A contractor walks into your living room, glances at the square footage on the listing sheet, does some arithmetic in his head, and tells you that you need a 4-ton system. He has been doing this for 22 years. He is confident. Wrong, but confident. A growing stack of federal research says the numbers are not gentle.
A 2024 report from the National Renewable Energy Laboratory found that roughly 70 to 90 percent of residential HVAC systems in the United States exhibit energy-wasting installation faults, including incorrect airflow, bad refrigerant charge, duct defects, and improper sizing. A separate DOE review of field studies pegged the rate of improperly installed systems at 65 percent, with 31 percent specifically oversized. NIST quantified the damage: improper installation increases household energy use for heating and cooling by roughly 30 percent over what it should be.
Run those numbers across the installed base. There are 130 million residential HVAC systems in the United States, according to Carrier CEO Dave Gitlin. If 31 percent are oversized, that is 40.3 million homes running equipment too large for their actual loads. HVAC represents 48 to 51 percent of residential energy consumption, or roughly $980 per year for the average household. A 30 percent penalty on that spending hits $294 per home, per year.
That calculation is ours, and it relies on three inputs with different confidence levels. Gitlin's 130 million figure comes from an earnings conference and is plausible but unaudited. DOE's 31 percent oversizing rate synthesizes multiple field studies spanning different regions and vintages. NIST's 30 percent energy penalty was modeled in a controlled lab environment, not measured across a representative field sample. Multiply three estimates and you get an estimate, not a census. But even if the real number is half of $11.8 billion, it dwarfs the entire residential HVAC energy-efficiency rebate budget by an order of magnitude, and nobody is fixing it because the root cause is not technology. It is the tape measure and the gut instinct.
Why Contractors Oversize
Fear of the callback.
That is the honest answer every contractor will eventually give you if you push hard enough, and it is rational behavior given the incentive structure: an oversized system keeps the house cool on the hottest day of the year, nobody complains, and the contractor moves on to the next job, cashing a check for equipment that was bigger and more expensive than what the home actually needed, which means the fear also happens to be profitable. An undersized system produces a phone call from an angry homeowner at 3 p.m. on a 106-degree Saturday in July, and that phone call costs more than the profit margin on the install.
Manual J, the ACCA protocol for residential load calculations, exists precisely to solve this problem, and it is thorough, accounting for orientation, insulation values, window U-factors, infiltration rates, duct losses, and local design temperatures down to the specific county and altitude of the home. Done correctly, it produces a BTU-per-hour load for every room in the house. Almost always lower than the gut number.
Allison Bailes at Energy Vanguard ran Manual J on 40 new-construction projects and found an average load of 1,431 square feet per ton. A contractor using the industry rule of thumb, 500 square feet per ton, would install a system two to three times larger than the home actually needs. On a 2,500-square-foot house, that is the difference between a 1.75-ton system and a 5-ton monster that short-cycles every eight minutes, never dehumidifies properly, and wears out its compressor in nine years instead of fifteen.
A Steven Winter Associates study published by NREL made the cost concrete. Two identical multifamily buildings with the same equipment brand, the same floor plans, the same climate. One sized to 90 percent of peak load, the other ballooned to 240 percent. Cooling electricity in the right-sized building was six times lower, not 20 percent, not half, but six times less energy consumed for identical cooling output. Manufacturer data showed proper sizing would have saved 24 percent in upfront equipment cost alone, before a single kilowatt-hour was consumed.
Why Manual J Doesn't Get Done
Time. Two to four hours of on-site measurement and data entry with traditional software. Every window measured. Every wall assembly identified. Every duct run estimated. ACCA says it should happen before every install. The International Residential Code requires it for mechanical permits. In practice, the vast majority of replacement jobs skip it entirely because the contractor needs to close the sale in the living room while the homeowner is sweating through a broken AC, not come back next Tuesday with a 14-page engineering report.
So the contractor eyeballs the tonnage on the existing nameplate, bumps it up half a ton "just to be safe," quotes a price, and drives away with a signed contract. Done in fifteen minutes. The homeowner gets a functioning system that is too large, too expensive, and will run inefficiently for 15 years, but the compressor kicks on and cold air comes out of the vents, so nobody complains and the cycle repeats itself across eight million installs a year, which is how an industry spends $11.8 billion in energy waste without anyone noticing they have a problem.
What Changed: LiDAR in Your iPad
Conduit Tech, founded in 2022 by Marisa Reddy and Shelby Breger and acquired by ServiceTitan in 2025, built a tool that uses the LiDAR sensor in an iPad Pro to scan a home, generate a 3D model with measured dimensions, and run an ACCA-certified Manual J calculation on-site in 10 to 15 minutes. Scan the rooms, tag the windows and insulation types on the model, and the software calculates heating and cooling loads room by room, then recommends equipment sizing based on the actual data.
Fifteen minutes instead of four hours changes the entire economics of the sales call. A technician can walk in, scan, size, recommend, and present an AR visualization of the equipment in the homeowner's space before the competitor finishes his rule-of-thumb quote. Conduit reports a 21 percent higher close rate and a 32 percent increase in average ticket size among contractors using the platform, because showing someone a 3D model of their own house with equipment rendered in place builds trust that a napkin sketch never will.
Conduit is not the only tool in this space. Cool Calc, which powers the LennoxPros Manual J calculator, offers ACCA-approved reports through a mobile interface. Wrightsoft's Right-Suite Universal remains the gold standard for engineering-grade residential load calculations. Kwik Model 3D accepts LiDAR point-cloud imports and runs Manual J against scanned geometry. What Conduit did differently was embed the scan, the calculation, and the sales presentation into a single workflow that a 23-year-old technician with no engineering background can execute in the living room while the homeowner watches, which is the difference between a tool that exists and a tool that gets used.
The Numbers That Actually Matter
Approximately 8 million residential HVAC systems are replaced each year in the United States, with another 1 to 1.5 million installed in new construction. If even 20 percent of those replacement installs are oversized by one ton, which is conservative against the DOE's 31 percent finding, that represents 1.6 million homes per year receiving equipment that costs roughly $1,000 more than the right-sized alternative. That is $1.6 billion in unnecessary equipment spend annually, before you count the decade-plus of inflated energy bills that follows.
An ENERGY STAR certified new home allows no more than 15 percent cooling oversizing. The program sets the bar there because the data proves that anything beyond 15 percent degrades humidity control, increases short-cycling, and shortens equipment life. A system 50 percent oversized, which is common, reaches thermostat setpoint so fast that the evaporator coil never gets cold enough to pull moisture from the air, leaving the house at 73 degrees and 68 percent relative humidity, which is a mold factory masquerading as comfort.
The Counterargument Nobody Wants to Make
Right-sizing works in theory, and the physics is clear. But physics assumes you can measure the inputs, and in a 40-year-old house sitting in front of you, you often cannot. Insulation R-values degrade over time, and the contractor scanning your 1987 ranch has no way to know whether the R-19 batts in your attic have compressed to R-11 without pulling back the access hatch and putting hands on fiberglass. Infiltration estimates depend on a blower-door test that almost nobody orders for a replacement install. Duct leakage, the single largest source of HVAC inefficiency according to DOE, lives inside walls and ceilings where no LiDAR scanner can reach.
Manual J itself builds in conservative assumptions, typically producing loads 10 to 15 percent above actual measured performance according to ACCA's own guidance. A contractor who runs a perfect Manual J and selects equipment at exactly 100 percent of calculated load is already slightly oversized. The danger of right-sizing evangelism is that it pushes inexperienced technicians toward undersizing, which produces the exact callback problem that created the oversizing epidemic in the first place.
Variable-speed equipment complicates the picture further. A modern inverter-driven heat pump runs at partial capacity most of the time, modulating output to match the load rather than cycling on and off. Oversizing a variable-speed system by 30 or even 50 percent does not produce the same short-cycling and humidity problems as oversizing a single-stage unit because the compressor simply throttles down. According to a University of Maryland study published in March 2026, AI-hybrid models can predict VRF system behavior with just 5 to 6 percent error, which means the performance penalty of moderate oversizing in inverter systems may be far smaller than the DOE data, collected largely on older single-stage equipment, implies.
What You Should Do
If you are replacing your HVAC system, require the contractor to perform a Manual J load calculation before you sign anything. LiDAR scan, traditional software, or a free ACCA-approved tool. Just demand it. The IRC requires it for permitted work. If a contractor refuses or says he doesn't need one because he has been "doing this for 20 years," that refusal is the most valuable information you will receive in the entire sales process, and you should walk.
Ask what the calculated load is in BTU per hour and compare it to the proposed equipment capacity. Anything over 125 percent of calculated cooling load should require an explanation. In humid climates like Houston, Miami, or Savannah, closer to 100 percent is better because longer run times mean more dehumidification, which is worth more to indoor comfort than an extra half-ton of cooling you will never need.
If you are an HVAC contractor, the math favors adoption: an iPad Pro with LiDAR runs $799. Conduit Tech or comparable software costs a few hundred dollars per month. A single properly sized install that avoids one callback recoups the entire annual software cost, and Conduit's data shows that contractors who present 3D models and certified reports close at 21 percent higher rates than those who show up with a clipboard and a gut number. Right-sizing is not a sacrifice. It is a competitive advantage hiding in plain sight behind a piece of technology your competitors are still ignoring.
What We Did Not Prove
Our $11.8 billion estimate multiplies three independently sourced figures, each with its own uncertainty band. DOE's 31 percent oversizing rate draws on field studies that skew toward older homes and certain regions, so the national figure could be meaningfully higher or lower. NIST's 30 percent energy penalty was measured under controlled lab conditions with specific fault combinations and may not reflect real-world interactions between oversizing, duct leakage, and refrigerant charge errors, which compound in ways the lab tests did not fully replicate. Carrier's 130 million installed-base figure is a management estimate disclosed during an investor presentation, not a Census Bureau count.
Conduit Tech's performance metrics, including the 32 percent ticket increase and 21 percent close rate improvement, are company-reported and appear on marketing materials. No peer-reviewed study has independently verified these figures across a controlled sample of contractors. ServiceTitan's acquisition of Conduit may accelerate adoption, but the deal had not closed as of this article's publication, and integration outcomes remain uncertain.
We also cannot quantify how much of the oversizing problem is already being solved by the market transition to variable-speed equipment, which tolerates moderate oversizing without the catastrophic efficiency penalties of single-stage systems. If the installed base shifts to 60 or 70 percent inverter-driven over the next decade, the energy penalty per oversized system will decline, and the $11.8 billion figure will shrink even without any improvement in sizing practices. That is a legitimate pathway to the same destination, and it does not require any contractor to change anything about how they measure a room.