When you replace or upgrade your heating and air system, the smartest first step isn’t choosing a brand—it’s figuring out how much heating and cooling your home actually needs. That sizing step is called a load calculation. Do it right and you get even temperatures, quieter operation, lower utility bills, and longer equipment life. Do it wrong and you get hot/cold rooms, short cycling, and a system that never feels quite right.
At Caledonian Mechanical, we use the ACCA standards:
- Manual J for the load calculation (how many BTUs your home needs)
- Manual S for equipment selection (matching the gear to the load)
- Manual D for duct design (moving the right air to the right rooms)
People sometimes call it “Manual Jack”—it’s actually Manual J, but we know what you mean. Here’s how we do it, in plain language.
What a Load Calculation Really Is
A load calculation estimates the BTUs per hour your home needs on the hottest and coldest design days.
- BTU (British Thermal Unit) is just a unit of heat.
- Cooling: 1 ton of AC = 12,000 BTU/hr of cooling.
- Gas furnaces: We look at output BTUs (heat delivered), not just input.
- Heat pumps: Heating output changes with outdoor temperature; we use manufacturer data, not guesses.
Right-size means the system runs steady and smooth. Over-size and you get short blasts of air, humidity or comfort issues, and higher bills. Under-size and the system struggles on peak days.
The Caledonian Walk-Through: What We Measure (Room by Room)
We don’t “eyeball it.” We come out with tape, tablet, and a checklist:
- Room dimensions: length, width, ceiling height, and total square footage
- Orientation: which way each room faces (afternoon sun matters)
- Windows & doors: size, frame type, glass type (single pane vs. low-E), shading/coverings
- Insulation: attic and walls (R-values), plus duct insulation (R-6 vs. R-8)
- Infiltration (drafts): attic access, recessed lights, weather-stripping, leaky chases
- Occupancy & internal loads: people, lighting, cooking, home office gear
- Age & layout of the home: additions, vaulted ceilings, long hallways, closed-door bedrooms
- Existing ductwork: sizes, condition, leakage, kinks, crushed runs, location (attic vs. conditioned)
- Equipment & electrical: current system type/size, breaker/fuse sizing, panel capacity
- Location & climate: local summer and winter design conditions (we use the standard engineering weather data for our area)
All of that goes into our Manual J software to produce room-by-room heating and cooling BTUs. That’s the “map” we design from.
From Load to Equipment (Manual S): Matching the Gear to Your Home
With the load known, we pick equipment that will actually deliver those BTUs in real life.
- Cooling capacity (tons): We select models whose performance tables match your home’s cooling load—not just the nameplate tonnage.
- SEER / SEER2: Higher ratings mean less electricity for the same cooling. But only if the ductwork and airflow are right. A “20-SEER” unit connected to undersized, leaky, poorly insulated ducts won’t perform like a 20-SEER system.
- Gas furnaces: We size to output BTUs and match the blower to your duct design.
- Heat pumps: We use the manufacturer’s cold-weather tables to ensure you have the heating capacity you need (and we discuss auxiliary heat if it makes sense).
- Staging & variable speed: We often recommend 2-stage or variable-speed systems for comfort, quiet, and efficiency.
Ductwork & Airflow (Manual D): Getting Air Where It Needs to Go
Great equipment can’t fix bad ductwork. We design for proper CFM, low static pressure, and quiet, balanced airflow.
- Airflow target: A common design rule is ~400 CFM per ton of cooling (varies by equipment).
- Static pressure: Think of it as air “back-pressure” in the ducts. Lower is better for efficiency, noise, and reliability. Most residential systems are happiest at ≤ 0.5 in. w.c. total external static.
- Supply register locations: We place supplies near heat loads (exterior walls/windows) so cool air “washes” the hot surfaces in summer; in heating, the same layout helps prevent cold drafts.
- Return air: We like central returns for circulation and additional returns for large or closed-door rooms.
- Balancing dampers: These let us trim airflow run-by-run so each room gets its fair share.
- Duct size & insulation: We size trunks and branches for the target CFM at low static, and we prefer R-8 duct insulation in hot attics to protect efficiency.
- Filters & grilles: We keep filter face velocity reasonable (often ~300 fpm). Example: a 3-ton system (≈ 1,200 CFM) needs about 4 ft² of filter area (≈ 576 in²) for low noise and easy breathing.
BTUs, SEER, R-Value—Quick Plain-English Guide
- Cooling BTUs: How much heat your AC removes per hour. Tonnage is just BTUs divided by 12,000.
- Heating BTUs (gas): An 80,000 BTU input furnace at 80% efficiency gives ~64,000 BTU output. We size to output.
- Heating BTUs (heat pump): Varies with outdoor temperature; we use performance tables.
- SEER/SEER2: Seasonal efficiency rating for cooling. Higher number = fewer kWh for the same comfort (again, only with proper ducts/airflow).
- R-value: Insulation’s resistance to heat flow. Higher R = better insulation. For ducts in attics, R-8 beats R-6 and helps preserve that high SEER in real conditions.
Why Register Placement and Balancing Matter
- Supplies near the heat load: Windows and exterior walls are where heat sneaks in. Putting supplies there controls the problem at the source.
- Returns in the right spots: Good returns keep air moving gently through the whole home, not just the hallway.
- Right sizes, right counts: Too few or too small returns make the blower work harder (noisy, inefficient). Too small supplies create drafts and starve rooms. We size both based on your room-by-room CFM.
Insulation, Ducts, and “Real-World” Efficiency
A high-efficiency unit can’t overcome bad ductwork or thin insulation. If your system is rated 20 SEER but the ducts are undersized and poorly insulated, you won’t get 20-SEER comfort or bills.
Fix the air highway (ducts), then the engine (equipment) can shine.
How We Know if You Need Insulation or Duct Upgrades
During our visit we:
- Measure duct sizes, check for kinks/crushes, and test/inspect for leakage
- Note attic and wall insulation levels and any obvious gaps
- Read static pressure and temperature splits to see how the current system is actually performing
- Fold those findings into the Manual J/S/D design so your new system isn’t held back by old bottlenecks
If the numbers say the ducts are undersized or insulation is weak, we’ll show you exactly how fixing them improves comfort and reduces kWh.
What About Panels, Breakers, and Wire Gauges?
Bigger or more advanced HVAC sometimes means checking the electrical side:
- Correct breaker size and wire gauge for the air handler, heat strips, condenser, or heat pump
- Panel capacity and available spaces
- Clean, code-compliant disconnects and grounding
If something needs attention, we handle permits and coordinate the licensed electrical work so your upgrade is safe, legal, and future-ready.
The Caledonian Process (Start to Finish)
- Listen & measure: Comfort complaints, hot/cold rooms, dust/noise—plus all the room sizes, windows, insulation, and duct details.
- Manual J: Room-by-room heating and cooling BTUs based on your home, not a rule of thumb.
- Manual S: We match equipment to the actual load—SEER/SEER2, staging, and performance tables.
- Manual D: We size and lay out ducts, supplies, returns, dampers, and target static pressure.
- Proposal: Clear options (often our Bronze / Silver / Gold tiers), what’s included, and financing if you want it.
- Permits & install: Professional, tidy work; then commissioning—we verify airflow, charge, temps, and static so you get what you paid for.
- Handoff: Photo-rich report, thermostat training, and maintenance plan options.
Our Stamp, Warranty & Mission
We stand behind our work with a one-year workmanship warranty (plus full manufacturer coverage). More importantly, we stand on our reputation—clear communication, clean work, and systems that perform the way the paperwork says they should.
Our mission: deliver the most efficient, best-performing system possible for every home we touch—and earn your trust as the #1 HVAC/R provider in Fresno County and the Central Valley.
Ready for a real load calculation?
We’ll measure, calculate, and show you a simple side-by-side plan for equipment, ducts, airflow, insulation, and controls—so you can choose with confidence.
No pressure. Just solid numbers and a comfortable home.