5.6 Ductless Mini-Split & Variable Refrigerant Flow Systems

Ductless Mini-Split & Variable Refrigerant Flow Systems

"Mini splits" appear by name in PSI's HVAC Equipment content outline, and they now represent a large share of Michigan retrofit work: additions without duct capacity, converted attics and garages, sunrooms, server closets, and whole-house replacements in homes that never had ducts.

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What Makes a Mini-Split Different

A conventional split system is a fixed-capacity machine that satisfies the load by cycling. A mini-split is an inverter-driven, variable-capacity machine that satisfies the load by modulating.

Two practical consequences for Michigan:

1. Cold-climate (hyper-heat) models change the conversation. A conventional air-source heat pump in Marquette needs a big strip-heat or fuel backup. A cold-climate inverter unit rated to hold capacity near 5°F and to operate to -13°F or lower can carry a much larger fraction of the season. Always select from the manufacturer's extended heating capacity table at the design temperature, not from the nominal tonnage.
2. Oversizing hurts. An inverter compressor at the bottom of its modulation range is less efficient and dehumidifies poorly. Sizing a 12,000 Btu/h head for a 4,000 Btu/h room produces short cycling and a clammy space.

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Line Sets: The Numbers That Fail Inspections

Every manufacturer publishes four limits, and a Michigan mechanical inspector on a rough-in will look for them:

Worked additional-charge example

A single-zone 18,000 Btu/h heat pump is precharged for 25 feet. The installed line set measures 62 feet. The manufacturer specifies 0.22 oz/ft of additional charge.

$62 - 25 = 37 \text{ ft beyond precharge}$ $37 \times 0.22 = 8.14 \text{ oz added}$

Weigh it in with a scale accurate to a tenth of an ounce. Charging a mini-split by superheat or subcooling is generally not valid — the EEV is actively controlling superheat, and the correct procedure is weigh-in per the manufacturer's table.

Assembly practice

• Flares are the number one leak source. Use a rigid (clutch-type) flaring tool, deburr the tube, use the manufacturer's flare dimension, apply a trace of approved refrigerant oil, and tighten with a torque wrench to the published value — not by feel. Over-torquing a 1/4-inch flare cracks it just as reliably as under-torquing leaks.
• Nitrogen purge while brazing any non-flare joint to prevent cupric oxide scale from forming inside the tubing and migrating into the EEV.
• Evacuate to 500 microns and hold a decay test with the service valves still closed, then open the valves. Never use the system's refrigerant charge to "purge" the line set.
• Insulate both lines separately for their full length, including outdoors, where UV-stable jacketing or wrap is required.

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Condensate

Wall and ceiling cassettes rarely have gravity fall to a drain. Most installations use a small condensate lift pump with an integral float switch. Two rules:

1. The pump's safety switch must be wired to shut the unit down on a high-level condition. An unwired float switch is the most common cause of ceiling damage on cassette installations.
2. Condensate discharge must terminate at an approved point per the Michigan Mechanical Code condensate provisions — not onto a roof deck, into a wall cavity, or at a doorway where it will glaze in a Michigan January.

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Multi-Zone and VRF Architecture

Multi-zone mini-split: one outdoor unit, a branch manifold or multiple ports, two to eight indoor heads. All heads are in the same mode (heating or cooling) at the same time unless the system is heat-recovery capable.

VRF (variable refrigerant flow): the commercial extension of the same idea.

• Heat pump VRF (2-pipe) — all indoor units in the same mode at any moment.
• Heat recovery VRF (3-pipe, or 2-pipe with branch selector boxes) — some indoor units cool while others heat, with heat moved from the cooling zones to the heating zones. This is extremely efficient in buildings with simultaneous loads — a Michigan office with a hot server room and a cold north perimeter is the textbook case.

VRF brings a large refrigerant charge and long piping, which raises the code issue below.

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Refrigerant Concentration Limit (MMC Chapter 11 / ASHRAE 15)

This is the code item most often missed on mini-split and VRF work, and it is squarely inside the Michigan Mechanical Code's refrigeration chapter.

The rule: for a refrigerating system serving an occupied space, the total refrigerant in the circuit must not exceed the quantity that, if it all leaked into the smallest occupied space served by that circuit, would exceed the refrigerant's Refrigerant Concentration Limit (RCL). RCL values come from ASHRAE Standard 34 and are expressed in pounds per thousand cubic feet.

Practical application:

$\text{Allowable charge (lb)} = \text{RCL} \left(\frac{\text{lb}}{1000\,\text{ft}^3}\right) \times \frac{\text{Volume of smallest occupied space (ft}^3)}{1000}$

The word smallest is what catches installers. A VRF riser with a 90-pound charge serving twelve offices is evaluated against the smallest of those twelve rooms, not against the building volume. The usual remedies are to split the circuit into smaller charges, to add a listed refrigerant detection and alarm system with mechanical ventilation, or to relocate a head out of the undersized room.

A2L refrigerants raise the stakes

New mini-split and VRF equipment increasingly ships with R-32 or R-454B, which are A2L (lower toxicity, lower flammability) under ASHRAE 34. A2L equipment carries its own charge-limit rules, requires listed leak detection and mitigation on many configurations under UL/CSA 60335-2-40, and demands A2L-rated recovery equipment, vacuum pumps, and leak detectors. Note that UL/CSA 60335-2-40-2022 and UL/CSA 60335-2-89-2021 are both on LARA's published reference list for the refrigeration classification — the state has already put the A2L standards on the table.

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Troubleshooting Quick Reference