Contractors are already well aware of temperature sensors, but humidity sensors and CO₂ sensors are also making in-roads into smaller applications. Many types of air-handling equipment are now coming with sensor-ready contacts, making it even easier for a contractor to suggest additional sensors to keep a space comfortable.

Why bother?

In a small space, you might be wondering just why anything beyond measuring temperature would be required.

The reason is that while high humidity or insufficient ventilation can make any building space uncomfortable, it can make a small space stifling. Adding sensors can help increase comfort while saving energy, which is usually of interest to every building owner.

As Dennis Snow, manager of technical service, and John Sartain, marketing manager, White-Rodgers (St. Louis, MO), note, the market will continue to drive sensors into smaller and easier-to-install packages.

CO₂ sensors can help save energy by being used for demand-controlled ventilation, in which ventilation is controlled based on occupancy. Basically, the ventilation system delivers fresh air when a higher concentration of CO₂ is detected, usually meaning that there are more people in the space.

CO₂ sensors also keep from overventilating a space, further saving energy. Most manufacturers quote a two-year payback period on their CO₂ sensors in both small and large applications.

Some are also starting to use CO₂ sensors to detect the presence of combustion fumes. “Most combustion sources actually give off a high amount of CO₂, related to almost any other contaminant,” says Mike Schell, director of marketing and business development, Telaire (formerly Engelhard Sensor Technologies), Santa Barbara, CA.

“In normal conditions, concentrations in outside air should be fairly low. So if elevated levels are measured, the sensor can actually sense for the CO₂ and close down the air intake for a period of time.”

Humidity sensors are also beneficial for smaller applications. They can be used to control economizer operation or the amount of dehumidification or humidification provided by a system.

Overwhelming humidity

Schell notes that one of the problems that’s happening in all types of buildings is that too much humid air is, at times, introduced through the air intake. The result is that incoming air may be cooled to such an extent that water will condense in the duct downstream of the coil.

This water combines with nutrient sources (such as dust and dirt), dormant spores, and bacteria already in the duct to create mold and mildew growth.

“To get temperatures down enough to cool the space, temperatures may be in the 50°s at and beyond the cooling coil, making the walls of the duct like one giant beer glass on a humid day,” says Schell.

Sensors can help control the amount of outside air so that too much humid air does not overwhelm the system and make the space uncomfortable. More importantly, they can help make sure that the system isn’t so overwhelmed with humid air that water condenses in the wrong places beyond the coil.

One issue with relative humidity sensors, notes Schell, is that while they can work well in conditioned spaces, they can be overwhelmed or have limited life when measuring the humid air at air intakes or in the saturation conditions that occur when return air and outside air mix before the coil.

Schell suggests the use of absolute humidity or dewpoint sensors rather than relative humidity sensors. “Absolute humidity is an absolute measure of water in the air, regardless of the temperature and no matter where you measure it, indoors or outdoors, there is only a single level that you have to control to ensure comfort and mold, mildew, and fungus control.

“In contrast, because relative humidity is always changing with temperature, it is very difficult to effectively utilize a percent-rh sensor with the type of simple control systems available today on most unitary equipment.”

Cost coming down

Schell notes that the traditional problem with absolute humidity and dewpoint sensors has been that their cost has traditionally been three to five times more than rh sensors.

“New developments in sensor technology have now brought the cost down so that a dewpoint or absolute humidity sensor can be as inexpensive as a regular humidity sensor. These sensors also appear to be more durable in tolerating the extremes of conditions found in air intakes and outside air.”

He adds that the problem with traditional humidity control is that the only way to control humidity has typically been with controlling the outside air damper. Traditionally, most unitary equipment could not vary the amount of cooling vs. the amount of dehumidification they provided to a space.

“That’s starting to change now. A lot of companies are making equipment that can vary their sensible-latent cooling ability using mechanical cooling or desiccant-based technologies.”

Humidity sensors may still be more of a difficult sell to building owners, simply because the benefit may not be as apparent as with CO₂ sensors. To prove the benefits, contractors need to become detectives, looking around a space for any evidence of peeling wallpaper or mold and mildew growth.

Showing a building owner that money will be saved over the long term by not having to replace interior furnishings every few years is one of the better arguments for humidity sensors. Health is another consideration. Symptoms resulting from exposure to mold, mildew, and fungus can range from allergic reactions to more serious illnesses.

No bas required

The nice part about some of the new “smart” sensor technology is that you do not need a centralized bas to provide effective control.

Schell notes that a lot of work has been done in this area, “So with a couple of potentiometers and some off-the-shelf control equipment that’s already available, you can achieve good control of the space.”

He notes that all economizer control manufacturers now have the ability to take inputs from both humidity and CO₂ sensors to control space conditions and save energy.

Traditionally it has been difficult to integrate CO₂ sensors into smaller systems because of the lack of a control system to interpret what to do with a sensor signal. This type of equipment is basically “dumb” in that it will modulate a damper based on a voltage input or turn off or on based on a relay input, but it cannot really make decisions on when, how, and how much to control.

Some CO₂ sensors now have built-in programmability, so the voltage output of the CO₂ sensor signal can be scaled. This means that the amount of outside air can be exactly controlled to ensure that the required amount of outside air on a cfm-per-person basis (i.e., 15 cfm) is provided at all times, based on actual occupancy.

Probably 20% of those buildings with CO₂ sensors — with or without a bas — aren’t using them to control anything, Schell estimates. “They’re just taking the signal and logging what’s going on. They keep ongoing records that they can check and verify that things are operating properly.”

In some cases, outside service centers are collecting that data for building owners. These centers analyze what’s going on in the building space, then report back to the owner whether or not the system is working as it should. That can be a beneficial use of the sensor technology as well.

Sidebar: Learning curve

If you’re used to working only with controlling temperature, additional sensor technology can be a challenge.

With CO₂ sensors, you need to understand how the CO₂ level determines the ventilation rate. You also need to understand how a CO₂ sensor can modulate a damper and override an economizer cycle, or vice versa.

However, “It’s not rocket science,” notes Schell. “You just have to know a little bit more” about how CO₂ sensors work.

Sartain and Snow add that contractors should feel comfortable with sensors and install them just like they would any thermostat.

Schell adds that there are manufacturers out there that will help contractors understand and apply sensor technology. “Instead of looking on it as a burden, it really is offering up myriad new opportunities for contractors to go into buildings, be heroes, and solve problems that they just ignored before.”