By John West
Chester, Va.

My question involves a CVHE 450 Centravac Trane low-pressure chiller (serial number L95EO4820). When I check the display on the front of the machine and get to refrigerant pressure, it reads -8.3 psig. Shouldn’t it read in inches of mercury instead of psig?

The machine has R-123 in it. What can you tell me about the toxicity of 123? Also, please explain the “approach” method of checking for a proper charge.

From Peggy Weller
Marketing Communications Manager
Trane Commercial Systems

The machine you referenced has a Unit Control Panel (UCP-2) controller, so it does provide vacuum readings in negative psig units.

The U.S. EPA and others have stated that HCFC-123 in emergencies is as safe or safer than the refrigerant it replaced, which is CFC-11. Its allowable exposure limit is 50 parts per million (ppm), while typically equipment room concentrations are less than 0.5 ppm. Today there are over 25,000 HCFC-123 chillers installed and running, some for over a decade, with an outstanding safety track record. For a full and complete description of the safety of all alternative refrigerants, we encourage readers to ask their local Trane sales representative for a 24-minute video entitled “The Safety of Alternative Refrigerants” (CFC-Vid-27).

The approach temperature is, by definition, the difference between the leaving water temperature and the refrigerant temperature. Measuring these approach temperatures and contrasting them to the design approach temperature can be used as an indicator of fouling and/or proper charge size. These readings are available on both the evaporator and the condenser with the UCP-2 chiller control panel.

High Pressure

By Terry (no last name given)
Heating & AC Consultants
Richmond Hill, Ga.

Two years ago I installed a Unico high-pressure system in a nice house on the coast of Georgia. The system works great. The downstairs duct system is run through the web truss on the first floor. The homeowner parks his car under the floor. The house is 10 feet off the ground. The house is finished in concrete and lattice panels all the way around the bottom provide plenty of ventilation.

The problem is that there is condensation on the bottom of the sheetrock. I have cut out some of the rock. The ductwork above the rock has signs of sweating. The entire duct system installation has been removed, sealed with mastic, and reinsulated. The branch lines have been removed and replaced with R-6 flex. The R-4 duct system does not sweat. Why is there condensation forming?

From Dan Kramer, P.E.
Specialist Grade Member of RSES
Professional Engineer

In order for a panel or any surface to sweat, it must have a temperature that is lower than the dewpoint of the air surrounding it. While that statement sounds simplistic, I believe it contains the directions for a solution to your problem. I will use the term “garage” to refer to the space below the sweating panel. Your question does not state (but implies) that the sweating occurs only when the house air conditioning is running. Of course, if there is an unvented clothes drier or washing machine in the garage area, the steam or water vapors they discharge would raise the air dewpoint temperature sufficiently high that the ceiling panel would sweat, even if the ceiling was not colder than the garage. An idling auto also discharges water vapor along with carbon monoxide and other combustion products from its exhaust pipe.

Also, the garage panel would sweat if a very humid morning followed a cold night. Then the panel would chill during the night and sweat in the morning. This is the same thing that happens to a car left outdoors on a clear night.

If you decide that none of the above is to blame, I would do the following: During a hot afternoon when the air conditioner has been running for a few hours, measure both the temperature of the sweating panel using your scanning (infrared) thermometer and, at the same time, the garage air temperature using a glass tube thermometer with an etched scale (presumed more accurate). If the panel is cooler than the garage ambient, even if it is not sweating at the time, I would have to assume the adjacent duct installed over the panel, however well insulated, was cooling the stagnant air around the duct. This would mean it was cooling the panel enough to lower its temperature below the ambient temperature in the garage.

If your measurements suggest that to be the cause, I would take steps to help ensure that the ceiling panel and the garage ambient temperature were the same. I would install a small fan or blower in the cutout in the ceiling panel that would bring garage air into the space above the panel.

An alternate solution would be to install a fan in the garage area that continuously blows on or across the garage side of the panel. Either step will raise the panel temperature to very near the temperature in the garage and should stop the sweating. Either of these fans could be wired to run only when the home air conditioner runs.

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Publication date: 01/13/2003