5.3 Chimneys

5.3 Chimneys

Quick Answer: A chimney is a listed factory-built, metal, or masonry shaft, not a leftover hole in the brick. Masonry flues that serve gas must be lined and have unused openings closed. Match size to appliance input: too small and the flue cannot carry the load; too large and the gases cool, lose draft, and condense. 248 CMR 5.12(3) is the Massachusetts reuse test — and it forbids using an existing chimney when you replace an atmospheric appliance with a fan-assisted one. Inspect before you connect. Cap and cleanout belong on the chimney. Gas-only liners stay gas-only. Abandoned flues get sealed, not ignored.

PSI lists Chimneys next to natural draft. In Massachusetts the chimney question is often a 248 CMR 5.12(3) question wearing an NFPA 54 costume. 248 CMR 4.02 still points you at NFPA 54-2012 Chapter 12 (chimneys live in 12.6 in that edition). 248 CMR 7.02(4) adds chimney language for large gas utilization equipment. The Board’s Venting of Flue Gases policy puts liners, metal chimneys, and factory-built chimneys under the same licensee-and-permit umbrella as Type B.

Masonry, metal, and factory-built — the three legal chimneys

NFPA 54 12.6 (2012 numbering) splits chimneys three ways:

Masonry liners are approved clay flue lining, a listed chimney lining system, or another approved material that resists corrosion, erosion, softening, or cracking from vent gases at temperatures up to the figure the national chimney section prints (1,800°F / 982°C for the general clay/listed-liner sentence). The exception you will see on gas-only jobs: a masonry flue may instead be lined with a system listed only for draft-hood, Category I, and Type B-listed appliances. That liner is installed to its instructions, and a permanent label at the connection point must read that the liner is for appliances that burn gas only and that solid- or liquid-fuel appliances or incinerators shall not be connected.

That label is the mixed-fuel rule in one sentence. If the liner is gas-only, do not later “save the flue” by teeing in an oil boiler or a wood stove. Appliances that can be converted to solid fuel, unlisted combination gas-and-solid units, and indoor incinerators go to a chimney built for those fuels — not to a Type B and not to a gas-only liner.

Unused openings in the chimney (old cleanout doors, former thimbles, abandoned fireplace throats) must be closed with materials that belong on a chimney. An open thimble is a false draft break: it cools the flue, spills, and can feed a fire into the room.

248 CMR 5.12(5) deletes the last sentence of NFPA 54 12.9.4. That is a through-the-wall termination subsection, not a masonry-height subsection. Do not invent the deleted sentence. Use the remaining 12.9 text plus Massachusetts 12.9.7 and 12.9.8 when the job leaves the chimney and goes out a wall.

Size versus appliance input — and the Massachusetts seven-times rule

Chimney capacity must remove the maximum volume of flue gas the connected equipment can produce, under the least draft you will actually see (cold start, wind, competing exhausts, other appliances on the same shaft). 248 CMR 7.02(4)(a) says that in plain language for equipment in the large-gas book: secure connection, good condition, proper construction, ample size, all weather; design for maximum volume and other combustion equipment on the same chimney; resist wind, weather, interior corrosion, and temperature; pre-inspect and test if needed; correct defects before use; check an existing chimney’s area, arrangement, and height against the new load, including the temperature and pressure of gases entering the chimney.

For ordinary 248 CMR 5.00 jobs, NFPA 54 Chapter 13 (next chapter) supplies the Category I tables. This chapter needs the failure modes:

• Too small: high pressure at the connector, spillage at the draft hood, rollout, incomplete combustion.
• Too large: slow velocity, cooled walls, condensate, rusted connectors, collapsed draft on mild days, especially on exterior masonry.

248 CMR 5.12(3) replaces the exception to NFPA 54 12.6.4.2. Tile-lined chimneys, or existing chimneys that do not meet 12.6.4.2, may be used with a new or replacement gas appliance — including a gas appliance replacing oil — only when all of the following are true:

1. The existing appliance operated with non-positive vent static pressure and did not create condensation in the chimney.
2. You are not replacing an atmospheric appliance with a fan-assisted appliance. Existing chimneys may not be utilized for that swap.
3. The chimney’s interior area is not greater than seven times the area of the smallest flue connected to that chimney.

Worked area check. The smallest connector is 4 inch round. Area is π × 2² ≈ 12.6 in². Seven times that is about 88 in². A sound, lined flue whose interior is under that figure can pass the area test. A big unused fireplace shaft — often well above 88 in² — fails 5.12(3)(c) even if someone “always vented the heater there.” Passing the area test does not override (a) or (b). A fan-assisted Category I furnace replacing an atmospheric furnace cannot use that existing chimney under (b), period.

Why (b) exists: fan-assisted Category I appliances send less dilution air and cooler products than a draft-hood unit of the same input. An oversized, cold masonry flue that “worked” on an atmospheric heater becomes a condenser and a downdraft machine on FAN equipment. Massachusetts did not leave that to installer judgment.

Exterior chimneys

A chimney with one or more walls exposed to the outdoors below the roof line is an exterior chimney. It is a heat sink. National Category I tables that assume an interior chimney do not apply the same way; later NFPA 54 Chapter 13 text even gives separate clay-tile exterior masonry tables and extra conditions (Type B double-wall connector, at least one draft-hood appliance, combined input under a maximum, each space-heating appliance above a minimum). You will use those tables in Chapter 6. Here, remember the inspection story: exterior flues condense, lose draft, and fail reuse more often. Type B or a listed lining system passing through an unused masonry flue is not treated as exposed to the outdoors — that is how you turn a dead exterior shaft into an interior-equivalent Category I vent without pretending the raw brick is fine.

Roof termination for a residential or low-heat chimney, in the national chimney section, is the 3-2-10 picture: at least 3 feet above the highest roof point it passes through, and at least 2 feet higher than any part of the building within 10 feet horizontally. Factory-built chimneys follow their listing if the listing is tighter.

Inspection, caps, cleanouts, abandoned flues

Before any new connector enters an existing chimney, look and, if needed, test. The Board policy and 7.02(4)(a)3–4 are the large-equipment version of the same duty; Chapter 12 expects it on dwelling jobs too. Check:

• Blockage (nests, broken tile, collapsed liner, leftover oil-vent debris).
• Liner continuity and whether the liner is gas-only or a full NFPA 211 clay/listed liner.
• Openings that should have been sealed.
• Area and height against 5.12(3) and the Chapter 13 capacity you will apply next.
• Moisture and condensate stains — a chimney that already condensed fails 5.12(3)(a).
• Connector entry: thimble or listed fitting, not a raw hole; Massachusetts 12.11.6 fastening still applies at the collar.

A cleanout at the base lets you inspect and remove debris without cutting the connector. A cap keeps out rain, snow, and animals. The cap must not reduce the chimney below the capacity the appliance needs; a decorative plate that chokes the outlet is a failed draft test waiting for a CO call.

Abandoned chimneys are still building shafts. If you permanently disconnect the last appliance, close the connector opening and any unused thimbles so the flue is not an open path for fire, vermin, or leftover combustion products from another floor. Do not leave a “maybe we’ll use it” hole behind a new sealed Category IV vent. If the shaft will be reused later, it goes back through inspection + 5.12(3), not through memory.

Combining fuels. Do not connect a gas appliance to a flue that still serves oil, coal, or wood unless the chimney is designed and listed for that combination. The gas-only liner label is a hard stop. Decorative gas appliances in vented fireplaces still use the fireplace chimney, and 248 CMR 5.10 requires the fireplace damper to be removed or welded fully open (and bedroom/bath decorative units to be direct-vent). That is chimney use, not Type B.

Licensee duty, large equipment, and sidewall leftovers

Venting of Flue Gases says chimney lining systems, metal and factory-built chimneys, combustion hoods, and Type B may be assembled by qualified persons only when a master or journeyman plumber or gas fitter supervises and holds the 248 CMR 3.00 permit. The licensee remains responsible and must attend the final. For 400,000 Btu/h and greater, 248 CMR 7.00 is the book, but the chimney still has to be pre-inspected, sized for the worst draft, and sound.

If the job abandons the chimney for a through-the-wall special vent, you have left 12.6 and entered 12.9 as amended: three feet from the gas meter, snow height from the manufacturer, a local ordinance, or NWS ten-year statistics, and the eight-foot identification plate with 1/2-inch letters reading GAS VENT DIRECTLY BELOW. KEEP CLEAR OF ALL OBSTRUCTIONS when the terminal is under seven feet above finished grade. The old chimney, if abandoned, still gets sealed.

248 CMR 5.09 / 9.1.23 still requires the CO detector on the appliance floor (and on every habitable level in a dwelling) when you install or replace the vented appliance that now uses that chimney or that new sidewall.

Field scenario

A Quincy two-family has a tile-lined interior masonry chimney that once served an atmospheric gas boiler and still serves a draft-hood water heater. The boiler is being replaced with a fan-assisted Category I boiler of similar input. 248 CMR 5.12(3)(b) ends the masonry plan. You do not “check the seven-times rule and proceed.” You install a Type B or other Category I system that Chapter 12 allows — often Type B inside the unused flue, labeled gas only, with the water heater reconnected as a legal FAN+NAT common-vent if the new Type B is sized for both (Chapter 6). If the replacement had been another atmospheric Category I boiler, you would still prove non-positive, non-condensing history, area ≤ 7 × smallest flue, a sound liner, sealed unused openings, cleanout, cap, and a passing draft test.

Official resources

• 248 CMR 5.00 PDF — 5.12(3) existing-chimney exception; 5.12(6)–(7) sidewall plate and meter/snow rules
• 248 CMR 7.00: Large Gas Utilization Equipment — 7.02(4) chimney condition, capacity, and pre-inspection
• Venting of Flue Gases — liners and factory-built chimneys under the licensee’s permit
• Policies, regulations, and statutes for plumbers and gas fitters — 5.00, 7.00, and the flue-gas policy on one list