HVAC Sizing Terminology Glossary: Key Terms Every Homeowner Should Know

BTUs & Tonnage Capacity in Plain Numbers

BTU (British Thermal Unit) is the unit of heating/cooling work. One ton of cooling equals 12,000 BTU/h. Typical residential systems run from 18,000 BTU (1.5 ton) to 60,000 BTU (5 ton). As a ballpark, occupants contribute ~100–400 BTU each to the cooling load and windows/exterior doors ~1,000 BTU each.
Visual:

LOAD (BTU/h) = Envelope + Solar + People + Appliances ± Infiltration

TONS = LOAD ÷ 12,000

Table (context only):

Manual J Load Calculation What Actually Drives Size

Manual J is the residential standard to calculate the exact cooling/heating load. It factors square footage, ceiling height, insulation, window count/type/orientation, shading, local weather, occupants, appliances, and duct condition.
Why it matters: Oversizing leads to short cycles and poor dehumidification; undersizing runs the unit to death without hitting setpoints.
Workflow Visual:

Measure Envelope → Inputs (U-values, ACH, SHGC) → Weather Data → Manual J

→ Sensible Load (BTU/h) + Latent Load (BTU/h)

After Manual J, use Manual S (equipment selection) to match sensible/latent capacity at your design conditions, then Manual D for ducts. If you want help turning a survey into a real design, start with our Sizing Guide.

CFM (Airflow) The Other Half of Capacity

Capacity is useless without airflow. A common baseline is ~400 CFM per ton:

• 2-ton ≈ 800 CFM

• 3-ton ≈ 1,200 CFM

• 4-ton ≈ 1,600 CFM
  Room method:

CFM = (Room Volume × ACH) ÷ 60

Bathrooms often need 6–8 ACH; bedrooms 4–6 ACH.
Why pros care: Low CFM means poor coil heat transfer, low sensible capacity, and high humidity. High CFM can sacrifice latent removal.
Field check: Measure external static pressure and fan speed, then compare delivered CFM to the blower table.
Variable-speed air handlers let you fine-tune CFM for sensible vs. latent targets and zoning. Browse air handlers to keep airflow and coil sizing aligned.

Sensible vs. Latent Heat Temperature vs. Moisture

Sensible heat changes air temperature. Latent heat changes moisture content (humidity) without changing temperature. Your AC must handle both:

• Sensible load: solar gain, conduction through walls/attic, people, lighting, appliances.

• Latent load: infiltration, showers, cooking, wet basements, high outdoor dew points.
  Visual:

Total Cooling = Sensible (BTU/h) + Latent (BTU/h)

SHR (Sensible Heat Ratio) = Sensible ÷ Total

In humid climates, aim for a slightly lower CFM per ton or consider equipment with longer dehumidification cycles (variable capacity). Check versatile ductless options for zone-by-zone latent control.

Static Pressure & Duct Design Where Sizing Fails First

External static pressure (ESP) is the resistance your blower works against. Excess ESP chokes airflow and collapses capacity. Keep friction rates in a practical band (often ~0.05–0.10" w.c. per 100 ft equivalent length) and size ducts accordingly.
Diagnostic loop:

Measure ESP → Compare to blower table → Verify CFM → Inspect restrictions

(Filters, coils, dampers, flex kinks, undersized returns)

If Manual J/S say 3 tons but ducts only support ~1,000–1,100 CFM at acceptable ESP, fix ducts or select equipment/air handlers that can move air efficiently at lower noise. See line sets & accessories to complete upgrades correctly.

Energy Efficiency SEER, EER, and HSPF Explained

• SEER: Seasonal cooling efficiency (higher is better). Typical minimum ~14; high-efficiency ≈ 16–21+; some commercial gear tests higher.

• EER: Point efficiency at a fixed condition—useful for hot-climate peak checks.

• HSPF: Heat pump heating season efficiency (higher = lower winter cost).

Reality check: A high SEER unit sized or ducted poorly won’t deliver comfort or savings. Prioritize load match + airflow first, then compare efficiency tiers.
Visual:

Operating Cost ∝ Load ÷ Efficiency

Improve both sides: reduce load + raise SEER/EER/HSPF

Variable-capacity systems often shine on humidity control and part-load efficiency. If you’re considering a heat pump swap, browse our R-32 heat pumps.

People, Windows, and Appliances Quiet BTUs That Add Up

Estimating internal gains prevents “mystery” oversizing. Use conservative but realistic adders: ~100–400 BTU per person depending on activity and time at home; ~1,000 BTU per exterior window/door (type and exposure matter). Kitchen ranges, dryers, and electronics add sensible heat; showers and cooking add latent load.

In highly glazed spaces, consider zoning or a ductless head serving that area to avoid upsizing the whole system. See wall-mounted mini-splits for targeted control.

Room-by-Room Sizing, ACH & Distribution

A whole-house tonnage is only half the story. Comfort is delivered room-by-room. Use volume and air changes per hour (ACH) targets to allocate supply CFM:

Room CFM = (Room Volume × ACH) ÷ 60

Bedrooms usually land in 4–6 ACH; baths 6–8 ACH. Match supply/return paths, limit pressure imbalances, and verify throw patterns at registers.
Zoning note: Multi-stage or variable systems plus dampers can manage diverse room loads—but only when duct friction, leakage, and bypass strategies are engineered correctly.
For retrofits with hard-to-reach rooms, a small ductless mini-split can solve a chronic hot/cold room without wrecking the main system sizing.

Oversized vs. Undersized Symptoms & Field Diagnostics

Oversized: Short cycles, temperature swings, poor dehumidification, higher peaks on utility bills, noisy starts, rooms that cool fast but feel clammy.
Undersized: Long runtimes that never hit setpoint, comfortable humidity but insufficient capacity on design days, premature wear from continuous operation.
What to test:

• Compare thermostat cycles to design expectations.

• Measure ESP and deliver CFM vs. target (400 CFM/ton baseline).

• Check coil delta-T (and wet-coil behavior under humidity).

• Verify return/supply temperature splits versus load.

If humidity is high with a “right-sized” unit, reduce airflow slightly, enable dehumidify-on-demand modes, or consider equipment with a lower sensible heat ratio. Explore air handlers that support enhanced dehumidification controls.

Putting It Together A Practical Sizing Workflow

1. Survey: Measurements, window schedule, insulation levels, infiltration clues, occupancy, appliances.

2. Manual J: Calculate sensible + latent loads at your design temps.

3. Manual S: Select equipment whose actual capacity at your conditions matches the load (don’t use only “nameplate tons”).

4. Manual D: Design ducts to deliver target CFM at acceptable ESP and noise.

5. Commission: Verify ESP, CFM, superheat/subcool, and dehumidification.

6. Mini example: A 1,800 sq ft, well-insulated home with moderate glazing might start around 3 tons by square-foot rules, but a proper Manual J could justify 2.5–3.0 tons depending on windows, orientation, and climate. Confirm the duct system can move 1,000–1,200 CFM quietly.

If you need a packaged solution, review residential packaged systems. For room-by-room solutions, see through-the-wall.