2021 IMC Chapter 11
Some refrigerants are harmless in the open and dangerous in a small closed room, and a few burn. Chapter 11 of the 2021 IMC asks what refrigerant is in the system, how much sits in one circuit compared with the room people occupy, and, if the charge is too large, what the machinery room must look like. It also governs refrigerant piping and tests. Houston enforces the Uniform Mechanical Code rather than the IMC (Lesson 1), so check the adopted code before you write a correction.
Chapter 11 covers systems that vaporize and liquefy a fluid in the refrigerating cycle. Systems using anything other than ammonia follow this chapter plus ASHRAE 15 (1101.1.1). Ammonia systems follow the IIAR standards and are not required to meet this chapter (1101.1.2), so an ammonia cold-storage plant is a different review. Listed factory-built equipment, such as split systems and walk-in coolers, is deemed to meet design and factory testing when installed to its listing (1101.2). Fuel gas devices follow the IFGC (1101.5), and every system must be kept free of leaks, oil and corrosion (1101.6).
Each refrigerant has a two-part safety group. The letter is toxicity: A is low, B is high. The number is flammability: 1 means no flame propagation, 2 is low flammability, 2L is low flammability with a low burning velocity, and 3 is high flammability (Section 202). Table 1103.1 also gives the amount allowed per occupied space, in pounds per 1,000 cubic feet. These are the rows you will meet most.
| Refrigerant | Safety group | lb per 1,000 cu ft |
|---|---|---|
| R-134a | A1 | 13 |
| R-410A | A1 | 26 |
| R-407C | A1 | 18 |
| R-404A | A1 | 31 |
| R-22 (older equipment) | A1 | 13 |
| R-744 (carbon dioxide) | A1 | 4.5 |
| R-32 | A2L | 4.8 |
| R-1234yf | A2L | 4.7 |
| R-290 (propane) | A3 | 0.56 |
| R-123 | B1 | 3.5 |
R-32 and R-1234yf are printed in the table as A2 with a footnote giving their ASHRAE 34 class as 2L, so they are the A2L refrigerants of this lesson. Ammonia belongs to IIAR rather than this table, and old refrigerants such as R-11 and R-12 are marked as prohibited for new installations.
Locations are sorted by how well people can respond to a leak: institutional (people who cannot leave without help, such as nursing homes), public assembly, residential, commercial, large mercantile (more than 100 people above or below street level), industrial, and mixed. Outdoor equipment within 20 feet of a building opening takes the building's class, and in a mixed building not sealed apart by tight construction and self-closing doors, the strictest occupancy governs. Systems are also classed by leak probability: direct systems, where the refrigerant itself circulates to the coils that condition the air, are high probability, while indirect closed systems such as chilled-water loops are low probability when their refrigerant piping is isolated.
Every refrigerant-containing component must be outdoors or in a machinery room where the charge in one independent circuit exceeds the Table 1103.1 amount (1104.2). The amount is figured against the volume of the smallest enclosed occupied space the refrigerant could reach (1104.4.1). For a coil in a duct system, that is the smallest space the ducts serve, unless airflow to a space cannot fall below one-quarter of its maximum, and a ceiling plenum that is part of the air system counts (1104.4.2, 1104.4.3). Two adjustments matter. In institutional occupancies the table amounts are cut by 50 percent, except in kitchens, laboratories and mortuaries (1104.2.1). Listed equipment holding 6.6 pounds or less needs no machinery room when installed to its listing (1104.2, exception 1). The total of all A2, B2, A3 and B3 refrigerant is also capped at 1,100 pounds except where approved, and at 550 pounds in institutional occupancies (1104.3.3, 1104.2.1).
Worked example. A 30 pound R-410A circuit serves a small office suite whose smallest occupied space is 12 ft x 14 ft x 9 ft = 1,512 cubic feet. At 26 lb per 1,000 cu ft the allowance is 26 x 1.512 = 39.3 pounds, so 30 pounds passes this test. Put the same circuit in a nursing home with the same room size and the allowance drops to half, 19.7 pounds. Thirty pounds now exceeds it, so the components go outdoors or in a machinery room. ASHRAE 15 still applies to the design.
A machinery room is built to the IBC and Section 1105, and A2, B2, A3 and B3 rooms add Section 1106. It must be mechanically ventilated to the outdoors, with openings placed by the refrigerant's density relative to air, so a refrigerant heavier than air is drawn from low in the room (1105.6, 1105.6.1.1). Normal ventilation while occupied exhausts the larger of 0.5 cfm per square foot of room or 20 cfm per person, or whatever holds the temperature rise to 18°F (1105.6.3.1). For the 1,200 sq ft room in the figure the area basis gives 600 cfm, far above the 40 cfm for two people. A refrigerant detector starts a higher emergency rate (1105.6.3.2). The free opening area and the emergency rate come from Equations 11-1 and 11-2, which grow with the largest charge; read them in the adopted code. Makeup air is required, the supply and exhaust ducts may not serve any other area, and the openings are covered with corrosion-resistant screen of not less than 1/4 inch mesh (1105.6.2). Exhaust discharges at least 20 feet from a property line or building openings (1105.6.1).
A machinery room over 1,000 sq ft needs at least two exits or exit access doorways, separated by at least half the room's maximum horizontal dimension, and every point must be within 150 feet of one (1105.9). Exit and exit access doors swing in the direction of egress, carry panic hardware regardless of occupant load, and are tight fitting and self-closing. Pressure relief devices, fusible plugs and purge outlets terminate outdoors at least 15 feet above grade and 20 feet from any window, ventilation opening or exit (1105.7). Open-flame appliances that draw room air are not allowed in the room, unless the refrigerant is water or carbon dioxide, combustion air is ducted from outside and sealed, or a detector shuts the burner off (1105.5).
No open flame or continuously operating surface hotter than 800°F may be permanently installed in the room (1106.2). Rooms for A2, A3, B2 and B3 refrigerants must meet the Class I, Division 2 hazardous location rules of NFPA 70, with one exception for A2L rooms (1106.3). An A2L room may skip that if a supervised detection system, set to act at or below 25 percent of the lower flammable limit, starts emergency ventilation that meets ASHRAE 15 or Table 1106.4.2, discharges outdoors 15 feet above grade and 20 feet from any window, opening or exit, and is shut down by manual means only (1106.4). Remote controls are required for the room equipment: a clearly identified break-glass or tamper-resistant switch gives off-only control of compressors, pumps and normally closed automatic refrigerant valves, and a second gives on-only control of the ventilation fans (1106.5).
Table 1107.4 allows copper tube and pipe, steel pipe (ASTM A53 or A106) and aluminum tube, with the outside protected from corrosion. Steel pipe for A2, A3, B2 and B3 refrigerants is Schedule 80 in sizes 1 1/2 inches and smaller (1107.4.1). Brazing is done with the pipe purged with oxygen-free nitrogen, helium or argon: five volume changes at 100 feet per minute or more, then 1.0 to 3.0 psi maintained during the work (1108.3.1). Soldered joints are limited to A1 refrigerants at 200 psi or less (1108.3.3). Exposed piping in an open passage must be at least 7 feet 3 inches above the floor (1109.2.1) and may not run exposed in a rated exit access corridor or an exit stair (1109.2.3). Pipe outside the equipment room is labeled at intervals of 20 feet or less, in letters at least 1/2 inch high, with the refrigerant and its safety group (1109.2.7). Stop valves go at each compressor suction and discharge and each receiver outlet on systems over 6.6 pounds, with more over 100 pounds, unless a pumpdown or recovery provision exists (1109.8).
In the field
A senior housing rehab adds a heat pump system, and the drawings call the building residential. Ask what the wing really is: if residents need help to leave, it is institutional under 1103.2, the table amounts are halved, and a charge that looked fine may now call for a machinery room. At rough-in, look for pipe left exposed for the test and labels on pipe leaving the equipment room. At the machinery room door, look for the swing, the panic hardware and the emergency switches.
Field-erected systems are strength tested and leak tested before operation, high and low sides, and the pipe and joints stay exposed until the test is done (1110.1, 1110.2). The medium is oxygen-free nitrogen, helium or argon (carbon dioxide for R-744, water for R-718), never air, oxygen or combustible gas, though tubing up to 5/8 inch outside diameter may use the nameplate refrigerant (1110.3). Two tests follow (1110.5). First, hold pressure for at least 60 minutes at not less than the lower of the design pressures or the relief setting, adding no gas and showing no pressure loss. Second, pull a vacuum to 500 microns, isolate the pump, and confirm the pressure stays at or below 1,500 microns for at least 10 minutes. Parts inside ducts that carry air to occupied spaces are tested at 150 percent of the higher of design pressure or relief setting (1110.5.1). For systems holding 55 pounds or more, a signed certificate of test goes to the code official (1110.8).
1. A designer lists R-32 as the refrigerant for a Group B office building. Table 1103.1 prints it as A2 with a footnote giving its ASHRAE 34 flammability class as 2L. What does 2L tell you?
2. A cold-storage building in a rehab uses an ammonia (R-717) system. Which document does the 2021 IMC send you to for that system's design and installation?
3. A Group B office suite has a smallest occupied space of 2,000 cubic feet. Under Table 1103.1 (R-410A at 26 lb per 1,000 cu ft), what is the most R-410A one independent circuit may hold before a machinery room is triggered?
4. A 12 ft x 12 ft x 10 ft resident room in a nursing home is the smallest occupied space served by a R-410A circuit (26 lb per 1,000 cu ft). What is the allowance after the institutional reduction in 1104.2.1?
5. A tenant space in a Group M store gets a listed self-contained refrigerated case holding 5 pounds of an A3 refrigerant, installed exactly to its listing and instructions. Under the exception to 1104.2, is a machinery room required?
6. A machinery room serves a system with an A2L refrigerant and is not built to Class I, Division 2 requirements. At what refrigerant level must the supervised detection system activate the emergency ventilation?
7. A machinery room in a small commercial building measures 30 ft x 40 ft with one person normally present. What is the minimum normal ventilation rate on the area basis of 1105.6.3.1, before checking the temperature-rise alternative?
8. An inspector finds the exhaust fan discharge of a refrigerant machinery room 12 feet from an operable window of the next building. Which requirement is not met?
9. A machinery room in a hospital wing measures 1,400 sq ft. Which door arrangement complies with 1105.9?
10. A relief valve serving a refrigerating system in a machinery room is piped to terminate outdoors. Under 1105.7, where must the termination be?
11. A contractor wants to install a gas-fired unit heater with an open flame and room-air combustion inside an R-134a machinery room. Under 1105.5, this is acceptable only if:
12. Field-brazed copper refrigerant piping is finished above a ceiling. The contractor plans to close the ceiling over the joints before they are inspected and tested, arguing the factory equipment was already tested. What does Section 1110.2 require?
13. After a field pressure test on a commercial split system, the installer pulls a vacuum. Under 1110.5, which result passes?
14. A contractor proposes to pressure test refrigerant piping in a new Group B building with compressed shop air. Under 1110.3, what is the code position?