Engineering Firm Specifies Humidity Control

August 11, 2008
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The Saxony Lutheran High School in Saxony, Mo., previously installed light commercial equipment that was yielding too much outside air and creating a major humidity concern.

SAXONY, Mo. - The Saxony Lutheran High School in Saxony, Mo., presented Mark Strickland, principal of Strickland Engineering LLC, in Jackson, Mo., with a significant problem - humidity. The school’s previously installed light commercial HVAC equipment was delivering too much outside air.

Looking for a solution, Strickland turned to Cris Meyer, a consultant at Rollie Johnson Inc., Chesterfield, Mo., the regional distributor for Rawal Devices Inc. Meyer recommended Rawal’s APR-410 Control.

Designed for HVAC systems in various commercial buildings, the APR Control provides full capacity modulation for all sizes of direct-expansion air conditioning systems. According to Rawal, the control continuously and automatically matches a/c system capacity-output to changing load conditions, resulting in improved humidity control without overcooling the space. It also eliminates multiple common maintenance problems associated with excess system capacity such as coil icing, liquid slugging, and excessive compressor cycling.

“Once the APR valve was installed, it was able to help control humidity by loading the compressor properly so that it would run longer to take the humidity out of the air,” said Strickland. “Since then, we’ve pretty well made the APR Control a standard specification for smaller systems that we’re not putting elaborate controls on.”

Strickland’s design firm specializes in specifying HVAC equipment for commercial and non-profit buildings. It looks to provide maximum system control benefits while requiring minimal cost and maintenance staffing.

“We will specify the APR Control on smaller projects where we don’t have to put in a high-end control system with re-heat strips or other elaborate controls,” explained Strickland. “These are places that don’t have big maintenance staffs. Thanks to the APR Control, on a smaller-scale project where there is not a high budget, the customer can gain reliable humidity control that doesn’t require additional systems.”

In his experience, Strickland has also found that the APR Control eliminates the need to run a hot gas line to remote evaporators, reducing installation time.

“We don’t install things ourselves, but I don’t have building owners calling me back later with humidity issues. That’s a good thing.”

Designed for HVAC systems in various commercial buildings, the APR Control, like the one installed in this building, provides full capacity modulation for all sizes of direct-expansion air conditioning systems.


Strickland’s use of the APR Control at Saxony Lutheran isn’t the only application benefitting from this technology. Bob Reichenbach, HVAC service manager for the Warko Group in Reading, Pa., has also begun to specify and install the control in multiple applications. Reichenbach, however, made an interesting observation when discussing some of the customers that he deals with.

“Building owners and tenants don’t understand that many of the problems with their a/c systems are due to worst-case design assumptions and excess system capacity, nor do they seem to care,” he noted. “I deal with the consequences of system overcapacity and unhappy customers every day.”

Reichenbach’s solution centers not only on the APR Control, but also on the approach that system designers are taking when drawing up system installation plans.

According to him, engineers often design HVAC systems for maximum load conditions using a “design day” formula - a sunny, 95°F day with 95 percent relative humidity, and assuming maximum building occupancy.

“Unfortunately, these conditions seldom occur, and most of the time HVAC systems have more capacity than needed to satisfy actual load,” said Reichenbach. “And since most direct expansion (DX) systems lack the sophistication to run at less than full capacity, they cycle on and off constantly.”

This can result in uncomfortable conditions in the building. When the a/c system shuts off, humidity levels increase, causing occupants to feel uncomfortable even when temperatures are within the desired range. These problems are further exacerbated when portions of a building are vacant or when occupants change thermostat settings to compensate for a/c system shortcomings.

“When it comes to design solutions in my company, I am really excited about the APR Control,” said Reichenbach. “The APR Control basically takes an HVAC system designed for on/off operation and allows it to track the load - to operate at the same capacity as the load,”

While compressors pump only what they are rated for, the APR Control “de-rates” the evaporator coil for a reduced load. Especially suited for muggy weather regardless of the temperature outdoors, it maintains an a/c system in a dehumidifying mode while allowing it to run at partial capacity. If the system is not performing properly, it forces a service call, but without the emergency of an iced-up coil.

“For customers under contract, the APR Control eliminates a long list of problems and a lot of service calls,” noted Reichenbach. “For those not under contract, it corrects recurring problems and makes for happy customers.”

For more information, visit

Publication date: 08/11/2008  

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