Cooling coil condensate system design
Cooling coil condensate is an important aspect of HVAC system design and should be carefully considered to avoid major issues in the future
- Know how to size the condensate trap right for blow– and draw–through fan configurations.
- Learn about sizing the condensate pumps and condensate pipes.
- Understand code requirements for condensate disposal per International Mechanical Code.
One afternoon a mechanical contractor reported a situation on a project. He said the cooling coil drain pan inside the air handler was flooding. He also advised that the condensate pipe was not draining any water. We were surprised and couldn’t think about what could have gone wrong. We went to the site and found that drain pan and the air handling unit floor was flooded and the fan inside the air handler was wet. When the coil section door was opened, however, the water in the pan drained very quickly.
We had studied the design of the condensate trap. We inspected the trap to figure that the difference in elevation between the AHU drain pan outlet and the exiting end of the U trap (termed here “H”) was zero. The reason this was happening was the coil section was at negative pressure since this was a draw through AHU. Since the “H” of the trap was zero, the suction pressure through the U trap inhibited the draining of any water. This leads to the accumulation of water in the drain pan and resulting flooding inside the AHU (see Figure 1).
Once we fixed the trap, the problem went away. It came at the cost of $2,000, which included the cost to raise the AHU by 4 inches. The client was not happy about the change. There have been other cases where the AHU had to be raised, or condensate pumps had to be installed because the details were not thought out during design. The cost of these changes could be in the tens of thousands of dollars.
Ignoring coil condensate design makes engineers look incompetent. It can cause health issues because of mold and algae growth when not noticed early. Sizing the condensate trap is commonly overlooked, and there is no good literature that covers all aspects of design.
What is condensation?
When people think of condensation, it’s common to think of water droplets accumulating on a glass of water with ice or mist accumulating on a car windshield. Humid air condenses easily, which means condensation is much more common in Miami as compared to Phoenix. An air conditioner that moves air at higher velocity produces condensate at a higher rate because condensate volume is proportional to the supply flow rate and the air density. Lower density air will result in a lower condensation rate. In engineering terms, condensation occurs when air hits a surface cooler than its dewpoint temperature. This is what happens at coil surfaces inside an AHU.
Latent heat is transferred by moisture in the air to the coil via the process of condensing on the surface of the cooling coil. Upon completing a psychrometric chart, we can see when relative humidity increases from 60% to 70%, dewpoint increases (see Table 1).
As a result, it becomes easier for water to condense on the cooling coil. The higher relative humidity also means that the moisture content of the air is higher (expressed as the humidity ratio or specific humidity) and thus more moisture can potentially be condensed on the coil.
The condensate generated gets collected in a drain pan, which is placed below the cooling coil. The pan must be drained continuously to prevent overflowing and causing any equipment damage. Failing to do so leads to unwanted problems, including biological growth such as algae. Using a mathematical formula to calculate condensate volume can help users find the right medium.
Condensate volume can be calculated for specific situations. This is based on 2017 ASHRAE Fundamentals, Chapter 1 Section 8: numerical calculation of moist air properties.
From the above equation, we can conclude the volume of condensate generated is a measure of the specific humidity of air entering the AHU, which is a function of the dry bulb and wet bulb of the air and local elevation. To put this in perspective, condensate volume generated for an identical building with identical occupancy and orientation in five different climate zones was compared. A commercial–grade load calculation software to perform these calculations was used. Ventilation rate was calculated per Chapter 4 of 2015 International Mechanical Code. Assumptions are listed below. See results in the Table 2:
Use: Office building
Building size = 10,000 square feet, Occupancy = 250 square feet/person, Miscellaneous load = 1 workstation/person
The identical orientation of the building for all zones
U values for roof, walls, windows as per the 2013 edition of ASHRAE 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings.
Table 1 gives an overview of how the amount of condensate collected varies in different climatic conditions.
There could be various scenarios where incorrectly installed trap and condensate piping can lead to issues. Following are most common causes issues observed:
No trap/trap too short: The water in the drain pan will not drain, causing flooding and air spray effect inside the AHU (see Figures 4B and 4C). The negative pressure will cause the air to backflow into the system. This incoming air stream due to the negative pressure created by the draw–through fan could have enough velocity to pick up droplets from the water at the bottom of the drain pan and cause a spray/mist (see Figure 4A). The mist carried by air can make the fan and ductwork wet and create humidity issues in space.
Shared trapping: Having a single trap for multiple units is a poor way to design a condensate trap. There can be a scenario where the fans of connected units are operating at different static pressures. The unit operating with greater static pressure will result in drawing air through the drain line of another unit. It can lead to a similar effect as described in Figure 4B.
Air locking in the pipe due to improper supports: Incorrectly supporting of condensate pipe will result into sagging of the pipe creating an airlock, which can, in turn, result in flooding of the pan as shown in Figure 4C.
Inadequate slope: Condensate usually travels by gravity, and therefore piping should be pitched in the direction of flow per IMC (see code requirement section below). This is a commonly observed issue. Designers usually find during construction that condensate piping cannot be pitched due to several site-specific conditions. Engineers often specify condensate pump during construction to resolve this issue.
Condensate pump integral to precision cooling units: Several times the condensate pumps inside the computer room air conditioning or other precision cooling units do not have enough head to pump water to the floor drain that is 100 feet away. Evaluating this during design can prevent a headache and costly change orders.
Designing a trap
For blow-through units, the trap must be designed as shown in Figures 5B and 5C to avoid the problems associated with condensate traps. Ideally, it is recommended to have ½ inch of safety factor for any unaccounted increase in pressure. This also would take care of any increase in pressure drop due to dirty filters over the period. When the fan starts, it would create a positive pressure (blow-through fan) and pushes the water away from the pan, resulting into proper draining of the system.
For draw-through units, the trap must be designed as shown in Figures 6B, 6C and 6D to avoid issues discussed earlier. The recommended safety factor of 1 inch is a perfect balance between the increase in pressure due to any unaccounted components and efforts to keep the overall length feasible. When the fan starts, it creates a negative pressure (draw-through fan) and the size of the trap, “H+1” (inches water column) provides enough head to make sure water is not backed up into the system, thereby ensuring proper functioning of the system.
Code requirement and enforcement within the U.S. varies from one location to another. The authority having jurisdiction has the governing authority and has the authority to override or modify requirements listed in the national codes. IMC is a widely referenced code in the U.S. Most AHJs have adopted the code with some region-specific modifications.
In addition, industry good practices have been used to complement it so they can use good judgment, sound engineering principles, and local practices instead of blindly following suggestions.
The IMC code is vague as it relates to condensate disposal. It says that condensate must be disposed into an “approved location” and that it should not dump on walkways, streets or alleys as to “cause a nuisance.” This leaves a lot of wiggle room for interpretation and much authority to the AHJ and design professionals to establish what is and what isn’t an “approved location.”
Here are a few guidelines:
- Do not dump condensate around foundations, basements or other areas that could cause ponding, erosion and/or leakage.
- Do not dump condensate from a large rooftop units to prevent pounding. Route it to nearest roof drain.
- When discharging into a shared drain or sewer system ensure that it is not piped in such a way that waste fumes could enter the system or occupied space.
- Don’t dump condensate in places that could create trip hazards.
2015 IMC 307.2.2 requires that an air conditioning condensate drain inside diameter should not be smaller than ¾ inch and should not be smaller than the drain pan outlet diameter. Three-quarters of an inch is sufficient for up to 20 tons according to the IMC unless the drain outlet size is larger than ¾-inch. Use Table 307.2.2 for condensate pipe sizing.
2015 IMC dictates a 1% minimum pitch of the drain which is equal to 1/8 inch fall for every 12 feet of horizontal run. Wherever practical, it is safer to use ¼ inch of fall per foot to ensure proper drainage.
Drains can be made from many materials such as acrylonitrile butadiene styrene, chlorinated polyvinyl chloride, polyvinyl chloride, steel and copper. However, PVC is by far the most common. When a drain line is PVC, the IMC dictates that it should be supported every 4 feet when horizontal (while maintaining proper pitch) and every 10 inches of vertical run.
2015 IMC 307.2.5 states that the condensate assembly must be installed in such a way that the drain line can be “cleared of blockages and maintained” without cutting the drain.
Connecting condensate line to a sewer pipe in the building shall be carefully evaluated for approval and compliance by AHJ. Where connecting to sewer line is allowed, an air gap fitting should be provided at the connection.
Venting after (downstream of) the trap is a really good idea in most applications because it helps prevent airlock that can occur due to double traps and shared drains as well as prevent syphoning. This vent is after the trap and must remain open to be effective. The vent opening should always rise above the trip level of the condensate overflow switch when it is in the primary drain line or pan or above the secondary/auxillary overflow port on the primary drain pan. This helps ensure that if a backup occurs, the water properly trips the switch instead of overflowing out of the vent. While venting is a common best practice, it is not part of the IMC.
2015 IMC does not directly state that the drain line must be insulated. When routing condensate pipe through concealed areas, it is a good practice to insulate them to eliminate any chances of condensation. In mechanical rooms, insulation will prevent any sweating, and prevent potential trip hazards. In most situations, ½–inch fiberglass or flexible elastomeric pipe insulation (minimum R-2) with a vapor barrier would be sufficient. Some municipalities do require drain inside the structure be insulated to prevent condensation.
Four additional best practices to consider
1. Trap with tees
It is always a good practice to provide drain pipes with tees. Tees can be used for inspection purposes and used for priming the pipe.
2. Condensate pumps
Condensate pumps shall be used where gravity drains are impossible to install. This is commonly seen in residential and commercial sites, where no floor drains were provisioned to drain condensate. The piping from the cooling coil to the condensate pump reservoir should be installed with minimum 1/8-inch slope to enable gravity flow. The condensate shall be collected into the reservoir. Once the water reaches a certain level in the reservoir, the float switch within the pump will turn it on, and water is pumped from the reservoir to a safe location.
3. Auxiliary drain pan
Water leaks in applications such as data centers can be very costly. To offset this, users should provide an auxiliary/emergency drain pan under the cooling equipment. The pan can have a water leak detection sensor installed at the lowest point. The sensor can be tied to a building management system and send water alarms. Where required, the signal can also be used to turn off the cooling equipment when water is detected.
4. Condensate headers
When multiple units with pumped condensate are headered together, use inverted traps and pitch header in the direction of flow.