Between Walls and Windows: The Heat We Keep and the Cold That Gets In
Before we talk compressors, condensers, or the guts of the machine, we need to talk about the space we’re trying to protect. The fortress. The home. Understanding heat gain and heat loss is like understanding how the tide moves before setting sail. It’s not glamorous, but it’s the foundation of all comfort.
Heat Gain vs. Heat Loss
- Heat Gain is the amount of heat that enters a building—through windows, walls, roofs, doors, infiltration, and internal loads like people, lights, and appliances.
- Heat Loss is the amount of heat that escapes a building, typically in the winter, through similar paths in reverse.
Both tell us what size, type, and efficiency rating of system we need—not just to maintain comfort, but to avoid overworking the equipment and running up energy bills.
Why Heat Calculations Matter
If you don’t know what’s coming in or going out, you’re guessing with thousands of dollars. Oversize the system? You waste energy and never properly dehumidify. Undersize it? You chase your tail in July and freeze in February.
Four Key Factors in Heat Calculations
- Building Orientation and Location
- Insulation Values (U and R values)
- Construction Materials
- Windows and Doors (Size, Orientation, Type)
Interior vs. Exterior Walls
- Interior Walls separate rooms inside a structure and typically don’t contribute to heat loss or gain.
- Exterior Walls are in direct contact with the outside and significantly affect thermal transfer.
Using Charts for Design Temperatures
Design temperature charts (from ASHRAE or local climate data) give you the average low and high temps for heating and cooling loads in your area.
Example: Miami might use 91°F for cooling and 47°F for heating.
Temperature Differentials
Cooling and heating load calculations are based on the difference between indoor design temp and outdoor design temp.
- Cooling Differential = Indoor Temp (75°F) – Outdoor Design Temp (91°F) = 16°F
- Heating Differential = Indoor Temp (70°F) – Outdoor Design Temp (47°F) = 23°F
U-Value and R-Value
- U-Value: Measures how well a material conducts heat. Lower = better insulation.
- R-Value: Measures resistance to heat flow. Higher = better insulation.
They are inverses. R = 1/U. Think of U-value as how much heat sneaks in, and R-value as how stubborn the wall is about letting it.
Using Tables to Determine U-Values
ASHRAE or manufacturer tables list typical U-values for materials like:
- Brick veneer with batt insulation
- Wood frame walls
- Double-glazed windows Match your construction style to the table, find your U, and use it in your heat transfer equations.
Heat Transfer Through Construction Members
Q = U x A x ΔT
- Q = Heat transfer rate (BTUs/hour)
- U = U-value of the material
- A = Surface area in square feet
- ΔT = Temperature difference across that surface
Heat Loss Factors
- Window and door leakage
- Poor insulation
- Large surface areas
- High ΔT
Net Wall Area vs. Gross Wall Area
- Gross Wall Area = Total area of a wall, including windows and doors.
- Net Wall Area = Gross wall area minus doors and windows. Used when calculating heat loss through just the wall material.
Heat Gain Factors
- Solar radiation through windows
- Internal loads (people, lights, equipment)
- Orientation (south-facing glass gets cooked)
- Ventilation/infiltration
How Window Orientation Affects Load
- East-Facing: Morning sun, short but intense load
- South-Facing: Consistent sun all day (in northern hemisphere)
- West-Facing: Afternoon blaze—worst for cooling loads
- North-Facing: Minimal direct solar gain
Heat gain and heat loss aren’t just numbers—they’re the story of a building’s relationship with its environment. Before you can fix the system, you’ve got to know what it’s up against. Next up: fuel, flame, and the soul of the furnace.