Heat loss and heat gain analysis is essential in HVAC design to determine the heating and cooling loads of a building. This ensures that the HVAC system is properly sized, leading to better comfort, energy efficiency, and cost savings.
In this article:
1. What is Heat Loss & Heat Gain in HVAC?
Heat Loss: The amount of heat that escapes a building during cold weather, requiring heating.
Heat Gain: The amount of heat that enters a building during hot weather, requiring cooling.
Both must be analyzed to select the right HVAC system size (Manual J calculation).
2. Factors Affecting Heat Loss & Heat Gain
| Factor | Impact on Heat Loss | Impact on Heat Gain |
|---|---|---|
| Building Size & Layout | More exterior surface area increases heat loss | Large windows & open spaces increase heat gain |
| Insulation (Walls, Roof, Floors) | Better insulation reduces heat loss | Helps reduce cooling load |
| Windows & Doors | Poorly insulated windows lose heat | Sunlight through windows adds heat (solar gain) |
| Climate & Outdoor Temperature | Colder climates = higher heat loss | Hot climates = more cooling needed |
| Air Infiltration (Leaks, Ventilation) | Gaps around doors/windows increase heat loss | Uncontrolled ventilation can bring in warm air |
| Internal Heat Sources | Not a major factor | Appliances, lighting, and occupants generate heat |
| HVAC Ductwork | Poorly insulated ducts lose heat | Leaky ducts allow hot air into cool zones |
3. Heat Loss Calculation (Winter Load Analysis)
🔹 Formula for Heat Loss:
Q=U×A×ΔT
Where:
- Q = Heat loss (BTU/hr)
- U = Overall heat transfer coefficient (depends on material insulation)
- A = Surface area (sq. ft)
- ΔT = Temperature difference (inside vs. outside)
Example:
- A 200 sq. ft. wall with U = 0.5 BTU/hr·ft²·°F
- Indoor temp: 70°F, Outdoor temp: 30°F
To calculate the heat loss (Q) through the wall, use the formula:
Q=U×A×ΔT
Where:
- U = 0.5 BTU/hr·ft²·°F (overall heat transfer coefficient)
- A = 200 ft² (wall area)
- ΔT = (Indoor temp – Outdoor temp) = 70°F – 30°F = 40°F
Calculation:
Q=0.5×200×40Q
Q=4000 BTU/hr
Final Answer:
The heat loss through the 200 sq. ft. wall is 4,000 BTU/hr.
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So, 4,000 BTU/hr is lost through this wall, requiring heating to maintain indoor comfort.
✅ Total heat loss = Sum of losses through walls, roof, windows, and air leakage.
4. Heat Gain Calculation (Summer Load Analysis)
Types of Heat Gain:
1️⃣ Sensible Heat Gain – From walls, windows, people, equipment, and lighting.
2️⃣ Latent Heat Gain – From moisture in the air (humidity, people breathing, cooking).
Formula for Sensible Heat Gain:
Q=U×A×ΔT
(Same as heat loss formula, but applied to heat entering the space.)
Formula for Latent Heat Gain (Moisture Load):
QL=0.68×CFM×ΔW
Where:
- Q_L = Latent heat gain (BTU/hr)
- 0.68 = Conversion factor (BTU per cfm per grain of moisture)
- CFM = Air infiltration or ventilation rate (cubic feet per minute)
- ΔW = Humidity ratio difference (grains of moisture per pound of dry air)
- 1 grain = 1/7000 lb of moisture
Example:
- A 100 sq. ft. west-facing window (U = 1.2, Solar heat gain factor = 200 BTU/hr·ft²)
Solution:
1. Sensible Heat Gain (Conduction)
Heat gain through the window due to the temperature difference between indoor and outdoor air is calculated using:
Qconduction=U×A×ΔT
Where:
- U = 1.2 BTU/hr·ft²·°F (window heat transfer coefficient)
- A = 100 ft² (window area)
- ΔT = (Outdoor temp – Indoor temp)
- Assume Outdoor temp = 95°F, Indoor temp = 75°F
- ΔT = 95 – 75 = 20°F
Calculation:
Qconduction=1.2×100×20
Qconduction=2,400 BTU/hr
2. Solar Heat Gain
Solar radiation passing through the window is given by:
Qsolar=SHGF×A
Where:
- SHGF = 200 BTU/hr·ft² (Solar Heat Gain Factor for west-facing window)
- A = 100 ft²
Calculation:
Qsolar=200×100
Qsolar=20,000 BTU/hr
Total Heat Gain
Qtotal=Qconduction+Qsolar
Qtotal=2,400+20,000
Qtotal=22,400 BTU/hr
Final Answer:
The total heat gain through the 100 sq. ft. west-facing window is 22,400 BTU/hr.
5. How to Use Heat Loss & Heat Gain Analysis in HVAC Design
1️⃣ Perform Manual J Calculation – Determines the correct heating & cooling capacity (tons of AC or BTU of furnace).
2️⃣ Select HVAC Equipment (Manual S) – Choose an AC unit, furnace, or heat pump that matches the calculated loads.
3️⃣ Design the Duct System (Manual D) – Ensures air is evenly distributed to all rooms for comfort.
4️⃣ Improve Energy Efficiency – Add insulation, better windows, and air sealing to reduce heating & cooling loads.
Also Read: How to Calculate Manual J, S and D in HVAC?
Note:
📌 Oversized HVAC units cycle on/off too frequently, wasting energy.
📌 Undersized systems can’t maintain comfortable temperatures.
6. Tools for Heat Load Calculations
🔹 ASHRAE Handbook – Load Calculations
🔹 Manual J (Residential) & Manual N (Commercial)
🔹 HVAC Software:
✔ Elite Software RHVAC
✔ Wrightsoft Right-J
✔ CoolCalc
Other Courses:
