Central split air conditioner
An outdoor condensing unit works with an indoor evaporator coil and blower. A separate furnace or electric heater may provide heat. The indoor and outdoor equipment must be properly matched.
Systems and terminology
The outdoor unit is only one part of home comfort.
Equipment, ducts, airflow, controls, drainage, insulation, air leakage, and moisture all interact. Understanding the system helps you compare proposals that solve the house—not just replace the box.
Common system types
Know the basic arrangement.
Heat pump
The refrigeration cycle moves heat in either direction, providing cooling and heating. Many systems include electric or fuel-fired backup heat for low temperatures or special operating conditions.
Gas furnace
A burner and heat exchanger warm air moved by the blower. Combustion, venting, gas pressure, safeties, and carbon-monoxide protection require qualified inspection and service.
Electric resistance heat
Electric heating elements warm the airstream. It is mechanically simple but can draw substantial power, making controls, staging, wiring, and heat-pump coordination important.
Ductless mini-split
One or more indoor units connect to an outdoor unit without conventional central ductwork. Placement, condensate routing, line installation, filtration, and maintenance access still matter.
Packaged system
Heating and cooling components share an outdoor cabinet, often on a roof or pad. Duct connections, weather exposure, drainage, structural support, and service access deserve attention.
Whole-house thinking
Five layers affect the result.
Load
Sun, windows, insulation, air leakage, occupants, and internal heat determine what the home needs.
Equipment
Capacity and performance must fit the load and the climate.
Air distribution
Duct size, leakage, insulation, returns, grilles, and blower settings determine what reaches each room.
Controls
Thermostat location, staging, schedules, sensors, and humidity logic determine when equipment runs.
Drainage and moisture
Condensate removal, infiltration, ventilation, runtime, and surface temperatures determine moisture risk.
Sizing
Bigger equipment is not automatically better.
Oversized cooling equipment may satisfy the thermostat quickly while running too briefly for good moisture removal. Undersized or poorly distributed systems may struggle at design conditions. The old equipment size is a clue—not a load calculation.
Questions to ask about a proposalAsk the contractor:
- Was a room-by-room load calculation completed?
- What indoor and outdoor design conditions were used?
- How were windows, insulation, ducts, leakage, and additions represented?
- How does the selected equipment capacity compare with the sensible and latent loads?
- What airflow and duct changes are required?
Proposal glossary
Twelve terms worth recognizing.
A rating is useful only when it describes the actual matched equipment being proposed and the installation supports that performance.
- BTU per hour
- A rate of heating or cooling capacity.
- Ton
- In air conditioning, 12,000 BTU per hour of nominal cooling capacity—not equipment weight.
- SEER2
- A seasonal cooling-efficiency metric used for rated equipment comparisons under current test procedures.
- EER2
- A cooling-efficiency metric at a specified test condition; useful context for hot-weather performance comparisons.
- HSPF2
- A seasonal heating-efficiency metric for heat pumps.
- AHRI reference
- A directory record that helps confirm the indoor and outdoor equipment combination has a published matched rating.
- Manual J
- A room-by-room heating and cooling load calculation based on the home—not simply the old equipment size.
- Manual S
- A process for selecting equipment that can meet the calculated loads under design conditions.
- Manual D
- A duct-design method used to size and lay out air distribution for the required airflow.
- MERV
- A filter-efficiency rating. Higher capture can also mean greater airflow resistance if the system and filter area are not suited to it.
- Static pressure
- Resistance the blower works against in the duct and filter system. Measurement helps identify airflow constraints.
- Commissioning
- Measured startup and verification that installed equipment, airflow, controls, drainage, and operation meet the design and manufacturer requirements.
Indoor air and humidity
Start with the source and the airflow.
Filters, ultraviolet devices, air cleaners, dehumidifiers, ventilation, and controls can each have a role. None replaces moisture-source control, clean equipment, sound ducts, proper airflow, or appropriate system operation.
Filtration
Match capture goals to the equipment’s airflow capability and provide enough filter area.
Humidity control
Investigate sizing, runtime, airflow, drainage, infiltration, and duct conditions before adding a device.
Ventilation
Outside air must be intentional, filtered as appropriate, and managed for a hot-humid climate.
Contaminant sources
Combustion, leaks, pests, stored chemicals, construction dust, and water damage require source-specific action.