The Ultimate AC Cheat Sheet — HVAC Systems, Terminology & Quick Reference
The definitive AC cheat sheet and HVAC quick reference. Covers system types, key terminology, refrigerant charts, and common specs every contractor and technician should know.
Whether you are a seasoned HVAC contractor brushing up on specs, a technician studying for certification, or a business owner who needs to speak the language confidently in proposals and customer conversations, this HVAC cheat sheet covers the essentials. Bookmark this page - it is the quick reference you will come back to again and again.
The 4 Basic Categories of HVAC Systems
Every HVAC system falls into one of four fundamental categories. Understanding these categories is essential for recommending the right solution, writing accurate proposals, and communicating clearly with customers.
1. Heating Systems
Systems designed primarily to generate heat. Includes gas furnaces (most common in residential), oil furnaces, boilers (hydronic heating), electric furnaces, and heat pumps operating in heating mode. Gas furnaces remain the dominant residential heating source, typically rated by AFUE (Annual Fuel Utilization Efficiency) - modern units range from 80% to 98.5% AFUE.
2. Cooling Systems
Systems that remove heat from indoor spaces. Includes central air conditioners (split systems), packaged air conditioners, heat pumps operating in cooling mode, ductless mini-split systems, and window/portable units. Efficiency is measured in SEER2 (Seasonal Energy Efficiency Ratio 2) - residential units currently range from 13.4 to 26+ SEER2.
3. Ventilation Systems
Systems that exchange, filter, or condition indoor air. Includes exhaust fans, supply fans, balanced ventilation systems, energy recovery ventilators (ERVs), and heat recovery ventilators (HRVs). As building codes push for tighter construction, mechanical ventilation has become increasingly important for indoor air quality.
4. Combined and Packaged Systems
Systems that integrate heating, cooling, and sometimes ventilation into a single unit. Includes packaged rooftop units (RTUs - common in commercial), packaged heat pumps, and dual-fuel systems (heat pump + gas furnace backup). These are popular in commercial applications and in residential homes without basements or utility closets.
| Category | Common Types | Primary Metric |
|---|---|---|
| Heating | Gas furnace, boiler, heat pump | AFUE / HSPF2 |
| Cooling | Central AC, mini-split, heat pump | SEER2 / EER2 |
| Ventilation | ERV, HRV, exhaust fans | CFM / ACH |
| Combined/Packaged | RTU, packaged heat pump, dual-fuel | Varies by function |
HVAC vs AC - What Is the Difference?
This is one of the most common points of confusion for homeowners - and even some newer technicians. The difference between HVAC and AC is simple: AC (air conditioning) is one component within the broader HVAC system.
HVAC stands for Heating, Ventilation, and Air Conditioning. It is the umbrella term that covers everything related to climate control in a building - furnaces, air conditioners, heat pumps, ductwork, thermostats, ventilation systems, and air quality equipment.
AC refers specifically to the cooling component - the air conditioner itself. When a homeowner says “my AC is broken,” they mean their cooling system is not working. When they say “I need a new HVAC system,” they typically mean both heating and cooling.
In practice, most homeowners use these terms interchangeably. As a contractor, it helps to clarify what the customer actually needs - are they looking for cooling only, heating only, or a complete system replacement? This distinction directly impacts your proposal scope and pricing.
Furnace vs HVAC - Understanding the Distinction
A furnace is a heating-only appliance - it is one piece of a complete HVAC system, not a system by itself. In a typical residential setup, the furnace works alongside several other components:
- Furnace - Burns fuel (gas, oil, or propane) or uses electric resistance to generate heat
- Evaporator coil - Sits on top of or downstream from the furnace, provides cooling when the AC runs
- Blower motor - Located inside the furnace cabinet, pushes conditioned air through the ductwork for both heating and cooling
- Condenser unit - The outdoor unit that pairs with the evaporator coil for cooling
- Ductwork - The distribution system that delivers conditioned air to each room
- Thermostat - The control interface that tells the system what to do
When homeowners say “I need a new furnace,” they usually mean just the heating component. But since the furnace houses the blower motor that drives the entire system, replacing a furnace often affects AC performance too. This is why many contractors recommend replacing the furnace and AC together for optimal efficiency and compatibility.
Essential HVAC Terminology Cheat Sheet
Here are the terms every HVAC professional should know cold - and that you may need to explain to customers in proposals and conversations:
SEER2 (Seasonal Energy Efficiency Ratio 2)
Cooling efficiency rating. Higher is better. Minimum residential standard is 13.4-15 SEER2 depending on region. Replaced the original SEER metric in 2023.
HSPF2 (Heating Seasonal Performance Factor 2)
Heating efficiency rating for heat pumps. Minimum is 7.5 HSPF2. Higher numbers mean lower heating costs.
AFUE (Annual Fuel Utilization Efficiency)
Heating efficiency for furnaces and boilers, expressed as a percentage. A 96% AFUE furnace converts 96% of fuel to heat. Minimum standard is 80%.
BTU (British Thermal Unit)
Unit of heat energy. One BTU is the energy needed to raise one pound of water by one degree Fahrenheit. Used to rate heating and cooling capacity.
Ton (of cooling)
Unit of cooling capacity. One ton equals 12,000 BTU/hour. A typical residential AC is 1.5 to 5 tons. Named after the cooling effect of melting one ton of ice over 24 hours.
Static Pressure
Resistance to airflow in the duct system, measured in inches of water column (iwc). Ideal total external static pressure is 0.5 iwc or less for most residential systems. High static pressure reduces efficiency and shortens equipment life.
CFM (Cubic Feet per Minute)
Airflow measurement. Rule of thumb: residential systems need approximately 400 CFM per ton of cooling. A 3-ton system should deliver about 1,200 CFM.
Delta T (Temperature Differential)
The temperature difference between supply and return air. For cooling, a healthy delta T is 14-22°F. Outside this range indicates an airflow or charge issue.
Superheat and Subcooling
Refrigerant diagnostic measurements. Superheat measures heat added to refrigerant after it boils (evaporator). Subcooling measures heat removed after it condenses (condenser). Used to verify proper refrigerant charge.
Manual J Load Calculation
The industry-standard method for calculating heating and cooling loads for a building. Considers square footage, insulation, windows, orientation, climate zone, and occupancy. Required for proper system sizing.
TXV (Thermostatic Expansion Valve)
A metering device that regulates refrigerant flow into the evaporator based on superheat. More precise than a fixed orifice, improving efficiency and performance across varying conditions.
Pro Tip
When writing proposals, using clear terminology helps homeowners understand what they are paying for. ProposalKit auto-fills equipment specs and descriptions so your proposals include the right technical details without extra typing.
Refrigerant Quick Reference Chart
The refrigerant landscape is changing rapidly. Here is a quick reference for the refrigerants you are most likely to encounter in the field:
| Refrigerant | Type | Status | GWP | Notes |
|---|---|---|---|---|
| R-22 | HCFC | Phased out (2020) | 1,810 | No longer manufactured. Reclaimed only. Very expensive. Found in older systems. |
| R-410A | HFC | Being phased down | 2,088 | Current standard but being replaced. Higher operating pressures than R-22. Equipment production phasedown began 2025. |
| R-454B | HFO blend | Next standard | 466 | Replacing R-410A in new equipment. Mildly flammable (A2L). Requires updated safety training and handling procedures. |
| R-32 | HFC | Growing adoption | 675 | Used in some ductless and VRF systems. Mildly flammable (A2L). Higher efficiency than R-410A. |
All technicians handling refrigerants must hold EPA Section 608 certification. The transition to A2L refrigerants like R-454B also requires additional safety training for handling mildly flammable substances.
Common HVAC Equipment Specs at a Glance
These are the typical specifications for residential HVAC equipment. Use these as quick reference points when sizing systems, writing proposals, or answering customer questions:
| Spec | Typical Range | Rule of Thumb |
|---|---|---|
| System size (residential) | 1.5 - 5 tons | ~1 ton per 500-600 sq ft (varies by climate and insulation) |
| Airflow per ton | 350 - 450 CFM | 400 CFM/ton is standard; adjust for humidity |
| Duct sizing (trunk line) | 0.08 - 0.10 iwc/100ft | Size for friction rate; lower is better |
| Return air grille | 1 sq ft per ton minimum | Undersized returns cause high static pressure |
| Electrical (AC condenser) | 20-60 amps, 240V | Always check nameplate; never assume |
| Gas line (furnace) | 1/2" - 3/4" pipe | Size based on BTU input and run length |
| Condensate drain | 3/4" PVC minimum | Always install a safety float switch |
Download the HVAC Cheat Sheet PDF
Want a printable version of this cheat sheet to keep in your truck, office, or toolbox? We are putting together a downloadable PDF with all the tables, terminology, and reference charts from this page in a print-friendly format.
In the meantime, bookmark this page for quick access from your phone or tablet - it is updated regularly as standards and specifications change.
Running an HVAC Business?
Knowing the specs is only half the job - you also need to present them professionally to win work. If you are dealing with operational challenges like slow quoting or unprofessional proposals, check out ProposalKit - it lets you build branded proposals with auto-filled equipment specs in minutes. Free 14-day trial.
For more on writing proposals that communicate this information clearly, see our guide to writing HVAC proposals and our contractor's guide to pricing HVAC jobs.