Cooling Coil Condensate Engine

What is HVAC Psychrometrics & Condensate Physics?

When warm, highly humid air passes over an active AC coil, it drops below its latent dew point. The air molecules physically cannot hold the moisture anymore, causing water vapor to aggressively condense into liquid water and rain down into the drain pan. This engine quantifies that exact destruction of vapor into gallons of liquid.

Mathematical Foundation

Stage 1: Dry Air Mass Flow

\dot{m}_{air} = Q_{cfm} \times 60 \times \rho_{air}

\dot{m}_{air}

= Mass flow rate of dry air (lbs/hr) slamming into the coil.

Q_{cfm}

= Airflow volume (CFM — cubic feet per minute).

60

= Time conversion multiplier (minutes to hours).

\rho_{air}

= Standard air density constant = 0.075 lbs/ft³ at sea level.

Stage 2: Water Extracted By Weight

\dot{m}_{water} = \dot{m}_{air} \times \frac{W_{in} - W_{out}}{7000}

\dot{m}_{water}

= Absolute weight of water ripped out of the air (lbs/hr).

W_{in}, W_{out}

= Entering and leaving humidity ratios, measured in Grains of Moisture.

7000

= The universal psychrometric constant: There are exactly 7,000 grains of moisture in one pound.

Stage 3: Fluid Condensate Volume

GPH = \frac{\dot{m}_{water}}{8.33}

GPH

= Final Gallons Per Hour of liquid condensate filling the drain pan.

8.33

= The physical weight of liquid water: 1 US gallon weighs exactly 8.33 lbs.

Laws & Principles

THE DEW POINT THRESHOLD: Condensation is not guaranteed. It physically only occurs when the metal surface temperature of the cooling coil drops strictly below the dew point temperature of the entering air mass.
THE P-TRAP VACUUM LAW: The rapidly rushing air inside an air handler creates a powerful negative static vacuum. If the condensate P-trap is improperly sized or runs completely dry, that vacuum will literally suck the water backward up the drain pipe, overflowing the pan and destroying the ceiling below.
THE ACIDITY CORROSION FACTOR: Because it scrubs CO2 out of the building air, cooling coil condensate is naturally acidic (pH 4.0 to 6.0). Standard carbon-steel drain pans will ruthlessly rust out. HVAC engineers must heavily mandate PVC or stainless-steel primary pans for structural longevity.

Step-by-Step Example Walkthrough

" A commercial makeup air unit (MAU) in humid Florida pulls 4,500 CFM of outdoor air. The entering swamp air holds 120 Grains/lb. The chilled-water coil cools the air to 55°F, dropping the moisture to 64 Grains/lb. "

1. Calculate dry air mass flow: 4,500 CFM × 60 min/hr × 0.075 lbs/ft³ = 20,250 lbs/hr of dry air.
2. Find the moisture drop: 120 Grains − 64 Grains = 56 Grains of moisture removed per pound of air.
3. Convert to pounds of water: 20,250 lbs/hr × (56 ÷ 7,000) = 162.0 lbs/hr of liquid water extracted.
4. Convert to Liquid Gallons: 162.0 lbs/hr ÷ 8.33 lbs/gallon = 19.44 GPH of condensate payload.

Final Result: This specific AHU acts like a rain machine, dumping nearly 20 Gallons Per Hour of acidic water down the plumbing lines. If the 3/4-inch PVC drain line clogs with algae, it will overflow a standard 5-gallon safety pan in exactly 15 minutes.

Quick Answer: How do you calculate AC condensate generation?

To calculate how many gallons of water an air conditioning unit produces, you must first calculate the Mass Flow of Air by multiplying CFM by 60 and the air density (0.075). Next, measure how many Grains of Moisture the coil removes from the air, and divide that number by 7,000. Finally, multiply your mass airflow by that moisture fraction, and divide by 8.33 (the weight of water). This gives you the exact Gallons Per Hour (GPH) pouring into your drain pan.

The Complete Condensate Formula

GPH = [ CFM × 4.5 × (Grains In - Grains Out) / 7000 ] / 8.33

Scaling Variables:

The Constant 4.5: This is simply the 60 minutes/hour multiplier combined with the 0.075 lbs/cu.ft standard air density multiplier.
The Constant 7000: This converts the tiny metric 'Grains' back into standard HVAC Pounds.

Total Cooling Capacity	Minimum PVC Pipe Diameter	Maximum Estimated GPH
Up to 20 Tons (240,000 BTU/h)	3/4 Inch (0.75")	~25 GPH
21 to 40 Tons	1 Inch (1.00")	~50 GPH
41 to 90 Tons	1-1/4 Inch (1.25")	~115 GPH
Over 90 Tons / Massive Custom AHUs	1-1/2 Inch (1.50")	120+ GPH

Total Cooling Capacity

Minimum PVC Pipe Diameter

Maximum Estimated GPH

Up to 20 Tons (240,000 BTU/h)

3/4 Inch (0.75")

~25 GPH

21 to 40 Tons

1 Inch (1.00")

~50 GPH

41 to 90 Tons

1-1/4 Inch (1.25")

~115 GPH

Over 90 Tons / Massive Custom AHUs

1-1/2 Inch (1.50")

120+ GPH

Catastrophic Failures & Design Mistakes

The Dry P-Trap Reversal

During the cool, dry winter, an air handler continues to circulate 10,000 CFM of air but the chilled water coil is off. The water resting in the condensate P-trap slowly evaporates over a few months. When the unit kicks on in the spring, the heavy static vacuum of the fan directly sucks raw, unfiltered mechanical room air (or sewer gas) straight up the dry condensate pipe, dispersing it continuously onto the building occupants.

The Humidifier Wars

A facility manager installs a massive steam humidifier grid to maintain 50% relative humidity in a data center. However, they placed the steam grid directly *upstream* of an active chilled water cooling coil. The humidifier injects 40 GPH of steam into the air, and the active cooling coil immediately violently condenses 40 GPH of water directly back out into the drain pan. Both systems fight each other endlessly, wasting thousands of dollars in energy.

Field Design Best Practices & Pro Tips

Do This

✓Always mandate secondary float switches. No matter how perfectly you calculate the GPH load, algae and biofilm WILL eventually clog the primary PVC line. A secondary, slightly elevated drain pan float switch must be wired to cut power to the 24V control circuit, shutting down the compressor before it causes multi-level water damage.

Avoid This

✗Never undersize your condensate pump. If you calculate a 15.0 GPH load under design conditions, do not buy a 16.0 GPH pump. Condensate pumps must push water heavily uphill (head pressure). The 16.0 GPH rating is usually at 0 feet of head. If it has to push up 15 feet to the drop ceiling, its actual capacity may plummet to 5.0 GPH, causing it to overflow immediately.

Frequently Asked Questions

How much condensate does a 3-ton AC make?

It heavily depends on local humidity. In a dry desert like Arizona, it may produce less than 0.5 GPH. In an incredibly humid area like Houston or Miami running at 1200 CFM, a 3-ton unit actively stripping moisture can easily generate 5 to 7 Gallons Per Hour of continuous liquid condensate.

Can I route condensate drains into the sanitary sewer?

Yes, but only indirectly via an 'Air Gap'. You can never hard-pipe an HVAC condensate drain directly into a building's sanitary waste plumbing. If the main sewer backs up, it would push raw sewage up into the AC unit and blow it out the ductwork. It must drop over an open air gap into a floor sink or funnel drain.

What are grains of moisture in HVAC?

A 'grain' is a microscopic unit of weight measurement dating back to antiquity. In standard US psychrometrics, there are exactly 7,000 grains in one pound of water. We use it because the amount of water vapor suspended in a pound of dry air is so small that using standard decimal pounds is mathematically annoying. A comfortable room usually sits around 65 Grains.

Why does my drain pan build up pink slime?

The 'pink slime' or algae growing in the pan is usually airborne bacteria (like Serratia marcescens) that gets trapped in the dark, constantly-wet environment of the AC chassis. Dropping condensate pan treatment tablets (biocides) into the pan every 6 months prevents this bacterial colony from growing large enough to clog the PVC drain pipe.

Psychrometric Condensate Engine