24.3 Filters, Flues, Warm-Air, Gas & Fuel-Oil Appliances

24.3 Filters, Flues, Warm-Air, Gas & Fuel-Oil Appliances

The last Trade D cluster in this chapter is air filters, flues, warm-air appliances, gas appliances, and fuel-oil appliances. The CBT books are FBC Mechanical 2023 Chapters 6, 7, 8, and 9, NFPA 90B (2024) for dwelling warm-air systems, NFPA 90A (2015) for other occupancies, Refrigeration & Air Conditioning Technology, 9th Edition (2021), and F.S. 489.105 for who may touch a gas pipe. Class B installs the same appliances up to 25 tons cooling / 500,000 Btu heating in any one system. An 80,000 Btu/h (80 kBtu) gas furnace is inside Class B heating. A 600,000 Btu rooftop gas module is Class A even if cooling tons look small. Boilers and unfired pressure vessels remain Class A statutory scope; this section is warm-air furnaces and similar appliances, not Chapter 18 boilers.

Quick Answer: Filters go upstream of the coil (605.1). MERV up means ΔP up. HVAC Class A/B may disconnect/reconnect LP or natural-gas appliances on changeouts and shall not install gas fuel lines within buildings (F.S. 489.105). NFPA 90B 2024 is dwelling warm air; 90A 2015 is everything else. Type B vents listed Category I gas; Type L vents oil. Connectors slope up 1/4 inch per foot.

Air filters — MERV versus the fan curve

FBC Mechanical 605.1: heating and air-conditioning systems shall have approved filters. All return, outdoor, and makeup air is filtered upstream from any heat exchanger or coil. Filters sit in an approved convenient location. Liquid adhesives on filters have a flash point not lower than 325°F. 605.2: media and electrostatic filters are listed and labeled; media UL 900, HEPA UL 586, electrostatic UL 867. Dwelling-unit filters shall be designed for the application and are not required to be listed. 605.3: ducts shall spread air evenly over the entire filter — a 20×20 filter sitting in a 16-inch return with a 4-inch dead band is a 605.3 miss.

Minimum Efficiency Reporting Value (MERV) is ASHRAE 52.2. Higher MERV captures smaller particles and adds pressure drop, especially as the media loads in a humid Florida return. Typical 1-inch throwaway (MERV 4–6) might drop 0.1–0.2 in. w.g. clean. A 1-inch MERV 13 can land in the 0.3 in. w.g. neighborhood clean and climb from there. Many residential furnaces and air handlers are selected at about 0.50 in. w.g. total external static pressure (TESP) for the entire duct, coil, and filter. Steal 0.35 for a dirty high-MERV 1-inch and the coil plus attic flex no longer fit on the fan curve: low CFM, high superheat or a frozen coil in cooling, a cracked heat exchanger or high-limit trips in heating.

Worked static. Nameplate TESP 0.50 in. w.g. Coil wet 0.28. Metal plus flex 0.18. Remaining for the filter: 0.04 in. w.g. A 1-inch MERV 13 that needs 0.30 clean does not fit. A 4- or 5-inch media cabinet with the same MERV 13 at lower face velocity (design throwaway face velocity is often kept near 300–350 fpm; deeper media lowers ΔP) might drop 0.10 and leave the blower alive. ECM motors that “hold CFM” will raise watts against that extra static until they fault — they do not repeal 605.1 or the nameplate TESP. NFPA 90B still wants filters that will not burn freely or emit large volumes of smoke; a foam “cut-to-fit” from a craft store is not a 90B filter.

Flues and metal chimneys

A flue is the passage that carries combustion products. A vent is the listed factory-built system (Type B, Type L, Type BW, polypropylene, etc.). A chimney may be masonry or factory-built metal (typically UL 103). FBC Mechanical Chapter 8 governs chimneys, vents, and connectors. Type B (double-wall gas vent, typically UL 441) is for listed, draft-hood and other Category I gas appliances — not for oil, coal, or wood. Type L (typically UL 641) is the oil-appliance vent (and some gas). Single-wall connectors need larger clearance to combustibles (often on the order of 6 inches unless the listing says otherwise); Type B commonly lists about 1 inch. Read the listing on the pipe.

Connector slope: chimney and vent connectors rise toward the chimney or vent not less than 1/4 inch per foot. A sagging connector is a condensate/soot pocket and a failed draft. Termination: masonry and factory-built chimneys commonly use the 3-2-10 idea — at least 3 feet above the roof penetration and 2 feet higher than any portion of the building within 10 feet — while Type B gas vents use the Fuel Gas termination table (height above roof versus roof pitch, plus distance from walls). Do not apply a B-vent 12-inch-above-roof figure to an oil Type L or a masonry chimney by habit. FBC 601.4: chimneys and vents shall not extend into or pass through ducts or plenums unless they are listed for positive pressure and sealed, or enclosed. Category IV condensing furnaces use listed PVC / polypropylene vents — that plastic is not Type B.

Warm-air appliances — NFPA 90B 2024

NFPA 90B, 2024 edition (on the 2026 CBT list) is the standard for warm-air heating and air-conditioning in one- and two-family dwellings and manufactured homes: ducts, filters, clearances, return air, and combustion air for those occupancies. NFPA 90A, 2015 is the air-conditioning and ventilating standard for other occupancies (the corridor, shaft, and damper world of Section 24.2). Do not open 90B for a hospital shaft damper, and do not open 90A as if it rewrote a 90B dwelling filter.

A warm-air appliance is a furnace or similar machine that heats air (gas, oil, or electric) and delivers it in ducts. Upflow, downflow, and horizontal are cabinet orientations, not license classes. Listed clearances to combustibles on the nameplate are code (304.1 / 603.11). Return air still follows FBC 601.5 (not from garage, toilet, unconditioned attic, etc.). Combustion air is Chapter 7 — confined closets need the two openings or a listed outdoor-air kit; a “hot closet” with a louvered door that pulls from a garage is a 601.5 and a combustion-air failure. Category I natural-draft furnaces want a Type B vent and an intact draft hood or listed inducer; Category IV condensing furnaces want the listed plastic vent, a condensate drain, and no draft hood.

Electric warm-air (heat strips, Chapter 20) is still a warm-air appliance. FBC Energy R403.7.2 still forbids electric resistance as the primary heat in Climate Zone 2; Zone 1A counties are the exception on that particular energy item — do not confuse it with 90B.

Gas appliances — the 489.105 fence

F.S. 489.105(3)(f) and (g) (2026): the Class A and Class B air-conditioning scopes do not include liquefied petroleum or natural gas fuel lines within buildings, except for disconnecting or reconnecting changeouts of LP or natural-gas appliances within buildings. You may set a listed gas furnace or package gas module, disconnect the existing appliance union or connector, reconnect that changeout, hang the vent, the ducts, the filter, the controls, and the condensate. You may not run new CSST, black iron, or CSST bonding as a “gas fitter” inside the building. Mechanical contractors (489.105(3)(i)) may install LP and natural-gas fuel lines within buildings. Plumbing contractors (489.105(3)(m)) also include natural gas, LP, and related venting. Subcontract the pipe; keep the HVAC ticket.

Manifold pressure (typical listed values — always use the rating plate): natural gas about 3.5 in. w.c.; LP about 10–11 in. w.c. Orifices and conversion kits are listed pairs. Lighting an LP furnace on a natural-gas orifice (or the reverse) is a CO producer, not a field hack. Clocking the meter and a manometer on the manifold are the commissioning tools. Category I appliances use Type B (or a listed chimney). Category IV use listed positive-pressure plastic. Combustion air, drip legs, sediment traps, and shutoff valves at the appliance are part of a legal reconnect — you still do not extend the fuel line through the building.

Fuel-oil appliances

Fuel-oil warm-air furnaces typically burn No. 2 heating oil through a high-pressure nozzle (gph stamped on the nozzle), with an oil filter, fuel pump, cad cell, and a barometric draft regulator on the vent connector. Type L vent or a listed chimney — not Type B gas vent. Combustion air still follows Chapter 7. Fuel-oil piping and tanks sit in the plumbing definition (fuel oil and gasoline piping) and in mechanical contractor fuel-piping language; they are not a Class A/B air-conditioning grant the way refrigerant pipe is. Treat a changeout like gas: set the listed appliance, reconnect the existing oil line only to the extent the listing and local AHJ treat it as an appliance connection, hang the Type L vent, the barometric, the filter, and the ducts. Do not invent a new buried oil line on an HVAC permit.

Florida HVAC scenario

Treasure Coast Heating, a certified Class B shop in St. Lucie County, change-outs an 80,000 Btu/h LP upflow furnace (under the 500,000 Btu cap) and the matching 3-ton coil. The helper (1) runs new CSST from the tank to a relocated furnace because “489 lets us reconnect gas,” (2) vents the LP furnace with leftover Type L because it was on the truck, (3) stuffs a 1-inch MERV 13 into a 1-inch rack on a 0.50 in. TESP blower, and (4) sags the Type B connector he eventually installs so it drains back toward the draft hood. Four independent failures: 489.105 reconnect is not a license to install fuel lines within the building; Type B is the Category I gas vent, Type L is oil; 605.1 / fan curve — the MERV 13 belongs in a deep media cabinet; connectors slope up 1/4 inch per foot. Subcontract the CSST to a mechanical or plumbing contractor, use the listed B-vent, size the filter for ΔP, and pitch the connector up. NFPA 90B 2024 still applies to this dwelling warm-air system; it does not repeal the statute.

Traps: (1) MERV 16 1-inch as a default “IAQ upgrade.” (2) Filter downstream of the coil “so it stays dry.” (3) Class A running new black-iron gas because the job is over 25 tons. (4) Type B on an oil furnace. (5) Using 90A damper rules as if they were 90B dwelling filter rules, or the reverse.