Moisture In Homes

By Dawn R. Snyder
Allentown, Pa.

I am writing to you in hopes that your experience can assist my family with a moisture problem. Our home tends to have an above-average humidity level. In the past, our air conditioning ducts would fill with water during the winter months, when we were not using the central air. Once the ducts got too full, water would drip from the air conditioning vents in the ceiling. My husband now disconnects the ducts from the vents when the fall months arrive. He then blocks the vents with insulation so that heat is not lost into the attic. Also, he wraps the return filter with plastic.

Our system is only about six years old and we have already replaced all the ductwork due to the water buildup. Because of allergy problems, the return that was installed is all metal rather than the standard construction materials. Even with the return filter wrapped in plastic, there is so much condensation forming within the return that it is causing our ceiling to become wet around the return.

Is there anything else we can do to minimize the damage that is being caused? We have contacted HVAC companies and they all seem to be at a loss. Can some type of dehumidifier system be installed within the central air conditioning unit? I assume that the main reason for the condensation is the difference in temperature between the air inside the house and the colder air in the attic, where the system is located.

Is there anything else we can do to minimize this temperature difference?

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

My first home had an attic-mounted air conditioner with both supply and return ducts in the ceiling. It cooled only the second floor. The attic ducts were essentially uninsulated. We never had a problem such as you describe. With both the air outlet and the air return registers at the same level, there was no incentive for air circulation.

The most likely way that so much water could collect in your attic ducts would be for humid home air to circulate through the ducts. The warm, humid home air would reach the cold attic ducts and cool, thereby causing the excess moisture in the home air to condense in the ducts.

You have described your home as having an above-average humidity level. In homes without a mechanical humidifier or other sources of moisture input (such as frequent showers, much cooking, or the moisture emitted from the breath and bodies of many people), the amount of moisture in the air is substantially the same as the amount of moisture in the outdoor air. Therefore, even if some air circulation through attic ducts did take place, the only internal effect might be cold drafts from the cold air leaving the upstairs ducts. Naturally, if there is no artificial humidification, the indoor humidity will be low and the health problems of people with colds or other problems such as asthma sensitive to low humidity may be exacerbated. Then a humidifier may be necessary. However, your humidifier control may be misadjusted or not operative, thereby allowing the home to become excessively humid.

A visible indication of excess interior humidity would be moisture condensation on the inside of the exterior (single-pane) windows.

If it is necessary to continue with the high indoor humidity, there may be several directions you can instruct contractors to take, perhaps simultaneously.

I recommend taking steps to curtail winter time air circulation through the air conditioning ducts. If your attic-mounted air conditioner also serves to cool a lower floor, then it is almost certain that some of the return air ducts are quite a bit lower than the upstairs ceiling outlets. This elevation difference would generate a thermal (or so-called "gravity") air circulation where the warm, humid home air enters the lowest air return outlet and rises to the uppermost point in the duct system and eventually flows out of the highest inlets and outlets. During the air conditioning process (no fans required), the moist air will cool within the attic ducts and drop its moisture, thereby causing your problem.

Your husband's wrapping the inlet air filter in plastic sheet is in exactly the right direction. However, the air filter may be far from the actual air inlet. Since air return registers normally do not have closure dampers, I would tape plastic sheets all over the low air return registers, and I would find a way to positively close the air conditioning inlet and outlet registers.

While insulating the air conditioning ducts in the attic will ensure less heat gain and provide better cooling in summer, it probably won't affect the condensation problem.

Your problem with the attic ducts dripping water during winter could only arise if there was circulation of humid house air through the ducts.

As a very last resort, you might have to accept the air circulation and resultant heat loss from the home and drafts but attempt to overcome the water problem by pitching the ducts to a common low point. Form a drain outlet in the ducts at the low point so the condensation will drain to a nondestructive place, such as outdoors or to a sewer. You should expect that there will be frost or ice formation within the ducts during freezing weather. But attics usually warm daily from sun effect, and the frost should thaw, allowing the drainage to a safe outlet to take place.

I hope these ideas help.

Sensor Testing

By John West
Chesterfield, Va.

My question involves Cope-land compressors and troubleshooting the motor protector. The applications manual instructs you to use 6 V or less when checking the thermistors and then it warns you not to check continuity in them at all.

My Fluke 12 has a 9-V battery in it. How much of that is used for continuity and would that be safe to use on the thermistors?

By Dave Bell
Copeland Corp.
Sidney, Ohio

The sensors are sensitive and can be damaged easily. Voltage or current applied to the sensors may cause damage requiring compressor replacement.

You are correct in what you stated about checking the sensors and 6 V and the continuity check. However, the Application Engineering (AE) bulletin you are referencing has been superceded. Look at Application Engineering Bulletin 10-1264, revised March 2000. It states: "Use an ohmmeter with a maximum of 9 VAC for checking. ...and no attempt should be made to check continuity through them with other than an ohmmeter."

The Fluke 12 should be fine for making this check. To verify this, you can check your Fluke 12, or any ohmmeter, as follows. Set your ohmmeter on the ohm scale you wish to use. With another voltmeter, check the voltage on the leads coming from your ohmmeter. Even though a 9-V battery powers the meter, the voltage output will only be about 1 or 2 V. The voltage output of the ohmmeter is what is being used to check the resistance (ohms) of the sensors. As long as the ohmmeter output does not exceed 9 VAC, you will be OK.

As for the continuity check, we say only to use the ohmmeter. Other ways of performing continuity checks may apply excessive voltage, which would damage the sensors.

The most current Application Bulletins can be found on the Copeland Web site at

From the home page, select "Online Product Information" and look under "Publications."


By David C. Long
DeSoto, Mo.

I am writing in reference to the Hotline exchange concerning pick-up problems with different-size contactors.

I had the same problem some 30 years ago. We found the cause to be the fact that some contactors had a 2-ohm resistance and some have 4 ohms or greater. The contactor with 2-ohm resistance will not always pick up on a 40-VA transformer and requires a 75- to 100-VA transformer.

We found that a contactor with 4-ohm resistance would pick up regardless of the contactor's rated amps. After figuring this out, we always stocked our trucks with contactors that had a 4-ohm resistance.

I have heard discussions that the reason for the contactor problems was either the length or the size wire in the low-voltage wire. Our experience has been very good when we use the 4-ohm resistance contactors.

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Publication date: 05/03/2004