What is Heat Loss and Air Flow Rate

Heat loss is the thermal energy lost from a building through windows/doors, walls, ceilings, and floors, calculated as the product of area (ft²) and Heat Multiplier (H*M, BTU/hr-ft²). The air flow rate (\( L \)) is the volume of air required per second to compensate for this heat loss, based on the temperature difference between the heating air (\( t_h \)) and room temperature (\( t_r \)), along with air's specific heat (\( c_p \)) and density (\( \rho \)).

For example, a room with 100 ft² of windows/doors (Single Insulated), 300 ft² of walls (2 inch insulation), 100 ft² of ceiling (6 inches insulation), and 100 ft² of floor (Overhang 3 inch), with heating air at 40°C and room temperature at 20°C, has a heat loss of 6800 BTU/hr and requires approximately 0.0820 m³/s of air flow.

How to Calculate Heat Loss and Air Flow Rate

Formulas:

\( L = \frac{Q}{c_p \rho (t_h - t_r)} \)

Where:

• \(\text{Area}\) = Area in ft²
• \(\text{H*M}\) = Heat Multiplier (BTU/hr-ft²)
• \( Q \) = Heat loss (kW)
• \( c_p \) = Specific heat of air = 1.005 kJ/(kg°C)
• \( \rho \) = Density of air = 1.2 kg/m³
• \( t_h \) = Heating air temperature (°C)
• \( t_r \) = Room temperature (°C)
• \( L \) = Air flow rate (m³/s)

Example Calculation: For a room:

• Windows/Doors: 100 ft² × 41 = 4100 BTU/hr
• Walls: 300 ft² × 6 = 1800 BTU/hr
• Ceiling: 100 ft² × 4 = 400 BTU/hr
• Floor: 100 ft² × 5 = 500 BTU/hr
• Total Heat Loss = 4100 + 1800 + 400 + 500 = 6800 BTU/hr
• Heat Loss in kW = \( \frac{6800}{3412.14} \approx 1.993 \, \text{kW} \)
• \( t_h = 40°C \), \( t_r = 20°C \), \( L = \frac{1.993}{1.005 \times 1.2 \times (40 - 20)} \approx 0.0820 \, \text{m}^3/\text{s} \)
• In m³/h: \( 0.0820 \times 3600 \approx 295.2 \, \text{m}^3/\text{h} \)
• In L/s: \( 0.0820 \times 1000 \approx 82.0 \, \text{L}/\text{s} \)

Use the form above to input areas, select component types, and provide temperatures to calculate heat loss and air flow rate.

Heat Multipliers (H*M) Table

The following table lists heat multipliers (H*M in BTU/hr-ft²) for different components:

Component	Type	H*M (BTU/hr-ft²)
Windows/Doors	Single	67
	Single Insulated	41
	Storm	34
	Double Insulated	30
Wall	No Insulation	15
	2 inch	6
	4 inch	5
	6 inch	4
Ceiling	3 inches	5
	6 inches	4
	9 inches	3
	10 inches	2
Floor	No Insulation	4
	Overhang 3 inch	5
	Overhang 6 inch	3
	Overhang 9 inch	2

Air Flow Rate Units and Conversions

The calculator displays results in the following units:

• Air Flow Rate:
  • Cubic Meters per Second (m³/s): Base unit
  • Cubic Meters per Hour (m³/h): 1 m³/s = 3600 m³/h
  • Liters per Second (L/s): 1 m³/s = 1000 L/s

Results are provided in all listed units after calculation.

FAQs

How do you calculate heat loss and air flow rate?

To calculate heat loss and air flow rate:

1. Input the area (ft²) for each component (Windows & Doors, Net Walls, Ceiling, Floor).
2. Select the corresponding type for each component from the dropdowns to determine the Heat Multiplier (H*M).
3. Input the heating air temperature (\( t_h \)) and room temperature (\( t_r \)) in °C, °F, or K.
4. The calculator computes the heat loss for each component (\(\text{Area} \times \text{H*M}\)), sums them to get total heat loss in BTU/hr, converts to kW, and applies \( L = \frac{Q}{c_p \rho (t_h - t_r)} \) with \( c_p = 1.005 \, \text{kJ}/(\text{kg°C}) \) and \( \rho = 1.2 \, \text{kg}/\text{m}^3 \) to calculate the air flow rate in m³/s, then converts to m³/h and L/s.

What is the air flow rate for a room with specific areas and temperatures?

For a room with:

• Windows/Doors: 100 ft², Single Insulated
• Walls: 300 ft², 2 inch insulation
• Ceiling: 100 ft², 6 inches insulation
• Floor: 100 ft², Overhang 3 inch
• Total Heat Loss = 6800 BTU/hr, \( Q = \frac{6800}{3412.14} \approx 1.993 \, \text{kW} \)
• \( t_h = 40°C \), \( t_r = 20°C \), \( L = \frac{1.993}{1.005 \times 1.2 \times (40 - 20)} \approx 0.0820 \, \text{m}^3/\text{s} \)
• In m³/h: \( 0.0820 \times 3600 \approx 295.2 \, \text{m}^3/\text{h} \)
• In L/s: \( 0.0820 \times 1000 \approx 82.0 \, \text{L}/\text{s} \)

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