Service Hotline: 11/04/2002

November 3, 2002
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Residential Unit

COMMENT:
Name Withheld by Request

I would like to comment on the Dec. 3, 2001 Service Hotline question concerning a residential unit. There was a problem matching house temperature with set temperature.

With all due respect to the answer, you are not supposed to install an evaporator of lesser capacity than that of the condensing unit. With that said, it is not uncommon to increase the size of the evaporator (by no more than a half ton) larger than the condensing unit as this increases latent cooling (dehumidification).

This equipment pairing works well here in the Midwest, where we see hot, humid periods. (I’m talking residential comfort cooling here.)

I surmise that the evaporator coil slowly begins to frost, then it begins to ice over, and by afternoon, airflow is low and the temperature in the space is high and the compressor is being overtaxed because the equipment is applied incorrectly.

Plugged Cap Tubes

COMMENT:
By Greg Snyder
President
Edwards Refrigeration
Loveland, CO

There were several comments dealing with plugged cap tubes in the Feb. 11, 2002 Hotline. I would also like to offer some thoughts.

Something that was overlooked in the answer regarding R-134a and R-404A is the idea of stopping the sludge before it hits the inlet of the capillary tube.

Early in the introduction of small, self-contained units using R-22, our company’s techs noticed an increase in the number of units with restricted cap tubes to the point we would not replace a compressor without replacing the cap tube also. When units with R-134a were introduced and the problem of restricted capillaries hit epidemic proportions, we began using Sporlan C-052S driers. This almost eliminated the problem. Sporlan now offers a solid-core drier with an access fitting made specifically for capillary tube applications. We have used those with excellent results.

I believe the problem starts with poor soldering and housekeeping habits with certain manufacturers and is aggravated by the fact that the molecular screens in the spun driers used by OEMs do not trap the sludge. The fate is sealed by the severe operating conditions in most restaurants. The fact that health departments today want the units sealed to the wall also elevates condensing temperatures to the point that everything inside the compressor breaks down, inducing heat-related failures.

In summary, to prevent plugged cap tubes:

  • Use a solid-core drier.

  • Keep the heat as low as possible when soldering components in the system.

  • Ensure good airflow by ventilating cabinets, keeping fan shrouds in place, and installing baffles to prevent recirculation of discharge air.

    Advise your customers not to buy any equipment using R-134a in a freezer application. This is an even shorter trip to frustration.

    High-Pressure System

    QUESTION:
    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 a R-6 flex. The R-4 duct system does not sweat.

    Why is there condensation forming?

    ANSWER:
    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 dryer or washing machine in the garage area, the steam or water vapors discharged 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.

    Oil Logging

    COMMENT:
    From Robert Hollembeak
    Staunton, VA

    There was a question on oil logging in the Feb. 11, 2002 Hotline, to which I would like to add some comments.

    Here are some possible causes of oil loss or low oil pressure safety failures:

  • Short-cycling compressors due to low load, improper control staging, oscillating load controls, valve leaks, or improper pumpdown control operation causing excess compressor cycling or no cooling demand;

  • Floodback due to rapid load changes, improper expansion valve setting, improper bulb installation, improper load cycling or shut down, improper piping, improper pumpdown control, excess oil in the system, or a valve that loses control;

  • Flooded start caused by sump heaters not operating, control power loss, poor piping design, or poor pumpdown control;

  • Compressor pumping oil with excessive clearances or wear;

  • Low discharge gas or suction gas velocities because of piping sizing, hot gas bypass sizing/calibration errors, or improper component selection and/or installation;

  • Compressor loses oil at low load due to piping design problems or hot gas bypass or low lift;

  • Oil suction strainer fouled; and

  • Improper oil for application.

    Contact The Service Hotline
    Do you have a technical question for the pros? Submit your questions directly by clicking on Service Hotline on The News home page.

    Publication date: 11/04/2002

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