Model numbers decoded: how pros read the nameplate
Manufacturers hide a lot in a model number. The first letters point to the series and equipment type (furnace, air conditioner, heat pump). The numbers usually flag capacity in thousands of BTUs 18, 24, 30, 36, 42, 48, 60 (12,000 BTU = 1 ton), so a “36” is commonly 3 tons. Suffixes identify efficiency tiers, blower style, orientation (upflow/downflow/horizontal), and often a dash code for design revisions. The nameplate also lists refrigerants (R-32, R-410A, or older R-22) and electrical requirements. Before buying, confirm the refrigerant family across all pieces and that your electrical service matches. If you’re pairing a new outdoor unit with a new coil, browse compatible AC & Coil combos.
Nominal vs. rated vs. actual capacity (the truth about “tons”)
Nominal capacity is a rough label used for cataloging. Rated capacity comes from AHRI under standard test conditions (e.g., 95°F outside, 80°F inside for cooling). Actual capacity is what you get at your house—affected by ductwork, static pressure, airflow, line length, charge, and maintenance. That’s why a “3-ton” system can behave like 2.5 tons on a muggy August afternoon if airflow is low or ducts are undersized. When reading spec sheets, look for the performance tables that show capacity across different indoor/outdoor conditions. Tie those numbers back to your home’s Manual J load. If you’d like options that hold capacity at heat or maintain high efficiency, compare our R32 condensers.
AHRI certificates: the proof behind matched systems
An AHRI certificate verifies that your outdoor unit, indoor coil or air handler, and (if applicable) furnace were tested together. It confirms the published SEER/EER/AFUE and capacity so rebates and warranties stick. Mixing random parts can chop efficiency by 20–30% and void claims. On spec sheets, match exact model numbers—even small dash codes matter. Your contractor (or we) can pull the AHRI reference with those models. Keep the certificate for rebates and permits. If you’re building a system from scratch, browse factory-matched options like R32 AC + Air Handler systems.
Load-matching tables: sizing by math, not square feet
Forget “500 square feet per ton.” Proper sizing starts with a Manual J load calculation. Once you have the load (e.g., 28,000 BTU sensible at 95°F), open the manufacturer’s load-matching table and find the model that delivers that capacity at your indoor setpoint and outdoor design temperature. Look at sensible vs. latent capacity (humidity removal), blower speed options, and coil selection. If your design temp is hotter than AHRI test conditions or your ducts are restrictive, you may need to step up or pick a different coil to hit the target. For quick ballparks, start with our Sizing Guide, then compare against matched AC & gas furnace systems.
Airflow and static pressure: the hidden capacity killers
Spec sheets list blower performance tables showing airflow (CFM) at different external static pressures. Aim for ~400 CFM per ton in cooling unless the table or humidity goals suggest otherwise. If your ducts push static above the blower’s limit, airflow drops, coils get cold, and capacity/efficiency suffer. That’s when you see long run times, coil icing, or rooms that won’t cool. Check the maximum ESP rating on the air handler or furnace, and compare it to your duct design. If your supply/return trunk is undersized, fix that before blaming the equipment. When in doubt, look at systems with stronger blowers or consider a ductless option like our mini-splits that avoid duct losses entirely.
Refrigerant, line sets, and electrical: match or pay later
Modern systems specify refrigerant type (R-32 vs. R-410A), charge, acceptable line-set lengths/diameters, and voltage/phase. If you reuse existing lines, verify they meet diameter and length limits from the spec sheet; long or undersized lines hurt capacity and compressor life. Replace kinked or contaminated lines and install new filter-driers per manufacturer guidance. Check breaker size and MCA/MOP on the nameplate to match wiring and overcurrent protection. Need materials? See our Line Sets and Accessories.
Derating factors: altitude, heat, and long runs
Spec sheets include correction factors for altitude, ambient temperature, and line length. At higher elevations, thinner air means less heat transfer—capacity drops and you may need to upsize. In extreme heat, some condensers re-rate significantly; that 3-ton at 95°F might deliver much less at 110°F. Long vertical line sets add lift and oil return challenges; always check maximum rise, traps, and additional charge per foot. Indoor airflow also changes capacity: too low and the system loses sensible performance; too high and humidity control suffers. If your site is in a tough hot climate, long lines, or high altitude consider variable-speed equipment and review the performance tables carefully. Not sure? Our Design Center can help engineer the right solution.
Matching coils, furnaces, and condensers (cooling & heat pump)
Cooling performance depends on the coil. A mismatched coil can bottleneck airflow, mis-set the metering device, and tank efficiency. For AC + furnace systems, match the outdoor unit, indoor coil, and furnace blower as shown on the spec sheet and AHRI listing. For heat pumps, pair the outdoor unit with the correct air handler or furnace (dual fuel) and confirm defrost, refrigerant metering, and heat strip options. If you prefer pre-matched kits, browse R32 AC & Coils or complete R32 packaged heat pumps. Good match = quieter operation, proper humidity control, and the efficiency you paid for.
A real-world example: when “3 tons” works like 2.3
The home needs 34,000 BTU sensible at 98°F. The homeowner installs a “3-ton” condenser with a random coil and keeps old ducts. Blower delivers only 325 CFM/ton at 0.9 in. w.c. static (too high). On the performance table, that airflow and temperature push the system’s actual capacity down near 27–28k BTU. Humidity climbs because the coil runs too cold and freezes intermittently. Energy bills spike, comfort drops. Fix: resize the return, add additional supply runs, set blower tap for target CFM, and install the matched coil listed on the AHRI certificate. After duct corrections, the same equipment delivers close to spec. If ducts can’t be corrected, a ductless mini-split zone for the worst rooms is often smarter.
FAQs on reading-spec-sheets (for quick searches)
What’s the fastest way to tell system size?
Look for 18/24/30/36/42/48/60 in the model number divide by 12 for tons. Always verify with the performance table; conditions can change actual capacity.
Is an AHRI certificate really necessary?
Yes. It proves your indoor and outdoor pieces were tested together and locks in ratings, rebates, and warranty. Keep the reference number with your records.
Why does my new “3-ton” struggle in heat waves?
At high outdoor temps, capacity drops. If ducts are restrictive or airflow is low, the system may only deliver 2.5 tons (or less). Check CFM/ESP against the blower chart.
What’s “external static pressure”?
It’s the resistance your ducts/filters/registers create. Higher static = lower airflow, less capacity, more noise. Stay within the unit’s max ESP on the spec sheet.
Can I reuse my old line set?
Only if it meets diameter/length limits and is clean. Many installs require replacement. See approved sizes.
How do I size without Manual J?
You shouldn’t. Use our Sizing Guide to start and then confirm with Manual J. It prevents oversizing, short cycling, and humidity issues.
Where can I get help picking a matched system?
Use our Design Center or send pictures via Quote by Photo.
