HVAC System Types

HVAC stands for heating, ventilation and air conditioning. Those three jobs can be done by very different equipment. Before you can read a work order, order a part or diagnose a problem, you need to know which kind of system you are looking at and which parts it should have. This guide is a map of the common types and the ratings printed on them.

What You Will Be Able to Do

• Identify a split system, a package unit, a mini-split, a heat pump and a boiler by what you can see.
• Name the main parts each system has and where they sit.
• Explain how a heat pump differs from a standard air conditioner.
• Read SEER2, HSPF2 and AFUE ratings and say what each measures.

The Quick Map

Split Systems

A split system puts part of the equipment outside and part inside. The outdoor unit, called the condensing unit, holds the compressor, the outdoor coil and a fan. The indoor side holds the evaporator coil and a blower, usually sitting on top of a furnace or inside an air handler.

Two copper lines connect the halves. The larger insulated one is the suction line. The smaller one is the liquid line. Low-voltage control wire runs alongside them. Together, the copper pair is called the line set.

This is the most common cooling setup in U.S. homes, so learn it first.

Package Units

A package unit holds the compressor, both coils and the blower in one cabinet outdoors. Ducts connect straight to it. Rooftop units on stores and offices are the most familiar example. Some homes on slabs use ground-mounted package units. Some package units include a gas heating section, and these are often called gas packs.

Forced-Air Furnaces

The most common home heating system in the United States is a forced-air furnace, and natural gas is its most common fuel. The furnace burns fuel inside a sealed heat exchanger, and a blower pushes house air across the outside of that exchanger and through the ducts. Burned gases never mix with the house air as long as the heat exchanger is sound.

Furnaces are often paired with a split air conditioner. The blower does double duty, moving air across the heat exchanger in winter and across the evaporator coil in summer.

Heat Pumps

A heat pump is a refrigeration system that can run in both directions. A reversing valve switches the direction of refrigerant flow, so one machine can cool in summer and heat in winter. A standard air conditioner has no reversing valve and only cools.

Even cold outdoor air holds heat, and the heat pump pulls it indoors. Its output drops as outdoor temperature drops. Many systems add backup heat, such as electric strips or a gas furnace. A system that pairs a heat pump with a gas furnace is called dual fuel.

In heating mode the outdoor coil runs cold enough to collect frost. So the unit routinely runs a defrost cycle, briefly reversing to send heat to the outdoor coil and melt the ice. Defrost is normal operation, not a failure. You may see steam rising from the unit during it.

Mini-Splits

A mini-split, also called a ductless system, connects an outdoor unit to one or more indoor heads with refrigerant lines and a control cable. Each head conditions its own space, so no ductwork is needed. A multi-zone system runs several heads from one outdoor unit. Most use an inverter-driven compressor, which changes speed to match the load rather than cycling on and off. Most mini-splits sold today are heat pumps.

Boilers and Hydronic Systems

A boiler heats water or makes steam, and pipes carry it to radiators, baseboard or tubing in floors. There is no ductwork, and therefore no air conditioning from the same system. Hydronic means water-based. Large buildings often use chilled water in the same way for cooling, with a chiller making cold water and air handlers using it.

Ventilation

The V in HVAC is fresh air. Homes often rely on bath and kitchen fans, while tighter homes may use an energy recovery ventilator or heat recovery ventilator, which brings in outdoor air and exchanges heat with the stale air going out. Commercial buildings bring in outdoor air through the rooftop unit or air handler to meet building code.

Reading Efficiency Ratings

Older equipment carries SEER and HSPF. SEER2 and HSPF2 have been the rating system since January 1, 2023. The test is tougher, so a SEER2 number runs lower than the old SEER for the same machine, and you cannot compare them directly.

Federal minimums vary by equipment and region. As of 2023 a split central air conditioner needs at least 13.4 SEER2 in the northern region and 14.3 SEER2 in the South for most home sizes, and a split heat pump needs 14.3 SEER2 and 7.5 HSPF2 nationwide. These numbers can change, so check the current standard before you quote one.

An 80% AFUE furnace turns 80 cents of each fuel dollar into heat in the house and loses the rest, mostly up the vent. Condensing furnaces reach above 90% by capturing extra heat from the flue gas.

How to Identify a System in the Field

1. Walk outside and look for an outdoor unit. No outdoor unit may mean a boiler, a furnace with no cooling, or a package unit on the roof.
2. Read the data plate. It gives the model, serial number, refrigerant type and electrical ratings.
3. Look for the reversing valve symbol or the words "heat pump" on the data plate and the model label.
4. Go inside and find the air handler, furnace or boiler. Note whether ducts or pipes leave it.
5. Check the thermostat. An O/B terminal in use usually means a heat pump.

Check Your Understanding

You see one big cabinet on a flat roof with ducts going straight down through the roof deck. What is it? A package unit, also called a rooftop unit. All the refrigeration parts are in that one cabinet.

A homeowner has steam rising from the outdoor unit on a frosty morning and thinks it is on fire. What is likely happening? The heat pump is in its normal defrost cycle, melting frost from the outdoor coil.

A customer asks why their new unit's SEER2 number is lower than the SEER on their old unit, even though the new one is better. What do you tell them? The test changed in 2023. SEER2 is measured under tougher conditions, so the two numbers do not compare directly.

In heating mode, which coil on a heat pump is the evaporator? The outdoor coil. It absorbs heat from outdoor air.

Key Takeaways

• A split system has an outdoor unit and an indoor coil joined by a line set. A package unit keeps everything in one cabinet.
• Forced-air furnaces are the most common home heating system in the United States.
• A heat pump uses a reversing valve to heat and cool, and defrost is a normal part of its winter operation.
• Mini-splits skip ducts, and boilers use pipes instead of air.
• SEER2 measures seasonal cooling efficiency, HSPF2 measures heat pump heating efficiency, and AFUE measures furnace fuel efficiency.