4.3 Refrigeration Systems, Refrigerant Classes & Machinery Rooms

Refrigeration is the rest of the 2 percent Cooling & Refrigeration task, and it is where M2 candidates lose time in the book. 2024 IMC 1101.1.1: systems using a refrigerant other than ammonia comply with this chapter, ASHRAE 15, and the IFC. Carbon dioxide systems also comply with IIAR CO2. Ammonia is different: 1101.1.2 sends those systems to IIAR 2, 3, 4, 5, and 6 and does not require compliance with Chapter 11. Do not apply 1105 ventilation equations to an ammonia engine room as if it were R-410A.

1102.1 is the inspection sequence. (1) Classify the system under 1103.3. (2) Classify the refrigerant under 1103.1 / Table 1103.1. (3) Find the maximum quantity under 1104. (4) Find enclosure requirements under 1104. (5) Apply Chapter 3 location limits. (6) Pressure-test nonfactory-tested field-erected equipment under 1110 (the 2024 field-test section; older memory of “1108” is the joints section in this edition).

Refrigerant safety groups and occupancy

A refrigerant safety group is a letter plus a number from ASHRAE 34, printed in Table 1103.1. The letter is toxicity: A is lower toxicity, B is higher toxicity. The number is flammability: 1 is no flame propagation, 2L is lower flammability with a burning velocity at or below the 2L threshold (mildly flammable), 2 is flammable, 3 is higher flammability. Common plates you will actually see:

RCL is the refrigerant concentration limit in pounds per 1,000 cubic feet of occupied space (lb/MCf). That is the number 1104.2 compares to the independent-circuit charge. Table 1103.1 is long; the exam skill is how to use it, not memorizing every blend. Find the refrigerant designation on the nameplate, read the safety group and the RCL, then compute:

Allowable charge (lb) = RCL (lb/MCf) × occupied volume (ft³) ÷ 1,000.

1104.4.1: if refrigerant-containing parts are in spaces that do not communicate through permanent openings or HVAC ducts, use the smallest enclosed occupied space. 1104.4.2: if an evaporator or condenser is in an air-duct system, use the smallest enclosed occupied space served by that duct system, unless airflow to every enclosed space cannot be reduced below one-quarter of its maximum — then use the entire served volume. 1104.4.3: a continuous above-ceiling supply or return plenum counts in the volume.

1103.2 occupancy for refrigeration is not the IBC Group letter. It is a Chapter 11 list that measures how fast people can leave: institutional (cannot leave without help — hospitals, locked cells), public assembly, residential, commercial, large mercantile (more than 100 persons above or below street level buying merchandise), industrial (not open to the public, access controlled), and mixed. Equipment outdoors within 20 feet of a building opening is governed by that building’s occupancy. 1104.2.1: for institutional occupancies except kitchens, laboratories, and mortuaries, cut Table 1103.1 amounts by 50 percent. The total of all Group A2, B2, A3, and B3 refrigerants shall not exceed 550 pounds in occupied areas or machinery rooms. 1104.3.3: in all occupancies that total shall not exceed 1,100 pounds except where approved.

System classification (1103.3) is probability that leaked refrigerant can enter an occupied area. Low-probability systems are double-indirect open-spray, indirect closed, and indirect-vented closed systems, provided refrigerant piping and fittings are isolated where Table 1103.1 quantities are exceeded. High-probability systems are direct systems and indirect open-spray systems (with a pressure-of-secondary-coolant exception). Packaged DX and typical split systems that put refrigerant in the occupied airstream are high-probability.

1104.3.1 is the comfort-cooling sentence. High-probability systems used for human comfort shall use Group A1 or A2L refrigerant. Exceptions: listed residential equipment with a maximum of 6.6 pounds; listed commercial equipment with a maximum of 22 pounds; industrial occupancies. 1104.3.2: Group A2 and B2 refrigerants shall not be used in high-probability systems; Group A3 and B3 shall not be used except where approved — with listed small-charge and occupancy exceptions printed in that section.

When a machinery room is required

A machinery room is the dedicated enclosure Chapter 11 requires when the charge is too large for the occupied space. 1104.2: except as provided in 1104.2.1 and 1104.2.2, all components containing the refrigerant shall be located either outdoors or in a machinery room where the quantity of refrigerant in an independent circuit exceeds the amounts shown in Table 1103.1. For unlisted blends, the same rule applies if any component, or the combined blend at 69,100 ppm by volume, exceeds its table limit. Machinery rooms for Group A1 and B1 are built to 1105. Groups A2, B2, A3, and B3 add 1106. (A2L/B2L rooms use 1105 plus the 1106.4 package discussed below.)

Two exceptions sit under 1104.2. (1) Machinery rooms are not required for listed equipment and appliances containing not more than 6.6 pounds (3 kg) of refrigerant, regardless of safety classification, installed to the listing and instructions. (2) Piping complying with 1107 may connect indoor machinery-room components to outdoor components.

Worked example 1 — R-410A A1 in an office. Table 1103.1 RCL = 26 lb/MCf. Smallest occupied space served is 20 feet × 20 feet × 10 feet = 4,000 ft³ = 4 MCf. Allowable independent-circuit charge = 26 × 4 = 104 pounds. A 90-pound packaged rooftop circuit that keeps all refrigerant-containing parts outdoors is outside 1104.2’s indoor trigger. A 120-pound indoor chiller circuit in that same 4,000 ft³ room exceeds 104 pounds: the refrigerant-containing components go outdoors or into a machinery room, unless a 1104.2.2 industrial refrigerated-room path applies (it does not — this is an office).

Worked example 2 — R-32 A2L, same 4,000 ft³ office. RCL = 4.8 lb/MCf. Allowable = 4.8 × 4 = 19.2 pounds. A 25-pound high-probability split exceeds 19.2 pounds. The 6.6-pound listed-equipment exception does not save a 25-pound charge. If the same 4,000 ft³ space is institutional (not a kitchen, lab, or mortuary), 1104.2.1 cuts the table amount by 50 percent: 19.2 × 0.5 = 9.6 pounds.

1104.2.2 can avoid a separate machinery room in a qualifying industrial refrigerated room (industrial occupancy, evaporator in the room, maintained below 68°F, used for manufacturing, food prep, meat cutting, process, or storage) if every listed condition is met: tight separation and doors, authorized access only, 1105.3 detectors, no open flames or surfaces over 800°F when A2/B2/A3/B3 is used, Class I Division 2 electrical where a single circuit could exceed 25 percent of the LFL, and refrigerant-containing parts on systems over 100 horsepower (with printed exceptions) still outdoors or in a machinery room.

Machinery-room construction, ventilation, and A2L differences

1105 is the general machinery-room chapter. 1105.2: ducts and air handlers in the room that operate at a lower pressure than the room shall be sealed so leaked refrigerant cannot enter the airstream. 1105.3: refrigerant detectors as required by IFC 608.9 and 608.18. 1105.5: fuel-burning appliances with open flames that use combustion air from the machinery room shall not be installed there, except R-718/R-744 or where combustion air is ducted from outside and sealed, or a detector shuts off combustion.

1105.6: machinery rooms shall be mechanically ventilated to the outdoors. Makeup air shall replace the exhaust; supply and exhaust ducts shall not serve any other area, shall follow Chapter 5, and shall be covered with corrosion-resistant screen of not less than ¼-inch mesh (1105.6.2). Exhaust discharges not less than 20 feet from a property line or openings into buildings (1105.6.1). Indoor exhaust inlets sit where leaked refrigerant will concentrate, based on density relative to air (1105.6.1.1).

Normal occupied ventilation (1105.6.3.1) is the larger of (1) 0.5 cfm per square foot of machinery-room area or 20 cfm per person, and (2) the airflow that limits room temperature rise to 18°F given the machinery’s heat. Emergency ventilation (1105.6.3.2) starts when the 1105.3 detector actuates. 2024 IMC Equation 11-2 (confirmed on the ICC 2024 Digital Codes rendering of 1105.6.3.2) is:

Q = 100 × √G

where Q is airflow in cfm and G is the design mass of refrigerant, in pounds, in the largest system any part of which is in the machinery room. (SI: the code publishes the companion Q = 0.07√G form on the same equation line.)

Worked example 3 — emergency exhaust. Largest circuit in the room, G = 100 pounds. Q = 100 × √100 = 100 × 10 = 1,000 cfm. If G = 36 pounds, Q = 100 × 6 = 600 cfm. Multiple fans or multispeed fans may provide the emergency rate and a reduced normal rate (1105.6.3).

The 1105.6 exception for an outdoor penthouse, lean-to, or other open structure more than 20 feet from any building opening allows natural or mechanical ventilation. Equation 11-1 publishes F (free opening area, square feet) as a function of the same G (pounds in the largest system). Confirm the printed formula on the 1105.6 equation line in the bound 2024 IMC — the IP variables are F in square feet and G in pounds. 2024 reprints that render the equation in text use F = √G (SI F = 0.138√G). For G = 100 pounds that is 10 square feet of free opening, located using the refrigerant’s density relative to air.

1105.7: pressure-relief devices, fusible plugs, and purge systems in the machinery room terminate outside the structure, not less than 15 feet above adjoining grade and not less than 20 feet from any window, ventilation opening, or exit. 1105.8 sends emergency pressure-control systems to IFC 608.11. 1105.9 means of egress: machinery rooms larger than 1,000 square feet shall have not less than two exits or exit-access doorways (one may be a fixed ladder or alternating-tread device). Separate those doorways by a horizontal distance equal to one-half the room’s maximum horizontal dimension. All portions of the room shall be within 150 feet of an exit or exit-access doorway (IBC 1017.1 may increase travel). Doors swing in the direction of egress, have panic hardware regardless of occupant load, and are tight-fitting and self-closing.

1106 adds special requirements where 1104.2 requires them. 1106.2 / 1106.3: for A2, A3, B2, and B3, no permanently installed open flame or continuously operating hot surface over 800°F, and the room conforms to NFPA 70 Class I, Division 2. 1106.4 Group A2L and B2L is the 2024 inspection difference you must not collapse into the A2 rule:

• 1106.4.1: the hot-surface / open-flame limit is 1290°F (700°C), not 800°F.
• 1106.4.2: detectors, in addition to 1105.3, shall signal an alarm and start ventilation on Table 1106.4.2 timing. At or below the OEL in Table 1103.1: 300 seconds maximum response, ASHRAE 15 ventilation level 1, automatic alarm reset, trouble alarm. At or below the RCL in Table 1103.1: 15 seconds maximum response, ventilation level 2, manual reset, emergency alarm.
• 1106.4.3: mechanical ventilation shall comply with ASHRAE 15.

1106.5 remote controls. 1106.5.1: a clearly identified break-glass or approved tamper-resistant switch provides off-only control of refrigerant compressors, refrigerant pumps, and normally closed automatic refrigerant valves in the room. That equipment also shuts off automatically when vapor exceeds the detector’s upper limit or 25 percent of the LEL, whichever is lower. 1106.5.2: a clearly identified break-glass or tamper-resistant switch provides on-only control of the machinery-room ventilation fans. Do not reverse those switch actions. 1106.6: emergency signs, charts, and labels per the IFC.

Piping, valves, relief discharge, and A2L field marks

1107.4 / Table 1107.4: copper tube (including linesets to ASTM B280 / B1003), copper pipe, aluminum tube, brass pipe, and listed steel pipe/tube. Soft annealed copper larger than 1⅜ inch O.D. shall not be used for field-assembled refrigerant piping unless protected from mechanical damage. 1107.4.1: steel pipe for A2, A3, B2, and B3 shall be not less than Schedule 80 for sizes 1½ inches or less. 1101.9: outdoor refrigerant access ports get locking-type tamper-resistant caps (or equivalent security), except equipment in controlled areas such as roofs with locked hatches.

1109.2.3 prohibited piping locations: exposed in a fire-resistance-rated exit-access corridor; exposed in an interior exit stairway; in an interior exit ramp; in an exit passageway; in an elevator, dumbwaiter, or other shaft containing a moving object. 1109.2.5: piping that penetrates two or more floor/ceiling assemblies goes in a fire-resistance-rated shaft (IBC 713), with printed exceptions for R-718, limited A1 direct systems, and exterior vented piping. 1109.2.7 pipe identification: outside the equipment room, identify piping at intervals not exceeding 20 feet, letters not less than ½ inch high, with the refrigerant designation and safety group. Group A2L and B2L labels add “WARNING—Risk of Fire. Flammable Refrigerant.” Group A2, A3, B2, and B3 add “DANGER—Risk of Fire or Explosion. Flammable Refrigerant.” Any Group B refrigerant also adds “DANGER—Toxic Refrigerant.”

1109.3.2: A2L/B2L pipe shafts may be naturally or mechanically ventilated; A2/A3/B2/B3 shafts are continuously mechanically ventilated and include a detector. Mechanical ventilation may be continuous or detector-activated at a maximum of 25 percent of the LFL. Table 1109.3.2 sets minimum shaft velocities (100 to 400 fpm by shaft area).

Stop valves. 1109.6.1: systems containing more than 6.6 pounds get stop valves at each compressor/condensing-unit suction inlet, discharge outlet, and each liquid-receiver outlet (with printed exceptions). 1109.6.2: more than 100 pounds adds receiver inlets and, where condensers run in parallel, each condenser inlet and outlet.

1110 field tests (field-erected piping): pressure-test for strength and leak-test for tightness on both high- and low-pressure sides before operation. Test medium is an inert gas — oxygen-free nitrogen, helium, argon, or premixed nonflammable oxygen-free nitrogen with a hydrogen or helium tracer. R-744 may use carbon dioxide; R-718 may use water. 1110.3.1 forbids oxygen, air, other refrigerants, and combustible mixtures as the test gas.

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