Source: https://guthlabs.ai/docs/instruments/hvac-heating-cooling-load-estimator-envelope-method/mechanism/

instrument mechanism / verified-clean

# HVAC Heating/Cooling Load Estimator (Envelope Method) mechanism

The inspectable method record for HVAC Heating/Cooling Load Estimator (Envelope Method), kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The arithmetic algorithms are standard for a rough envelope-only estimate, but the spec needs explicit unit selection, heating/cooling sign convention, air-property convention for the infiltration factors, and validation rules. Verdict is NEEDS_REVIEW because those ambiguities materially affect computed outputs. The stated honest label is appropriate but should also explicitly say that solar, internal, latent, duct, ground, and thermal-bridge loads are excluded.

- Q=U·A·ΔT is the standard steady-state envelope conduction method when U, area, and temperature difference use compatible units.

- Heating and cooling loads must be separated by direction: use indoor minus outdoor for heating and outdoor minus indoor for cooling, with the non-applicable load reported as zero. The specification does not explicitly define this sign convention.

- The infiltration constants are approximate air-property conversions, not universal constants. 1.08 requires CFM and degrees Fahrenheit and assumes approximately 0.075 lb/ft³ air density and 0.24 Btu/(lb·°F) specific heat. 1.23 requires L/s and degrees Celsius or kelvin temperature difference and assumes approximately 1.225 kg/m³ air density and 1005 J/(kg·K) specific heat.

- A typical U-value table is construction-dependent; U-values must identify assembly, insulation level, framing, glazing, films, and whether the value is center-of-glass or whole-assembly.

- The method omits solar gains, internal gains, latent loads, thermal bridges, ground-contact conduction, duct losses, ventilation requirements, thermal storage, and equipment/system effects; it is not a complete ACCA Manual J calculation.

## Units and conventions

- US conduction: area in ft², U in Btu/(h·ft²·°F), temperature difference in °F, output in Btu/h.

- US infiltration: airflow in CFM, temperature difference in °F, output in Btu/h; 1.08 is an assumed standard-air conversion factor.

- SI conduction: area in m², U in W/(m²·K), temperature difference in K or °C difference, output in W.

- SI infiltration: airflow in L/s, temperature difference in K or °C difference, output in W; 1.23 assumes approximately 1.225 kg/m³ air density and 1005 J/(kg·K) specific heat.

- The vectors use heating load = positive conduction and infiltration when indoor temperature exceeds outdoor temperature, and cooling load = positive conduction and infiltration when outdoor temperature exceeds indoor temperature.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "system": "US heating",
    "surfaces": [
      {
        "name": "wall",
        "area_ft2": 800,
        "U_Btu_h_ft2_F": 0.07692307692307693
      },
      {
        "name": "window",
        "area_ft2": 100,
        "U_Btu_h_ft2_F": 0.5
      },
      {
        "name": "roof",
        "area_ft2": 800,
        "U_Btu_h_ft2_F": 0.05263157894736842
      }
    ],
    "indoor_temperature_F": 70,
    "outdoor_temperature_F": 20,
    "infiltration_CFM": 100
  },
  "expected_outputs": {
    "temperature_difference_F": 50,
    "conduction_Btu_h": 7682.186234818,
    "infiltration_Btu_h": 5400,
    "total_heating_load_Btu_h": 13082.186234818,
    "total_cooling_load_Btu_h": 0
  },
  "reference_data": {
    "wall_nominal_R_value_h_ft2_F_per_Btu": 13,
    "wall_U_Btu_h_ft2_F": "1/13 = 0.07692307692307693",
    "wall_source_convention": "Widely tabulated standard nominal insulation value, R-13",
    "roof_nominal_R_value_h_ft2_F_per_Btu": 19,
    "roof_U_Btu_h_ft2_F": "1/19 = 0.05263157894736842",
    "roof_source_convention": "Widely tabulated standard nominal insulation value, R-19",
    "window_U_Btu_h_ft2_F": 0.5,
    "window_source_convention": "Widely tabulated standard value for a representative modern double-glazed window",
    "US_infiltration_factor_Btu_h_per_CFM_F": 1.08,
    "infiltration_factor_source_convention": "Widely tabulated HVAC standard-air conversion value"
  },
  "working": "ΔT = 70 − 20 = 50°F. Wall = (1/13)·800·50 = 3076.923076923 Btu/h. Window = 0.50·100·50 = 2500 Btu/h. Roof = (1/19)·800·50 = 2105.263157895 Btu/h. Conduction = 3076.923076923 + 2500 + 2105.263157895 = 7682.186234818 Btu/h. Infiltration = 1.08·100·50 = 5400 Btu/h. Total heating = 7682.186234818 + 5400 = 13082.186234818 Btu/h. Since outdoor temperature is lower, cooling load = 0."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The extreme infiltration input produces a mathematical result of 1.08e310 Btu/h, which overflows ordinary IEEE-754 floating-point arithmetic to a non-finite value. The convention probe also exposes a sign ambiguity: reversing the definition of ΔT changes the reported result from +8960 to -8960 Btu/h even though the physical condition is heating.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/hvac-heating-cooling-load-estimator-envelope-method/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/hvac-heating-cooling-load-estimator-envelope-method/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/hvac-heating-cooling-load-estimator-envelope-method/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche7/tranche7-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`