Reviewed against ANSI / ACCA Manual J 8th Edition / Standard 2-2016 (Residential Load Calculation

HVAC Load Calculation (Rough Manual J) Calculator

Rough whole-building residential heating and cooling load estimate using a simplified ACCA Manual J approximation. Inputs: conditioned square footage, IECC climate zone (1-8), wall and ceiling insulation tier, total window area and window type, occupants, and internal-gain appliance count. Outputs: cooling load in BTU/hr and tons, heating load in BTU/hr and kBTU/hr, recommended ACCA Manual S equipment-size band, and a rule-of-thumb sqft-per-ton cross-check. EXPLICITLY NOT A SUBSTITUTE FOR FULL ACCA MANUAL J — for permitting under IRC 2024 Chapter M, equipment selection per ACCA Manual S, and duct design per ACCA Manual D, commission a full Manual J calculation from a licensed HVAC contractor using ACCA-approved software (Wrightsoft Right-J, Elite RHVAC, Cool Calc Manual J). Tool, not advice.

Calculator

Adjust the inputs below; the result updates instantly.

Building

Conditioned square footage

IECC climate zone (1-8)

International Energy Conservation Code climate zone. 1 = very hot (south Florida, Hawaii); 2 = hot (south Texas, Gulf Coast); 3 = warm (north Florida, Atlanta, Las Vegas); 4 = mixed (mid-Atlantic, Pacific Northwest); 5 = cool (Chicago, Boston, Denver); 6 = cold (Minneapolis, Burlington); 7 = very cold (Duluth, northern Maine); 8 = subarctic (interior Alaska). Drives both the cooling and heating climate factors used in the calculation.

Wall insulation tier

Wall cavity insulation R-value. Code-minimum varies by climate zone (R-13 typical for warm zones; R-21 for cool / cold zones; R-30 for very cold zones per IECC 2024). The calculator uses the entered tier as a load multiplier — higher R-value reduces the load.

Ceiling / attic insulation tier

Attic / ceiling insulation R-value. Code-minimum is R-30 in warm zones and R-38 in cool / cold zones per IECC 2024. The calculator uses the entered tier as a load multiplier.

Windows

Total window area (sqft)

Window type

Glazing type drives the U-value (conductive heat transfer coefficient) and SHGC (solar heat gain coefficient). Single-pane U≈1.0 is pre-1990 vintage; double-pane U≈0.5 is 1990-2010 typical; double-pane low-E U≈0.30 is current code-minimum; triple-pane low-E U≈0.20 is high-performance (typical in cold-climate Passive House construction).

Internal gain

Number of occupants

Significant internal-gain appliances

Cooling load (tons)

4.31

Cooling load (BTU/hr): 51,700
Heating load (BTU/hr): 60,100
Recommended cooling tonnage (low): 4
Recommended cooling tonnage (high): 4.5
Recommended heating capacity (low, kBTU/hr): 54
Recommended heating capacity (high, kBTU/hr): 69
Rule-of-thumb sqft per ton (cross-check): 465
Window component of cooling load (BTU/hr): 5,400
Internal-gain component of cooling load (BTU/hr): 2,920
ACCA Manual J disclaimer (REQUIRED for permitting): ACCA MANUAL J REQUIRED FOR PERMITTING. This calculator is a simplified whole-building approximation suitable for first-pass sizing discussion only. It does NOT account for orientation, shading, infiltration rate per blower-door tested ACH50, latent vs sensible load split, room-by-room load distribution, ductwork heat loss in unconditioned spaces, or local design temperature data. For permitting under IRC 2024 Chapter M, equipment selection per ACCA Manual S, and duct design per ACCA Manual D, commission a full ACCA Manual J calculation from a licensed HVAC contractor using ACCA-approved software (Wrightsoft Right-J, Elite RHVAC, or Cool Calc Manual J). For 26 USC § 25C / 25D energy-credit eligibility, consult a licensed CPA familiar with energy-credit practice.
Summary: At 2,000 sqft in IECC climate zone 4 (cooling factor 22 BTU/hr/sqft, heating factor 28 BTU/hr/sqft) with R-13 walls and R-30 ceiling (combined insulation factor 0.99), and 300 sqft of double-pane-low-e windows (U=0.30, SHGC=0.40): Cooling load ≈ 51,700 BTU/hr ≈ 4.31 tons (envelope 43,300 BTU/hr + windows 5,400 BTU/hr + internal 2,920 BTU/hr from 4 occupants and 5 appliances). Heating load ≈ 60,100 BTU/hr ≈ 60.1 kBTU/hr (envelope 55,200 BTU/hr + windows 5,000 BTU/hr at design delta T of 55F). Recommended equipment size: 4.0-4.5 tons cooling, 54-69 kBTU/hr heating (ACCA Manual S band: ±5% cooling, −10% to +15% heating with oversize bias). Rule-of-thumb cross-check: 465 sqft per ton of cooling (typical 400-800 by climate zone). ACCA MANUAL J REQUIRED FOR PERMITTING. This calculator is a simplified whole-building approximation suitable for first-pass sizing discussion only. It does NOT account for orientation, shading, infiltration rate per blower-door tested ACH50, latent vs sensible load split, room-by-room load distribution, ductwork heat loss in unconditioned spaces, or local design temperature data. For permitting under IRC 2024 Chapter M, equipment selection per ACCA Manual S, and duct design per ACCA Manual D, commission a full ACCA Manual J calculation from a licensed HVAC contractor using ACCA-approved software (Wrightsoft Right-J, Elite RHVAC, or Cool Calc Manual J). For 26 USC § 25C / 25D energy-credit eligibility, consult a licensed CPA familiar with energy-credit practice.

Tools to go with this

Sizing residential HVAC equipment? The rough number is a conversation starter — Manual J is the permitting requirement.

Fennec Press s HVAC operations bundle includes the ACCA Manual J / S / D field-cheat reference (room-by-room load worksheet, infiltration ACH50 lookup, design-temperature table by city), the Manual J vs rule-of-thumb cross-check audit, the Manual S equipment selection guidance with SEER2 / HSPF2 thresholds for 26 USC § 25C credit eligibility, the Manual D duct-design quick reference for retrofit replacements, and the IRC 2024 Chapter M permitting-requirement summary by adopting jurisdiction — built for HVAC contractors, designers, and the project managers who specify residential equipment.

Open Fennec Press HVAC operations bundle→

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Published by

The Fennec Lab Editorial Team

Statute-cited · Unit-tested

Reviewed against: ANSI / ACCA Manual J 8th Edition / Standard 2-2016 (Residential Load Calculation; the analytical basis for the simplified whole-building approximation in this calculator); ACCA Manual S (Residential Equipment Selection; the source for the recommended equipment-size band of ±5% cooling and −10% to +15% heating with oversize bias); ACCA Manual D (Residential Duct Design); IRC 2024 Chapter M (International Residential Code mechanical-systems chapter; requires Manual J / S / D for new construction and equipment replacement in adopting jurisdictions); IECC 2024 (International Energy Conservation Code climate zone classifications 1-8 used in the calculator s climate-factor lookup); 10 CFR Part 430 (DOE residential equipment energy-conservation standards); ASHRAE Handbook of Fundamentals (heat transfer coefficients and design temperature data); 26 USC § 25C (Energy Efficient Home Improvement Credit; requires elevated SEER2 / HSPF2 / EER2 efficiency thresholds that depend on Manual J / S sizing for credit eligibility verification)

Last reviewed: 2026-05-17

How this calculator works

This calculator produces a rough whole-building residential heating and cooling load estimate using a simplified ACCA Manual J approximation. Inputs: conditioned square footage, IECC climate zone (1-8), wall insulation tier, ceiling insulation tier, total window area in square feet, window type (single-pane through triple-pane low-E), number of occupants, and count of significant internal-gain appliances. Outputs: cooling load in BTU per hour and tons, heating load in BTU per hour and kBTU per hour, recommended ACCA Manual S equipment-size band (low and high), the rule-of-thumb sqft-per-ton cross-check, individual component contributions (envelope, window, internal gain), and the required ACCA Manual J disclaimer.

EXPLICITLY NOT A SUBSTITUTE FOR FULL ACCA MANUAL J. For permitting under IRC 2024 Chapter M, equipment selection per ACCA Manual S, and duct design per ACCA Manual D, commission a full ACCA Manual J calculation from a licensed HVAC contractor using ACCA-approved software (Wrightsoft Right-J, Elite RHVAC, or Cool Calc Manual J).

The framework — simplified Manual J approximation

The Air Conditioning Contractors of America (ACCA) Manual J, Residential Load Calculation, is the residential HVAC industry s required heat-loss / heat-gain analysis. Manual J performs a room-by-room, surface-by-surface heat transfer calculation that accounts for orientation, shading, infiltration, ventilation, latent load, internal gain, and design temperature conditions per ANSI / ACCA Standard 2-2016. Manual J is required for permitting in most U.S. jurisdictions for new construction and equipment replacement and is the analytical basis for ACCA Manual S (equipment selection) and Manual D (duct design).

This calculator implements a simplified whole-building approximation. The simplification uses:

A climate-zone cooling factor (BTU per hour per sqft) that varies from 40 in zone 1 (very hot) to 11 in zone 8 (subarctic) at baseline insulation.
A climate-zone heating factor (BTU per hour per sqft) that varies from 10 in zone 1 to 60 in zone 8 at baseline insulation.
An insulation factor that averages wall and ceiling R-value multipliers (R-13 walls produce a slight uplift; R-30 walls produce a 22 percent reduction).
Window heat gain: U-value times area times design delta T (20F at cooling design) plus SHGC times area times peak solar (30 BTU per hour per sqft).
Window heat loss: U-value times area times design heating delta T (25F in zone 1 ramping to 95F in zone 8).
Internal gain on cooling: 230 BTU per hour per occupant (per ACCA Standard 2 sensible-gain figure) plus 400 BTU per hour per appliance.
No internal-gain credit on heating (per Manual J peak-heating convention).

The output is a building-level cooling load in BTU per hour and tons and a heating load in BTU per hour and kBTU per hour. For a typical single-family residential building (1,500 to 3,500 sqft, code-minimum envelope, balanced glazing), the calculator typically falls within plus or minus 15 to 20 percent of a full Manual J result.

Inputs explained

Conditioned square footage. Heated and cooled spaces only. Exclude unconditioned garage, basement, and attic. Use measured floor area, not county property record gross figures.

IECC climate zone (1-8). International Energy Conservation Code classification by heating-degree-days and cooling-degree-days. Zone 1 is very hot (south Florida, Hawaii); zone 8 is subarctic (interior Alaska). Most of the populated U.S. is in zones 2-6. The DOE Building America Solution Center publishes a county-by-county map.

Wall insulation tier. Cavity insulation R-value. Code-minimum varies by climate zone per IECC 2024: R-13 in warm zones, R-21 in cool zones, R-30 in very cold zones. The calculator uses a load multiplier — higher R reduces load.

Ceiling / attic insulation tier. Code-minimum is R-30 in warm zones and R-38 in cool / cold zones per IECC 2024.

Total window area. Sum across all orientations. Typical residential glazing is 12-18 percent of conditioned floor area. The calculator does not break out by orientation; a full Manual J does.

Window type. Drives U-value (conduction) and SHGC (solar). Single-pane clear has U around 1.0 and SHGC around 0.85; double-pane low-E (current code) has U around 0.30 and SHGC around 0.40; triple-pane low-E (high-performance) has U around 0.20 and SHGC around 0.30.

Number of occupants. Drives the 230 BTU per hour per person sensible internal gain figure per ACCA Standard 2.

Significant internal-gain appliances. Kitchen and laundry equipment that contributes meaningful heat (refrigerator, range, oven, dishwasher, dryer in conditioned space). Typical residential count is 4-6. Drives the 400 BTU per hour per appliance estimate.

Industry benchmarks (ACCA, NCI, RSES, BLS)

| Reference | Value | | --- | --- | | ACCA Manual J 8th Edition / Standard 2-2016 | Required residential load calc standard | | ACCA Manual S | Equipment selection from Manual J output | | ACCA Manual D | Duct design from Manual S equipment | | IRC 2024 Chapter M | Permitting backdrop in adopting jurisdictions | | IECC 2024 climate zones | 1-8 classifications by HDD / CDD | | 10 CFR Part 430 | DOE energy-conservation standards (SEER2 / HSPF2 / EER2) | | ASHRAE Handbook of Fundamentals | Heat transfer coefficients and design temperatures | | ACCA Standard 2 sensible internal gain | 230 BTU/hr per occupant | | ASHRAE 62.2 ventilation rate | 7.5 cfm per occupant + 0.03 cfm per sqft | | Manual S cooling band | Load to 115 percent of load | | Manual S heating band | 90 to 115 percent of load with oversize bias allowed | | Rule of thumb sqft per ton | 400-500 hot-humid, 500-700 moderate, 700-900 cold (unreliable across zones) |

The NCI (National Comfort Institute) and RSES (Refrigeration Service Engineers Society) publish operational training materials that translate Manual J output into commissioning practice: airflow verification at design CFM per ton (typical 400 CFM per ton for cooling), static pressure verification at design external static (typical 0.5 in. wc), and refrigerant-charge verification at the manufacturer s subcooling / superheat target. A correctly-sized system that fails commissioning operates at field efficiency 10-25 percent below rated efficiency.

What this calculator does NOT model

Orientation. A south-facing window gains roughly 2 times more solar than a north-facing window of the same area and type. The calculator uses an averaged peak solar value across orientations.
Shading. Existing trees, neighbor structures, and overhangs reduce solar gain significantly; the calculator assumes no shading.
Infiltration rate. Blower-door tested ACH50 vs assumed natural infiltration produces a 10-30 percent load difference. The calculator s climate-zone factor implicitly assumes typical code-minimum infiltration.
Latent vs sensible load split. Manual J separates the two because cooling equipment is rated separately for sensible and latent capacity, and oversizing on sensible produces a system that short-cycles and fails to dehumidify.
Room-by-room load distribution. Manual D duct design requires the room-by-room breakdown.
Ductwork heat loss in unconditioned spaces. Ducts in unconditioned attic can lose 15-30 percent of their delivered capacity; the calculator does not model.
Mechanical ventilation (ASHRAE 62.2). 7.5 cfm per occupant plus 0.03 cfm per sqft of mechanical ventilation contributes to load; the calculator assumes implicit natural-infiltration ventilation.
Thermal mass. Heavy masonry construction has very different load dynamics than light-frame; the calculator assumes light-frame.
High-performance envelope buildings. Passive House construction (ACH50 less than 0.6, R-40 walls, R-60 ceiling, triple-pane low-E) diverges from the calculator s factor approach; commission a Passive House-aware Manual J for binding design.
Local design temperature. The calculator uses climate-zone averages; actual local design temperatures vary by 5-15F within a zone.

For permitting and binding design, commission a full ACCA Manual J calculation from a licensed HVAC contractor using ACCA-approved software (Wrightsoft Right-J, Elite RHVAC, or Cool Calc Manual J). For 26 USC Section 25C / 25D energy-credit eligibility on heat pump and high-efficiency equipment replacements, consult a licensed CPA familiar with energy-credit practice.

Sources

ANSI / ACCA Manual J 8th Edition / Standard 2-2016 (Residential Load Calculation).
ACCA Manual S (Residential Equipment Selection).
ACCA Manual D (Residential Duct Design).
IRC 2024 Chapter M (International Residential Code mechanical-systems chapter).
IECC 2024 (International Energy Conservation Code climate zones).
10 CFR Part 430 (DOE residential equipment energy-conservation standards).
ASHRAE Handbook of Fundamentals (heat transfer coefficients, design temperature data).
ASHRAE Standard 62.2 (residential ventilation and indoor air quality).
NCI (National Comfort Institute) airflow / static-pressure commissioning standards.
RSES (Refrigeration Service Engineers Society) service-life and commissioning data.
DOE Building America Solution Center (climate-zone HVAC design guidance).
BLS Occupational Employment and Wage Statistics SOC 49-9021 (the workforce that implements Manual J / S / D).

NO. This calculator is a simplified whole-building approximation suitable for first-pass sizing discussion only. It does NOT account for orientation (a south-facing window gains roughly 2× more solar than a north-facing window of the same size and type), shading (existing trees, neighbor structures, overhangs), infiltration rate (blower-door tested ACH50 vs assumed natural infiltration), latent vs sensible load split (Manual J separates the two because oversizing on sensible alone produces a system that short-cycles and fails to dehumidify), room-by-room load distribution (which Manual D duct design requires), or ductwork heat loss in unconditioned spaces. For permitting under IRC 2024 Chapter M, equipment selection per ACCA Manual S, and duct design per ACCA Manual D, commission a full ACCA Manual J calculation from a licensed HVAC contractor using ACCA-approved software (Wrightsoft Right-J, Elite RHVAC, or Cool Calc Manual J). Use this calculator for educational and first-pass sizing discussion only.

The rule of thumb produces wildly different right answers depending on climate zone and envelope tightness. A 2,000 sqft home in IECC climate zone 1 (south Florida) with R-13 walls and single-pane windows might need 4-5 tons of cooling (400-500 sqft/ton). The same 2,000 sqft home in climate zone 5 (Chicago) with R-21 walls and double-pane low-E windows might need 2-2.5 tons (800-1,000 sqft/ton). The same home in climate zone 7 (Duluth) might need only 1.5-2 tons. The rule of thumb fails because it ignores climate zone (the dominant variable), envelope tightness (the second-most-important variable), and window orientation. The actual industry-standard sizing rule is: do a Manual J. The rule of thumb is for back-of-envelope conversations only, not for equipment selection.

SEER (Seasonal Energy Efficiency Ratio) is the efficiency RATING of the equipment — cooling output in BTU per watt-hour of electrical input, averaged across a season. The load calculation determines the SIZE of the equipment in BTU/hr (or tons), independent of efficiency. The two are decoupled: a 3-ton SEER 14 unit and a 3-ton SEER 20 unit deliver the same cooling capacity (36,000 BTU/hr) but the SEER 20 unit consumes 30% less electricity. The load calc tells you how big a unit you need; the SEER tells you how efficiently that capacity is delivered. ACCA Manual S takes the load output and matches it to actual equipment by both capacity and efficiency.

An oversized cooling system short-cycles — it cools the air quickly, satisfies the thermostat, shuts off, and restarts before it has run long enough to dehumidify the space. The result is a cool but clammy interior, condensation problems, and a poorly-conditioned space at the same energy cost as a correctly-sized unit. ACCA Manual S explicitly limits oversizing on cooling (the recommended band is the load to 115% of load; 125%+ is treated as a design defect). Heating-side oversizing is less pathological because furnaces and heat pumps modulate down at part-load conditions in modern equipment, but very large oversizing still produces short-cycling and higher cycling losses. The calculator s recommended-band output reflects the Manual S band — ±5% on cooling with a slight oversize bias allowed on heating.

IECC climate zones 1-8 categorize the U.S. by heating-degree-days and cooling-degree-days. Zone 1 is very hot (south Florida, Hawaii); zone 8 is subarctic (interior Alaska). Most of the U.S. populated landmass is in zones 2-6. The DOE Building America Solution Center publishes a county-by-county map; the IECC publishes a comprehensive jurisdiction lookup. Common cities by zone: zone 1 = Miami, Honolulu; zone 2 = Houston, Phoenix, Orlando; zone 3 = Atlanta, Las Vegas, Charlotte; zone 4 = Washington DC, Seattle, St. Louis; zone 5 = Chicago, Boston, Denver; zone 6 = Minneapolis, Burlington, Anchorage coastal; zone 7 = Duluth, northern Maine, interior Alaska; zone 8 = far-northern Alaska. The climate zone drives both the cooling factor (declining toward cold zones) and the heating factor (rising toward cold zones) used in the calculation.

The Manual J peak heating load is computed at the design heating condition — a cold, cloudy, occupied-but-not-heat-generating night. Internal gains from occupants, appliances, and solar are explicitly excluded because (a) the design condition may occur at 3am when the household is asleep and minimally heat-generating; (b) the peak heating load needs to be served by the heating system alone, without relying on intermittent internal gain; (c) sizing the heating system at the internal-gain-credited load produces a system that fails on the coldest nights. The calculator follows the same convention: heating load does not include the 230-BTU/hr-per-occupant credit. Cooling load DOES include internal gain because the design cooling condition is a hot, sunny, fully-occupied summer afternoon when internal-gain sources are simultaneously contributing to the cooling demand.

For a typical single-family residential building (1,500-3,500 sqft, single-story or two-story, code-minimum envelope, balanced glazing), this calculator typically falls within ±15-20% of a full Manual J result. The accuracy degrades on (a) buildings with heavily-imbalanced glazing (heavy south or west glass), (b) very small buildings under 500 sqft (whole-building approximation is unreliable for accessory dwelling units and additions), (c) very large buildings over 5,000 sqft (residential approximation breaks down at light-commercial scale), (d) high-performance envelope buildings (Passive House construction has infiltration rates and thermal mass that diverge from the standard assumptions), and (e) any building where ductwork runs through unconditioned spaces (the calculator does not model duct gain / loss). The accuracy band is good enough for first-pass sizing conversation but not for permitting or equipment selection.

SHGC (Solar Heat Gain Coefficient) is the fraction of incident solar radiation admitted through a window. A single-pane clear window has SHGC of approximately 0.85 (admits 85% of incident solar); a double-pane low-E window has SHGC of approximately 0.30-0.40 (selectively coated to reflect infrared while admitting visible light). On a sunny summer afternoon, peak solar irradiance is roughly 250 W/sqm (~80 BTU/hr/sqft) on a south-facing vertical surface; with a 0.85 SHGC that translates to ~68 BTU/hr/sqft of cooling load, and with a 0.30 SHGC to ~24 BTU/hr/sqft. The window-type difference is large — replacing single-pane with double-pane low-E on a 300-sqft glazed home reduces cooling load by roughly 13,000 BTU/hr (1+ ton). The calculator uses an averaged peak solar value across orientations because it does not break out by direction; a full Manual J does the orientation breakdown.

No. ASHRAE Standard 62.2 (Ventilation and Acceptable Indoor Air Quality in Residential Buildings) requires mechanical ventilation at 7.5 cfm per occupant + 0.03 cfm per sqft for residential buildings; the ventilation air contributes both sensible and latent load to the conditioning system. A full Manual J adds the ventilation load explicitly. This calculator does not separately model ventilation load because (a) ventilation strategy varies widely (exhaust-only, supply-only, balanced HRV / ERV), (b) the implementation depends on the building s air-tightness target, (c) the calculator s building-factor approach implicitly assumes typical infiltration that approximates code-minimum ventilation. For high-performance / tight-envelope buildings (ACH50 ≤ 3.0) the ventilation load is a non-trivial fraction of total load and must be explicitly modeled in Manual J.

The 26 USC § 25C Energy Efficient Home Improvement Credit (post-IRA 2022 expansion) requires equipment to meet elevated SEER2 / HSPF2 / EER2 efficiency thresholds set annually by the DOE — for example, central air conditioning must meet SEER2 ≥ 17 (or the equivalent regional efficiency) and split-system heat pumps must meet HSPF2 ≥ 9.0 to qualify for the 30% credit up to the $2,000 heat-pump cap. The CREDIT itself does not depend on the load calculation, but the equipment SIZING does — credit eligibility requires that the equipment be correctly sized per ACCA Manual J / S, and many state programs require a Manual J calculation to be on file. This calculator s output is not sufficient for credit verification; commission a full Manual J / S from a licensed HVAC contractor as part of the credit-eligibility documentation, and consult a licensed CPA familiar with energy-credit practice.

Resources

Links marked sponsoredmay earn The Fennec Lab a commission. They do not affect the calculator's output. See disclosures.

ACCA — Manual J Residential Load Calculation (Standard 2-2016) — ACCA Manual J / ANSI Standard 2-2016 — the residential HVAC industry s required heat-loss / heat-gain analysis; the analytical basis for the simplification implemented in this calculator and the binding standard for permitting under IRC 2024 Chapter M.
ACCA — Manual S Residential Equipment Selection — ACCA Manual S — the binding equipment-selection standard that converts Manual J load output to actual equipment selection; the source for the ±5% cooling / −10% to +15% heating recommended-band logic used in this calculator s output.
ACCA — Manual D Residential Duct Design — ACCA Manual D — residential duct-design standard; the third leg of the Manual J / S / D stack required for new-construction and replacement-equipment permitting under IRC 2024 Chapter M.
IECC 2024 — Climate Zone Map and Insulation Requirements — International Energy Conservation Code 2024 — the climate-zone classifications used in this calculator (zones 1-8) and the code-minimum wall and ceiling insulation R-values by zone.
IRC 2024 Chapter M — Mechanical (Residential) — International Residential Code 2024, Chapter M (Mechanical) — the permitting backdrop that requires Manual J / S / D for new construction and equipment replacement in adopting jurisdictions.
ASHRAE — Handbook of Fundamentals — ASHRAE Handbook of Fundamentals — the source for the heat transfer coefficients (window U-values, SHGC, infiltration rates) and design temperature data underlying Manual J and this calculator s simplification.
DOE — Building America Solution Center — DOE Building America Solution Center — climate-zone-specific HVAC design guidance, including load calculation methodology, equipment selection, and duct-design best practices for residential construction.