2021 IMC Chapter 3
Chapter 3 of the IMC covers the physical facts of a mechanical job: whether the equipment is approved, where it sits, how close it is to something that burns, and whether a technician can actually reach it in August. Most corrections from this chapter are measurements. This lesson is built around an attic furnace, because one attic shows almost everything at once. Houston follows the Uniform Mechanical Code (Lesson 1), so confirm local values.
Appliances must be listed and labeled for the application unless the code official approves something else under Section 105 (301.7). The label comes from an approved agency that tests a representative sample and inspects periodically (301.8). Equipment is installed as its listing, the manufacturer's instructions, and the code require, and the instructions must be on the job site at inspection (304.1). Where the code and the instructions conflict, the code applies, except where the code is less restrictive than the listing or instructions, in which case the listing and instructions apply (304.2).
Mechanical work must not weaken the building, and anything cut or replaced must be left structurally safe (302.1). Penetrations of rated floor, ceiling, and wall assemblies are protected under IBC Chapter 7 (302.2). For wood floor joists, notches at the ends may not exceed one-fourth the depth, notches in the top or bottom may not exceed one-sixth the depth and stay out of the middle third of the span, and bored holes may be no larger than one-third the depth and no closer than 2 inches to the top or bottom (302.3.1). In studs, cuts and notches are limited to 25 percent of the depth in exterior and bearing walls and 40 percent in nonbearing partitions, and bored holes to 40 percent (302.3.2, 302.3.3). Trusses may not be cut or altered, or loaded with new HVAC equipment or water heaters, without a registered design professional's approval (302.4).
Fuel-fired appliances may not be located in, or take combustion air from, sleeping rooms, bathrooms, toilet rooms, storage closets, or surgical rooms (303.3). Three exceptions apply: direct-vent appliances that take all combustion air from outdoors, solid fuel appliances with air provided per the manufacturer, and appliances in a dedicated enclosure with all air from outdoors and a weather-stripped, self-closing solid door. Appliances in hazardous locations must be listed and approved for them (303.2), furnaces and boilers in closets and alcoves must be listed for it (303.5), and no mechanical system may sit in an elevator shaft (303.8). In garages and hazardous locations the ignition source must be at least 18 inches above the floor unless the appliance is listed as flammable vapor ignition resistant (304.3).
Equipment at grade sits on a level concrete slab or approved material at least 3 inches above adjoining grade, or is suspended at least 6 inches above it (304.10). Guards are required where serviceable components or a roof hatch are within 10 feet of a roof edge that is more than 30 inches above the surface below. The guard extends 30 inches past each end, is at least 42 inches high, and stops a 21-inch sphere. Fall-arrest anchorage that meets ANSI/ASSP Z359.1 can replace the guard (304.11). Appliances serving another area must be marked with the area served (304.12).
Equipment must be reachable for inspection, service, repair, and replacement without disabling a rated assembly or removing permanent construction, other appliances, or unrelated piping and ducts. A level working space at least 30 inches deep and 30 inches wide is required in front of the control side (306.1). A room containing appliances needs a door and passage at least 36 inches wide and 80 inches high (306.2).
For attics (306.3), the opening and passage must let the largest appliance out. The passage must be at least 30 inches high and 22 inches wide, no more than 20 feet long along the centerline, with continuous solid flooring at least 24 inches wide, and a level service space of 30 by 30 inches at the appliance. The clear opening must be at least 20 by 30 inches. Two exceptions: no passage or service space is needed if the appliance can be serviced and removed through the opening, and a passage 6 feet high and 22 inches wide for its full length may run up to 50 feet. A switched light at the opening and a receptacle at or near the appliance are required (306.3.1). Under-floor spaces follow the same pattern (306.4), but the opening is at least 22 by 30 inches, passage walls deeper than 12 inches below grade must be lined with concrete or masonry, and the long-passage exception (6 feet high, 22 inches wide) has no length limit under the floor, unlike the 50-foot cap in an attic.
Where personnel must climb higher than 16 feet above grade to reach roof or elevated equipment, a means of access is required that does not use portable ladders (306.5). The path may not require climbing over obstructions taller than 30 inches or walking roofs steeper than 4 in 12. Group R-3 occupancies are excepted. Permanent ladders have numbered limits: side rails 30 inches above the parapet or roof edge, rungs no more than 14 inches on center, 18 inches between rails, 30 inches of climbing clearance, and a 30 by 30 inch clear landing at the bottom. Catwalks are at least 24 inches wide.
On roofs sloped 3 in 12 or more with an edge over 30 inches above grade, each serviced item needs a level platform on each side, at least 30 inches in any dimension, with 42-inch guards (306.5.1). A receptacle is required at or near roof equipment (306.5.2).
Heat-producing equipment keeps the clearances in its listing and instructions. Door swings, drawer pulls, shelving, and drapes count, and doorstops or limiters may not be used to create clearance. Clearance is reduced only under Section 308 (304.9). A reduced-clearance protective assembly must be noncombustible, must allow air to circulate by convection, and must sit at least 1 inch from the appliance regardless of the reduction (308.3). A listing that prohibits reduction wins (308.2). The assembly may be listed to UL 1618 or built to Table 308.4.2 (308.4.1, 308.4.2). Where the required clearance falls between table values, interpolate linearly, and never extrapolate below the table. The rows below are one common method, 24-gage sheet steel spaced 1 inch off the combustible surface.
| Clearance required with no protection | Combustible above the heat source | Combustible beside or below |
|---|---|---|
| 36 in | 18 in | 12 in |
| 18 in | 9 in | 6 in |
| 9 in | 5 in | 3 in |
| 6 in | 3 in | 2 in |
Worked example. A gas-fired unit heater in a community center storage room is listed for 27 inches to a combustible wall beside it, and the listing does not prohibit reduction. The table has no 27-inch row, so interpolate between 18 (reduced to 6) and 36 (reduced to 12). Twenty-seven is halfway, so the reduced clearance is halfway between 6 and 12: 9 inches. The steel sits 1 inch off the wall and at least 1 inch from the heater. If the listing asked for only 4 inches, the table could not help, because extrapolating below the 6-inch row is not allowed.
In the field
On a rehab job a clinic's old attic furnace was installed before anyone floored the attic. The replacement is deeper than the old unit, so the inspector starts at the hatch: is the opening at least 20 by 30 inches, and does the new unit fit through it? Then the walk: 30 inches of headroom, 22 inches of width, and 24 inches of solid decking all the way in. Finally the service side: a level 30 by 30 inch space, a switch at the hatch, a receptacle at the unit. It is cheaper to measure before the crew orders the furnace than to reframe afterward.
Any equipment with cooling coils or evaporators that can produce condensate needs a condensate drain system (307.2). Piping slopes at least 1/8 inch per foot (1 percent) toward the discharge point, and condensate may not run into a street or alley or create a nuisance (307.2.1). The drain may not connect directly to a plumbing drain, waste, or vent pipe, and it may discharge only into fixtures such as a floor sink, floor drain, mop sink, or standpipe within the same tenant space, unless it goes to grade outdoors (307.2.1.1). The line is at least 3/4 inch and never reduces in size. Where several units are manifolded, Table 307.2.2 sets the minimum: 3/4 inch up to 20 tons, 1 inch over 20 to 40 tons, 1 1/4 inch over 40 to 90, 1 1/2 inch over 90 to 125, and 2 inch over 125 to 250 (307.2.2). Where overflow could damage the building, auxiliary protection is required: a secondary pan at least 1 1/2 inches deep and 3 inches larger than the unit, a separate overflow line, or a water-level shutoff device (307.2.3).
Heating and cooling loads used to size equipment follow ASHRAE/ACCA Standard 183, or an approved equivalent procedure (312.1).
1. Which of these gas-fired installations is NOT permitted in a storage closet under IMC Section 303.3?
2. An inspector measures an attic passage to a furnace: 26 inches high, 22 inches wide, 15 feet long, with 24-inch-wide solid flooring. No exception applies. What is the deficiency under IMC Section 306.3?
3. What is the minimum clear access opening to an attic containing an appliance under IMC Section 306.3?
4. What is the minimum clear access opening to an under-floor space containing an appliance under IMC Section 306.4?
5. A furnace is installed in a mechanical room. What is the minimum level working space required in front of its control side under IMC Section 306.1?
6. A rooftop unit sits on the roof of a one-story retail building where the climb from grade is 14 feet. Does IMC Section 306.5 require a permanent means of roof access?
7. A roof hatch is 9 feet from a roof edge that is 20 feet above the ground below, and no fall-arrest anchorage is installed. What does IMC Section 304.11 require?
8. An exhaust fan needing service sits on a roof sloped 4 units vertical in 12 horizontal, with an edge 25 feet above grade. What must be provided under IMC Section 306.5.1?
9. An inspector measures rung spacing of 15 inches on center on a permanent ladder to a roof hatch. What is the finding under IMC Section 306.5?
10. A unit heater is listed for 27 inches to a combustible wall beside it. Using 24-gage sheet steel spaced 1 inch off the wall, the table gives 6 inches at an 18-inch listing and 12 inches at a 36-inch listing. What is the reduced clearance by linear interpolation?
11. A contractor mounts a noncombustible shield 1/2 inch from a unit heater so that a reduced wall clearance is met. Is this acceptable under IMC Section 308.3?
12. An inspector finds the condensate line from an air handler in a clinic tenant space piped directly into a sanitary drain line. What is the problem under IMC Section 307.2.1.1?
13. Condensate drains from several units totaling 30 tons of refrigeration are manifolded together. What is the minimum pipe size under IMC Table 307.2.2?
14. A mechanic wants to bore a hole through a 9-inch-deep wood floor joist for a duct riser. What is the largest permitted hole diameter under IMC Section 302.3.1?