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Finding heat leaks with an infrared thermometer Print E-mail
Tuesday, 06 October 2009 12:38

In my article on thermography, you learned how a home energy auditor could take a picture showing the places where heat leaks into or out of your house. They often use a camera that works in the far infrared (the range of light that is what we call radiant heat) and produces a picture in which the colors indicate the precise temperature of every surface in the picture. If you want to do this yourself, you, too can lay out a few thousand dollars for the camera and learn how to interpret these images.

But what if you can't afford an audit, much less afford to buy the camera? Without such high technology, how on earth could you possibly find the areas of your walls that are, say, especially cool in heating season (indicating poor insulation), or especially warm (suggesting a leaking heat duct)?

The Manual Approach

Here's an approach that's within your reach: use your hand. Run your hand all over each wall and find the cool and warm spots. (Note: wash hand first.) You can even feel breezes where there are air leaks to the outside or heating duct leaks. Amazing, huh? Like you really needed me to point that one out.

This literally manual technique, while cheap, has a few limitations. For one, your hand is not especially sensitive to changes in wall temperature of a couple of degrees. I tried comparing a sun-warmed section of wall to a shaded one with my hand; the two areas differed by about two degrees, but I could feel no difference. (I temporarily shaded the sunny area while I took the measurement to avoid sunlight complicating the experiment.)

For another limitation, you may not be able to reach the top of a wall easily, and you almost certainly can't scan your ceiling without a moving a ladder around. For another, your hand will get cold; I recommend warming it up on the back of your spouse's or friend's neck, which will also encourage a sense of camaraderie (French for homicide). The method does have the advantage of removing cobwebs, if done thoroughly. Don't be surprised if your spouse hands you a dustrag to use while he or she is calling the folks from the mental health service.

The Infrared Thermometer

If you're a bit more serious about finding these heat leaks, consider an infrared thermometer. This is a gadget you simply have to point at an area, and it gives you that area's temperature.

Do be careful, in that the farther the thermometer is away from the surface, the broader the area measured, so you can miss small leaks. Small variations will be averaged out. The ratio of distance from the surface to width of area measured is called the "distance to spot ratio." Mine has a ratio of 5:1, so at five feet away, I'd be measuring a spot one foot wide. (Mine is an LA Crosse Technology Infrascan I received as a holiday gift  -- thanks, FIL!)

Some of them have a laser pointer built in so that you can see where you're pointing it, but don't confuse the size of this spot to the width of the area being measured. You could dead-center the aiming spot on, say, a light fixture, but if you're five feet away, you might be averaging the temperature over a foot or more, and miss the cold spot where the fixture leaks outdoor air. I've seen at least one model that displays a circle of laser dots corresponding to the area measured, which is useful.

One of these gadgets will run you about $50. One clever design that came out late in the northern hemisphere's heating season is the Black & Decker TLD100 Energy Series Thermal Leak Detector. I haven't tried one, but I like two things about it for checking home heat leaks:

1. It shines a visible light on the wall or ceiling that is red, green, or blue, indicating that the measured area is warm, neutral, or cool, respectively. This is considerably easier to read than a continuous digital readout. Digital readouts are sensitive enough that they change constantly, and your brain may be so overloaded noticing the fractions of a degree change that it misses a change of a single whole degree: 74.7, 74.5, 74.6, 74.7, 74.8, 75.9, 74.9 ...

2.  The red, green, or blue signal is indicating a change in temperature, which is really more useful than knowing the actual temperature when it comes to finding heat leaks. You set the instrument to be green when aimed at a part of the wall you consider reasonably normal in temperature. You then set it to sense a change of 1, 5, or 10 degrees (Fahrenheit). Obviously, if it thereafter shows red or blue most of the time, you chose a misleadingly cool or warm place to start with. An instrument that gives you absolute temperature, like 65 degrees F (no, not absolute as in degrees Kelvin, you geeks -- get a grip) may mislead you to thinking an entire wall is less well insulated than another wall at 70 degrees, when it is quite possible that there is a slight breeze on the cooler wall, or that the sun warmed the warmer wall all day.

The best time to do this is on a still, cold night, indoors. This should help eliminate confusing factors like wind, sun, and sun shadows. To check for leaks in a forced air heating system, make sure the system is on and leave open any interior door that leads to the furnace area. (I'm assuming you have a properly working furnace and, of course, CO2 detectors in your house.) Leaking ducts will warm a small area of a nearby wall; cold air leaks will similarly show up around a door, window, or outlet because the furnace's combustion creates a slight vacuum in the house that draws in air.

Before you get carried away aiming your new toy at the refrigerator door, your car's passenger compartment (visible through the window, warming up in the driveway) your dog's inner ear, or your spouse's reportedly cold feet, also note that these things don't work on shiny stuff like stainless steel, or through glass, or on uncooperative animals or people that have an unreasonable aversion to scientific inquiry such that they won't take their shoes and socks off. If you have to measure something shiny or glass, put a piece of opaque tape (like masking, painter's or electrical tape) on the surface, and be sure you read only the area covered by the tape. This corrects for something called emissivity, and helps avoid errors due to reflections.

Have fun, draft fighters!

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