Most homeowners and many home builders give little thought to the air conditioning condensate line or trap. The primary purpose of a condensate trap is to prevent air from moving in or out of the air handler during operation. Traps must be installed in a manner that will stop the air from passing through, but still allow the condensate to drain from the condensate pan.
When the trap is omitted, efficiency is lowered as air is lost through the condensate drain in blow-through systems. Failure to install a trap on a blow-through system is the same as drilling a hole in the ducts for each drain connection. The pressure around the pan on a blow-through system guarantees the pan will drain whether it is trapped or not.
Trapping is a major issue on draw-through systems. Untreated air can be drawn into the airstream while the system is running. If the coil is located in an attic or other warm space, there is even greater reason for concern. As on a blow-through system, a drain on a draw-through system that is not trapped is an efficiency issue. But more importantly, the air being sucked through the drainpipe can prevent the pan from draining, causing it to run over.
Without proper trapping, air pulled back into the equipment can lift the water up from the condensate pan, like sucking water up into a straw. This can cause the insulating liner material and many of the components located nearby to become wet. Microbes in the water can become a problem if they grow on the insulation. As the microbes multiply they can release spores into the airstream, raising the probability of health risks.
When a trap seal is too shallow, it dries out during each cooling cycle startup. Running traps are prone to fail in this manner. Some manufacturers specifically recommend against the use of running traps.
If the seal is too deep, it actually can cause condensate to be held in the pan. A trap with an excessively deep seal also is prone to clogging. Because fan speed, duct size, coil condition or other issues are specific to unit configurations, the trap design should be specific to the unit as well. Rarely, if ever, do static pressure considerations come into play when condensate traps are installed.
Equipment manufacturers often recommend a drop from the condensate pan at a minimum of 2 inches and then a minimum of a 2-inch trap seal. If your trap is outside these parameters, you should look more closely for signs of moisture concerns in or around the equipment.
There are a number of piping mistakes made when installing a condensate trap. An open cleanout between the trap and coil is one of the most common mistakes. It could be that installers think leaving the pipe open will help the system drain, working much as a vent does on the house plumbing. As we now know, an open cleanout at this location allows air to bypass the trap altogether. This mistake is easily corrected by placing a cap over the cleanout pipe and only removing it for cleaning purposes. It should be easy to remove and does not need to be glued. If you are concerned about a clogged trap, you can glue a threaded adapter on the end of the pipe and screw in a plug.
Connecting multiple units to one trap often allows air to be pulled through one or more of the units, bypassing the trap as well. Each unit should have its own individual trap, and some equipment makers recommend against installing the trap more than 4 feet away from the coil.
Many manufacturers and trap makers recommend using removable caps on tees and crosses for cleaning purposes. Not necessarily a mistake, you should install a cleanout at the end of your cooling season. There are products on the market now which have clear fittings so you can see if it starts to clog and clean it out before you have problems.
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