Project Report

Investigation of Coil Failures: A Whole Systems Approach

Prepared for: Geothermal Heat Pump Consortium
Report dated: July 23, 1998
Prepared by Proctor Engineering Group
Contributor: John Proctor

Executive Summary

The coil failures at this subdivision were similar to previous failures that have occurred sporadically in other locations. The earlier coil failures had been traced to corrosion from the exterior surface of the tubes. Metallurgical investigation had found chlorine present and the probable cause of the corrosion. The source of the chlorine had not been determined.

Proctor Engineering Group reviewed the existing data, visited the site, and performed diagnostic testing on four homes. In particular, the investigators were interested in determining how the entire system (home, equipment, distribution system, and occupants) interacted. Each home provided insight into the problem.

The home with the highest failure rate had been a model home for the development and, since that time, had been continuously occupied by the same family. The family complained about high bills, the repeated failures, and comfort problems.

The kitchen, bath, and utility rooms of this house contained a wide variety of chemicals. Of particular interest was the presence of a dry cleaning fluid often used on the carpet. Chlorine bleach was used for regular cleaning as well as in clothes washing.

The washing machine and clothes dryer were located in a utility room in the center of the house. This room had an exhaust fan vented into the attic space. When the heat pumps were on, pressure measurements showed that the dryer vent was connected into the return system. This connection occurred via leakage of the dryer vent and return leakage from the interstitial spaces. The investigators found dryer lint on the coil.

Investigators found a high rate of return system duct leakage (656 cfm of unbalanced return leakage). Most of this leakage is in the return that makes extensive use of interstitial spaces. The most likely cause of the coil failures in this home is the combination of:

• the presence of corrosive household chemicals
• the delivery of clothes washing chemicals and moisture through leakage
• the dryer vent into the return system
• the excessive return leakage bringing in moisture from outside
• the excessive return leakage increasing the run time of the heat pump

Some of the causes of the high bill complaints are:

• the excessive return leakage bringing in moisture from outside
• the excessive return leakage bringing in hot air in summer and cold air in winter
• the excessive house leakage allowing high levels of summer and winter infiltration
• the increased air infiltration in the summer due to the attic power ventilators

Considering the metallurgical reports, the field data on all four investigated homes, the warranty data, the dates of failure, and the climate data, Proctor Engineering Group concludes that:

• The indoor coil failures are most likely caused by corrosion aggravated by stress as the temperature and pressure of the coil are cycled.
• Ordinary and, in some cases, extraordinary household pollutants are being brought to the coil by the return system. The dryer in some cases is supplying both pollutants (chlorine and fabric softener for example) and moisture to the coil. In other cases, the chemicals and moisture may be bypassing the vent due to restrictive venting.

Proctor Engineering Group recommends that the basic design of the heat pump reduce the corrosion rate to an acceptable level. We recommend that an accelerated test be developed which will expose the coil to high concentrations of airborne moisture and household chemicals.

Proctor Engineering Group further recommends that changes in home design and construction are necessary to achieve proper function of the entire system. Specifically: ducts must leak less than 3% of air handler flow, building cavities must not be used as air returns, and exhaust vents must be leak free and terminate outside the building shell.

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