the manufacturer’s instructions.
3. Other approved means.

Three screws is the single-wall exam number. Tape alone is not a fastening device. Listed Type B joints follow the Type B manufacturer, not a random sheet-metal habit.

248 CMR 5.04 added 4.4 Workmanship. Gas piping and vent piping shall be installed straight, level or plumb and shall meet the applicable grade or slope. A roller-coaster connector that ponds condensate or hangs off a water-heater draft hood fails workmanship even if the diameter later “fits a table.”

Rise, pitch, and clearance

Natural draft is a heat engine. The vertical column is the engine. The horizontal connector is a tax.

• Rise. Take as much vertical rise as you can from the draft hood or flue collar before the first elbow toward the chimney. A connector that leaves the hood and immediately travels 12 feet sideways is the usual spillage complaint. On a common-vent, each appliance keeps its own rise into the junction; you do not dump the second connector downward into the first.
• Pitch. Pitch horizontal connectors upward toward the vent or chimney. The national figure candidates are expected to know is 1/4 inch per foot. Massachusetts 5.04 does not replace that grade; it requires you to actually build it.
• Length versus height. Chapter 12 limits how much horizontal connector (and, for draft-hood appliances, how much horizontal vent-plus-connector) you may have relative to vertical height. One national statement still printed in later NFPA 54 errata is that for draft-hood appliances the total horizontal vent plus connector shall not exceed 75 percent of the vent’s vertical height. Chapter 6 will put numbers on FAN and NAT laterals. For this section, remember the physics: too much horizontal, not enough vertical, draft dies.
• Clearance. Hold the listed Type B clearance. Hold the larger single-wall clearance. Keep connectors out of concealed spaces that the listing does not allow. Do not bury single-wall in an exterior wall as if it were Type B.
• Support. Support connectors so joints do not sag and so the pitch does not flatten after the first heating season. 5.04’s “generally accepted good mechanical work practice” is the hook inspectors use.

Through-the-wall Category I terminals are uncommon compared with Category IV sidewalls, but if you do have a low sidewall terminal, 248 CMR 5.12(6) / 12.9.7 and 5.12(7) / 12.9.8 still apply: plate at eight feet when the vent is under seven feet above grade, three feet from a gas meter, and snow height from the manufacturer’s specified method, a city ordinance, or the National Weather Service ten-year average monthly maximum snow depth.

Common venting, in words

Two or more Category I appliances may share one vent or chimney when the common portion is built as a multiple-appliance Category I system. National tables later split NAT+NAT, FAN+NAT, and FAN+FAN. This chapter only needs the eligibility rules.

May share: two draft-hood (NAT) appliances; a fan-assisted Category I furnace plus a draft-hood water heater; two fan-assisted Category I appliances — each still non-positive, each still non-condensing.

May not share that Type B or Category I chimney: any Category II, III, or IV appliance; a positive-pressure special vent; a solid- or liquid-fuel appliance on a gas-only liner; an appliance whose listing says “individual vent only.”

Join connectors with a method that keeps flow up and into the common vertical — a 45-degree wye or a listed common-vent fitting — not a tee that fires one appliance’s products down the other connector. The smallest connector is never smaller than that appliance’s draft-hood outlet or listed flue collar unless an engineering method says otherwise. The common vent is never smaller than the largest connector. Exact diameters are Chapter 6.

Worked common-vent scenario (eligibility only). A Needham ranch basement has an 80,000 Btu/h fan-assisted Category I furnace and a 40,000 Btu/h draft-hood water heater. Both nameplates say Category I. You install a Type B common vent in an interior chase that stays inside the thermal envelope up to the roof. Each connector gets a vertical rise above its hood or collar, then a short pitched run to a wye. The water-heater connector does not drop into the furnace connector. The furnace is the FAN appliance; the water heater is the NAT appliance. In Chapter 6 you will open the FAN+NAT common-vent capacity and the two connector capacities. In this chapter you only need to know why that pairing is legal: both appliances are Category I, the vent is Type B, the chase is not an exterior masonry dump, and nothing Category IV is teed in.

Now change one fact. The owner also wants a new Category IV condensing boiler on the same Type B. Stop. Positive, condensing products do not belong in that common vent. Run a separate listed special vent. If someone instead tries to keep the old oversized masonry chimney and only swap the atmospheric furnace for the fan-assisted furnace, section 5.3’s 248 CMR 5.12(3) answer is no — Massachusetts will not use that existing chimney for an atmospheric-to-fan-assisted replacement.

248 CMR 5.12(3)(c) also caps a reused masonry chimney at seven times the area of the smallest flue connected. That is a chimney-reuse number, but it is how common-vent thinking dies on an old 8×12 fireplace flue with a 3-inch water-heater connector.

Dampers, hoods, and the Massachusetts solenoid

National Chapter 12 already forbids a casual manual damper in a Category I connector. 248 CMR 5.12(1) is the Massachusetts overlay when the appliance is vented through a ventilating or exhaust system that has a damper or power exhaust (a kitchen hood over a gas range is the picture most shops know; the text is broader).

Build this checklist:

1. Gas to the main burners only when the damper is open enough to vent the appliance or the power exhaust is operating.
2. Solenoid on all gas lines serving that equipment.
3. Solenoid may not be bypassed (no jumper, no ball valve around it).
4. Manual-reset control from either a hard-wired listed CO detector that closes the valve on high CO or detector failure, or a draft-proving switch that proves the damper is 100% open or the exhaust is fully operational.
5. Replacements on those hoods or exhaust systems must meet 12.4.4.2; the installer does not hide behind the model-code retroactivity section.

That solenoid is not the same device as a listed automatic vent damper on a Category I draft-hood heater, and it is not the barometric on a boiler. If the question says “ventilating hood” or “power means of exhaust,” reach for 5.12(1).

Official resources

• 248 CMR 5.00 PDF — 5.12(1) damper/power-exhaust solenoid; 5.12(9) connector joints; 5.04 vent-piping workmanship
• Venting of Flue Gases — Type B, connectors, and special vents as permitted gas-fitting work under a licensee
• 248 CMR 4.00 — 2012 NFPA 54 adoption