VRF design: refrigerant piping, leak limits and CSA B52

A variable refrigerant flow system is unusual among commercial HVAC systems because the refrigerant itself is the distribution medium. Instead of chilled water or air, refrigerant runs through pipes into ceilings, corridors and occupied rooms. That brings efficiency and compact distribution, but it also means the whole refrigerant charge of a system could, in principle, leak into a single room.

CSA B52, the Mechanical Refrigeration Code, sets rules to keep the concentration of leaked refrigerant in occupied spaces below safe limits. Getting this right at design stage avoids expensive changes later, when an authority having jurisdiction or a reviewer finds that a small office or hotel room cannot legally be served as drawn.

VRF piping basics

Layout

A typical VRF piping network runs from the outdoor unit through a main line, then splits at branch fittings (refnets or headers) to feed individual indoor units. Heat recovery systems add branch controllers or mode-change boxes that direct refrigerant to units in heating or cooling.

Manufacturers publish strict limits for each product line, including:

• Maximum total piping length
• Maximum length from the outdoor unit to the farthest indoor unit
• Maximum length from the first branch to the farthest indoor unit
• Maximum vertical separation between the outdoor unit and indoor units, and between indoor units
• Combination ratio: the sum of indoor unit capacities relative to outdoor unit capacity

These limits differ between manufacturers and models, so the piping design must be based on the selected product and checked with the manufacturer's selection software.

Sizing and capacity corrections

Pipe sizes are chosen by the manufacturer's software based on connected capacity downstream of each section. Long runs and large elevation differences reduce capacity, because of pressure drop and the effect of the refrigerant column. The software applies correction factors; designers should check corrected capacities, not nominal ones, against loads.

Installation requirements

1. Use refrigerant-grade copper of the specified wall thickness and temper.
2. Braze with a nitrogen purge to prevent internal oxidation.
3. Support and anchor piping to allow for thermal expansion.
4. Insulate both liquid and gas lines where required, with vapour-tight joints to prevent condensation.
5. Pressure test with dry nitrogen at the manufacturer's specified pressure and hold time.
6. Evacuate to the specified vacuum and confirm the hold.
7. Charge by weight, adding the calculated additional charge for the installed pipe lengths.
8. Record actual pipe lengths and total charge for the refrigerant concentration check and the O&M manual.

Why refrigerant concentration matters

Most VRF systems use refrigerants that are low in toxicity, but at high concentrations any refrigerant can displace oxygen, cause cardiac sensitization or other effects, or, for flammable refrigerants, create a flammable mixture. A leak into a large open office will dilute quickly. A leak into a small hotel room, private office, washroom or storage room may not.

CSA B52 addresses this by limiting the quantity of refrigerant that can be released into an occupied space, based on the refrigerant's concentration limit and the volume of the space. Refrigerant data, including safety classifications and concentration limits, are derived from ASHRAE Standard 34 and related standards.

The concentration check

The basic check compares the potential concentration in the smallest occupied space served by a system against the allowable limit for that refrigerant and occupancy.

1. Determine the total system charge. Include the factory charge of the outdoor unit plus the additional charge for installed piping, based on the manufacturer's calculation.
2. Identify each space served by the system. Every space containing an indoor unit or refrigerant piping is a potential release location.
3. Calculate the effective volume of each space. Volumes of connected spaces can sometimes be combined if there are permanent openings or transfer paths between them that meet the code's conditions.
4. Divide the charge by the volume to get the potential concentration.
5. Compare with the allowable limit for the refrigerant and occupancy type in the edition of CSA B52 adopted in your province. Some occupancies, such as institutional ones where occupants may not be able to evacuate quickly, have stricter limits.

The smallest room on a system is usually the critical one. A system serving a whole floor might pass in every open office area and fail only in one small office or meeting room.

Because concentration limits, occupancy factors and A2L requirements differ between editions of CSA B52, always confirm the edition in force with the authority having jurisdiction before relying on a specific number.

When a space fails the check

If a room exceeds the allowable concentration, there are several ways to bring the design into compliance. Options and their acceptance depend on the code edition and local authority.

Designers usually address the problem at the layout stage by grouping small rooms onto systems with lower charge, or by serving them with ducted units from a larger zone.

A2L refrigerants and VRF

As manufacturers move from R-410A to A2L refrigerants such as R-32 or R-454B, additional requirements apply because these refrigerants are mildly flammable. These can include:

• Charge limits based on flammability as well as toxicity
• Requirements for refrigerant detection and automatic mitigation such as circulation fan operation or ventilation
• Restrictions on potential ignition sources near indoor units and piping
• Product safety requirements under the applicable CSA and UL equipment standards

The 2023 edition of CSA B52 includes provisions for A2L refrigerants, but adoption timing varies by province. Our guide to refrigerant regulations and low-GWP options summarizes the transition.

Practical design tips

• Run the concentration check early, as soon as rooms and zones are laid out, not after the piping design is finished.
• Keep a room-by-room table of volumes, connected charge and calculated concentration in the design file.
• Avoid routing refrigerant piping through stairwells, exits and other spaces where codes restrict it.
• Coordinate pipe routes with fire separations, using approved firestop systems for penetrations.
• Record final installed charge and update the concentration table if pipe lengths change during construction.

Frequently asked questions

Does CSA B52 apply to every VRF system?

CSA B52 is adopted by provincial regulation and generally applies to refrigeration systems in buildings, including VRF. Specific scope, exemptions and editions vary by province, so check with the local authority.

Can leak detectors alone solve a failed room?

Detectors with automatic shut-off valves or ventilation can be an accepted mitigation under some code provisions. Alarms alone are generally not considered sufficient. The detectors must be maintained and tested regularly to remain effective.

Key takeaways

• VRF piping must follow manufacturer limits for length, elevation and combination ratio, with capacity corrections checked.
• CSA B52 limits refrigerant concentration in occupied spaces; the smallest room on each system is usually critical.
• Compare total system charge against effective room volume using the CSA B52 edition in force in your province.
• When a room fails, options include splitting systems, re-zoning small rooms, increasing effective volume, or detection with shut-off or ventilation.
• Track installed charge during construction and verify it through building commissioning.