Methodology & sources
Every formula, constant, and limitation of the sizing calculator, in the open. If you find an error, email support@whetstonetools.com.
1 · Heating load: a banded envelope-factor estimate
Design load = floor area × envelope factor × (70°F − design temp) × draftiness × duct factor
The envelope factor (Btu/h per sq ft per °F) comes in four tiers, each calibrated to published anchors — not retail folklore:
| Tier | Factor range | Calibration anchor |
|---|---|---|
| Tight (2015+ code / deep retrofit) | 0.10–0.14 | "Pretty Good House" ≈7.5 Btu/h·sqft at ΔT≈65–70°F (Energy Vanguard) |
| Code-built 1990s–2010s / older weatherized | 0.15–0.20 | ≈12–14 Btu/h·sqft at ΔT=70°F for an IRC-2018-level home (GreenBuildingAdvisor) |
| Average older (1960–1990) | 0.21–0.28 | CEE Minnesota field data: median pre-1990 home ≈41,000 Btu/h at ΔT≈83°F, ~1,800–2,000 sqft → ≈0.25 |
| Old & leaky (pre-1960) | 0.29–0.40 | upper practitioner band; verify with the fuel-bill method |
Draftiness: ±10–15% (NEEP's infiltration guidance: weatherized ACHnat < 0.4, efficient new < 0.1). Ducts in unconditioned space: +20% (the +15–25% practitioner band). The result is always shown as a range — the spread is the honesty: a single number from a web form is false precision.
Applicability: heat loss scales with envelope surface, not floor area. Outside ~1,200–3,500 sq ft — or for 1.5-story, cathedral-ceiling, or heavily glazed homes — the tiers degrade; trust the fuel-bill cross-check and a professional Manual J instead.
2 · Cross-checks (measured beats estimated)
- Fuel-bill method:
fuel used × heat content × AFUE ÷ HDD65 ÷ 24 × (65 − design temp). Heat contents per EIA: therm = 100,000 Btu; ccf = 103,600 Btu; oil = 138,500 Btu/gal; propane = 91,452 Btu/gal. AFUE tiers include 0.65 for pre-1985/standing-pilot equipment. Caveats the tool states: using climate-normal HDD adds ±10–15% in unusual winters; thermostat setbacks and wood stoves deflate the estimate; the result includes your current duct losses. Practitioners find this method reads low vs Manual J (solar/internal gains) by up to ~1.4× — so when the envelope estimate runs more than 1.4× the fuel-derived load, the tool flags the discrepancy and tells you to trust the lower, fuel-derived number. Measured fuel use is the stronger evidence; the envelope band yields to it, never the other way around. - Furnace bracketing (NRCAN): if the existing furnace runs near-continuously in cold snaps, design load ≈ furnace output ÷ 1.25–1.40.
3 · Design temperature
The 99% heating design temperature for your county, embedded from the public-domain EPA ENERGY STAR County-Level Design Temperature Reference Guide (derived from ASHRAE 2013 Handbook of Fundamentals and ACCA Manual J 8th-ed conditions; lowest-station-in-county rule, which can run conservative in large mountainous counties — hence the manual override). Indoor design temperature: 70°F (NEEP). The 99% temp is exceeded ≈88 hours/year; the tool says so wherever it matters.
4 · Load line & capacity interpolation
Heating load line: linear from the design load at design temp to zero at 60°F (NEEP Sizing Support Tools User Guide; PNNL/ACEEE: "often estimated to be about 60°F"). Unit capacity: piecewise linear interpolation between the certified 47/17/5°F points (PNNL Building America formula). Below 5°F the tool extrapolates with a declining slope floored so a flattened (clamped) curve can't stay flat and overstate capacity, and floored again at zero — for verdicts only — and flags that some units shut off entirely below their minimum operating temperature, a field public data does not include.
Capacity-data quirk we correct for: the ENERGY STAR dataset mixes rated and maximum capacity semantics between temperature points (a unit can show 5°F capacity above its 47°F rating). Interpolating across mixed points overstates capacity exactly where cold-climate buyers live (5–17°F). The tool therefore monotonizes conservatively — each colder point is clamped to no more than the next warmer point — for all verdict math, displays the raw published values separately with a flag, and never presents 5°F/47°F as an official "capacity retention" metric.
5 · Verdict bands
| Test | Band | Source |
|---|---|---|
| Sole-source pass | capacity at design temp ≥ 100% of the high end of the load band and ≤120% of the band midpoint | NEEP 100–115% sole-source. Conservative asymmetry is ours: adequacy is judged against the high end. Oversize is judged against the midpoint, not the low end — with bands this wide (~1.33–1.40× low-to-high) a low-end oversize test makes "pass" mathematically unreachable. |
| Well-sized range | 90–120% of design load | NEEP Sizing Support Tools User Guide |
| With-backup target | 75–100% of the band midpoint | NEEP sizing guide 75–85% supplemental approach |
| Oversize warning | >120% of the band midpoint | NEEP "avoid oversizing"; ACCA Manual S 3rd ed heating size factor limits |
Backup heat, stated precisely: with a dual-fuel furnace, the furnace carries the entire load below the switchover temperature (heat pump and furnace generally can't run together on one air handler). Electric strips top up only the gap — and gap-sized strips will not keep the house warm if the compressor fails; emergency-heat sizing is a separate decision. Strip heat of 10–15 kW is a 200-amp-panel conversation: involve an electrician.
Defrost: certified points are steady-state. In humid 20–40°F weather, defrost cycles reduce delivered capacity, so computed balance points are optimistic — the tool shows this caveat whenever it applies, not just in coastal areas.
6 · What the tool refuses to do (and why)
- Room-by-room loads, duct design (Manual D), or multizone head selection — NEEP: each zone must be analyzed independently; this tool can't.
- Cooling-side verification — a heating-sized unit is commonly 1.5–2× the cooling load, and public data lacks minimum-capacity (turndown) figures, so Manual S cooling limits (≤130%) and the minimum-compressor rule (≤0.80) cannot be checked here. The result card says so every time.
- Below-5°F capacity claims from public data, or any guarantee for non-standard homes.
- Final equipment selection. The standard is ACCA Manual J (8th ed) + Manual S (3rd ed) by a qualified contractor, ideally with blower-door-informed inputs.
Data sources
- Model data: EPA ENERGY STAR Certified Air-Source Heat Pumps (data.energystar.gov, dataset w7cv-9xjt; US-government open data, refreshed daily; cold-climate = capacity@5°F ≥ 70% of rated 47°F and COP@5°F ≥ 1.75). One row per outdoor unit (best-HSPF2 certified pairing); HSPF2 shown is Region IV.
- Design temperatures: EPA ENERGY STAR County-Level Design Temperature Reference Guide (public domain).
- Methodology: NEEP Cold-Climate ASHP Sizing Support Tools User Guide (Feb 2024); NEEP Guide to Sizing & Selecting ASHPs in Cold Climates; PNNL Building America Solution Center; ACEEE 2024 Summer Study (PNNL); ANSI/ACCA Manual S-2023 Addendum C; NRCAN ASHP Sizing and Selection Guide; EIA fuel heat contents; Energy Vanguard / GreenBuildingAdvisor practitioner calibration anchors.
- For richer min/rated/max capacity curves and below-5°F points, verify any unit on NEEP's ccASHP product list (data shown there is NEEP's; we link rather than embed it).
Disclaimer: estimates only — not engineering, installation, or purchasing advice. Heat pump performance varies with installation quality, refrigerant charge, ductwork, controls, and defrost behavior. Final selection requires a professional load calculation.