Calculate the BTU (British Thermal Unit) requirements for heating and cooling your space.
Results:
Room Area: 144 square feet
Room Volume: 1,152 cubic feet
Required BTU: 7,200 BTU/hr
Recommended AC Size: 0.6 tons
Energy Efficiency: Look for units with SEER 14+ rating
HVAC Load Optimization & BTU Sizing Framework
Core Concept
Correct BTU sizing balances comfort, humidity control, runtime efficiency, and lifecycle cost. Oversize → short cycling & moisture issues. Undersize → continuous runtime & inadequate setpoint recovery.
Cooling Load Heuristics
- Baseline: 18–25 BTU per sq ft (moderate climate, 8 ft ceilings)
- Ceiling Factor: BTU × (Actual Height ÷ 8)
- Solar Gain: +10–18% (high sun), −8–10% (deep shade)
- Occupancy: +600 BTU per person above 2
- Appliance / Internal Gains: Add 1–4 kBTU for high‑load kitchens/equipment
Heating Load Bands (Per Sq Ft)
- Warm Zone: 20–30
- Moderate: 30–40
- Cold: 40–50
- Very Cold: 50–60+
Adjustment Matrix
Adjusted BTU = Base × InsulationFactor × SolarFactor + OccupantGain + ApplianceGain
- Insulation: Poor 0.85 • Avg 1.00 • Good 1.12 • Excellent 1.22
- Solar: Shaded 0.92 • Neutral 1.00 • Sunny 1.15
AC Tonnage Mapping
- < 9,000 BTU → 0.75 ton (mini split / single room)
- 9k–12k → 1.0 ton
- 12k–18k → 1.5 ton
- 18k–24k → 2.0 ton
- 24k–30k → 2.5 ton
- 30k–36k → 3.0 ton
Efficiency Targets
- Cooling: SEER2 ≥ 14 (baseline), 16–18 (optimized), 20+ (premium inverter)
- Heating (furnace): AFUE 80% (legacy) → 92–96% (condensing)
- Heat Pump HSPF2 ≥ 8.0 (cold climate models for northern zones)
When to Get Manual J
- Whole‑home retrofit or new build
- High performance envelope (spray foam, triple glazing)
- Oversizing suspicion (short cycles & humidity >60%)
Rapid Diagnostic Signals
- Short Cycle < 8 min runtime: Oversized cooling stage
- Run 100% on design day yet misses setpoint: Undersized or duct loss
- Even temps but sticky air: Latent removal deficit (oversize)
Quick FAQ
- Do I round up capacity? Only after adjustment & latent review.
- Is higher SEER always ROI? Diminishing return above ~18 without high runtime hours.
- Can ceiling fans reduce BTU? They reduce perceived temp, not actual load.
Action Tip: Recalculate after envelope upgrades (windows, insulation, air sealing) before replacing equipment.