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Underfloor Air Distribution Temperature Control

Underfloor Air Distribution Temperature Control


Underfloor Air Distribution Time Modulation Temperature Control

Johnson Controls is the world leader in providing HVAC solutions to building designers and owners. The Johnson Controls FlexSys brand underfloor air distribution systems and associated controls are among the most innovative technologies on the market today. These air systems help owners secure their lead ratings while providing superior occupant comfort.

Thermal Comfort Factors

Thermal comfort, as provided in modern buildings, is defined in ASHRAE Standard 55. The standard states that humans are sensitive to rising temperatures from toe to head. And recommends an optimal difference of no more than 5.4 degrees Fahrenheit.

While there are other factors, in an underfloor air distribution system, diffuser design and capacity modulation, are keys to achieving suitable comfort levels in the breathing zone. Namely, the floor, to two meters.

Underfloor Air Distribution Temperature Control

When it comes to air distribution system effectiveness, we will define perfect air distribution as air temperature at set point, well mixed throughout the lower two meters, consistent throughout the floor plate, and all these must occur at full and part load conditions.

The Johnson Controls facility in Kansas City is one of the most advanced, technically capable labs in use today. It's completely dedicated to underfloor air distribution testing and product development. Five separate temperature control loops allow the modulation of weather chamber conditions from -20 degrees to +140 degrees Fahrenheit. At the same time, it can independently control subfloor, occupied space, above-ceiling and upper slab temperatures. Temperature trees in the occupied floor space measure space temperatures at varying heights across the entire floor plate.

Let's now consider a comparison of throttling, as an air controlled method, versus time modulation. With throttling of air, a diffuser's throw height and distance drop as the volume is modulated. At low or part loads, these diffusers pool cold air, causing a large head to toe differences. In reality, how often are space conditions at part load? Nearly all the time.

This diffuser is operating at about 25% air flow. As you see, the cold air, represented here by smoke, pools on the floor with virtually no mixing into the top half of the occupied zone. We believe this contributes to cold draft complaint calls. This pooling of cold air not only affects occupant comfort, it inhibits thermostatic control. Note that most of the mixed air pools below the thermostat.

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A Unique Temperature Control Solution

Johnson Controls has developed a better way. With the introduction of the time modulation process. This concept has already been successfully applied in compressor modulation and heating control systems. Instead of throttling the air like a VAV box, the air valve has only two positions, open or closed. To achieve 50% load, the valve is open for six seconds and then closed for six seconds. To achieve 33% load, the valve is open for six seconds then closed for 12 seconds. These open/close cycles vary in length to exactly match load conditions.

As you can see, the cycles are kept short so that occupants have a sensation of continuous air movement. The diffusers take turns opening, so plenum pressures can be maintained. The diffuser you now see is also operating at about 25% air flow, and is virtually the same as the first example. Except this one has a time modulation air valve. The air valve is the defining difference. The diffuser is delivering the same amount of air, but you'll notice that the throw is through the entire breathing zone height and always the same throw distance. The result is much better mixing in the breathing zone and a much more consistent temperature for the occupants.

Our diffusers are designed to throw conditioned air throughout the breathing zone, but not too high, so that beneficial stratification can still be maintained. We measure success not just by diffuser throw patterns, but by the end result, effective three-dimensional temperature control. By monitoring temperatures at different heights and at different locations in a floor plate, we directly measure effectiveness and optimize diffuser designs.

Linear diffusers can throw the air past the optimal height of two meters. For ideal air distribution patterns, we recommend MIT or PCD style diffusers, as demonstrated here, which direct air only to the top of the breathing zone.

FlexSys diffusers are all metal and designed to be as strong as the floor. The diffusers install in seconds. Many FlexSys diffusers feature user adjustable volume control and/or VAV. They're available in square, round, large, small and in colors to match the architect's color palette.

In the HVAC industry, basic innovation rarely comes along. We believe time modulation is one of those rare product developments that can significantly improve the air quality in indoor environments.

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