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Rob Dumont

posted Mar 1, 2011, 8:05 PM by ASHRAE Saskatoon
If you could have only one instrument to help tune up a house's energy use, what would it be?

A thermocouple temperature sensor would be my choice.

Most of the energy consumption in houses is used to maintain a temperature difference. For instance, house temperature in winter is usually held around 22 C, the hot water around 55 C, the refrigerator at about 4 C, the freezer at -18 C, etc. If these values are not right, you may be wasting a considerable amount of energy.

For measuring temperature, my favourite device is a thermocouple based digital readout. The thermocouple has the advantage that it is quite rugged, can measure temperatures over a wide range,  is not very sensitive to liquid water, and  is relatively inexpensive. Thermocouples operate on the Seebeck effect. A junction of two dissimilar metals will generate a voltage if the junction is at a different temperature than the readout device.  The readout device converts this voltage signal into a temperature. A schematic of the thermocouple and readout is shown in Figure 1.  The voltage output from most thermocouples is quite small—about 40 microvolts per degree C-- thus a sensitive readout is needed. 


Figure 1. Thermcouple wires and a temperature readout

For about $150 you can get a dual input thermocouple readout with decent accuracy. Omega.ca has an Omegaette HH300 model that uses type K thermocouples, and I have been satisfied with their performance over the years. Some other suppliers include Fluke, Extech, and Cole-Parmer.

Here are some temperatures that can be checked using a thermocouple:

Refrigerator temperature.  The desirable temperature should be about 4 C. A higher temperature will increase food spoilage, and a lower temperature will increase energy use.

Freezer temperature. The desirable temperature should be about -18 C. As with a refrigerator, a higher temperature will increase food spoilage, and a lower temperature will increase energy use.

Room thermostat.  Use the thermocouple readout to check that your thermostat is reading properly.

Water heater temperature.  The water temperature should be about 55 C coming out of the unit. Higher temperatures will waste energy and can cause scalding, and lower temperatures increase the risk of legionella and other bacteria growing in the unit.

Oven temperatures. The thermocouple can be used to cross-check the temperature settings on the oven dial and prevent uncooked or burnt food.

Furnace exhaust temperatures.  A high efficiency condensing natural gas furnace will have an exhaust gas temperature of about 50 C or lower. A low efficiency conventional natural draft gas furnace should have an exhaust gas temperature of about 175 C as the exhaust gas leaves the heat exchanger. (We have, however,  measured some exhaust gas temperatures as high as 400 C in cases where the warm air flow through the furnace was  seriously impeded by bad ductwork or a dirty air filter.) A high exhaust gas temperature is a ready indicator of low efficiency.

Air to Air Heat Exchanger (Heat Recovery Ventilator).  Use the thermocouples to check out the temperatures at the inlets and outlets of the HRV.  For example, if the outdoor air temperature is -10 C and the indoor air temperature is  +22 C, the temperature of the outside air should be about +13.4 C after it has been passed through the HRV (this assumes that the sensible heat recovery effectiveness is about 70%). In addition, the temperature drop on the exhaust air side of the heat exchanger should be roughly equal to the temperature rise on the outside air side of the heat exchanger. For the above example the temperature rise of the outside air would be  11.4 -(-10) = 23.4 C. The temperature fall on the exhaust air side of the heat exchanger should also be about 23.4 C if the air flows on each side of the heat exchanger are equal. (This assumes that condensation effects can be neglected.) Thus the exhaust air temperature after it has been cooled should be about 22 -23.4 = -1.4 C.
If the temperature rise and temperature fall values are quite different, the most likely reason is that the flows on each side of the exchanger are unequal. Adjust the flows to bring the HRV into balance.

Skin temperature.  Your skin temperature is usually about 30 C. If it is less than or equal to  room temperature, you are likely dead, and you need read no further.
Thermocouples can be fooled if not used properly. Here are some common errors with using thermocouples:
1 The junction of the thermocouple must be in thermal equilbrium with the temperature of the object being measured.  Make sure that the thermocouple wire is in contact for at least an inch (25 mm) with the surface being measured. For instance, if you place only the tip of a thermocouple on your skin, the temperature will read about 25 C, which is clearly too low. If, however, you tape the thermocouple tip and an inch of wire near the tip to your skin, you will get a temperature of about 30 C, which is much more accurate.
2 Ensure that the insulation on the two wires is intact. If the wires are shorted at a place other than the tip, the thermocouple will give an inaccurate reading. 
3 Periodically check the calibration of the thermocouple. An accurate check can be made by placing the thermocouple junction and about an inch of the wire into an ice bath. The ice bath should consist of a mixture of water and crushed ice. The thermocouple should read very close to 0 C.
4 Watch out for radiation sources, as they can give a false reading of temperature.  For instance, a thermocouple that is exposed to direct sun outdoors will give an inaccurate reading of the air temperature. A radiation shield such as a toilet paper cardboard wrapped with aluminum foil will work well. A thermocouple inside an oven will give an inaccurate reading of temperature if the thermocouple is placed near the hot element.

Temperature is the probably the most important parameter in a dwelling. It pays to have a good temperature sensor.