You can put a furnace or a condenser in a shopping cart the same way you buy a coffee maker, and the price you see will usually look better than the quote in your hand. The difference is that a coffee maker arrives finished. Heating and cooling equipment arrives as one part of a system that has to be sized, matched, installed, permitted, and tested before it does what the label says, and most of that cost sits on the installation side of the line rather than the equipment side.
This is written for homeowners in Pasadena, CA and around the San Gabriel and San Fernando Valleys who are weighing it up. It is not an argument against buying online. It is the list of things that decide whether it works out.
The price on the box is not the price of the job
Roughly speaking, what you pay a contractor covers the equipment plus the labor, the materials, the permit, the code-required testing, and the risk that the job turns up something the quote did not cover. Buying the equipment yourself removes one line. The rest stays, and a few items get more expensive because the installer is now working with a box they did not choose and cannot return.
A contractor who supplies the equipment carries the failure. If the coil is wrong for the condenser, or the unit arrives damaged, that is their problem to sort out with their supplier. When you supply it, that becomes your problem, usually on the day the old system is already out of the house.
Sizing is a calculation, not a guess from square footage
A Department of Energy guide for contractors says equipment should be sized with an ACCA Manual J load calculation and selected with Manual S, and the ducts sized with Manual D. Manual J works off your house: orientation, insulation levels, window area and type, air leakage, ceiling height, how much of the duct run sits in a 130-degree attic.
Two houses on the same street in Altadena with the same floor plan can land on different numbers because one has original single-pane windows and the other does not. Why it matters is short: an oversized unit short cycles and leaves the house uneven, and an undersized one cannot keep up on the hottest days. Online listings sell by tonnage, and tonnage is an output, not an input.
The efficiency rating belongs to a pair, not to the outdoor unit
A split system is an outdoor unit and an indoor coil, and as a rule the published efficiency number is measured for a specific combination of the two. Buy a high-efficiency condenser and hang it off whatever coil is already in the closet and you typically get neither the rated efficiency nor a clean warranty position. If you are buying online, buy the matched pair, and ask the seller for the certificate that shows the combination was rated together.
The same applies to the line set, the metering device, and the airflow the blower actually delivers. AC installation is where those get set, and a system that is charged and balanced to the manufacturer's numbers behaves very differently from one that is merely connected.
California adds a permit and a duct test
This is the part that surprises people. Under the 2025 Building Energy Efficiency Standards, when a space-conditioning system is altered by installing or replacing the air handler, the outdoor condensing unit of a split-system air conditioner or heat pump, or a cooling or heating coil, the connected duct system has to be sealed, confirmed through field verification and diagnostic testing (Section 150.2(b)1E). There are exceptions: ducts already documented as sealed and verified, systems with less than 40 linear feet of duct, and ducts containing asbestos.
Equipment changeouts are normally permitted work here, and the permit is what brings the rest of the energy code with it. None of that is optional because you bought the box yourself, and a licensed contractor is the one who pulls the permit and arranges the test.
Refrigerant is not a homeowner product
The EPA's Section 608 rules prohibit intentionally venting refrigerants while maintaining, servicing, repairing or disposing of air conditioning and refrigeration equipment, and require certification for technicians who maintain, service, repair, or dispose of equipment that could release those refrigerants. In practice, the refrigerant side of the job is a licensed, certified task from start to finish: recovering the old charge, brazing, pulling a vacuum, weighing in the new charge.
Warranty: read the registration terms before you buy
Manufacturer warranties usually turn on registration, and the window is short. Carrier, one of the brands we service, publishes registration terms for its Consumer Choice warranty program stating that the warranty option has to be selected at registration within the first 90 days after installation, and that registering later leaves the standard five-year parts warranty with no labor coverage. Parts-only is the normal shape of these warranties, so the labor to fit a covered part is a separate question, answered by whoever installed it. Ours is a one-year labor warranty on the work we do.
Where the equipment came from is worth asking about in writing before you pay. Terms vary by manufacturer and by seller, and a screenshot of a forum post is not the warranty document.
What you can check yourself
- The model numbers of the outdoor unit and the indoor coil, and whether the seller lists them as a rated combination.
- The data plate on your existing outdoor unit: refrigerant type, tonnage, and the manufacture year in the serial number.
- Whether the listing includes the coil, the pad, the line set, the disconnect, and the filter drier, or only the condenser.
- The return policy and the freight damage policy, and how long a replacement would take.
- Whether a contractor will install owner-supplied equipment before you order it. Ask first, not after it is on the driveway.
When to call a licensed tech
Before you buy. A load calculation, a look at the ducts, and a look at what is already in the closet will tell you what the right equipment is, and that answer is the same whether you or we supply it. If the ducts turn out to be the reason the back bedroom never cools, a bigger condenser will not fix it and the money is better spent elsewhere. We put every AC replacement quote in writing before any work starts, and if spreading the cost is the reason you were shopping online, financing through Wisetack is worth a look first. Checking your options there does not affect your credit score.
Common questions
Will a contractor install equipment I bought myself?
Some will and some will not, and those that do often quote a different labor rate, because the equipment warranty and the supply risk sit with you rather than with them. Ask before you order, and ask what happens if the unit arrives damaged or turns out not to match what you already have.
Is the online price really lower?
For the box alone, often yes. The comparison people get wrong is box price against installed price. Ask for the installed quote broken into equipment and labor, then compare equipment to equipment.
Do I still need a permit if I supply the equipment?
Yes. The permit and the energy-code testing attach to the work, not to who bought the parts.
If you want a straight answer on what size and what type your house actually needs before you order anything, book a $69 diagnostic. We tell you the fee before booking and credit it toward the work. Call Boost Services at 818-277-5056 or send us a message.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.