Calculating Hourly Heat Loss

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.

$$\left(8\times 10=80{\text{ ft}}^{\text{2}}\right)$$

So the area is 8 times 10 will be 80 foot 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.

$$\begin{array}{l}\text{Inside temp }=70°F\\ -\text{Outside temp}=-43°F\\ \Delta T=27°F\end{array}$$

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.

$$\begin{array}{l}\Delta T=27°F\\ A=80f{t}^2\\ R=\frac{22f{t}^2°Fh}{BTU}\end{array}$$

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

$$\frac{Q}{r}=\frac{Area\times \Delta T}{R}$$

Now in this case, it is:

$$\frac{80f{t}^2\times 27°F}{22f{t}^2°Fh/btu}=98.2BTU/h$$

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.