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Condensate blockages – fix now or pay later

Air conditioning units with blocked or poorly maintained condensate drain lines damage property and affect indoor air quality. Installing traps, overflow switches and switches with integral pumps, coupled with a regular maintenance regimen will prove economical in the long run, says Gerry Spanger.

| | Mar 20, 2011 | 11:16 am
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Air conditioning units with blocked or poorly maintained condensate drain lines damage property and affect indoor air quality. Installing traps, overflow switches and switches with integral pumps, coupled with a regular maintenance regimen will prove economical in the long run, says Gerry Spanger.

Some building maintenance departments do not realise the importance of air conditioning unit condensate maintenance until it is too late, and the property they are supposed to be maintaining incurs thousands of dollars worth of water damage or, worse yet, significant mould or mildew problems.

All air conditioning systems condense humidity from the airstream which gets collected in the drain pan. When a blockage prevents a drain pan from properly draining, it overflows into its surrounding area. While property damage is visually evident, the hidden danger of mould, mildew and other indoor air quality problems can lurk for months in air conditioning systems with blocked, partially blocked or poorly maintained condensate drain lines.

All this can cause substantial damage. However, many maintenance departments overlook the potential of overflowing external units on rooftops because they think the roof pitch will drain away the condensate. Unfortunately, many air conditioning units sit on a kerb and use bottom air discharge, a design where condensate can overflow the drain pan and drop into the building.

In my opinion, all air conditioners should include an adequately sized and pitched drain line to transport the condensate away from the unit. Drain lines are a necessity. However, biological or inorganic particulate matter in the unit’s airstream invariably washes down into the condensate, and eventually clogs the drain line. Units in high pollen areas are the most susceptible to biological growth. Therefore, air conditioner condensate drain lines should be checked by maintenance departments a minimum of every three months or simultaneously, when air filters are checked and changed.

Luckily, technology has developed solutions such as cleanable and transparent P-traps, overflow switches, overflow switches with integral evacuation pumps and other air conditioner condensate overflow prevention devices to minimise these problems.


Condensate traps are available in many varieties, but the best maintenance solutions are the transparent and cleanable models. The see-through feature allows easy blockage detection. The cleanable aspect offers up to three accessible cleaning ports on the trap for a brush, which is not included with most trap brands.

Generally, condensate traps are mandated by the International Mechanical Code and the Uniform Mechanical Code, as well as the air conditioner manufacturers. However, this does not necessarily mean the installer of an air conditioning unit has actually complied with, or was aware of, these code requirements.

Traps are easy to install and should be fitted to every unit, regardless of size, or whether the unit is under positive or negative pressure.

Good practice dictates that the first fitting off the drain pan should always be a capped cross, which provides permanent access into the drain line or trap. This also permits access into the drain pan itself without the necessity of opening the unit or removing access panels.

The mechanical codes mandate the use of rigid pipe, which may be PVC, CPVC, abs, copper or other metals. Coiled pipe is specifically prohibited, because it can develop sags which create secondary traps and inhibit water flow. The pitch of the condensate drain pipe is also mandated by mechanical codes and must not be less than ¼ inch per foot.

To check an air conditioning unit’s draining integrity, water is poured into the condensate pan to ensure proper drainage.

Condensate traps also offer an IAQ benefit that is not always appreciated. In negative pressure systems, the trap prevents the introduction of unconditioned ambient air into the airstream, which reduces efficiency as well as prevents dirty air, insects and small rodents from entering occupied spaces through the drain line and air handler.

In positive pressure systems, the trap prevents energy loss by eliminating an open drain line from which conditioned air can escape.

Another anti-microbial strategy is condensate tablets that dissolve in the drain pan over the course of several months. However, this method introduces unnecessary chemicals into the environment and, therefore, should be used only in locations with histories of extreme biological growths.


Maintenance departments planning a trap installation should know that there are different trap sizes and specifications to consider:

Diameter: The standard diameter is 3/4-inch for up to 15 tonnes; one inch for approximately up to 25 tonnes, and can vary from 1/¼ inch to two inches for larger tonnage units

Depth: Trap depth is critical as it is a direct function of negative or positive pressure in the unit usually expressed in inches of water (wg). Rule of thumb is that trap depth must be at least equal to inches of water plus 30% margin of error to allow for surges in pressure in the AC unit

WG capacity: Traps come in pre-engineered ratings ranging from ½ inch to six inches wg to match the static pressure of the AC unit

It is, therefore, important to choose the right trap as per the air conditioning unit manufacturer’s specifications. It is equally important to see that the unit is sufficiently elevated to a minimum height of six inches to insure there is room for a trap and adequate pitch in the drain line.


Conventional P-traps depend on residual water level for a seal. This usually dries up when the cooling season is over, creating what is known as ‘dry trap’. A dry trap will allow insect ingress and air infiltration, retard drainage on a negative pressure system and increase energy costs on a positive pressure system. A good solution to these problems is a waterless trap, which relies on a float (which may be spring-loaded) to act as a check valve and create a seal. Waterless traps are freeze-resistant, require no priming, do not allow air to flow in either direction and permit the free flow of water at all times. Waterless traps are available in ¾, 1 and 1¼-inch diameters.


An overflow float switch is useful because it deactivates the unit during an overflow event. The switch is typically mounted to the top of the cross fitting above the trap. During a trap blockage, water rises up into the cross and lifts the switch’s mechanical float, which cuts power to the 24-volt circuit that is wired in-line with the air conditioning unit. While there are many overflow switches on the market, it is important to note that switches with a capacity of less than four amps may not perform properly on many units, because they may not be able to handle the higher currents associated with modern air conditioning systems.

Overflow switches are invaluable, but not a cure. They should not be used as a substitute for the regular maintenance of traps and/or drain lines. An overflow switch is a second line of defense that only treats the symptoms of a blockage by cutting power to the unit. Once the water recedes, as in the case of a partial blockage, the unit activates and the cycle perpetuates until the drain line blockage is physically removed. There are many varieties of switches, of which some have visual or audible warning alarms.

While an overflow cut-off switch prevents the unit from operating during blockages, there is one disadvantage. Iced evaporator coils will continue to thaw and feed the slow receding or blocked drain pan even though the unit is deactivated.


For units up to five tonnes of capacity, the ultimate in overflow protection is an overflow switch with a self-evacuating pump. Regardless of thawing ice, which may form on evaporator coils, the pump will continue to pump out the drain pan and prevent an overflow, even through the switch has deactivated the air conditioning unit. Blocked drain pans are evacuated in under three minutes by the integral pump.

As previously mentioned, a simple overflow switch only momentarily deactivates the air conditioning unit in situations of partially blocked drains. Conversely, a combination switch and pump uses stainless steel electronic probes to sense rising condensate levels. It deactivates the unit while the integral mini-pump evacuates the excess condensate into a ¼-inch vinyl tubing drain line. The pump remains live, while the unit stays deactivated until maintenance removes the blockage and manually presses the integral pump’s reset button.

Traps, overflow switches and switches with integral pumps are inexpensive to purchase and install in-house when compared to the costs of replacing damaged property. These devices are invaluable when applied along with a periodic maintenance schedule. A maintenance crew well-versed in controlling condensate will save tenants and building managers a significant amount of money long-term, while also improving indoor air quality.

The author is Vice President of  Airtec Products, Fall River, Massachusetts, USA. He can be reached at gspanger@airtecproducts.com.

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One comment on “Condensate blockages – fix now or pay later”

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