This is the first post in a four-part series covering the topic of data logging in terms of Types of Monitoring, Products and Performance, and Indoor Air Quality.

“Data logging” is a term used to describe the recording of information about a home’s interior conditions—such as temperature and humidity—using a little device designed for this purpose. (I am partial to Onset Hobo loggers.) Logging allows us to solve several types of problems that are hard to analyze. We can see things that happen only late in the evening or early in the morning—the times when heating and air conditioning systems are under the highest loads—as well as things like moisture loads that happen over the course of months, or things that happen only on the hottest or coldest days. In short, they are useful for collecting data related to any number of conditions and time frames.

For example, data loggers can record information every minute for an hour and indicate how a complicated multi-speed, multi-zone HVAC system is operating, or they can record every six hours for three months and reveal how humid an attic gets. In the last few years, logging of electrical loads has become practical, which allows us to see where energy is going in a house. Similarly, inexpensive consumer-grade indoor air quality monitors can log various pollutants, allowing us to verify that the ventilation and filtration systems are working correctly.

Temperature and humidity

When I was a kid, a visit to a museum might include a bonus artifact: a paper disk with a pen slowly drawing a ring around it. The pen was attached to a humidity meter and the more humid, the farther from the center the pen would draw as the disk spun on a 30-day cycle. Today, we can buy devices that do the same job for the price of a fast-food lunch. In fact, this kind of data collection is now part of many thermostats, indoor air quality devices, and the like. I’ve been using loggers for two decades and have made enough discoveries and mistakes that I thought I’d share a few.

In my work at BOWA, the most common use for logging equipment is to measure temperature and humidity over time. Initially, we were collecting temperature information on the coldest mornings and hottest evenings—outside of working hours, or even waking hours—when the houses we were working on were having problems with temperature control. Data loggers are fantastic for this; we just “launch” a handful of them and place them around around the house, then wait for bad weather to show how the house misbehaves.

These days, I always recommend taking humidity readings even if you think you’re looking for temperature only. Humidity has a big impact on comfort in the air conditioning season—and even to a degree in winter—so it’s good to have the information. Monitoring humidity also provides insight in occupant behaviors like showering without using the exhaust fan, cooking without good ventilation, or leaving windows open all night—all which could be the real issue in a “bad air conditioning” scenario. In such cases, the humidity levels suddenly change in a way they wouldn’t at other times of the day. Armed with this information, you can have a conversation with clients about how they operate their house. To be able to say, “Here is what I’m seeing on this day at this hour” can make for a productive conversation when troubleshooting thermal control complaints.

Where to put data loggers

Usually there’s a part of the house that’s “the problem area”—the one bedroom at the end of the HVAC run, or the kitchen addition on piers. It’s critical to measure the temperature in that space. Ideally, it’s possible to find a location away from exterior walls where the data loggers will not be in direct sunlight. In the houses and condos where we work, there always seems to be a piece of furniture or set of bookshelves in a good location. Often, we use pictures, books, lamps, and other bric-a-brac to hide the loggers from the sun. In addition to the problem area, I recommend putting a logger in another similar room or two in order to see if the space beyond is misbehaving too.

Another key location is at the thermostat for the HVAC system serving these spaces. When I work on multi-zone HVAC systems, I try to put a logger near each thermostat on the system, and one in another part of the house that isn’t reported to be misbehaving. For this type of data project, I need a lot of loggers, which is why I am glad my old Onset Hobo loggers from 2001 are still working—I’ve had as many as 12 loggers running in a house at one time. In some cases, we have found the thermostat reading a comfortable temperature, but the room is too cold. This is more apt to happen in a house with multiple thermostats; the downstairs heat will keep the upstairs hall thermostat warm in some houses, while the upstairs bedroom gets colder and colder, for example. In a case like this, the loggers have discovered that the HVAC system isn’t undersized, it’s just not operating on cold nights. Now we know where to start to resolve the issue. (As a matter of practice, we now move thermostats out of the halls.)

Be careful with “smart” or “communicating” thermostats, which emit a fair amount of heat. Resting a logger on top of one will result in false readings. It’s better to tape a logger so it hangs from the bottom of the thermostat where the heat plume won’t affect it, or to rest it somewhere nearby. The powerful magnets on some of my loggers allow me to attach them to the metal corner bead commonly used in our area on drywall; this can be helpful for loggers in thermostat locations.

I also like to get outdoor temperatures for comparisons’ sake. One of these days, I’ll buy an outdoor-rated temperature and humidity logger, but to date (going on 20 years now), I’ve just put a regular logger into a zip-closure bag and found a shady spot outside to hide it. (I’ve had water get into the bag and stop the outdoor logger from working, so I use extra bags as a matter of practice now.) I use two or three nested bags and include a large label with my contact information, in case any landscapers find my hiding spot. In a pinch, I’ve downloaded local-ish weather information but finding and formatting it takes longer than recording it myself.

So far, I’ve pointed to the most common reason we use data loggers: to measure temperature and humidity—the two primary conditions at the root of occupant discomfort. In the next post, I will talk about the various other types of monitoring achievable with data loggers—from wood moisture to water flow rate to electrical consumption.

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Doug Horgan is vice president of Best Practices at BOWA in Washington, D.C. Photo by Peter Yost.