Measuring Heat Flow

Measuring the flow of heat (or energy, in general) is a tricky task. Generally speaking, one has to know the temperature on both sides of a wall, window etc. The temperature difference provides a measure of the drop in temperature and, consequently, the flow of energy through the object. So in principle one has to keep an eye on both sides of the object in question.

But what happens if only one side is accessible, like in a storage tank where measuring the temperature on the inner side of the vessel proves to be very difficult if not impossible? So how much energy does a hot water storage lose?

Luckily, there are solutions for measuring the heat flow by using a simple gadget. With a thermal flux sensor one may easily determine how much energy goes through a wall or a window or any other object. The sensor is attached to one side of the object and yields a signal which is directly proportional to the heat passing through the object and thus makes it possible to determine how much energy actually penetrates a window, wall, etc.

Performing such a measurement is easy and exciting as it may reveal some unexpected features of energy leaving or entering a dwelling.

We have performed a series of measurements in a house in Sweden over a couple of days and discovered some really interesting features. And yet, no sophisticated equipment was necessary. A heat flow sensor and a multimeter, that´s all it takes. The measurements took place during the last week of February 2012. Let us have a look at the results before we start discussing them in more detail (Fig. 1).

Fig. 1 Measuring the heat flow through a window.

The red curve shows the heat flow through a window facing a westerly direction. The outside temperature was approximately 7 °C in the morning. During the entire morning until 12:30 hrs the heat flow was relatively stable at about 3 W/sqm. Then shortly after 13:00 the sun slowly started finding its way through the window leading to a massive change in energy flow. In fact, we even observed an inversion of the energy flux, i.e. more energy was entering the living room than leaving it. This situation when the sun was delivering free energy through the window lasted until 15:30. Then it disappeared behind a nearby forest, and the energy flux got back to its normal behaviour which was governed by the temperature difference between the living room and the exterior. It goes without saying that during those hours of direct sun exposure the heating could effectively be switched off.

During the late afternoon a significant drop in outside temperature occurred (blue curve) while simultaneously the flow of energy was rapidly increasing reaching a plateau at almost 15 W/sqm after 18:00 hrs. Since at that time it was already dark outside the heat flow was no longer overlapped by indirect solar irradiation.

Now we are in a position to calculate the net heat flow during the measurement period. The heat loss was strongest in the evening when the outside temperature dropped drastically. During the early afternoon we had a net inflow of energy. The overall heat flow balance amounts to 0.063 kWh/sqm going through the window. Thus, by using relatively simple means we could perform a thermal analysis of a window.

The experimental setup is shown in Fig. 2.

Fig. 2  Measuring the heat flow via a heat flow sensor

More information and/or quotations please contact:  manfred.jacobi (at) gmail.com