What is ASHRAE Standard 15?

ASHRAE Standard 15 is the Safety Standard for Refrigeration Systems. It establishes minimum safety requirements for the design, construction, installation, and operation of refrigeration systems. It covers refrigerant detection, ventilation, pressure relief, and machinery room design.

What is an A2L refrigerant?

A2L is a safety classification under ASHRAE Standard 34 meaning 'lower flammability.' A2L refrigerants have a burning velocity below 10 cm/s and a heat of combustion below 19 kJ/g. They are considered 'mildly flammable' — significantly less flammable than A3 (propane) but still requiring detection and safety measures.

Do I need a refrigerant detector for an R-454B chiller?

Per ASHRAE 15-2024 Section 8.11, occupied spaces and machinery rooms always require refrigerant detection with a trouble alarm set at or below the OEL. Machinery rooms (and non-occupied spaces with potential for high refrigerant charges, such as shafts) additionally require an emergency alarm set at or below the RCL (29,000 ppm for R-454B).

What is the refrigerant concentration limit (RCL)?

The RCL is the maximum allowable refrigerant concentration in an occupied space, defined in ASHRAE Standard 34. It is based on toxicity (ATEL) and oxygen depletion (ODL), whichever produces the lower limit. For A2L refrigerants, the RCL is typically around 29,000–30,000 ppm.

At what concentration does an A2L refrigerant detector need to alarm?

Per ASHRAE 15-2024, the emergency alarm setpoint for A2L, A2, B2L, and B2 refrigerants must be no greater than the RCL, and detectors must alarm at no more than 25% of the Lower Flammability Limit (LFL). For R-454B, 25% of 115,000 ppm = 28,750 ppm. For R-32, 25% of 141,000 ppm = 35,250 ppm. A separate trouble alarm must be set at or below the OEL.

Does ASHRAE 15 apply to R-410A systems?

Yes, ASHRAE 15 applies to all refrigeration systems regardless of refrigerant type. R-410A is classified A1 (non-flammable, lower toxicity), but occupied spaces and machinery rooms still require OEL trouble-alarm detection per Section 8.11. Machinery rooms and non-occupied spaces with high-charge potential additionally require RCL emergency-alarm detection.

What's the difference between ASHRAE 15-2019, 15-2022, and 15-2024?

ASHRAE 15-2019 introduced A2L refrigerant provisions. ASHRAE 15-2022 expanded A2L charge limits, updated machinery room detection rules, and aligned with UL 60335-2-40. ASHRAE 15-2024 incorporates 14 addenda to the 2022 edition and is the current edition referenced by this tool. Always verify which edition your local code references.

Does the ASHRAE 15 detection requirement also apply to ammonia systems?

Yes. Ammonia (R-717, B2L) has a very low RCL of 320 ppm due to its toxicity. ASHRAE 15 and IIAR 2 both require ammonia detectors in machinery rooms, typically set to alarm at 25 ppm per IIAR 2-2021 §6.13 (the OSHA PEL-TWA is 50 ppm; the ACGIH TLV-TWA is 25 ppm). Emergency ventilation activates at 150 ppm (half the NIOSH IDLH of 300 ppm). Ammonia detection has been standard practice in industrial refrigeration for decades.

What is a machinery room under ASHRAE 15?

A machinery room is an enclosed space used primarily for housing refrigeration equipment. ASHRAE 15 requires machinery rooms when the refrigerant charge exceeds the RCL-based limit for the occupied space. Machinery rooms have specific requirements for ventilation, detection, emergency shutoffs, and access restrictions.

How does ASHRAE 15 relate to the International Mechanical Code?

The International Mechanical Code (IMC) Chapter 11 references ASHRAE 15 as the basis for refrigeration system safety. Many local jurisdictions adopt the IMC, which effectively makes ASHRAE 15 requirements enforceable by law. The IMC 2024 edition aligns with ASHRAE 15 provisions for A2L refrigerants.

ASHRAE 15 Refrigerant Detection Requirement Guide

Determine the ASHRAE 15-2024 refrigerant detection requirements for your system. Enter the refrigerant type, system charge, space type, and volume to identify the required OEL trouble-alarm and RCL emergency-alarm setpoints.

Refrigerant Type

Use metric units (kg, m³)

System Charge (lb)

Space Type

Occupied space (human comfort)

Machinery room (service access only)

Non-occupied space

Space Volume (ft³)

Concentration Check

If the entire charge releases: 5.00 lb/1000ft³

RCL for R-32: 4.8 lb/1000ft³

Exceeds RCL — concentration would reach 104% of the RCL. Machinery-room construction and emergency-alarm detection per ASHRAE 15-2024 Section 8.11 are required.

Required Detection Setpoints

Trouble alarm (OEL): ≤ 1,000 ppm

Required in occupied spaces per ASHRAE 15-2024 Section 8.11. Basis: OEL of 1,000 ppm for R-32.

Emergency alarm (RCL): ≤ 30,000 ppm

Required in occupied spaces where the releasable charge exceeds the RCL. Basis: RCL (30,000 ppm), which is lower than 25% LFL.

Advisory Summary

For a 25.0 lb system of R-32 in a 5,000 ft³ occupied space: ASHRAE 15-2024 Section 8.11 applies. A trouble alarm set at or below 1,000 ppm (OEL) plus an emergency alarm at or below 30,000 ppm (RCL) would meet the standard's minimum requirement. Verify against the full standard and your Authority Having Jurisdiction (AHJ).

ASHRAE 34-2022 Refrigerant Safety Data

RCL, LFL, OEL, and GWP values from ANSI/ASHRAE Standard 34-2022 and Addendum a

Refrigerant	Group	RCL (ppm)	RCL (lb/1000ft³)	LFL (ppm)	OEL (ppm)	GWP
R-32	A2L	30,000	4.8	141,000	1,000	675
R-454B	A2L	29,000	4.6	115,000	850	466
R-454C	A2L	29,000	4.6	77,000	620	148
R-455A	A2L	30,000	6.8	118,000	650	148
R-1234yf	A2L	30,000	7.6	62,000	500	4
R-1234ze(E)	A2L	29,000	6.9	65,000	800	6
R-717 (Ammonia)	B2L	320	0.014	162,000	25	0
R-744 (CO₂)	A1	40,000	4.5	N/A	5,000	1
R-290 (Propane)	A3	9,000	1.03	21,000	1,000	3
R-410A	A1	140,000	26	N/A	1,000	2,088
R-134a	A1	60,000	13	N/A	1,000	1,430
R-407C	A1	76,000	17	N/A	1,000	1,774
R-404A	A1	130,000	25	N/A	1,000	3,922

Sources: ANSI/ASHRAE Standards 15-2024 & 34-2022, UL 60335-2-40, IMC 2024 Chapter 11.

Frequently Asked Questions

Advisory Disclaimer

This tool is provided for educational and reference purposes only. It summarizes requirements from published standards as of their most recent edition date and does not replace the full text of the applicable standard, local code, or the judgment of a qualified design professional. Values shown here must be verified against the current edition of the relevant standard (ASHRAE 15, ASHRAE 34, IIAR 2, etc.) and the Authority Having Jurisdiction for the project location. Critical Environment Technologies assumes no liability for decisions made using this tool. Always consult a licensed mechanical engineer for system design and compliance determinations.