EGEE 102
Energy Conservation and Environmental Protection

Calculating Hourly Heat Loss

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As we have learned, most heat is lost through a house's walls through conduction. One of the three factors that affect heat loss is a wall's capacity to resist heat loss.

We will now look at how to calculate the rate of heat loss of the walls of a house, using the following formula:

Heat Loss ( BTUs h ) =  Area ( ft 2 )×Temperature Difference ( °F ) R-Value( ft 2  °F h BTUs )

From the above equation it can be seen that once the house is built, these two variables will NOT change:

  • The area of the walls
  • The R-value of the walls

The only variable that will change is the temperature difference between inside and outside.

Example

Calculate the heat loss for a 10 ft by 8 ft wall, insulated to R-value 22. The inside temperature is maintained at 70° F. The temperature outside is 43° F.

Please watch the following 2:25 presentation about Hourly Heat Loss:

Click here for a transcript of Hourly Heat Loss - Problem #1 video.

Lesson 7a, Screen 26: Calculating Hourly Heat Loss

Example 5

Calculate the heat loss for a 10 ft by 8 ft wall, insulated to R-value 22. The inside temperature is maintained at 70°F. The temperature outside is 43°F.

For this problem, we are trying to calculate the heat loss. We are given the dimensions; we are given the R-value and the temperature difference. Those are the quantities that we need to calculate the heat loss basically.

The dimensions are, the wall dimensions are 8ft and this side is 10 ft.

( 8 × 10 = 80  ft 2 )

So the area is 8 times 10 will be 80 square or 80 square feet. And we are also given the inside temperature is 70°F and outside temperature, average outside temperature, is 43°F. Therefore the difference or delta T (ΔT) is equal to 27°F.

Inside temp     = 70°F  Outside temp= 43°F                        ΔT = 27°F 

Now area is given, A is, we already calculated 80ft2 and R-value is also given as 22 which is 22 ft2 °F hour over BTU.

ΔT = 27°F  A = 80 f t 2   R =  22 f t 2  °Fh BTU   

Now, heat loss, BTUs per hour, is equal to area times ΔT divided by R-value.

Q r  =  Area×ΔT R

Now in this case, it is:

80f t 2  × 27°F 22f t 2  °F h/btu  = 98.2 BTU/h 

So this will be equal to, here you can cancel ft2 and this ft2 and this °F and this °F and we are left with BTUs per hour. And the heat loss comes out to be 98.2 BTU/h.