HVAC Load Calculator

-- BTU/hr

Enter building details to calculate

Building Parameters

IECC Climate Zone

Conditioned Area (sq ft)

Stories

Ceiling Height (ft)

Envelope Insulation

Wall Insulation

Ceiling Insulation

Floor / Slab

Window Type

Window-to-Floor Area Ratio

System Conditions

Infiltration Level

Internal Gains

Load Summary

Heating Load

BTU/hr

Cooling Load

BTU/hr

Heat BTU/sqft

Cool BTU/sqft

Window Area (sqft)

Wall Area (sqft)

Winter Design ΔT -- °F

Summer Design ΔT -- °F

✓ Load density within typical range

Heating Load Components

Cooling Load Components

🔥

Gas Furnace

-- BTU/hr

-- input BTU/hr at 80% AFUE

Oversizing by >15% reduces comfort and efficiency. Size to output, not input.

❄️

Central Air Conditioner

-- Tons

-- BTU/hr cooling capacity

Rule of thumb: 400–500 sqft per ton for well-insulated homes. Oversizing causes short-cycling.

♻️

Heat Pump

-- Tons

-- BTU/hr (heating + cooling)

Size to the larger load (heating or cooling). May need auxiliary heat in Zone 5+.

Sizing Comparison

IECC Climate Zone Design Temperatures

Zone	Region	Winter Design (°F)	Summer Design (°F)	ΔT Heat (70° indoor)	ΔT Cool (75° indoor)
1	Very Hot (Miami)	45°F	91°F	25°F	16°F
2	Hot (Houston)	30°F	96°F	40°F	21°F
3	Warm (Atlanta)	22°F	92°F	48°F	17°F
4	Mixed (Baltimore)	12°F	91°F	58°F	16°F
5	Cool (Chicago)	-4°F	91°F	74°F	16°F
6	Cold (Minneapolis)	-16°F	88°F	86°F	13°F
7	Very Cold (Fairbanks)	-47°F	78°F	117°F	3°F

IECC 2021 Minimum Insulation Requirements

Zone	Wall (cavity)	Ceiling	Floor	Window U-max
1	R-13	R-30	R-13	U-0.50
2	R-13	R-38	R-13	U-0.40
3	R-20 or R-13+5	R-38	R-19	U-0.30
4	R-20 or R-13+5	R-49	R-19	U-0.30
5	R-20+5 or R-13+10	R-49	R-30	U-0.27
6	R-20+5 or R-13+10	R-49	R-30	U-0.27
7	R-21+R-12ci	R-49	R-38	U-0.22

Typical Load Density (BTU/hr per sq ft)

Building Type	Heating	Cooling	Notes
High-performance (Zone 4–6)	15–25	10–15	R-21 walls, R-49 ceil, triple glazing
Code-minimum (Zone 4–6)	25–40	15–25	R-13–20 walls, R-38–49, double LowE
Older/poorly insulated (Zone 5–6)	40–60	20–30	R-11 walls, R-19 ceil, single/double pane
Hot-dry climate (Zone 2–3)	10–20	20–30	Cooling-dominant; low heating loads
Commercial office	25–35	25–40	Higher internal gains

U-Values by Assembly Type

Assembly	R-Value	U-Value	Notes
Wall — R-13 batt + framing	~R-12	0.082	Includes framing factor & air films
Wall — R-20 + framing	~R-17	0.059	Wood frame, typical sheathing
Ceiling — R-38 attic	R-38	0.026	Blown/batt, no framing factor
Ceiling — R-49 attic	R-49	0.020	IECC zone 5–8 minimum
Window — Double LowE	R-3.3	0.30	Standard ENERGY STAR zone 4–6
Window — Triple LowE	R-5.0	0.20	Premium; required zone 7–8
Floor — R-13 over crawl	~R-13.5	0.074	Includes air films

How to Use This HVAC Load Calculator

Select your climate zone — IECC zones 1–7 determine design temperatures. If you don't know your zone, look up your city on the IECC map or enter your zip code on energy.gov.

Enter conditioned floor area — Include only heated/cooled spaces. Exclude garages, unfinished basements, and attics.

Set ceiling height and stories — These determine building volume for infiltration calculations and wall area estimation.

Choose insulation levels — Match your actual or planned construction. IECC minimum is pre-selected for zone 4 as a baseline.

Set window-to-floor ratio — 15% is typical for residential. Energy codes often limit windows to 15–20% of floor area.

Review Equipment Sizing tab — See recommended furnace BTU/hr, AC tonnage, and heat pump sizing based on your loads.

Glossary

Manual J

ACCA Manual J is the industry-standard method for calculating residential HVAC loads. This calculator uses a simplified Manual J approach.

Design Temperature

The outdoor temperature used for load calculations — typically the 99% heating or 1% cooling design condition from ASHRAE weather data.

U-Value

Thermal transmittance in BTU/hr·ft²·°F. The inverse of R-value (U = 1/R). Lower U = better insulation.

ACH (Air Changes/Hour)

The number of times per hour the entire building volume of air is replaced by infiltration. Tight modern homes: 0.25–0.35 ACH.

SHGC

Solar Heat Gain Coefficient — fraction of solar radiation admitted through a window. Lower SHGC reduces cooling loads in sunny climates.

Tonnage

AC capacity unit equal to 12,000 BTU/hr (heat removed to freeze one ton of ice in 24 hours). Residential systems: 1.5–5 tons.

AFUE

Annual Fuel Utilization Efficiency — percent of fuel converted to heat. 80% AFUE = 80% efficient. Federal minimum is 80% for most climates.

CLF

Cooling Load Factor — accounts for the time lag between heat gain and peak cooling load due to building thermal mass.

Frequently Asked Questions

How accurate is this calculator vs. a professional Manual J?

This simplified calculator gives results within 15–25% of a full Manual J for typical rectangular homes. Professional Manual J software (e.g., Wrightsoft, Elite RHVAC) accounts for exact room-by-room layouts, orientation, shading, duct losses, and local weather data. For equipment purchasing decisions, hire an HVAC contractor to perform a full Manual J.

My load density seems high — what's normal?

For a code-minimum home in Zone 4–5, 25–40 BTU/hr per sq ft for heating and 15–25 BTU/hr per sq ft for cooling are typical. Older homes can reach 50–60 BTU/hr·sqft. High-performance homes can be as low as 15–20 BTU/hr·sqft. Very high densities usually indicate high infiltration, poor insulation, or large windows.

Should I size up for safety margin?

No — ACCA Manual J explicitly discourages oversizing. An oversized furnace short-cycles (turns on/off rapidly), reducing efficiency and comfort. An oversized AC cools too fast without dehumidifying adequately. Size to the calculated load ± 15%. Modulating variable-speed equipment can tolerate slightly more oversizing.

What about duct losses?

This calculator calculates building envelope loads only. Duct losses in unconditioned spaces (attics, crawlspaces) can add 15–30% to equipment sizing requirements. The ACCA Manual D standard recommends adding a duct loss factor if ducts run outside the conditioned envelope.

Does this account for humidity/latent load?

This calculator estimates sensible (temperature) loads only. Latent (humidity) loads can add 20–40% to cooling equipment sizing in humid climates (Zones 1–3A). Full Manual J includes latent calculations using wet-bulb design temperatures and psychrometric analysis.

How do I find my IECC climate zone?

IECC climate zones are determined by county. You can look up your county on the DOE Building Energy Codes Program map at energycodes.gov/about/climate-zones, or search your state's energy code office. Most US population centers are in Zones 3–6.