Solving Heat Transfer Equations for Rtot: Understanding Units and Formulas

In summary, the question presents a scenario of a room in a house with composite walls and asks for the heat resistance of the walls in W. The relevant equation is missing the total area of the walls and the units of the given answer are in K/W. The correct unit should be Km^2/W.
  • #1
stunner5000pt
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Homework Statement
Consider a room in a house that has composite walls with some heat resistance R(tot). The total cross sectional area of the walls in 35 m^2. During winter, the temp outside is -5 deg C. Inside there is an electric heater emitting heat at a rate of 400 W so the room temperature is constant at 18 deg C. The room is occupied by 2 people each emitting 80 W each. What is the heat resistance of the walls in W?
Relevant Equations
[tex] \frac{dQ}{dt} = \frac{T_{out} - T_{in}}{R_{tot}} [/tex]
I believe that just using the equation above should yield the following:

[tex] R_{tot} = \frac{18 -(-5)}{400 + 2(80)} = 0.041 [/tex]

but the units don't seem to match up... The units of the above answer are in K / W.

Is there a different formula i should be using?

Your help is appreciated!
 
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  • #2
stunner5000pt said:
Homework Statement:: Consider a room in a house that has composite walls with some heat resistance R(tot). The total cross sectional area of the walls in 35 m^2. During winter, the temp outside is -5 deg C. Inside there is an electric heater emitting heat at a rate of 400 W so the room temperature is constant at 18 deg C. The room is occupied by 2 people each emitting 80 W each. What is the heat resistance of the walls in W?
Your relevant equation is missing the total area of the walls of the room.
Also, the question seems to be mixing rate of heat transfer (watts) and R-value of the composite walls.

Please, see:
https://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html

:cool:
 
Last edited:
  • #3
Lnewqban said:
the question seems to be mixing rate of heat transfer (watts) and R-value of the composite walls.
Yes, it should ask for ##Km^2/W##.
 
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FAQ: Solving Heat Transfer Equations for Rtot: Understanding Units and Formulas

What is Rtot in heat transfer equations?

Rtot, or total thermal resistance, is a measure of the overall resistance to heat flow in a material or system. It takes into account all types of heat transfer, including conduction, convection, and radiation.

How is Rtot calculated?

Rtot is calculated by adding together the individual thermal resistances of each component in a system. This can be done using the formula Rtot = R1 + R2 + ... + Rn, where R1, R2, etc. are the thermal resistances of each component.

What are the units of Rtot?

The units of Rtot depend on the units used for the individual thermal resistances. In general, Rtot is measured in units of Kelvin per watt (K/W) or degrees Celsius per watt (°C/W).

How do I convert Rtot to other units?

To convert Rtot to other units, you can use conversion factors. For example, to convert from K/W to °C/W, you can use the formula °C/W = K/W x 1.8. It is important to note that the conversion factor may vary depending on the specific units being used.

What is the significance of understanding Rtot in heat transfer equations?

Understanding Rtot is crucial for accurately predicting and controlling heat flow in a system. It allows scientists and engineers to design and optimize systems for efficient heat transfer, which is important in various industries such as energy, manufacturing, and electronics.

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