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Heat Load Calculator: Heating Load by County

ENERGY STAR county design temperatures, IRC N1102.4.1.2 air leakage, conduction plus infiltration

A Manual J needs a design temperature, the envelope and the air a house leaks. ENERGY STAR publishes a heating and cooling design temperature for every one of 3,141 counties, the residential code caps air leakage at 5.0 air changes an hour at 50 pascals, and the heat air carries is a published constant. This page adds them up into a heating load, shows how much of it is air leakage, and names everything a real Manual J adds.

Single storey box: floor area from length and width, walls from perimeter and height. Indoor 70°F heating and 75°F cooling, as the ENERGY STAR HVAC Design Report requires.

Worked example

A 50 by 40 ft single storey house in Cook County, Illinois, 9 ft walls, R-19 walls and R-38 ceiling (the DOE guide's Chicago house), 240 sq ft of U-0.30 windows, at the code's 5.0 ACH50.

Dividing by 26.5 instead, the DOE guide's ratio, takes the air leakage part to 4,463 BTU/h. The DOE guide's own Chicago house, a different and larger design with a conditioned basement, came to 41,700 BTU/h in Manual J.

The formula

Two terms, both at the heating temperature difference:

No solar or internal gains are credited on the heating side; the DOE guide notes the peak heat loss comes at night.

Why this is a heating load and not a Manual J

Manual J is published by ACCA and is not free to read, so this page does not claim to follow it.

What it does use is public: ENERGY STAR's county design temperatures and indoor conditions, the residential code's air leakage limit, NREL's air properties and the DOE Building America guide to load calculations. Each is named where it is used.

The design temperature for every county, and why it is a limit

Air leakage: a code limit, and a conversion with no physics behind it

What one colder number does to the load

How to use the number

The sources, and what a Manual J adds

Frequently asked questions

What is a heat load calculation?

The heat a house loses on a design winter night: conduction through walls, ceiling, floor, windows and doors, plus the air that leaks in, each multiplied by the difference between indoors and the design outdoor temperature. ACCA Manual J is the residential method the code refers to.

What design temperature should I use for my county?

ENERGY STAR publishes a 99% heating and 1% cooling limit for every county, from ASHRAE and Manual J weather stations. It takes the coldest station within 40 miles and rounds down, so a designer may use a slightly milder figure from the nearest station, or your jurisdiction's own value.

How do you convert ACH50 to natural infiltration?

The common rule divides by 20. NIST and LBNL researchers traced it to early 1980s studies and found no physical basis for it. The DOE Building America guide's Chicago example pairs 5.04 ACH50 with 0.19 air changes, a ratio of 26.5. This page offers both.

How much heat does leaking air carry?

1.08 BTU per hour for every cfm and every degree of temperature difference: 0.075 lb per cubic foot times 0.240 Btu per lb per degree times 60 minutes, the air properties NREL's BESTEST-EX uses.

Is this a Manual J calculation?

No. It leaves out duct losses, mechanical ventilation, slab edge and basement wall losses, and on the cooling side solar gain, internal gains and moisture. It is a heating load built from public data, labelled as such.

Should I add a safety factor?

The DOE guide shows why not: moving only the design conditions on its Chicago house raised the heating load from 41,700 to 52,700 BTU/h, 26 per cent, and its Orlando house by 60 per cent. The county limit this page uses already leans cold.

Check these numbers yourself

• Design Temperature Limit Reference Guide (2019 Edition)ENERGY STAR Single-Family New Homes and Multifamily New Construction, US EPA, revised 4/27/2021. Exhibit 1, 1% cooling and 99% heating design temperature limits for every county, and the method used to assign them.
• National HVAC Design Report, Version 3.1 / 3.2 / 3.3 (Rev. 14)ENERGY STAR Single-Family New Homes, US EPA, revised 01/15/2025. Item 3.2, indoor design temperatures of 70 F heating and 75 F cooling.
• Strategy Guideline: Accurate Heating and Cooling Load CalculationsArlan Burdick, IBACOS, for the US Department of Energy Building America program, NREL, June 2011. Baseline and manipulated design conditions for a Chicago and an Orlando house, the resulting loads, and the baseline infiltration rates.
• The Origin and Application of Leakage-Infiltration RatiosBenjamin Jones, Andrew Persily (NIST) and Max Sherman (LBNL). Where the divide-by-20 rule came from and why it should be used for quick estimates at best.
• Building Energy Simulation Test for Existing Homes (BESTEST-EX), Phase 1Judkoff, Polly, Bianchi and Neymark, NREL/TP-550-47427, August 2010. Air properties for infiltration conductance: specific heat 0.240 Btu/(lb F), density 0.075 lb/ft3 at sea level.
• Utah Residential Code 2021, Chapter 11 Energy EfficiencyInternational Residential Code 2021 as adopted with amendments in Utah, read through UpCodes with the jurisdiction and edition stated on the page. Section N1102.4.1.2, the 5.0 ACH50 air leakage limit. Virginia's 2021 adoption also says five.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-13. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

This is not a Manual J load calculation. Design temperatures are ENERGY STAR limits, the air leakage limit is quoted from Utah's 2021 adoption of the residential code, read on 13 September 2026; your jurisdiction may publish its own values. Duct losses, ventilation and cooling gains other than conduction are not included.