Water heater or furnace closet: clearance and air requirements

Checklist

Find the clearance label on the appliance (Rheem, A. O. Smith, Goodman and Carrier point installers to it) and keep the distances it lists.
Leave 24 in. clear in front of a furnace for service. The manuals word it from "recommended" to "required", so check yours.
Do not block the combustion air openings on the tank or furnace.
If a gas appliance takes its air from the closet and the closet is a confined space, plan two permanent air openings: one within 12 in. of the ceiling, one within 12 in. of the floor.
Keep the gas shutoff valve, the relief valve and its discharge line reachable and uncovered.
Store no gasoline, paint, solvents or rags in the closet.

Each point is explained below with its source: the manual page or the government agency.

Manufacturers do allow gas water heaters and furnaces in closets and alcoves, on conditions printed in their installation manuals: minimum clearances around the appliance and room in front of it for service. A gas appliance that takes its combustion air from a small enclosed space also needs permanent air openings (Rheem AP23060, pp. 7–8; Carrier 58SC0A/58SC1A, pp. 6–8). The numbers differ by brand and series, so each one below comes with its manual and page. On your own appliance, find the clearance label first: Rheem, A. O. Smith, Goodman and Carrier all direct installers to it (Rheem AP23060, p. 7; A. O. Smith 315628-002, p. 8; Goodman IOG-2037D, p. 7; Carrier 58SC0A/58SC1A, p. 6).

Gas tank water heaters

Rheem and A. O. Smith give minimum clearances from combustible construction. Rheem defines the term: “Combustible construction refers to adjacent walls and ceilings and should not be confused with combustible or flammable products and materials” (Rheem AP23060, p. 7). Its drawings separate an alcove, shown with an open front, from a closet, shown with the front enclosed (Rheem AP22858, p. 7).

Brand and series	Top	Sides	Front	Service
Rheem atmospheric, 29–60 gal, alcove (Rheem AP23060, p. 7)	12 in.	0 in.	3 in.	24 in. front and top, “should be available”
Rheem atmospheric, 29–60 gal, closet (Rheem AP23060, p. 7)	12 in.	1 in.	3 in.	24 in. front and top, “should be available”
Rheem high-BTU atmospheric, 98 gal, alcove (Rheem AP22858, p. 7)	12 in.	1 in.	3 in.	24 in. front and top, “should be available”
Rheem high-BTU atmospheric, 48, 63 and 75 gal, alcove (Rheem AP22858, p. 7)	12 in.	0 in.	3 in.	24 in. front and top, “should be available”
Rheem high-BTU atmospheric, all sizes, closet (Rheem AP22858, p. 7)	12 in.	1 in.	3 in.	24 in. front and top, “should be available”
Rheem power vent, 40 and 50 gal (Rheem AP22510, p. 7)	12 in.	1 in.	3 in.	No number; checklist: “Sufficient room to service heater” (Rheem AP22510, p. 26)
Rheem PowerDirectVent, 40, 50, 65 and 75 gal (Rheem AP14896-2, p. 7)	12 in.	0 in.	5 in.	No number
A. O. Smith residential gas (A. O. Smith 315628-002, p. 8)	12 in. “on most models”	0 in.	4 in.	No number: “adequate clearances for servicing” (A. O. Smith 315628-002, p. 7)
A. O. Smith direct vent (A. O. Smith 326940-001, p. 12)	12 in.	0 in.	5.5 in.	24 in. “should be considered”; 5.5 in. “must be allowed” for access to parts

Rheem’s AP23060 manual measures the front clearance “from the Combination Gas Control (Thermostat) to the closet door”; the power vent and PowerDirectVent manuals measure it from the jacket to the closet door. The top clearance runs from the top of the jacket to the ceiling (Rheem AP23060, p. 7; Rheem AP22510, p. 7; Rheem AP14896-2, p. 7). The PowerDirectVent numbers come from the December 2010 manual; the June and August 2011 revisions print the same ones (Rheem AP14896-3, p. 7; Rheem AP14897-3, p. 7). A. O. Smith adds 6 in. from the vent pipe (A. O. Smith 315628-002, p. 8) and, on the direct vent model, 1 in. from the vent piping except where its Figure 21 notes otherwise (A. O. Smith 326940-001, p. 12).

If the label on the front of the heater shows different clearances, Rheem says to install by the label (Rheem AP23060, p. 7). A. O. Smith says “a lesser dimension may be allowed on some models” and points to the label next to the gas control valve (A. O. Smith 315628-002, p. 8).

The service space is worded more loosely than the fire clearances. Rheem’s atmospheric manuals say 24 in. at the front and top “should be available for adequate inspection and servicing” (Rheem AP23060, p. 7; Rheem AP22858, p. 7). A. O. Smith’s direct vent manual says 24 in. “should be considered” for jobs such as changing the anodes, and makes 5.5 in. a minimum for access to the thermostat, drain valve and relief valve (A. O. Smith 326940-001, p. 12). Rheem’s power vent and PowerDirectVent manuals give no service number.

In a closet or alcove, Rheem requires a wood or metal panel over the entire floor (Rheem AP23060, p. 7; Rheem AP22858, p. 7). The power vent, PowerDirectVent and A. O. Smith manuals state the same rule in their paragraph about carpeting (Rheem AP22510, p. 7; Rheem AP14896-2, p. 7; A. O. Smith 315628-002, p. 7).

Rheem’s atmospheric tanks take in combustion air through openings around the lower part of the jacket (Rheem AP23060, p. 14). In a closet, do not block the combustion air inlet openings, and keep at least 1 in. between them and any obstruction; the power vent and high-BTU manuals say the same (Rheem AP23060, p. 7; Rheem AP22510, p. 7; Rheem AP22858, p. 7). A drain pan under the tank must not restrict that air flow (Rheem AP23060, p. 7). If the inlets are blocked, a shutoff device stops the heater, and Rheem tells owners to contact a Qualified Service Technician (Rheem AP23060, p. 20). The power vent tank also has a flammable vapor sensor that the drain pan or anything else must not block, and the tank must not go where the surrounding air can exceed 125°F (52°C) (Rheem AP22510, pp. 7–8).

A. O. Smith’s direct vent tank takes all its combustion air from outside through the vent hood. Its sealed piping can get hot, though, and for a closet the manual recommends ventilating the space “to minimize the buildup of heat around the heater”, without giving an opening size (A. O. Smith 326940-001, p. 12). Rheem’s PowerDirectVent manual has no section on confined spaces.

On storage, Rheem says a gas water heater “should not be installed in a space where liquids which give off flammable vapors are to be used or stored”: gasoline, propane or butane, paint, adhesives and their thinners, solvents and removers. The vapors can travel “some distance” and be lit by the pilot or burner (Rheem AP23060, p. 7). Its installation checklist asks that clothing, cleaning materials and rags be kept clear of the base (Rheem AP23060, p. 15), and it warns against letting newspaper, rags or mops pile up near the heater (Rheem AP23060, p. 18). A. O. Smith: “Keep combustibles such as boxes, magazines, clothes, etc., away from the water heater area” (A. O. Smith 315628-002, p. 7). Rheem also rules out bathrooms, bedrooms and “any occupied rooms normally kept closed” (Rheem AP23060, p. 7).

Covers and insulation blankets

Rheem calls insulation blankets for gas water heaters “not necessary”. Where a local code requires one, keep it off the top, where it interferes with the draft hood, and off the bottom, where the combustion air inlets are. Do not cover the burner access door, the jacket door, the gas control or the relief valve (Rheem AP23060, p. 12). A. O. Smith adds that insulation must stay 2 in. above the floor (A. O. Smith 315628-002, p. 8).

Gas furnaces

Carrier’s 58SC0A/58SC1A (80%) and 59SC5B (condensing) furnaces are design-certified for installation in “alcoves, attics, basements, closets, utility rooms, crawlspaces, and garages” (Carrier 58SC0A/58SC1A, p. 3; Carrier 59SC5B, p. 4). Goodman: “The minimum clearance information for this furnace is provided on the unit’s clearance label. These clearances must be permanently maintained” (Goodman IOG-2037D, p. 7). The clearances change with the furnace’s position and, on Trane 80% models, with cabinet width.

Brand and series	Top	Sides	Rear	Front	Service (front)
Goodman MES80/CES80, 80%, upright or horizontal (Goodman IOG-2021H, p. 7)	1 in.	1 in.	0 in.	3 in.	24 in. “recommended”
Goodman R9S96, R9S92 and *D9S96, “Upflow” row (Goodman IOG-2037D, p. 7)	1 in.	0 in.	0 in.	3 in.	24 in. “required”
Goodman R9S96, R9S92 and *D9S96, horizontal (Goodman IOG-2037D, p. 7)	6 in.	6 in.	0 in.	3 in.	24 in. “required”
Carrier 58SC0A/58SC1A, 80%, any position (Carrier 58SC0A/58SC1A, p. 4)	1 in.	0 in.	0 in.	3 in.; 18 in. for an alcove	24 in. “MIN.” on the label
Carrier 59SC5B, condensing, all units (Carrier 59SC5B, p. 5)	1 in.	0 in.	0 in.	1 in. at the combustion air openings	24 in. “Required for service”, with a footnote to consult local building codes
Trane S8B1/S8X1/S8X2, 80%, vertical, all cabinet widths (Trane S8XB-SVX001-1D-EN, p. 14)	1 in.	0 in.	0 in.	3 in.	24 in. “must be provided”
Trane S8B1/S8X1/S8X2, horizontal, 14.5 and 17.5 in. cabinets (Trane S8XB-SVX001-1D-EN, p. 14)	2 in.	1 in.	0 in.	3 in.	24 in. “must be provided”
Trane S8B1/S8X1/S8X2, horizontal, 21.0 in. cabinet (Trane S8XB-SVX001-1D-EN, p. 14)	3 in.	1 in.	0 in.	3 in.	24 in. “must be provided”
Trane S8B1/S8X1/S8X2, horizontal, 24.5 in. cabinet (Trane S8XB-SVX001-1D-EN, p. 14)	5 in.	1 in.	0 in.	3 in.	24 in. “must be provided”
Trane S9V2, condensing, closet (Trane FNR-SVX001C-EN, p. 16)	1 in.	0 in.	1 in.	0 in.	24 in. “recommended” in the table, “must be provided” in the checklist
Trane S9V2, horizontal closet and alcove; horizontal flue on the left, closet (Trane FNR-SVX001C-EN, p. 16)	1 in.	0 in.	1 in.	Not listed	Same as above

The front fire clearance and the service space are two different numbers. Goodman: “ACCESSIBILITY CLEARANCE, WHERE GREATER, SHOULD TAKE PRECEDENCE OVER MINIMUM FIRE PROTECTION CLEARANCE” (Goodman IOG-2021H, p. 8). Its R9S96 manual says the same with “must”, and adds that the electrical, flue and drain connections may need more than the listed minimums (Goodman IOG-2037D, p. 7). Trane’s checklist asks for 24 in. of front access and continues: “Any access door or panel must permit removal of the largest component” (Trane S8XB-SVX001-1D-EN, p. 14; Trane FNR-SVX001C-EN, p. 16). All six furnace manuals name 24 in. in front; the wording runs from “recommended” to “required”.

Carrier’s 80% label has a line that raises the front figure: “18 inches front clearance required for alcove” (Carrier 58SC0A/58SC1A, p. 4). The manual does not define alcove or closet.

Several manuals protect the combustion air openings in the front panel. Trane asks whether there are “at least 3 inches of clearance between the Furnace combustion air openings in the front panel and any closed panel or door” (Trane S8XB-SVX001-1D-EN, p. 14; Trane FNR-SVX001C-EN, p. 16). Carrier’s condensing table gives the front clearance as 1 in. at the “Combustion air openings in furnace and structure” (Carrier 59SC5B, p. 5), and its 80% drawing for air from inside the building shows at least 3 in. in front of the combustion air openings (Carrier 58SC0A/58SC1A, p. 7). Trane also writes that openings in the casing “must be kept free of obstructions”, and: “Keep this in mind should you choose to remodel or change the area which contains your Furnace” (Trane S8XB-SVX001-1D-EN, p. 32).

Goodman’s 80% manual prints its closet rules under the clearance drawing. Adequate combustion and ventilation air must be supplied to the closet. The furnace must be completely sealed to the floor or base, and the combustion and ventilation air supply pipes must end 12 in. from the top and 12 in. from the floor of the closet. The return air ducts must be completely sealed to the furnace and end outside the enclosure (Goodman IOG-2021H, p. 7). Both Goodman manuals also say a room, closet or alcove must not be used as a return air chamber (Goodman IOG-2021H, p. 18; Goodman IOG-2037D, p. 33).

The tables also list vent clearances. The 80% furnaces need 6 in. from a single-wall vent and 1 in. from Type B-1 vent (Goodman IOG-2021H, p. 7; Carrier 58SC0A/58SC1A, p. 4); Trane gives 1 in. for Type B vent (Trane S8XB-SVX001-1D-EN, p. 14). The condensing tables list 0 in. for the vent (Goodman IOG-2037D, p. 7; Carrier 59SC5B, p. 5; Trane FNR-SVX001C-EN, p. 16).

On the floor: Goodman’s 80% manual allows an upflow or horizontal furnace on wood, but not on carpeting or any other combustible material (Goodman IOG-2021H, p. 6). Goodman’s R9S96 tables allow wood as the only combustible floor, and the *D9S96 “Upflow” row calls for a non-combustible floor or a subbase on combustible flooring (Goodman IOG-2037D, p. 7). Carrier’s 80% label allows downflow installation on a non-combustible floor, or on combustible flooring only when it sits on a special base or a listed coil assembly (Carrier 58SC0A/58SC1A, pp. 3, 4), and Trane requires its BAYBASE205 base for downflow on a combustible floor (Trane S8XB-SVX001-1D-EN, p. 14).

Indoor gas tankless water heaters

The tankless manuals give clearances on every side, including 12 in. below the unit in all three (Rinnai 100000508, p. 22; Navien NPE-A2/S2, p. 17; Navien NPN, p. 17). Rinnai’s indoor and outdoor versions are separate products: it says not to install the indoor RUR199i and RUR160i outdoors, or the outdoor RUR199e and RUR160e indoors (Rinnai 100000508, p. 18).

Brand and series	Top	Sides	Rear	Front	Service
Rinnai SENSEI RUR199i, RUR160i, indoor (Rinnai 100000508, p. 22)	2 in.; 0 in. from vent components	2 in.	0 in.	0 in.	24 in. in front
Navien NPE-A2/S2, indoor (Navien NPE-A2/S2, p. 17)	9 in.	3 in.	0.5 in.	4 in.	No separate number
Navien NPN-160U/180U/199U, indoor, to combustibles (Navien NPN, p. 17)	6 in.	2 in.	0 in.	6 in.	No separate number
Navien NPN-160U/180U/199U, indoor, to non-combustibles (Navien NPN, p. 17)	2 in.	1/2 in.	0 in.	6 in.	No separate number

Rinnai’s table applies to combustible and non-combustible surfaces alike; Navien’s NPE table does not say which it means (Rinnai 100000508, p. 22; Navien NPE-A2/S2, p. 17). Navien introduces both of its tables with the same purpose: install the heater “in an area that allows for service and maintenance access to utility connections, piping, filters, and traps” (Navien NPE-A2/S2, p. 17; Navien NPN, p. 17).

Rinnai’s indoor models can be installed direct vent, with combustion air piped from outside, or with room air (Rinnai 100000508, p. 26). Where indoor air is corrosive, Rinnai’s instruction is to “Install the water heater as direct vent in a sealed closet” (Rinnai 100000508, p. 20). With room air, Rinnai counts a closet as a confined space that needs two combustion air openings (Rinnai 100000508, p. 44); the next section covers their size.

Navien NPE units come from the factory as direct vent, drawing all combustion air from outdoors, and Navien recommends direct vent “whenever possible”. With a single pipe, the equipment room must have properly sized openings: from 30 to 50 sq in. for outdoor air and from 120 to 199 sq in. for indoor air, depending on the model (Navien NPE-A2/S2, pp. 47, 49). The NPN indoor models are direct vent only (Navien NPN, p. 7). Neither Navien manual uses the word closet.

Both Navien manuals ask that combustible materials be stored away from the heater and that “hanging laundry or similar items do not obstruct access to the water heater or its venting”, and both rule out areas where gasoline or other flammables are used or stored (Navien NPE-A2/S2, p. 17; Navien NPN, p. 17). The NPE manual says to avoid bathrooms, bedrooms and other occupied rooms that are normally kept closed or not adequately ventilated, and to install a carbon monoxide detector in the living area near the heater. It also says: “Do not enclose the vent termination” (Navien NPE-A2/S2, p. 16). Rinnai says to keep the area clear of combustible materials, gasoline and other flammable vapors and liquids, and not to install the heater in an area with negative air pressure (Rinnai 100000508, pp. 7, 18).

Rinnai’s accessory list includes a recess box that lets an outdoor model sit recessed in a wall, and a pipe cover that encloses the piping below the heater “for aesthetic purposes” (Rinnai 100000508, p. 15).

Electric tank water heaters

Rheem’s electric manuals AP21980 and AP17947-3 give no clearance numbers to walls, door or ceiling. AP21980 asks for access instead: place the heater so that “the upper electronic control and the lower access panels can be removed to permit inspection and servicing such as removal of elements or checking controls” (Rheem AP21980, p. 5). Where the manual gives no number, it does not say that zero clearance is acceptable; its checklist still asks for “Sufficient room to service heater” (Rheem AP21980, p. 11).

AP17947-3, for 15 to 50 gallon models, warns against installing the heater near appliances with high temperatures, “such as wood burning stoves, boilers, or furnaces”, because heat can damage its non-metallic construction (Rheem AP17947-3, p. 5). If the closet also holds a furnace, look for this warning in your own manual. If you insulate an electric tank, do not cover the labels, the access panels or the relief valve, and keep insulation off the top, where it could interfere with the electrical junction box (Rheem AP21980, p. 9).

Combustion air: confined and unconfined spaces

Gas burners need combustion air. A direct vent appliance gets it through a pipe from outdoors (A. O. Smith 326940-001, p. 12). A non-direct vent furnace may draw it from the surrounding area as long as the air requirements are met (Trane FNR-SVX001C-EN, p. 37). Goodman warns that insufficient combustion air “can expose building occupants to gas combustion products that could include carbon monoxide” (Goodman IOG-2021H, p. 6). A. O. Smith tells installers not to put a tank “in a confined area such as a closet, unless you provide air” as its manual shows, because too little air “can result in a fire or explosion” (A. O. Smith 315628-002, p. 8).

The manuals sort that space with one rule. It is unconfined if its volume is at least 50 cu ft per 1,000 Btu/h of the combined input of all appliances in it, and confined if it is smaller (Rheem AP23060, p. 8; A. O. Smith 315628-002, p. 8; Carrier 58SC0A/58SC1A, p. 6; Trane S8XB-SVX001-1D-EN, p. 32; Rinnai 100000508, p. 44).

Rinnai lists a small room, a closet, an alcove and a utility room as examples of confined spaces. Quoting the National Fuel Gas Code definition, it adds that rooms connected through openings “not furnished with doors” count as part of the unconfined space (Rinnai 100000508, p. 44). Goodman’s R9S96 manual writes “a confined space (i.e., a closet or utility room)” (Goodman IOG-2037D, p. 7). The arithmetic shows why: a closet 3 ft by 3 ft with an 8 ft ceiling holds 72 cu ft, which under the 50 cu ft rule covers about 1,440 Btu/h of input (our arithmetic). Trane’s table works the other way: with an 8 ft ceiling, a 40,000 Btu/h furnace needs 250 sq ft of floor area to count as unconfined, and a 120,000 Btu/h furnace needs 750 sq ft (Trane S8XB-SVX001-1D-EN, p. 32).

In an unconfined space in a building of conventional frame, masonry or metal construction, Rheem says infiltration air “is normally adequate”; in a confined space, “provisions for combustion and ventilation air must be made” (Rheem AP23060, p. 8).

Count every gas appliance in the closet. Rheem sizes the openings by “the total input rating of all the appliances in the enclosure” (Rheem AP23060, p. 8), and Goodman asks for enough air for all appliances when a water heater shares the furnace’s space (Goodman IOG-2021H, p. 10). Carrier’s worked example adds a 30,000 Btu/h water heater to a 110,000 Btu/h furnace: 140,000 divided by 4,000 gives 35.0 sq in. for each of two vertical ducts or openings (Carrier 58SC0A/58SC1A, p. 8).

The openings come in a pair: one starting within 12 in. of the ceiling and one within 12 in. of the floor (Rheem AP23060, p. 8; A. O. Smith 315628-002, p. 9; Carrier 58SC0A/58SC1A, pp. 7–8; Trane S8XB-SVX001-1D-EN, p. 33; Rinnai 100000508, p. 45). Carrier’s drawing for air from inside the building places them “in door or wall” and gives 100 sq in. as the minimum opening size, with no dimension under 3 in. (Carrier 58SC0A/58SC1A, p. 7). Air from inside the house works only if the closet and the rooms its openings connect to meet the 50 cu ft rule together (A. O. Smith 315628-002, p. 9; Trane S8XB-SVX001-1D-EN, p. 33; Rinnai 100000508, p. 44). Carrier’s manual sends spaces under 50 cu ft per 1,000 Btu/h to its outdoor combustion air method (Carrier 58SC0A/58SC1A, p. 6).

Free area as printed in each manual (Navien’s table does not say whether the figure is per opening):

Manual	Air from other rooms	Outdoors, direct or vertical duct	Outdoors, horizontal duct	One opening to outdoors
Rheem gas tanks (Rheem AP23060, p. 8)	1 sq in. per 1,000 Btu/h, at least 100 sq in.	1 sq in. per 4,000 Btu/h	1 sq in. per 2,000 Btu/h	Not given
A. O. Smith gas tanks (A. O. Smith 315628-002, p. 9)	1 sq in. per 1,000 Btu/h, at least 100 sq in.	1 sq in. per 4,000 Btu/h	1 sq in. per 2,000 Btu/h	1 sq in. per 3,000 Btu/h
Carrier 58SC0A/58SC1A and 59SC5B furnaces (Carrier 58SC0A/58SC1A, pp. 7–8; Carrier 59SC5B, pp. 9–10)	1 sq in. per 1,000 Btu/h, at least 100 sq in.	1 sq in. per 4,000 Btu/h	1 sq in. per 2,000 Btu/h	1 sq in. per 3,000 Btu/h
Trane S8X and S9V2 furnaces, furnace only (Trane S8XB-SVX001-1D-EN, p. 32; Trane FNR-SVX001C-EN, p. 49)	100 sq in. from 40,000 to 100,000 Btu/h; 120 sq in. at 120,000	10 to 30 sq in. by vertical duct (1 sq in. per 4,000 Btu/h)	20 to 60 sq in. (S8X); 20 to 50 sq in. as printed (S9V2); 1 sq in. per 2,000 Btu/h	Not given
Rinnai RUR with room air (Rinnai 100000508, p. 44)	1 sq in. per 1,000 Btu/h, at least 100 sq in.	Not given	1 sq in. per 2,000 Btu/h	Not given
Navien NPE with a single pipe (Navien NPE-A2/S2, p. 49)	1 sq in. per 1,000 Btu/h, printed as “indoor make up air”	Opening type not stated; 1 sq in. per 4,000 Btu/h is printed as “outdoor make up air”	Not given	Not given

Trane’s S9V2 manual prints 50 sq in., not 60, for the horizontal duct at 120,000 Btu/h, although its own rule of 1 sq in. per 2,000 Btu/h gives 60, and the S8X manual prints 60 (Trane FNR-SVX001C-EN, p. 49; Trane S8XB-SVX001-1D-EN, p. 32). Rinnai’s page says outdoor air can reach the space “by one of two ways” and then describes only horizontal ducts (Rinnai 100000508, p. 44). For a single outdoor opening, Carrier also requires at least the combined area of all vent connectors in the space (Carrier 58SC0A/58SC1A, p. 7).

Goodman’s R9S96 manual, which also covers the *R9S92 and *D9S96, gives a much smaller figure for a furnace in a closet or utility room: two ventilation openings with a total free area of 0.25 sq in. per 1,000 Btu/h of furnace input, one within 12 in. of the top and one within 12 in. of the bottom. It adds: “In a typical construction, the clearance between the door and door frame is usually adequate to satisfy this ventilation requirement” (Goodman IOG-2037D, p. 7). No other manual in this set says this. The manual does not say whether this applies to direct vent, single-pipe installation or both. For other gas appliances in the same space, such as a water heater, it refers to NFPA 54/ANSI Z223.1 and local codes (Goodman IOG-2037D, p. 8). Goodman’s 80% manual gives no opening sizes and refers to the same codes (Goodman IOG-2021H, p. 10).

Grilles and louvered doors

The areas above are net free area, the open part of a grille or screen. Rheem says to size by the net free area of any screen or grille and not to use screen with mesh smaller than 1/4 in. (Rheem AP23060, p. 8). When the free area of a louver is unknown, A. O. Smith allows assuming 20 to 25 percent for wood and 60 to 75 percent for metal, and requires louvers to be fixed open or interlocked with the appliance (A. O. Smith 315628-002, p. 9). Rinnai uses 25 and 75 percent and works an example: a 10 in. by 12 in. grille gives 30 sq in. in wood and 90 sq in. in metal (Rinnai 100000508, p. 45). No manual we read requires a louvered door for a house. For a manufactured home, Rinnai’s room-air installation enclosure must have an exterior access door with a single opening, “metal with no less than 1/4 in. (6.4 mm) mesh”, of at least 200 sq in. for the RUR199i and 160 sq in. for the RUR160i (Rinnai 100000508, p. 48).

When the air must come from outdoors

Rheem says buildings of “unusually tight construction” need combustion air from outdoors or from a freely ventilated attic or crawl space (Rheem AP23060, p. 8). Goodman writes that “Most homes require outside air be supplied into the furnace area” (Goodman IOG-2021H, p. 9), and Carrier that “Many homes” do (Carrier 58SC0A/58SC1A, p. 7). Carrier also requires enough air “to avoid negative pressure in the equipment room or space” (Carrier 58SC0A/58SC1A, p. 6), and Rinnai says not to take combustion air from an area that exhaust fans can put under negative pressure (Rinnai 100000508, p. 45).

The manuals refer to the National Fuel Gas Code, ANSI Z223.1/NFPA 54, for these rules; Carrier and Trane name its Section 9.3, Air for Combustion and Ventilation (Carrier 58SC0A/58SC1A, p. 6; Trane S8XB-SVX001-1D-EN, p. 32). The figures on this page are as printed in the manuals.

What direct vent changes

Carrier is the only manual in this set that says outright that direct vent removes the furnace’s own requirement: “When the furnace is installed as a 2-pipe furnace, no special provisions for air for combustion are required.” The next sentence matters for a shared closet: “However, other gas appliances installed in the space with the furnace may require outside air for combustion” (Carrier 59SC5B, p. 8). Carrier’s 26,000 Btu/h model can use the same 2-pipe system “but is not considered direct vent” (Carrier 59SC5B, p. 5).

The others describe direct vent without saying that the room air rules drop away. Trane’s S9V2 installed direct vent takes its combustion air “through the inlet air pipe from OUTSIDE AIR” (Trane FNR-SVX001C-EN, p. 37). Goodman’s R9S96 furnaces can be installed either way (Goodman IOG-2037D, p. 5), and its 0.25 sq in. closet rule has no exception for direct vent. Goodman’s 80% furnace “cannot be installed as a direct vent” furnace (Goodman IOG-2021H, p. 10). A. O. Smith’s direct vent tank still recommends ventilating a closet because of hot piping (A. O. Smith 326940-001, p. 12).

What to leave accessible

Gas shutoff valve

The Goodman and Carrier furnace manuals put a manual shutoff valve within 6 ft of the furnace, and Carrier calls it “accessible” and places it outside the furnace casing (Goodman IOG-2021H, p. 16; Goodman IOG-2037D, p. 29; Carrier 58SC0A/58SC1A, p. 15; Carrier 59SC5B, p. 23). A. O. Smith requires “A readily accessible manual shut off valve in the gas supply line serving the water heater” (A. O. Smith 315628-002, p. 13). Rheem tells owners to have the installer show where the gas shutoff is and how to close it, and to close it if the heater has been through overheating, fire, flood or physical damage, or if the gas supply fails to shut off (Rheem AP23060, p. 6). Rinnai and Navien call for a shutoff valve in the gas supply line (Rinnai 100000508, p. 57; Navien NPE-A2/S2, p. 21).

Relief valve and discharge line

On a tank, Rheem says a minimum of 1.5 in. “must be provided” between the temperature and pressure (T&P) relief valve and any wall or object (Rheem AP23060, p. 7; Rheem AP22510, p. 7). The discharge line runs downward to an open drain, and its end “should not be threaded or concealed” (Rheem AP23060, p. 10; Rheem AP21980, p. 7). On an electric tank, the discharge must not be able to reach live electrical parts or people (Rheem AP21980, p. 7). Rinnai says the relief valve “shall be accessible for servicing” and must be operated by hand once a year (Rinnai 100000508, p. 52). Navien says to operate it by hand at least once every three years and to end the discharge line 6 to 12 in. from a floor drain, where the discharge is clearly visible (Navien NPE-A2/S2, p. 37). The NPN manual: “Never cover the drain or pressure relief valve” (Navien NPN, p. 25).

Drain valve and service panels

Draining a tank means shutting off the gas and connecting a garden hose to the drain valve (Rheem AP23060, p. 20). A. O. Smith’s direct vent manual requires at least 5.5 in. for access to the thermostat, drain valve and relief valve (A. O. Smith 326940-001, p. 12). On an electric tank, the upper control and lower access panels must be removable (Rheem AP21980, p. 5), and Rheem asks you to learn where the circuit breaker is (Rheem AP21980, p. 4). Rinnai’s isolation valves are there to flush the heat exchanger (Rinnai 100000508, p. 51), and Navien needs the condensate discharge connection reachable for maintenance and pH testing (Navien NPE-A2/S2, p. 40). On a furnace, Trane says any access door or panel must allow removal of the largest component (Trane S8XB-SVX001-1D-EN, p. 14).

When to call a licensed professional

The manuals put gas work in a professional’s hands. Rheem: “Installation and service must be performed by a qualified installer, service agency or the gas supplier” (Rheem AP23060, p. 1). Rinnai: “A trained and qualified professional must install the gas supply” (Rinnai 100000508, p. 57). Navien: “ONLY a licensed professional should connect the gas supply” (Navien NPE-A2/S2, p. 14).

Call a licensed professional in these cases:

Before you build a closet around a gas appliance or move one into a closet, so the clearances and air openings are checked for your model.
To extend, reroute or replace a vent or a combustion air intake pipe.
To cut combustion air openings or run ducts to the outdoors, an attic or a crawl space.
Before you add a second gas appliance to a closet that already has one, since the openings are sized for the combined input.
When a Rheem tank shuts down because of blocked air inlets or flammable vapors. Rheem says to contact a Qualified Service Technician, and after flammable vapors the heater will not ignite until one has examined it (Rheem AP23060, pp. 7, 20).

The manuals defer to local codes, and where there are none, to the National Fuel Gas Code, ANSI Z223.1/NFPA 54 (Rheem AP23060, p. 7; A. O. Smith 315628-002, p. 4). A. O. Smith: “If you have any doubts or questions at all, call your gas supplier” (A. O. Smith 315628-002, p. 8).

What these manuals do not cover

None of them gives a minimum width or depth for the closet. They give clearances around the appliance and the volume rule.
We did not check Lennox, York, Bradford White or Noritz manuals, or Rheem furnaces and tankless heaters, for this page. Use the manual for your own model.
We quote the manuals’ references to ANSI Z223.1/NFPA 54 and did not read the code itself, or any state or local code.