HVAC System Types: How Split, Packaged & Ductless Systems Actually Work

The Vapor Compression Cycle: How Every AC Moves Heat

Every air conditioner and heat pump — regardless of brand, configuration, or price — uses the same four-stage thermodynamic cycle to move heat. Refrigerant circulates in a closed loop, changing between liquid and gas states to absorb heat indoors and reject it outdoors. The heat transfer rate Q (in BTU/hr) depends on the mass flow rate of refrigerant and the change in enthalpy across the evaporator coil:

Stage 1 — Compression: The compressor pressurizes low-pressure refrigerant gas into a high-temperature superheated vapor. This is the stage that consumes the most electrical power — typically 3,000 to 5,000 watts for a 3-ton residential unit while running.

Stage 2 — Condensation: The superheated vapor enters the outdoor condenser coil. A fan blows ambient air across the coil, pulling heat out of the refrigerant. As it cools, the refrigerant condenses into a high-pressure liquid. The outdoor unit is rejecting the heat that was absorbed inside your home.

Stage 3 — Expansion: The high-pressure liquid passes through a Thermostatic Expansion Valve (TXV) or electronic expansion valve. This is a physical restriction that causes a sudden pressure drop, flash-cooling the refrigerant to roughly 40°F before it enters the indoor coil.

Stage 4 — Evaporation: The cold liquid-vapor mix enters the indoor evaporator coil. Your blower fan pushes warm indoor air across this coil. Heat transfers from the air into the refrigerant, which boils back into a gas and returns to the compressor to repeat the cycle. The air that comes out the other side is what cools your home.

Split Systems: The Ducted Workhorse

Split systems are the most common residential configuration in North America — roughly 70% of US homes with central air use this design. The compressor and condenser live in an outdoor metal cabinet. The evaporator coil and blower fan (the air handler) sit indoors — typically in an attic, basement, or dedicated closet. The two halves are connected by a copper refrigerant line set: a small-diameter liquid line and a larger insulated suction vapor line.

The Physics of Ducted Airflow: Static Pressure

A split system pushes conditioned air through a network of sheet metal or flexible ducts. This network creates resistance — known as External Static Pressure (Pₛ, measured in inches of water column, or "in. w.g."). Every component in the airstream adds friction: the air filter, the evaporator coil face, the supply ducts, the return ducts, and every bend and transition.

For a system to deliver its designed airflow — typically 400 CFM per ton of cooling capacity — the blower fan must generate enough pressure to overcome all of these cumulative losses. ACCA Manual D, the industry standard for residential duct design, specifies a maximum external static pressure of 0.5 in. w.g. However, field measurements routinely find systems operating at 0.5 to 0.8 in. w.g., especially in older homes with undersized or kinked flex duct.

When static pressure exceeds the blower's capability, airflow drops. At reduced airflow, the evaporator coil gets too cold and can ice over — the refrigerant is absorbing less sensible heat from the slower-moving air, so the coil surface temperature drops below freezing. This is why a dirty filter or a crushed return duct can cause an AC to freeze up even on a hot day.

Packaged Systems: Everything in One Box

A packaged system consolidates the compressor, condenser, evaporator, blower, and heating elements into a single outdoor cabinet, typically mounted on a concrete slab at ground level or on a flat rooftop. There are no refrigerant lines running through the walls of the house — the entire refrigeration cycle completes inside the outdoor cabinet. Air is pulled from the home through a large return duct, conditioned inside the cabinet, and pushed back through a supply duct.

Packaged units make sense when interior space is tight — homes on slab foundations with no attic or basement, or commercial buildings where all mechanical equipment is kept on the roof. All the operational noise stays outside, which is a real quality-of-life advantage. The trade-off is that the supply and return ducts must run outside the home's thermal envelope before entering the conditioned space. Even well-insulated ductwork (code minimum is R-8 exterior insulation) will experience some conductive heat gain in summer and heat loss in winter. Packaged systems also tend to have lower efficiency ceilings — most top out around 16 SEER2 — because the entire unit sits in outdoor ambient conditions that vary from 0°F to 105°F.

Ductless Mini-Splits: Variable Refrigerant Flow Without Ducts

Ductless mini-splits represent a fundamentally different approach. Instead of moving conditioned air through ducts, they move refrigerant directly to individual indoor air handlers mounted on walls, ceilings, or floors in each room. There are no ducts, no static pressure losses, and no thermal losses through attic ductwork.

Inverter Compressor Physics

Traditional split and packaged systems use single-stage compressors that are either ON (100%) or OFF (0%). When the thermostat calls for cooling, the compressor slams on and draws a large startup current — Locked Rotor Amps (LRA) — which can be 3 to 7 times the normal running current. This binary cycling causes temperature swings of 2°F to 4°F and is electrically inefficient.

Ductless mini-splits use variable-speed inverter compressors. The inverter converts incoming AC power to DC, then uses a digital controller to modulate the motor speed across a wide range — typically 10 Hz to 120 Hz. Instead of cycling on and off, the system runs continuously at whatever speed matches the home's real-time heat load. The relationship between compressor speed and power draw is approximately cubic — meaning running at 50% speed uses roughly one-eighth the power of running at full speed. This is an idealized relationship from pump affinity laws; real-world inverter efficiency curves vary by manufacturer, but the principle holds: continuous low-speed operation is dramatically more efficient than start-stop cycling.

This is why top-tier mini-splits achieve efficiency ratings of 30+ SEER2. Mitsubishi's MSZ-FS series reaches 33.1 SEER2, and several manufacturers (Carrier, Gree, Fujitsu) have units in the 28-33 range.

HSPF2: The Heating Side of the Equation

SEER2 measures cooling efficiency. Its counterpart for heating is HSPF2 — Heating Seasonal Performance Factor, updated to the "2" standard in 2023 alongside SEER2 under the same DOE test procedure revision. HSPF2 represents the total heating output in BTU divided by the total electrical input in watt-hours over an entire heating season.

The rating matters because a heat pump moves heat rather than creating it. Even in cold outdoor conditions, a heat pump can deliver 2 to 4 units of heat for every unit of electricity consumed — a Coefficient of Performance (COP) of 2.0 to 4.0. By comparison, electric resistance heat strips deliver exactly 1.0 COP — every watt of electricity becomes exactly 3,412 BTU of heat, no more.

For split system heat pumps, HSPF2 ratings typically range from 7.5 to 10.5. Ductless mini-split heat pumps achieve 10.0 to 13.5 HSPF2, with some cold-climate units (Mitsubishi Hyper Heat, Fujitsu XLTH) maintaining usable capacity down to -13°F or lower without backup resistance heat. The HSPF2 standard requires testing at multiple outdoor temperatures including 47°F, 35°F, and 17°F, so the rating reflects real shoulder-season and mild-winter performance — not just ideal conditions.

When comparing systems, SEER2 tells you summer operating cost. HSPF2 tells you winter operating cost. A system with a high SEER2 but mediocre HSPF2 may cost more to run annually in a heating-dominated climate (like the northern Midwest) than a system with balanced ratings. For a deeper dive into how these numbers are tested and what changed in the 2023 update, see our SEER vs SEER2 physics guide.

Refrigerant: R-410A Is Phasing Out, R-32 Is Arriving

If you purchase an HVAC system in 2026 or later, you may encounter two different refrigerants. This is not a minor detail — it affects system availability, long-term service costs, and the environmental footprint of your equipment.

R-410A has been the standard residential refrigerant since the phaseout of R-22 in 2010. It has a Global Warming Potential (GWP) of 2,088 — meaning one pound of R-410A released into the atmosphere has the same warming effect as 2,088 pounds of CO₂ over 100 years. Under the AIM Act (American Innovation and Manufacturing Act of 2020), the EPA is enforcing a phasedown of HFC production. Starting in 2025, US manufacturing of new R-410A equipment began scaling down. R-410A systems will still be sold through existing inventory and will remain serviceable for years, but the refrigerant itself will become progressively more expensive as production quotas tighten — similar to what happened with R-22 between 2010 and 2020, when the per-pound cost roughly quadrupled.

R-32 (difluoromethane) is the primary replacement for new residential systems. It has a GWP of 675 — roughly one-third that of R-410A. It is a single-component refrigerant (R-410A is a blend), which makes it easier to recover and recycle. It also transfers heat slightly more efficiently, giving manufacturers an efficiency bump. Because R-32 is mildly flammable (A2L classification — the same classification as most home-use aerosol propellants), systems using it require specific safety sensors and installation protocols. This does not make them dangerous — the charge sizes in residential equipment are small compared to the volume of air in a mechanical room — but it does mean that refrigerant handling is not a DIY task.

What this means for you as a homeowner: If you are replacing a system today, both R-410A and R-32 systems are available. An R-410A system installed in 2026 will be serviceable for its full lifespan — but refrigerant costs will rise over time. An R-32 system future-proofs you against the phasedown and typically carries a modest efficiency advantage. Either way, handling refrigerant requires EPA Section 608 certification. The equipment, gauges, and recovery cylinders are not sold at retail to unlicensed individuals. When your contractor talks about refrigerant, the value of that conversation is understanding what they are installing and why — not trying to do it yourself.

Manual J: Sizing Based on Physics, Not Guesswork

An oversized air conditioner short-cycles. It blasts cold air, satisfies the thermostat in 8 minutes, and shuts off before the coil has run long enough to pull moisture out of the air. You end up with a cold, clammy house — and a system that wears out faster from repeated starts. An undersized system runs constantly on hot days and never reaches the setpoint. Both failures come from the same root cause: nobody did a load calculation.

The industry standard method is ACCA Manual J, which calculates the total heating and cooling load for each room based on the home's construction, orientation, insulation levels, window area, and local climate data. The rate of conductive heat transfer through a wall, ceiling, or window is:

But conduction is only one piece of the total load. A complete Manual J calculation sums four components:

Once you have the total cooling load in BTU/hr, convert to tons. One ton of refrigeration — the term dates back to the era of ice houses — is the rate of heat extraction needed to freeze one short ton (2,000 lbs) of 32°F water into 32°F ice in 24 hours:

A home with a calculated load of 34,500 BTU/hr needs a 3-ton system (34,500 ÷ 12,000 = 2.875, round up to the nearest available half-ton). You can run these numbers yourself using our HVAC sizing calculator.

A note on Manual J accuracy: the formula above is the physics. The quality of the result depends entirely on the accuracy of the inputs — wall R-values, window U-factors, infiltration rates, and design temperatures. These inputs should be measured or verified, not guessed. If a contractor gives you a sizing recommendation without measuring your windows, counting your supply registers, or inspecting your attic insulation depth, they are not doing a Manual J. They are applying a rule of thumb — and "500 square feet per ton" rules of thumb are why so many homes have oversized equipment.

System Comparison: By the Numbers

Which System Fits Your Situation?

Three diagnostic questions to narrow your options:

1. Do you have existing, functional ductwork? If yes — a split system replacement gives you the most capacity per dollar. Swap the outdoor unit and indoor coil, keep the ducts, and you have a 15+ SEER2 system with minimal labor.
2. Is interior space extremely tight — no attic, no basement, no closet for an air handler? If yes — a packaged unit on an exterior slab or rooftop removes all mechanical equipment from inside the home. Common in manufactured homes, small commercial spaces, and slab-on-grade construction.
3. Are you adding a room addition, converting a garage, or dealing with a house that has no ductwork? If yes — ductless mini-splits avoid the cost and mess of running ducts through finished walls while giving you independent temperature control per room. They are also the most efficient option by a significant margin.