Methodology & sources

Every number the software shows comes from the published methods below. Where the legacy Comfort Institute software used undocumented constants, this page says so and names the assumption we use instead. Retrotec administrators can tune the constants marked tunable.

1. Fan flow to CFM50

Readings taken at a house pressure other than 50 Pa are converted with the power law CFM50 = CFM × (50 / P)n, n = 0.65 tunable. Optional corrections (off by default, matching the legacy software): air density × √(Tout/Tin) for depressurization (inverse for pressurization; ideal-gas density ratio) and altitude × (1 + 0.000006 × ft).

2. Total leakage area

The legacy "Total Leakage Area" is the equivalent leakage area at 25 Pa: the sharp-edged orifice (C_d = 0.611, standard air 1.204 kg/m³) that passes the same flow at 25 Pa. It works out to 5.383 CFM per square inch, the same basis as the flow-hood duct test. For 1,900 CFM50 this gives 225 sq in, matching the legacy result of 224.91. The reports also list the CGSB EqLA at 10 Pa and the ASTM E779 / LBL ELA at 4 Pa (≈ CFM50 × 0.055).

3. Air changes per hour and natural air change

ACH50 = CFM50 × 60 / volume. Natural (annual average) air change uses the Lawrence Berkeley Laboratory correlation ACHnat = ACH50 / N, N = C × H × S × L: climate factor C by LBL zone (1: 15.5, 2: 18.5, 3: 21.5, 4: 24.5), height H (1 story 1.0, 1.5 → 0.9, 2 → 0.8, 3 → 0.7), wind shielding S (well shielded 1.2, normal 1.0, exposed 0.9) and leakiness L (default 1.0). "Air changes per day" = ACH_nat × 24. Source: Sherman, Lawrence Berkeley Laboratory, Estimation of Infiltration from Leakage and Climate Indicators (Energy and Buildings, 1987), and the widely published four-zone N-factor table. The published four-zone table below is exactly C × H × S.

The LBL zone is found from the client's location with polygons traced from the published LBL four-zone infiltration map (zone 1 = northern plains, zone 4 = mild coasts). Boundaries are approximate; the zone can always be overridden.

4. Recommended air changes and needed leakage area

The recommended fresh-air rate is ASHRAE 62.2 Qtot = 0.03 × floor area + 7.5 × (bedrooms + 1) CFM tunable, expressed as air changes per day. (With 3 bedrooms and 1,500 sq ft this reproduces the legacy 9.0 per day.) The "needed" leakage area is the measured area scaled by recommended ÷ measured air changes. When bedrooms are unknown the fallback is 0.35 ACH. The 62.2 fan requirement shown uses Qinf = 0.052 × Q50 × wsf × (H/8.2)0.4 and Qfan = Qtot − Φ(Qinf − Qtot/3) with a default wsf of 0.5.

5. Energy code check

ACH50 is compared with the statewide residential blower door limit for the client's US state (Retrotec's state energy-code table, 51 rows: limit, test requirement, code edition). Canadian provinces and states without a statewide limit show "no statewide requirement".

6. Design infiltration for Manual J

From the LBL infiltration model in ASHRAE Fundamentals (1997, ch. 25), as used for ACCA Manual J design infiltration: ELA = CFM50 × 0.055 sq in, Q = ELA × √(A·ΔT + B·V²) with stack coefficient A by stories (0.0156 / 0.0313 / 0.0471) and wind coefficient B by shielding class 1–5, V = 15 mph heating / 7.5 mph cooling, ΔT from the selected design-conditions location. A published worked example (2,350 CFM50, 2 story, class 4, ΔT 82 °F, 20 mph, 30,000 ft³ → 277 CFM, 0.55 ACH) is a unit test.

7. Flow hood duct leakage

Register readings are summed (converted to 25 Pa with n = 0.65 if the duct pressure differs) and turned into square inches with the same 25 Pa orifice basis. The new-construction standard is 6 % of 400 CFM per ton (24 CFM25 per ton); the alternative maximum is 3 CFM25 per 100 sq ft of living area tunable (legacy value; IECC 2015+ total leakage limit is 4).

8. Efficiency analysis

The legacy constants are not published, so this is a documented Retrotec model. Leak flow = sq in × CFM per sq in at the duct operating pressure (default 25 Pa). Return leaks pull unconditioned air: attic 130 / 115 / 100 °F for none / partial / heavy shading in summer, outdoor design temperature in winter; crawlspace ±10 °F from outdoor; basement 65 / 60 °F; garage and outdoors at design temperature. Cooling loss = 1.08 × CFM × ΔT (+ 0.68 × CFM × grains for latent) plus supply loss of conditioned air at 55 °F; heating loss uses 120 °F furnace / 95 °F heat pump supply air, and capacity = input × AFUE (0.70 / 0.80 / 0.92). Seasonal SEER and AFUE degrade = peak loss % × 0.75. All of these are tunable. For the legacy example (23 + 15 sq in, 3 tons, 80,000 BTU/h mid-efficiency furnace, Kalispell) the model gives 0.76 tons / 25 % cooling loss vs. the legacy 0.73 / 24 %, and a higher heating loss (23 % vs 18 %).

9. Data

• Outdoor design conditions: 2,125 US and Canadian locations (ACCA Manual J Table 1A / 1B layout: 99 % winter and 1 % summer dry bulb, coincident wet bulb, design grains at 55/50/45 % RH, daily range, elevation). Two wet-bulb typos in the source were corrected (Huntsville AP, Crossville). Station coordinates were geocoded with OpenStreetMap Nominatim for the "nearest station" lookup. © ACCA / ASHRAE for the underlying tables; used for members' load calculations.
• State energy codes: Retrotec compilation from ACEEE, DOE energycodes.gov and state code sites (2025).
• Maps: © OpenStreetMap contributors.

10. Current constants