Dry Trap Syndrome: A Cooling Startup Pitfall

May 29, 2000
“Dry trap syndrome” is not a newly discovered malady. It has been around for decades. Only recently, however, has its seriousness been fully recognized, the causes identified, and a remedy defined.

Dry trap syndrome has plagued building owners and managers since the inception of the draw-through hvac system more than 50 years ago. Today there are millions of such systems in use in this country.

Indeed, about 90% of all commercial systems are of the draw-through type, as are most of the heat pumps and electric furnace systems used in residences.

The draw-through hvac system is favored by equipment manufacturers and system designers because it affords an efficient airflow path, which yields better system performance than does the “so-called” blow-through system.

This performance advantage, however, comes with a costly penalty for building owners and managers, in the form of dry trap syndrome.

What Causes Dry Trap Syndrome

Dry trap syndrome stems from the unwise use of a plumbing (water) P-trap to form a seal on the condensate drain line of draw-through hvac systems, an application for which it is patently unsuitable.

The primary purpose of the P-trap in a plumbing system is to form an isolation seal between that system and the sanitary sewer, thereby preventing sewer gases and other contaminants from entering the building. For this application, the operating conditions are such that the P-trap provides an effective seal.

Generally, the only pressure across the P-trap seal is that resulting from the vapor pressure of water and sewer gases, and that is relatively small. P-traps in this type of system are continually supplied with adequate water to form a reliable seal. And, frequent surges of water through the trap that occur — for example, in sinks, lavatories, and water closets — purge the traps and minimize the potential for trap blockage.

The drain seal on the condensate drain line of a draw-through hvac system has an entirely different purpose. Moreover, the operating conditions are vastly different and much more demanding.

Contrary to what many in the industry seem to believe, the need for a seal on a drain of an hvac system has nothing to do with connection to the sanitary sewer. In fact, all three of the regional plumbing codes in this country prohibit direct connection of the condensate drain line to the sanitary sewer.

A drain seal is required on a draw-through system because the drain pan operates at a negative pressure (partial vacuum). When the system is operating without a drain seal, or with a trap that is dry, it acts much like a giant “Shop-Vac.” The fan draws outside air through the drain line into the system.

At the same time, the negative pressure acts to impede and/or prevent the flow of condensate from the drain pan. It is these conditions that cause the malady — property damage and health threats — referred to in the industry as dry trap syndrome.

No Water Source

Unlike the conditions that prevail in building plumbing systems, there is no source of water continually available to keep a P-trap filled. Thus, it is often dry and ineffective.

There are two primary reasons why conventional condensate traps are frequently dry and ineffective: condensate evaporation and condensate leakage caused by freeze-damaged traps (in outside locations).

Anytime an hvac system is not providing cooling — and no moisture is removed from the air — for a period of a few weeks, evaporation will destroy the seal formed by condensate.

In addition, at most latitudes in this country, condensate in a trap in an outside location is subjected to freezing conditions, which can damage the trap and destroy its seal.

Under these circumstances, the level of maintenance effort required to prevent dry trap operation is generally regarded as impractical. In fact, in some instances, successful maintenance is virtually impossible.1 Hence, most draw-through systems, which depend upon a trap for a drain seal, regularly experience dry trap syndrome.

Operating Conditions of Dry Trap Syndrome

There are two normal operating conditions where hvac systems experience dry trap syndrome: during winter heating operation and during startup for cooling operation.

During winter operation, dry trap syndrome has an effect on both human comfort and health. A dry trap allows the hvac unit — again, acting like a Shop Vac — to draw in outside air, which is sometimes contaminated with odorous and/or toxic gases.

The ingestion of odorous gas (sometimes from the sanitary sewer) occurs frequently. Reported incidences are widespread. The ingestion of toxic gas occurs less often but when it occurs, the consequences are far more serious.

Carbon monoxide (CO, a product of incomplete combustion in water heaters, furnaces, automobiles, etc.) is perhaps the most common of the toxic gases. Incidents of CO poisoning are reported frequently as news items.

Among these, there is documented evidence of persons being poisoned by carbon monoxide introduced into inhabited space through an hvac system operating without a drain seal. One remedy for this malady, although maintenance intensive, is to manually fill the trap with water — frequently.

Starting up an hvac system for cooling, when it has a dry trap, creates conditions that impose a very high cost on building owners and managers in terms of excessive service calls, maintenance efforts, equipment damage, and surrounding property damage.

What Dry Trap Can Do

Here is what happens. Immediately following startup with a dry trap, the internal negative pressure created by the hvac system acts to hold condensate in the drain pan. At the same time, it causes air to enter the condensate drain connection, often at a very high velocity.

Before condensate can flow from the drain pan, enter the trap, and form a seal, it must rise to a level in the pan which is slightly greater than the internal negative pressure. When the negative pressure (in inches of water, or in. wc) exceeds the depth of the drain pan, overflow occurs. It’s fairly common.

This condition accounts for the rash of service calls reporting flooded floors and ceiling drips (often erroneously attributed to roof leaks) every spring and summer when the cooling season begins.

Condensate overflow and the associated damage are not the only consequences of operating with a dry trap during startup.

High-velocity air entering the drain pan through the dry trap entrains condensate and propels it onto internal components, into the fan, and onto insulated walls and ducts. The resulting wetness not only damages the systems, it creates a fertile growth place for contaminating organisms, which in turn degrades indoor air quality (IAQ).

Observations show that condensate in the drain pan becomes entrained by the entering air when its velocity reaches about 16 mph. This value is exceeded in most systems operating with a dry trap. Relatively small, 5- to 10-ton hvac units frequently operate with internal pressures near 0.80 (and greater) in. wc. At this pressure, the velocity of the air entering through an empty trap is about 30 mph.

Some larger systems, with high-efficiency filters, operate with negative internal pressures near 5 in. wc. At a negative pressure of 3 in. wc (not an uncommon internal pressure for large air handlers), the entering air velocity through an empty trap is about 50 mph. At this velocity, a high volume of aerosol mist is generated and passed into the hvac system.

Under certain conditions, this may create a serious health threat, since an aerosol mist is a well-known mechanism for spreading Legionnaire’s disease bacteria.

Clearly, dry trap syndrome is a costly proposition for building owners and managers.

A Solution

Fortunately, there is at least one remedy to the dry trap syndrome malady.1, 2 It is a condensate drain seal defined generically as a “fluidic flow control device.”

This device makes use of an air seal instead of a water seal. Thus, in addition to eliminating dry trap syndrome, it eliminates all water problems common to the condensate trap. For example, it eliminates condensate flooding caused by flow blockage.

Since this seal traps no water, it does not trap debris to cause blockage. Nor does it support the growth of algae, a frequent cause of trap blockage. There may be other successful remedies for these persistent and troublesome condensate trap problems, but we know of none.

This drain seal offers building owners and managers immediate savings in terms of less maintenance, longer equipment life, and reduced damage to surrounding property. It has been estimated that this cost saving amounts to more that $80 per hvac unit per year.1

No effort has been made to evaluate the savings relating to improved IAQ, but the benefit may be even greater than the more tangible savings identified. Indeed, for some users, improvement in IAQ is the product’s most important attribute.

Produced under the trade name CostGard™, this new condensate drain seal is available in a wide range of sizes. It is simple (has no moving parts), effective, and reliable. Thousands are in use nationwide. To our knowledge, not one has failed to operate properly.

Well-known users include national chains of retailers and restaurants; hospitals and health care centers; manufacturers; major office buildings; and numerous Texas School Districts.

Explain the effects of dry trap to your customers; chances are, they will be interested in having you recommend a solution. Warren C. Trent, M.S. (Purdue University), P.E., is ceo of Trent Technologies, Inc., Tyler, TX, and an active member of ASHRAE. He has had more than 30 years of experience in fluid-flow research and development. C. Curtis Trent, Ph.D. (University of Wisconsin), is president of Trent Technologies. He has held tenured professorships and department head positions at Kansas State University, Washington State University, and North Carolina State University. The Trents may be reached at 903-509-4843 or online at