A2L Charge Limits and Minimum Room Area: Reading the OEM Table

Why charge and room area are linked

If the whole charge of an A2L system leaks into one small room, the concentration can reach the lower flammability limit (LFL). The product standard, UL 60335-2-40, and the residential installation standard, ASHRAE 15.2, prevent that by tying the allowed charge to the floor area the refrigerant can spread into. Above a threshold they require mitigation, usually a refrigerant detection system that starts the indoor blower to mix the leak into the room air.

UL Solutions notes that the 4th edition of UL 60335-2-40 (published December 15, 2022, mandatory from January 1, 2025) requires the detection system to respond when it senses 25% of the LFL. The 3rd edition allowed 100% of the LFL. The same article describes the charge limits as based on the minimum room volume, with a safety factor of 4.

For field work, the manufacturer has already done the math. Under IRC M1411.2 and M1411.6 in the ICC required changes to the 2021 IRC, A2L systems must be listed to UL/CSA 60335-2-40, installed per the listing, and charged per the manufacturer’s instructions and the listing. The installation manual’s charge and room area table is the document you work from.

m1 and m2: the charge levels in ASHRAE 15.2

ASHRAE 15.2 Table 9-1, “Flammable A2L Refrigerant Charge Levels for Mitigation,” lists two charge levels per refrigerant. The values below are from Addendum f to 15.2-2022 (approved July 31, 2024), which added R-1234yf.

Refrigerant	m1 charge level	m2 charge level	LFL conversion factor C (Table 9-2)
R-32	4.0 lb (1.8 kg)	35.0 lb (15.9 kg)	1.00
R-454B	4.0 lb (1.8 kg)	33.9 lb (15.4 kg)	0.96
R-452B	4.2 lb (1.9 kg)	35.4 lb (16.1 kg)	1.02
R-454A	3.7 lb (1.7 kg)	32.1 lb (14.6 kg)	0.90
R-454C	4.0 lb (1.8 kg)	33.4 lb (15.2 kg)	0.94
R-1234yf	3.8 lb (1.7 kg)	33.1 lb (15.0 kg)	0.93

In the ASHRAE Journal roundtable, UL’s Brian Rodgers described m1 as “the charge limit when only room area is used to determine the charge limit,” adding: “Above that we have to look at mitigation.” Do not use m1 as a field shortcut: Rheem’s R-454B table below starts at 50 oz, under the 4.0 lb m1 level, and already lists minimum room areas there. The OEM table governs.

How ASHRAE 15.2 sizes the maximum charge

Addendum a to 15.2-2022 revised Section 9.5. For A2L systems without ventilation (9.5.1), the maximum charge (mmax) is built from the LFL conversion factor C (Table 9-2) and M, “the allowable refrigerant amount allowed in a dispersal volume based on 25% LFL” (Table 9-3). Systems with ventilation (9.5.2) add MV from Table 9-4. The addendum also brought in the “gravity equation” from the 4th edition of UL 60335-2-40 to align the two standards.

The addendum prints its changes as underline and strikethrough markup, which is lost when the text is extracted, so the full equation is not reproduced here. Work from the current edition (15.2-2024) or the listing for design.

Table 9-3 is based on a 7.2 ft (2.2 m) dispersal height, with separate columns for systems with and without circulation. A few rows:

Area	M with circulation	M without circulation
100 sq ft	6.9 lb	6.9 lb
200 sq ft	13.8 lb	11.9 lb
300 sq ft	20.7 lb	14.6 lb
500 sq ft	34.4 lb	18.8 lb
525 sq ft and up	35.1 lb	rises slowly (19.3 lb at 525 sq ft)

The table notes allow dispersal heights from 2.0 ft to 9.0 ft by multiplying by hc = h ÷ 7.2 ft, and permit interpolation between rows; otherwise use the closest lower area. The point for the field: circulation (a blower started by a detector, or running continuously) lets a mid-size room take a larger charge, 20.7 lb instead of 14.6 lb at 300 sq ft before the C factor.

Reading an OEM minimum room area table

Rheem’s (-)P17AY R-454B instructions, section 5.8, show the usual layout. Selected rows:

Total system charge	Minimum indoor airflow	Smallest room, no sensor: release ≤2 ft	6 ft	≥9 ft	With sensor: total conditioned area
100 oz	333 CFM	781 sq ft	225 sq ft	150 sq ft	187 sq ft
200 oz	665 CFM	3,126 sq ft	451 sq ft	301 sq ft	375 sq ft
250 oz	831 CFM	4,884 sq ft	564 sq ft	376 sq ft	469 sq ft
400 oz	1,330 CFM	12,502 sq ft	1,389 sq ft	617 sq ft	750 sq ft
550 oz	1,829 CFM	not permitted, sensor required			1,031 sq ft

To use it:

Get the total system charge. Rheem: nameplate factory charge plus the field-added charge, written on the label next to the nameplate. The factory charge covers 15 ft of liquid line; the line-set charge calculator gives the adder.
Find the release height. The column is the distance from the floor to the bottom edge of the lowest supply or return opening, or of the indoor unit. A low return in a basement closet puts you in the ≤2 ft column, where the areas are largest.
Measure the smallest conditioned room the indoor unit serves. If it is below the table value, Rheem says a refrigerant leak detection sensor “will need to be installed.”
With a sensor, check the total conditioned area, the sum of all conditioned rooms, against the last column, and confirm the blower can deliver the minimum indoor airflow. The airflow calculator helps with the field check.
Correct for altitude. Multiply the minimum area by the altitude factor: 1.050 at 1,000 ft, 1.360 at 3,000 ft and 1.768 at 5,000 ft in Rheem’s table. Rheem allows interpolation or rounding up to the next altitude step.

At the top of the table the “without sensor” columns read “not permitted - sensor required,” so very large charges need detection regardless of room size.

Packaged and commercial units: TAmin

Packaged equipment usually lists a single minimum conditioned area (TAmin) per model. Carrier’s 50FE instructions say to add the floor areas of all spaces served by a common duct system and compare the total with Table 5. TAmin there ranges from 127 sq ft (50FE-A04) to 541 sq ft (50FE-N07), based on a minimum ceiling height of 7.2 ft and the worst-case charge. Below TAmin, Carrier says additional ventilation may be required and refers to local code, UL 60335-2-40 or ASHRAE 15.

Lennox’s LGX/LCX instructions list R-454B charge, TAmin and the minimum airflow during leak mitigation (Qmin) per model, with an altitude factor applied the same way.

Ignition sources in small spaces

Lennox: if the appliance conditions a space smaller than TAmin, or is stored in a space smaller than Amin, that space “must be without continuously operating open flames” or other potential ignition sources such as an operating electric heater, unless the flame-producing device has an effective flame arrest system. Carrier prints a similar caution. IRC M1411.4 separately bans accessories in A2L ductwork with electric heating elements, open flames, or devices switching more than 2.5 kVA.

Code cap

IRC M1411.6 (2024 A2L text) limits the A2L charge for an individual refrigeration system to 34.5 lb (15.7 kg), whatever the room size. How the 2024 codes reached your jurisdiction is covered in A2L codes and adoption.

Common mistakes

Using the nameplate charge alone instead of the total charge after the line-set adder.
Using the whole-house area to skip a sensor. Without a sensor, Rheem’s table uses the smallest conditioned room where the indoor unit is installed.
Reading the 6 ft column for a unit with a floor-level return.
Skipping the altitude factor in mountain jurisdictions.
Reusing an R-410A coil. Rheem says it “may lack a refrigerant detection system necessary for safe operation.”

A2L charge limits and minimum room area

Key facts