Free Cooling Coil Condensate Drain Calculator
Calculate moisture removal rates (GPH), minimum P-trap water seal depth for draw-through air handlers, and IPC code drain pipe sizing.
💧 Psychrometric Airflow & Coil Operating State
Daily Condensate Volume: 132 Gallons / Day
• Minimum Drain Pipe Size: 3/4" Schedule 40 PVC (Up to 20 Tons / IPC 307.2.2).
• Pitch Minimum: 1/8" per foot (1% slope) to open hub drain or dry well.
The Dry Trap Flood Trap: If the P-trap water seal $H$ is shallower than the fan suction pressure, air sucks up the drain pipe like a drinking straw! Water cannot drain, overflowing the drain pan into ceiling tiles below.
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Psychrometric Condensate Generation
When moist air passes through a chilled water or direct expansion (DX) coil operating below dew point, water vapor condenses out of the airstream. The mass rate of water removal is given by:
Flow (GPH) = Condensate (lb/hr) / 8.33 lb/gal
where W = grains of moisture per pound of dry air (7,000 grains = 1 lb H2O)
Draw-Through P-Trap Sizing Rules
In draw-through air handlers where the fan pulls negative pressure on the drain pan, atmospheric pressure outside the trap is higher than internal pressure. The P-trap must be configured with specific water columns:
- Water Seal Height ($H$): Equal to static pressure plus $1.0 ext{"}$ minimum ($H = P_{static} + 1 ext{"}$).
- Discharge Drop ($A$): Total drop from pan drain outlet to bottom of trap elbow: $A = 2 imes P_{static} + 1 ext{"}$.
Frequently Asked Questions
Why does water only overflow from the drain pan when the blower is running?
Because negative suction pressure inside the draw-through plenum holds the water in the pan like a finger over a straw. When the fan turns off, the suction vanishes and a huge wave of condensate drains out all at once.
Can a running trap use a mechanical ball float instead of water?
Yes, modern mechanical waterless condensate valves use an internal floating ball. Under negative pressure with no water, the ball seals the orifice to prevent air ingestion. When water enters, the ball floats, allowing drainage without ever needing manual trap priming.
What size drain line is required for a commercial 50-ton rooftop unit?
Under IPC Table 307.2.2, systems between 41 and 90 tons of cooling capacity require at least a 1-1/4" nominal diameter drain line.