HVAC 101

Types of Systems

Not All HVAC Equipment Does the Same Job

Each type solves a different problem. Here's what they do and who they're for.

Ductless Mini Split Most Flexible

An outdoor condenser connected to one or more indoor heads by a refrigerant line set — no ductwork required. Single-zone systems condition one room; multi-zone systems run several indoor heads off one outdoor unit, each with its own thermostat. Nearly all are inverter-driven heat pumps, so the same box both cools and heats.

Best for: Additions, garages, sunrooms, converted attics, older homes with no ducts, room-by-room control

6,000–60,000 BTU115V or 230VHeats & coolsNo ducts

Shop mini splits →

Central Air Conditioner

The traditional split system: an outdoor condenser paired with an indoor evaporator coil sitting on a furnace or air handler, distributing air through your existing ductwork. Cooling only — heat comes from the furnace or an electric heat kit.

Best for: Homes with existing ductwork, replacing a failed AC while keeping the furnace

1.5–5 tons208/230VCools onlyNeeds ducts

Shop central AC →

Heat Pump

Mechanically an air conditioner that can run in reverse, moving heat into the house in winter and out of it in summer. Efficient down to low outdoor temperatures; cold-climate and hyper-heat models keep useful capacity well below freezing. Often paired with a furnace as a dual-fuel system that switches to gas when it gets cold enough.

Best for: All-electric homes, mild to moderate winters, replacing AC + electric strip heat

1.5–5 tons208/230VHeats & coolsHSPF2 rated

Shop heat pumps →

Gas Furnace

Burns natural gas or propane to heat air, then pushes it through ductwork. Efficiency is measured in AFUE. An 80% AFUE furnace vents through metal pipe or an existing chimney; a 90%+ condensing furnace vents through PVC and produces condensate that needs a drain. That venting difference decides which one fits your house as much as the efficiency number does.

Best for: Cold climates, homes with an existing gas line and ductwork

40,000–120,000 BTU80% or 90%+ AFUENatural gas / LP115V control

Shop furnaces →

Air Handler

The indoor half of an all-electric system: a blower and coil in one cabinet, used instead of a furnace. Most accept an electric heat kit for supplemental heat. Multi-position cabinets install upflow, downflow, or horizontally, which matters in a tight attic or closet.

Best for: Pairing with a heat pump or AC where there's no gas service

1.5–5 tons208/230VOptional heat kitMulti-position

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Packaged Unit

Everything — compressor, coil, blower, and often gas heat — in a single cabinet that sits on the roof or a ground pad, with ducts running into the building. Common on manufactured homes and light commercial buildings where there's no room for indoor equipment.

Best for: Manufactured homes, light commercial, buildings with no indoor mechanical space

2–10 tons208/230V 1Ph or 3PhGas/electric or heat pumpRooftop or pad

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PTAC / Through-the-Wall

A self-contained unit that slides into a wall sleeve, conditioning a single room with no ductwork and no outdoor unit. Standard sleeve is roughly 42" × 16", so replacements usually drop into an existing opening. Heat is either electric resistance or a heat pump with electric backup.

Best for: Hotels, senior living, apartments, offices, additions

7,000–15,000 BTU208/230V or 265V42" × 16" sleevePer-room control

Shop PTAC units →

Dehumidifiers & IAQ

Air conditioning removes some moisture as a side effect, but a basement or crawl space often needs dedicated dehumidification. Whole-home units are rated in pints per day and can be ducted into the return or run standalone. Humidifiers do the opposite in dry winter climates.

Best for: Basements, crawl spaces, humid climates, homes with musty odors or condensation

50–110 pints/day115VDucted or freestandingDrain required

Shop dehumidifiers →

Sizing

What Size System Do You Actually Need?

Capacity is measured in BTU per hour. 12,000 BTU equals one ton. Bigger is not better — an oversized system short-cycles, never runs long enough to pull humidity out, and wears itself out faster.

Whole-home cooling, by conditioned square footage

Home size	Typical capacity	Tons
600 – 1,000 sq ft	18,000 – 24,000 BTU	1.5 – 2.0
1,000 – 1,300 sq ft	24,000 – 30,000 BTU	2.0 – 2.5
1,300 – 1,600 sq ft	30,000 – 36,000 BTU	2.5 – 3.0
1,600 – 2,000 sq ft	36,000 – 42,000 BTU	3.0 – 3.5
2,000 – 2,500 sq ft	42,000 – 48,000 BTU	3.5 – 4.0
2,500 – 3,300 sq ft	48,000 – 60,000 BTU	4.0 – 5.0

Single rooms and zones, for mini splits

Indoor head	Room size	Typical use
9,000 BTU	up to 400 sq ft	Bedroom, home office, small addition
12,000 BTU	400 – 550 sq ft	Master bedroom, garage bay, sunroom
18,000 BTU	700 – 1,000 sq ft	Open living area, large garage
24,000 BTU	1,000 – 1,400 sq ft	Great room, small open-plan home
36,000 BTU	1,400 – 2,000 sq ft	Whole small home, shop space

Furnace heating input

Furnaces are sold by BTU input. Multiply by the AFUE to get delivered heat — a 100,000 BTU furnace at 92% AFUE delivers about 92,000 BTU. Mild climates run roughly 30–35 BTU per square foot; cold northern climates 45–60. A 2,000 sq ft home lands somewhere between 60,000 and 120,000 BTU input depending on where you live and how well the house is sealed.

Square footage is a starting point, not an answer. Ceiling height, insulation, window area and orientation, air sealing, climate zone, and how many people live there all move the number. A Manual J load calculation is the only way to size a system properly — and in most jurisdictions it's required for a permit. Use the tables above to get in the right range, then confirm before you order. We're happy to sanity-check a model number against your situation.

Rather Not Do This On Paper?

Let the System Finder walk the table for you

Instead of reading a range off a chart, describe the space and look at it: the System Finder draws your rooms in 3D, asks about ceiling height, windows, doors and insulation, and looks up your IECC climate zone from your ZIP before it puts a number on it.

System Finder Build the space in 3D, get a capacity range, and see the in-stock systems that land in it. Size it in 3D →

It is a sizing estimate to shop with, on the same footing as the table above — not a Manual J. Have a qualified installer confirm the final capacity before you order.

Efficiency Ratings

What SEER2, HSPF2 and AFUE Actually Mean

The "2" ratings replaced the older SEER, EER and HSPF numbers in 2023. They're measured under higher static pressure, which is closer to a real duct system — so a SEER2 number looks slightly lower than the old SEER number on identical equipment. Don't compare across the two scales.

SEER2

Seasonal cooling efficiency across a whole season. Higher is cheaper to run. Current federal minimums are 13.4 in the northern region and 14.3 in the southern and southwestern regions for split air conditioners.

Typical range: 13.4 – 24

EER2

Cooling efficiency at a single hot design condition rather than averaged over a season. The number that matters most in a desert climate where it's near design temperature for months.

Typical range: 8 – 14

HSPF2

Seasonal heating efficiency for heat pumps. Higher means more heat delivered per kilowatt-hour over the heating season. Federal minimum is 7.5 for ducted systems.

Typical range: 7.5 – 11

AFUE

Annual fuel utilization efficiency for furnaces — the share of fuel converted to usable heat. 80% AFUE vents through metal; 90%+ condenses, vents through PVC, and needs a condensate drain.

Typical range: 80% – 98%

COP

Coefficient of performance — heat delivered divided by energy consumed at a given moment. A COP of 3.5 means 3.5 units of heat per unit of electricity.

Typical range: 2 – 4.5

Stages & Inverters

Single-stage runs flat out or not at all. Two-stage adds a low setting. Inverter (variable-speed) modulates continuously — quieter, steadier temperatures, better humidity control.

Single · Two-stage · Inverter

Refrigerants

R-410A, R-32 and R-454B

The industry is mid-transition. Knowing which refrigerant your equipment uses matters for compatibility, servicing, and what a technician needs on the truck.

Refrigerant	Status	What to know
R-410A	Being phased down	The standard for two decades. Still serviceable, and existing systems keep running, but new equipment has largely moved on. Higher global warming potential than the replacements.
R-32	Current	Single-component refrigerant, lower GWP, slightly higher capacity per pound. Mildly flammable (A2L), which changes handling and some installation rules.
R-454B	Current	A blend with low GWP, adopted widely by North American manufacturers for 2025 equipment. Also A2L.

Refrigerants are not interchangeable. An R-32 condenser cannot be paired with an R-410A coil or air handler, and the reverse is equally true. Line sets, service valves, and metering devices have to match the system. If you're replacing one half of an existing system, check the refrigerant on the data plate of the half you're keeping before ordering.

Will It Fit?

Electrical, Space and Site Requirements

Eight things worth confirming before the equipment shows up on a truck.

Voltage & phase

Small mini splits run on 115V. Most residential condensers, air handlers and heat pumps are 208/230V single phase. Light commercial packaged units are often 208/230V or 460V three phase — three-phase power is rare in homes.

MCA & MOCP

Every unit lists a minimum circuit ampacity and a maximum overcurrent protection on its data plate. Those two numbers tell your electrician the wire gauge and breaker size. A unit needing 30A won't run on an existing 20A circuit.

Line set & distance

Mini splits ship pre-charged for a set line length — often 15 to 25 feet. Beyond that, refrigerant is added per foot. Every system has a maximum line length and vertical separation between indoor and outdoor units. Measure the actual route, not the straight line.

Line sets, sized to the unit →

Cabinet size & access

Furnaces and air handlers come in different cabinet widths — commonly 17.5", 21" and 24.5". Check the closet or attic opening, and the path to it, before ordering. Multi-position cabinets solve orientation problems but not width problems.

Clearances & pad

Outdoor units need clear space around and above them for airflow and service access — typically 12 to 24 inches on the sides and several feet overhead. They also need a level pad or bracket, plus enough height above grade for snow and drainage.

Drainage & venting

Cooling produces condensate that needs a drain and, indoors, usually a secondary pan. Condensing furnaces and dehumidifiers produce condensate too. 90%+ furnaces need PVC venting to the outside — an existing chimney won't do.

Ductwork

Ducts sized for a 2-ton system won't move 4 tons of air. Residential blowers are typically rated near 0.5 in.WG external static pressure. Undersized or leaky ducts cost you more capacity than the difference between two efficiency tiers.

Gas supply

A furnace needs correctly sized gas piping and adequate meter capacity, plus combustion air. Converting between natural gas and propane requires the manufacturer's conversion kit — it isn't optional.

Matched Systems

Why the Outdoor Unit Alone Isn't a System

A condenser is rated with a specific indoor coil. Pair it with something else and you don't get the efficiency on the box.

What a complete split system includes

Outdoor condenser or heat pump
Indoor coil, air handler, or furnace with a coil on top
Refrigerant line set, correctly sized for the equipment
Thermostat compatible with the system's staging
Disconnect box, whip, breaker and pad or bracket
Condensate drain, and a pan where required

AHRI certified reference numbers

AHRI publishes the tested performance of specific outdoor/indoor combinations. The SEER2 on a condenser assumes one of those matches. Utility rebates and tax credits almost always ask for the AHRI reference number of your exact pairing, so keep it with your paperwork.

Our product pages list what each listing includes. If a listing is an outdoor unit only, it says so — check before assuming a coil is in the box.

The Matching, Done For You

You don't have to cross-check the match yourself

This is the part the Complete System Builder exists for. Pick the outdoor unit and it only ever offers you the indoor coil, air handler or furnace, heat kit, line set, thermostat and accessories that the manufacturer's own documentation or an AHRI certificate has put with it — and it shows you which of the two said so, on every part.

Complete System Builder Build the system one step at a time and see the evidence behind every match. Build a system →

Matching is not sizing. The Builder will not tell you what capacity you need — start with the System Finder, or a load calculation, for that.

Delivery

How HVAC Equipment Ships

Condensers, furnaces, air handlers and packaged units move by freight, not parcel.

1

Curbside freight

The carrier calls to schedule a window and brings the shipment to the curb or driveway. Getting it inside or into position is on you — plan for help or equipment.

2

Liftgate

Most residential addresses need a liftgate to lower the pallet. Say so up front — a truck arriving without one usually means a failed delivery and a re-delivery fee.

3

Inspect before signing

Count the pieces and look for crushed corners, punctures and dents. Note any damage on the delivery receipt before you sign, and photograph it.

4

Report within 48 hours

Damage or shortages need to reach us within 48 hours of delivery so we can file with the carrier while the claim is still open.

Full details are in our Shipping Policy and Return & Refund Policy.

Installation

Can You Install It Yourself?

Honestly: mostly no — and the exceptions are narrower than the internet suggests.

Needs a licensed pro

Anything requiring refrigerant handling, gas piping, or a new circuit. Opening a sealed refrigerant system requires EPA Section 608 certification. Most jurisdictions also require a permit and inspection for equipment replacement.

DIY-friendly with care

Pre-charged DIY mini splits with quick-connect line sets are designed for owner installation. You still need a correctly sized circuit, secure mounting, and a proper condensate route.

Warranty depends on it

Manufacturer warranties generally require installation by a licensed contractor, and many require product registration within 60 to 90 days of installation. Improper installation is the most common reason a claim gets denied.

Don't have a contractor?

That's the part most people get stuck on, because a lot of installers would rather sell you the equipment themselves. Give us your ZIP on the Installation Help page and we'll call licensed contractors in your area and send you the ones who confirm they'll install equipment you supplied. The same page covers the scope of work, what drives the labor price, and the questions worth asking before you book.

Avoid These

Five Mistakes People Make

Oversizing "to be safe"
An oversized system cools the air fast, shuts off before it removes humidity, and leaves the house cold and clammy. Short-cycling also shortens compressor life.
Mismatching refrigerants
Ordering an R-32 condenser for an R-410A coil. They don't pair, and finding out at installation costs you a return and a delay. Refrigerant is one of the things the Builder will not let you get wrong — a part on the other refrigerant is never offered in the first place.
Ignoring the electrical panel
Checking MCA and MOCP after delivery instead of before. A panel upgrade can cost more than the difference between two efficiency tiers.
Forgetting the indoor half
Buying a condenser and assuming a coil is included. Read what the listing includes — most outdoor units ship alone. If you would rather not keep track of the halves yourself, the Complete System Builder starts at the outdoor unit and walks you through the rest of it.
No plan for freight
A 300 lb pallet arrives at the curb and there's nobody home, no liftgate, and no way to move it. Re-delivery and storage fees follow.

Keeping It Running

Maintenance That Actually Matters

Filters, every 1–3 months

The single highest-value thing you can do. A clogged filter starves the blower, drops capacity, and can freeze the coil.

Keep the outdoor unit clear

Cut back vegetation, clear leaves from the coil, and keep the fan discharge unobstructed. Rinse the coil gently each season.

Clear the condensate drain

A blocked drain backs water into the pan and, eventually, your ceiling. Flush the line annually.

Annual professional service

Refrigerant charge, electrical connections, and combustion checks on gas equipment need a technician — and many warranties expect documented annual service.

Questions

Frequently Asked

How many tons do I need per square foot?

A common starting point is 20 BTU per square foot, so roughly one ton per 600 square feet. Treat that as a range check, not a specification — insulation, ceiling height, window area and climate move it substantially in both directions.

Is a higher SEER2 always worth it?

It depends on how many hours a year you actually run cooling. In a hot climate with a long season, the payback on a high-efficiency inverter system is real. In a mild climate with a short season, the extra cost may never come back — though variable-speed equipment is also quieter and controls humidity better, which is worth something on its own.

Can I replace just the outdoor unit?

Sometimes, but it needs to be a valid AHRI match with the existing indoor coil and the same refrigerant. Pairing a new high-efficiency condenser with an old coil usually gives you neither the rated efficiency nor the rated capacity, and can void the warranty.

Heat pump or furnace?

Heat pumps are more efficient in mild and moderate winters and run on electricity alone. Gas furnaces deliver more heat per dollar where gas is cheap and winters are severe. A dual-fuel setup uses the heat pump most of the year and switches to gas below a set outdoor temperature.

What's included with a mini split?

It varies by listing. Most single-zone systems include the outdoor condenser and the indoor head; the line set is sometimes separate and sometimes bundled. Every product page has a "This listing includes" line — check it before ordering.

Do I need a permit?

Usually yes for equipment replacement, and nearly always where gas piping or new electrical circuits are involved. Requirements are local — check with your building department, and expect a Manual J load calculation to be part of it.

Design Your System

Or Skip the Reading and Use the Tools

Everything on this page is here so you can make the call yourself. If you would rather the site did the arithmetic and the cross-referencing, that is what these two are for.

Two Doors

Start wherever you are

If you do not know the size yet, start with the Finder — it hands your sizing and your 3D room straight to the Builder when you are done. If you already know the size, go straight to the Builder.

System Finder Draw your space in 3D and get a capacity range, with equipment that fits it. Takes about a minute. Size it in 3D → Complete System Builder Start from the outdoor unit. Only the coil, air handler, heat kit, line set and thermostat matched to it are ever offered. Build a system →