Evaluating condensation and condensate

Learn methods to capture, control, dispose, and derive the condensation requirements within HVAC systems.


This article is peer-reviewed.Learning Objectives:

  • Evaluate the code requirements for condensate pipe sizing and how to adapt these for additional use.
  • Analyze psychrometrics and how this impacts condensation on building interiors and exteriors.
  • Illustrate dew point and how supply air temperatures impact condensation within buildings.
  • Explain the code requirements of cooling coil installations above ceilings.

Condensation is one of the simplest and most recognizable displays of psychrometrics in our daily lives. Most people can remember in their childhood seeing the “sweating” of a soda can or on the outside of an ice-cold glass of water on a summer day. At that time, we didn’t understand the physical factors behind this display. Mostly, we just tried to keep the glass from dripping on ourselves or on our mother’s table.

Figure 1: This psychrometric chart uses Macau, China, design conditions for outside air and inside air with 50% outside air. All graphics courtesy: JBA Consulting EngineersBasics of psychrometricsFrom a mechanical engineering practice in commercial buildings, condensation and condensate are found as part of the air and water systems. This article focuses on the air- and water-side components as they relate to the air conditioning. We typically refer to condensation as it forms on building elements (diffusers, windows, mirrors, etc.) and condensate as it forms on HVAC cooling coils (fan coil units, air handling units, etc.). These two are closest in relation and are due to the same psychrometric points used to provide conditioned air to a space.

Condensate is water that is drawn out of the air stream as it passes through the cooling coil to reach the required leaving-air temperature for space conditioning. It forms as air is cooled beyond its dew point where the dry bulb (DB) temperature equals the wet bulb (WB) temperature (see Figure 1). This figure indicates the standard psychrometric state points for air conditioning within Macau, China. This area possesses a high WB temperature and relative humidity year-round and especially during the summer months. These conditions result in a significantly higher condensate flow rate and formation of condensation on building elements present in other areas, such as Las Vegas.

The quantity of water condensed out of the airstream is equal to the difference in water mass flow rate between the two state points. In the case of Figure 1, condensate forms as the air is sensibly cooled from the mixed-air temperature (MAT) state point of (83.4°F DB/73.9°F WB) down to the respective dew point of 70.1°F. At this point, the air cannot be cooled further with the same quantity of moisture present (under standard HVAC conditions). The humidity ratio and absolute humidity must decrease to maintain the air state point within the bounds of the psychrometric chart. As the air moves down the saturation line to the leaving air temperature (LAT) 51.8°F DB/51.3°F WB) a mass flow rate of water is condensed out of the air.

The quantity of water formed into condensate is determined from the following physical properties of the airstream:

  • Mass of moisture in the air as part of the humidity ratio at each point between the mixed-air temperature and leaving-air temperature (kilogram/kilogram or pounds/pound of moisture per unit of dry air): ΔW
  • Specific volume of the air (cubic meter/kilogram or cubic foot/lbm): V
  • Airflow rate of supply air being cooled from the mixed-air condition to the leaving-air condition off the cooling coil (liters/second or cfm): Q

The equation will produce a flow rate in mass per unit of time by default (seconds or minutes based upon the airflow rate). This is due to the specific volume factor, but it can be easily converted to a water flow rate in l/s or gallons per minute by converting from mass to volume of water (1l = 1 kg and 1 gal = 8.34 lbm). Along with these volumetric flow rates, we can derive the code requirements and better explain the correlation between refrigeration tons (kilowatts) and condensate pipe sizing, which will be discussed in greater depth later in this article.

Figure 3: This shows water condensing on the exterior of windows due to the glass temperature being lower than the dew point of the ambient air conditions in Macau, China.

Code requirements

Below are excerpts taken from the 2015 International Mechanical Code (IMC), which directly relate to equipment requirements, pipe sizing, routing, and termination.

307.2 Evaporators and cooling coils: Condensate drain systems shall be provided for equipment and appliances containing evaporators or cooling coils. Condensate drain systems shall be designed, constructed, and installed in accordance with Sections 307.2.1 through 307.2.5.

Exception: Evaporators and cooling coils that are designed to operate in sensible cooling only and not support condensation shall not be required to meet the requirements of this section.

307.2.1 Condensate disposal: Condensate from all cooling coils and evaporators shall be conveyed from the drain pan outlet to an approved place of disposal. Such piping shall maintain a minimum horizontal slope in the direction of discharge of not less than 1/8-unit vertical in 12-units horizontal (1% slope). Condensate shall not discharge into a street, alley, or other areas so as to cause a nuisance.   

307.2.2 Drainpipe materials and sizes: Components of the condensate disposal system shall be cast iron, galvanized steel, copper, cross-linked polyethylene, polyethylene, ABS, CPVC, PVC (polyvinyl chloride), or polypropylene pipe or tubing. Components shall be selected for the pressure and temperature rating of the installation. Joints and connections shall be made in accordance with the applicable provisions of Chapter 7 of the International Plumbing Code relative to the material type. Condensate waste and drain line size shall be not less than 3/4-in. internal diameter and shall not decrease in size from the drain pan connection to the place of condensate disposal. Where the drainpipes from more than one unit are manifolded together for condensate drainage, the pipe or tubing shall be sized in accordance with Table 307.2.2. Please review Table 307.2.2 for specific refrigeration tonnage and condensate piping sizes.

The condensate drain piping must be provided with a U-type trap prior to being indirectly drained into the approved receptor (floor sink, planter, etc.). The trap height is as required to compensate for the negative fan pressure in draw-through units and allow for draining of condensate without backing up into the drain pan itself. This liquid trap prevents air from entering or leaving the equipment casing while allowing for condensate to drain away from the unit.

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