Heat loss calculator

What a heat loss calculation actually tells you

Heat loss is the rate at which your house bleeds energy when the outdoor temperature is at your local winter design value. The unit is BTU per hour. That number is the single most important input to picking a furnace or heat pump. Buy equipment too small and the house will not stay warm on the coldest days. Buy equipment too big (the more common mistake by a wide margin) and the system short-cycles, runs at low efficiency, costs more, and leaves rooms uncomfortable.

The calculation comes from ACCA Manual J 8th Edition, Chapter 6. It sums conductive heat loss through every above-grade surface, conductive loss through floor or foundation, slab edge loss for slab-on-grade homes, and infiltration loss from air that leaks in around windows, doors, and joints. The math itself is straightforward once you have the inputs. The hard part is getting the inputs right, which is what most online heat loss calculators get wrong.

How the heat loss formula works

Conductive loss through any surface uses the same formula:

• Q = U × A × ΔT where Q is heat loss in BTU/hr, U is the inverse of R-value (1/R), A is surface area in square feet, and ΔT is the difference between your indoor design temperature and the outdoor design temperature.

Infiltration loss uses the air-change method:

• Q = 0.018 × V × ACH × ΔT where V is the conditioned building volume in cubic feet, ACH is air changes per hour (natural, not ACH50), and 0.018 is the product of air density and specific heat at standard conditions (BTU per cubic foot per °F).

Slab edge loss is a different beast. Heat does not lose through the slab face the way it does through a wall. Instead, it loses around the perimeter where the slab meets the outdoors. ASHRAE Fundamentals Chapter 18 publishes F-factors (BTU/hr per linear foot per °F) for typical residential edge insulation levels. A bare slab on grade uses F = 0.84. A slab with R-5 perimeter insulation drops to F = 0.55. The calculator handles both.

Getting the outdoor design temperature right

Your outdoor design temperature is NOT the lowest temperature on record. It is the 99 percent design value: the outdoor temperature that is exceeded (warmer than) 99 percent of the heating hours in a typical year. The ACCA Manual J Table 1A and the IECC Appendix C publish these by ZIP code or city. Some examples to calibrate:

• Phoenix, AZ: 37°F
• Atlanta, GA: 23°F
• Dallas, TX: 22°F
• Seattle, WA: 26°F
• Denver, CO: 5°F
• Chicago, IL: -2°F
• Boston, MA: 9°F
• Minneapolis, MN: -11°F

Indoor design temperature is conventionally 68 to 70°F. Going higher (75°F bedroom setpoints in a cold climate) inflates your load and pushes equipment selection up a size for a comfort preference that almost no one actually wants once they see the operating cost.

Infiltration: the input that breaks most online heat loss calculators

Infiltration is the single largest source of error in DIY load calculations. Manual J Table 5A gives three default tightness levels for natural ACH: 0.4 for tight (post-2010 build, blower-door tested under 3 ACH50), 0.8 for average (1990s to 2009 construction, standard practice), and 1.2 for loose (pre-1990, single pane windows, no air sealing). For a 1,500 square foot 2-story home with 9-foot ceilings, that defaults span gives a 3x range in infiltration loss, which can swing total heat loss by 15 to 25 percent.

If you have a blower-door test result (ACH50), use the ACH50 mode in the calculator instead. The Sherman-Grimsrud N-factor converts blower-door ACH50 to natural ACH based on your climate zone and number of stories. N typically lands between 14 and 25 for residential. A house tested at 5 ACH50 in climate zone 4 with 1 story converts to roughly 0.28 natural ACH, which is tighter than the Manual J "tight" default. Tight houses use much less infiltration heat than the defaults assume.

Why the component breakdown matters more than the total

The total BTU/hr number tells you what size equipment to buy. The component breakdown tells you where your retrofit dollars should go. Most retrofits sold to homeowners are picked based on what is most visible (windows) rather than what actually loses the most heat. The calculator's breakdown lets you check before you spend.

Typical breakdowns we see in residential calculations:

• Walls: 15 to 25 percent. Hard to improve without invasive retrofit. Skip unless re-siding.
• Windows: 20 to 35 percent. High visibility but expensive per BTU saved. Storm windows are 80 percent of the benefit at 20 percent of the cost.
• Doors: 2 to 5 percent. Usually trivial. Replace only if rotted, never for energy alone.
• Ceiling / attic: 10 to 20 percent. The cheapest BTU you can save. Adding R-30 of blown cellulose over existing R-19 is often paid back in 3 to 5 years.
• Floor or slab: 5 to 15 percent for crawlspace floors. Up to 20 percent for slab-on-grade homes without edge insulation.
• Infiltration: 15 to 35 percent. Air sealing (caulk, weatherstrip, can foam around penetrations) saves 20 to 50 percent of infiltration loss at very low cost. Blower-door directed sealing is the highest-ROI retrofit on most homes.

Using heat loss output for equipment selection

Manual J calls for heating equipment sized at 100 to 115 percent of design heat loss. The calculator returns both numbers as a recommended range. Furnace and heat pump nameplate capacities are typically published at heating output, not input, so multiply gas furnace input by AFUE to get usable BTU/hr. A 80,000 BTU/hr input gas furnace at 95 AFUE delivers 76,000 BTU/hr of usable heat.

Heat pumps need a second look. Heat pump capacity drops as outdoor temperature falls. A 36,000 BTU/hr nominal heat pump might deliver only 22,000 BTU/hr at 17°F and even less at your local design temperature. Use the heat pump sizing calculator with this page's heat loss output as the input, then check the AHRI 17°F or 5°F capacity from the manufacturer for the specific model.

The oversizing flag and why most heating equipment is too big

If you enter your existing equipment's nameplate BTU/hr, the calculator flags anything that is 1.5x or more above your design load. Field studies consistently find that 70 to 90 percent of residential furnaces are oversized, often by 2x or more. The cause is usually one of three things: contractor used a rule of thumb instead of a load calculation, contractor matched the existing equipment without questioning the original sizing, or the homeowner picked the same size during a panicked emergency replacement.

Oversized equipment short cycles. A furnace runs for 4 to 6 minutes, hits setpoint, shuts off, and the house cools back down before the next call. AFUE ratings assume long cycles, so real-world efficiency drops 5 to 15 percent below nameplate. Heat pumps suffer even more because they need to ramp up to steady-state coefficients of performance, which short cycles never reach. The fix is right-sized replacement next time, not modulating the existing oversized unit (you cannot modulate a single-stage furnace).

Where heat loss fits in the load calculation family

Heat loss is the heating side of a full load calculation. The HVAC load calculator (Manual J) adds the cooling side: solar gain through windows, sensible and latent gain from people and appliances, and conduction in the cooling direction. For furnace-only sizing the heat loss page is sufficient. For heat pump sizing you need both heating and cooling load, because the heat pump has to handle both seasons with one piece of equipment.

Once you have heat loss, related tools become useful: the furnace sizing calculator uses heat loss + fuel type + AFUE to recommend specific nameplate sizes, gas pipe sizing uses BTU input to size the gas line, and operating cost uses annual BTU (heat loss x heating hours) to estimate annual fuel cost across heating technologies.