P-Traps

QUESTION:
By Michael Solow
Via E-Mail

How do you p-trap condensation lines when the fan pulls air through the cooling coil or the fan pushes air through the cooling coil? How do you p-trap refrigeration lines when the condensing unit (no compressor) is higher than the air handler and compressor or lower than the air handler and compressor? How do you p-trap refrigeration lines on a residential application condensing unit 35 to 60 feet higher than the air handler? Can the p-trap be eliminated if the line sizes are changed?

ANSWER:
From Gene Silberstein
Consulting Engineer

An important aspect of proper system installation is the proper installation of the lines that carry condensate from the air handler. The air handler can be either forced draft, where the air is pushed through the evaporator coil, or induced draft, where the air is pulled through the evaporator coil. These two situations are different and will be addressed separately.

We will first examine the forced-draft evaporator. It is good field practice to install a p-trap on the condensate line of a forced-draft evaporator to prevent the leakage of supply air. If the drain is located after the blower, supply air will be blown down the drain. It is important to note at this point that the air handler will drain fine without a p-trap as long as the drain is located after the evaporator coil.

The construction of the p-trap used in this application should have an offset of approximately 1-inch, meaning the outlet of the p-trap should be at least 1 inch lower than the inlet of the trap. The height of the trap, from the outlet to the bottom of the trap, is generally equal to the total static pressure in the duct system plus 1 inch.

If however, the condensate drain is located before the coil, a p-trap must be installed on the line. If the trap is not installed the following problems could result:

• Sewer line vapors can be drawn into the occupied space if the drain line is piped into a waste line (this depends on local codes, check codes for your area.)

• The unit will not drain properly (because the pressure in the drain line at the pan will be lower than atmospheric) and water will accumulate in the air handler leading to unpleasant odors and/or the introduction of harmful bacteria to the occupied space.

The construction of the p-trap used in this application should have an offset equal to the static pressure plus 1 inch, meaning the outlet of the p-trap should be lower than the inlet by an amount equal to the static pressure plus 1 inch. The height of the trap, from the outlet to the bottom of the trap, is generally equal to one-half of the offset from inlet to outlet. This larger offset prevents the condensate in the trap from flowing back into the pan on initial startup of the blower.

In either case, the material used in the condensate line must not be permitted to sweat. If local code permits the use of PVC piping, there is not a problem with sweating.

If however, the line is made of copper, it must be insulated with closed cell foam insulation, such as Armaflex. The lines must also be pitched away from the air handler to allow proper drainage. Usually a pitch of about 3/8 inch per foot is sufficient. (Check with the manufacturer of the specific piece of equipment).

Condensate drain lines should not be reduced. The size should be the same if not larger than the fitting on the drain pan.

Regarding your question on the condensing unit:

When there is a remote condenser located above the air handler and compressor, the discharge line should be trapped as follows:

• A p-trap should be installed at the bottom of the vertical run from the compressor up to the condenser.

• An inverted p-trap should be installed at the top of the vertical run from the compressor to the condenser.

• If the vertical run between the compressor and condenser is long, additional p-traps should be installed every 12-15 feet.

The reason for installing these traps is to collect any refrigerant that condenses in the discharge line before reaching the condenser. Since liquid refrigerant is denser than vapor, it will try to fall back down towards the compressor.

If enough refrigerant condenses in the vertical discharge line, the velocity of the refrigerant can drop. The installation of the traps helps minimize the effects of this problem. If the condenser is located below the air handler and compressor, no trapping is necessary.

Finally, regarding your question on residential units, I’m not quite sure what you’re asking for here. By definition, a condensing unit consists of the condenser as well as the compressor. In a previous question, you made reference to a condensing unit without a compressor, which is simply a condenser.

If the piece of equipment is a condensing unit (condenser and compressor) and is located 35 to 60 feet higher than the air handler, the following holds true:

• The suction line should be trapped at the base of the vertical run up to the compressor.

• There should be an inverted p-trap in the suction line at the top of the vertical run.

• After this inverted trap, the suction line should be pitched slightly downward towards the compressor.

• If the vertical run is in the range of 35 to 60 feet as you specified, additional p-traps should be installed every 15 feet.

This helps ensure proper oil return to the compressor. These traps cannot be eliminated if the line sizes are changed. Line sizes are reduced to maintain refrigerant velocity. The installation book provided with new equipment provides line-sizing charts that are to be used to determine the proper line sizes for both suction and liquid lines, depending on the length of the piping run as well as the vertical distance between the components.

Publication date: 03/12/2007