Heat Pump Sizing Quick-Check

The tool estimates a target capacity by starting with a baseline load factor per square foot (default 20 BTU/hr per sq ft), then applying adjustments for insulation quality, climate severity, and your comfort goal. It multiplies those factors to produce a target BTU/hr and an allowed “reasonable” range (±15%). Finally, it compares your proposed capacity to the target and reports the percent difference plus a verdict (likely undersized / likely oversized / likely reasonable).

Because this is a quick-check, it doesn’t include common drivers of real-world load like window area/orientation, air leakage, ceiling height, duct losses, zoning, humidity needs, or internal gains (sun, appliances, occupants). Also, heat pumps deliver different usable capacity at outdoor temperatures; a contractor’s “proposed capacity” should represent actual usable heating capacity for your conditions. If your home has unusual features (very high ceilings, leaky envelope, lots of south-facing glass), the target may shift and a Manual J becomes more important.

If the proposed capacity is more than 15% below the estimated target, the tool flags “Likely undersized”; more than 15% above, it flags “Likely oversized.” If it’s beyond the stricter thresholds (below 85% of the recommended range low, or above 115% of the range high), it marks as strongly undersized/oversized. For extreme home sizes (near the ends of the input range) or if your proposed system is far outside the expected window, treat this as a trigger to request a full Manual J load calculation rather than a final decision.

The calculator validates that home size and proposed capacity are positive and within the allowed ranges; invalid values won’t produce meaningful results. If you enter a home size or system size that’s extremely small/large relative to typical homes, it will show a caution that the quick-check is less reliable. If your selected insulation/climate/comfort options don’t match the allowed choices, the tool can’t apply the correct adjustments—so double-check your selections.