What should you ask before accepting a heat-pump size?
The right question is not “How many tons for this floor area?” It is whether the proposed system matches the home’s calculated heating and cooling loads, local design conditions, ductwork, and the equipment’s published performance.
The sizing evidence to request
| Question | Evidence that answers it | Why it matters |
|---|---|---|
| How was the heating and cooling load calculated? | A documented room-by-room and whole-house calculation using measured envelope, windows, orientation, infiltration, and design temperatures. | Floor area and rules of thumb do not show the actual peak demand. |
| What capacity does the model deliver at design temperature? | Manufacturer performance data at the local outdoor design condition, not only a nominal label. | Capacity changes as outdoor temperature changes. |
| What happens at the lowest load? | Minimum modulation or steady-state capacity and expected cycling behavior. | A system that cannot turn down may cycle and lose comfort. |
| Are ducts and airflow part of the design? | Duct condition, sizing, room airflow, return path, and any changes required. | Equipment capacity cannot be delivered through an unsuitable distribution system. |
Read the proposal in sequence
- Confirm the climate, indoor set points, insulation, windows, shading, and air leakage assumptions.
- Ask for heating and cooling loads separately. A heat pump must address both modes, including humidity where relevant.
- Compare the selected model’s capacity at design conditions with those loads. Ask how supplemental heat is controlled and when it runs.
- Check the minimum capacity, thermostat, defrost strategy, electrical service, and airflow requirements.
- Get the final model numbers, performance tables, duct changes, commissioning checks, and warranty terms in writing.
Worked example: why the label is not enough
A proposal may call a unit “three ton,” but that nominal number is not a complete heating answer. The model may deliver a different capacity at a cold outdoor temperature, and its minimum output may be too high for mild weather. Compare the load calculation and the performance table at the same conditions; do not compare a label with a whole-home square-footage estimate.
Stop signs
- A size selected solely from floor area or the old furnace nameplate.
- Unexplained safety factors or “bigger is safer” reasoning.
- No answer about ducts, airflow, backup heat, or low-load cycling.
- Capacity claims copied from a nominal rating without outdoor conditions.
Boundary: sizing and electrical work require a qualified designer or contractor familiar with local code. Do not alter refrigerant, wiring, combustion, or backup-heat controls from a worksheet.
Use the contractor question sheet to compare proposals and the moisture guide if the proposed work changes attic ducts or indoor humidity.
Sources: U.S. Department of Energy: accurate heating and cooling load calculations and DOE guide to sizing and selecting cold-climate air-source heat pumps.