A Manual J calculation is a room-by-room heat loss and heat gain estimate that determines how much heating and cooling a home needs, based on insulation, windows, air leakage, orientation, and local design temperatures. Developed by ACCA, it is the basis for correctly sizing furnaces, heat pumps, and air conditioners, instead of guessing from square footage.
Why it matters to you
Systems sized by rule of thumb or by the old unit's size are often oversized, which leads to short-cycling and poor humidity control. In Canada, residential heat loss and heat gain calculations are commonly done to the CSA F280 standard, which serves the same purpose as Manual J.
Practical guidance
Ask every contractor for a heat-loss calculation, or written confirmation that one was done, and ask how they arrived at the proposed size. Big Valley sizes from a heat-loss calculation, and its technicians are TECA certified.