Current assumptions
Modeled heat loss outside the house27–41%About 11,600–17,600 BTU/h at these operating assumptions.
Where could your duct heat be going?
Compare poor insulation and air leakage without a long questionnaire. Results update on this page. This first version covers heating, not cooling or humidity loads. No login, contact details or submission required.
Illustrative heating scenario—not a measurement of your home.
The starting example is 10 supply outlets, 2 returns, attic ducts, R-4 insulation and assumed 10–15% leakage on each side. Change it freely. Nothing here selects equipment size or prices the installation.
Counts create a rough example of duct surface area, not a survey. If unsure, keep the example or adjust the surface areas below. Count only visible grilles; do not open ducts or enter unsafe spaces.
Use current variable energy spending for heating with this one system only. Exclude cooling, hot water, other appliances, fixed utility charges and maintenance. Leave blank rather than using your whole utility bill.
Adjust technical assumptions · optional
These are operating scenarios, not recommended settings. Defaults are 1,000 CFM and a 40°F rise across the heating equipment; a heat pump may operate at a different rise. Change these only to compare assumptions or use verified readings.
Use operating air-handler flow, not a CFM25 duct-leakage test result. This tool does not convert test pressure to operating leakage.
Compare the same system.
Heating only. Scenario ranges, not confidence intervals or guaranteed savings.
Insulation only
Modeled heat loss outside the house23–36%About 9,700–15,400 BTU/h at these operating assumptions.
Potential reduction in heating energy cost6–8%Add heating-only energy spending above for a dollar scenario.
Sealing only
Modeled heat loss outside the house17–22%About 7,500–9,400 BTU/h at these operating assumptions.
Potential reduction in heating energy cost12–24%Add heating-only energy spending above for a dollar scenario.
Both improvements
Modeled heat loss outside the house13–16%About 5,400–6,700 BTU/h at these operating assumptions.
Potential reduction in heating energy cost17–30%Add heating-only energy spending above for a dollar scenario.
Modeled surfaces: 493 sq.ft. supply + 144 sq.ft. return. Assumed equipment heat output: 43,200 BTU/h. These values are not a load calculation.
The upgrade scenarios use R-8 insulation and/or 3% leakage per side—not a required specification or promised result. Dollar results annualize this operating scenario only if it represents your heating season, with the same useful heating load, equipment efficiency and energy rates. They are not a weather-calibrated annual forecast or a payback estimate.
A transparent scenario, not a promise.
One example network
Each supply count represents 25 feet of 6-inch round duct plus 100 sq.ft. of shared supply trunk. Each return represents 15 feet of 12-inch round duct plus 50 sq.ft. of shared return trunk. Trunks are added once, not per grille. These are editable geometric assumptions, not standard lengths or a real duct design. Counts do not set the equipment airflow.
Location changes the result
The room is modeled at 70°F. Attic scenarios use 30–45°F around ducts with no regained heat; unconditioned basements use 45–60°F with 50% of escaped heat regained by the house. Inside scenarios assume all heat is regained. “Not sure” spans 30–60°F and 0–100% regain. These are illustrative ranges, not local weather records.
Heat transfer and leakage interact
The model calculates return-air mixing, then heat transfer through return ducts, heating at the equipment and heat transfer through supply ducts, followed by supply leakage. Supply and return leakage fractions are assumed equal. It compares each improvement against the same baseline instead of adding two savings percentages.
Insulation assumptions
Effective thermal resistance is nominal insulation R-value plus an assumed R-2 for surface films. That addition is our simplification, not a universal rule. “Not sure” spans bare metal through R-6. The improved scenario uses R-8, without reducing existing insulation. Wet, crushed or damaged insulation needs inspection.
No leakage measurement is inferred
The selected percentages are operating assumptions on each side, not a diagnosis or a conversion from a duct test. CFM25 is leakage measured at a test pressure of 25 pascals and is not a direct operating-flow input here. Imbalanced supply/return leakage, envelope infiltration and fan-power changes are not modeled.
Money stays conditional
Optional dollar savings use current heating-only variable energy spending × (1 − baseline delivery efficiency ÷ improved delivery efficiency). Load, equipment efficiency and energy prices stay the same. This annualizes the selected conditions; actual seasonal results need weather, equipment operation and measured duct data. No cooling, rebates, installation cost or payback is calculated.
Heat transfer uses an exponential duct-temperature model with air heat capacity of 1.08 × CFM in BTU/h per °F. Invalid or out-of-scope combinations receive a review message rather than a forced result. This is not an ASHRAE 152 implementation, certified energy model or substitute for an inspection.
Technical background: Berkeley Lab — thermal distribution efficiency; PNNL — duct leakage tests. These references explain the underlying issues; they do not validate this calculator or your selected assumptions.