Heating System Fundamentals and Building Science
Systems, not boxes
A heating system is a boundary problem: the enclosure creates the load and the equipment only meets it. Candidates should be fluent in conduction, convection, and radiation; in R-value (resistance) versus U-factor (conductance, U = 1/R); in thermal bridging through framing; and in the difference between the air barrier and the vapor retarder. Pressure across a boundary is driven by stack effect, wind, and mechanical fans, and that pressure moves both heat and moisture.
Sensible heat changes dry-bulb temperature; latent heat changes moisture content. One Btu raises one pound of water 1°F, and one watt equals roughly 3.41 Btu/h. Design heating load is calculated at a local design outdoor temperature with stated indoor setpoints, not at the coldest day on record.
Traps
- Confusing AFUE (a seasonal, whole-system rating) with steady-state combustion or thermal efficiency measured in the field.
- Treating R-values as additive when framing, corners, and penetrations create parallel paths with much lower resistance.
- Assuming insulation alone stops air leakage, or that a vapor retarder is an air barrier.
- Oversizing: short cycling, higher distribution losses, and comfort complaints even when the load calculation is correct.
Worked example
A wall has R-13 cavity insulation plus R-5 continuous sheathing, so the nominal assembly is R-18 and U = 1/18 = 0.056 Btu/h·ft²·°F. Through 100 ft² at a 50°F indoor-outdoor difference: Q = U × A × ΔT = 0.056 × 100 × 50 ≈ 280 Btu/h. Real framing bridges raise the true U, so field performance is worse than the cavity math suggests. Rule of thumb: for sensible airflow, CFM = Btu/h ÷ (1.08 × ΔT).