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Calculator · Condensate
Condensate drain calculator
A cooling coil turns moisture in the air into water you have to get rid of. On the worked example, a 1,200 CFM coil pulling 80 °F, 67 °F wet-bulb air down to 55 °F makes about 1.57 gallons an hour. Enter your airflow and readings to get the rate, the daily total and the drain size and slope the 2021 IMC and IRC call for. For the moisture load of a whole space, the dehumidifier size calculator is the better fit.
- Type the CFM through the coil and say whether you measured it on the return side or the supply side.
- Add dry-bulb and wet-bulb for the air entering and leaving the coil, the elevation and the hours of cooling per day.
- Pick one unit or a shared drain with its total tons, and add the horizontal run if you want the minimum fall.
Example result for the default inputs
1,200 CFM measured in the air entering the coil. Entering 80 °F / 67 °F WB, leaving 55 °F / 54 °F WB, sea level, 12 hours of cooling a day.
Condensate
1.57 gallons per hour
Per day of cooling
18.83 gallons
Drain line
3/4 inch minimum
| Drain for one unit (IMC 307.2.2, IRC M1411.3.2) | 3/4 inch minimum, never smaller than the pan connection |
|---|---|
| Minimum slope (IMC 307.2.1, IRC M1411.3) | 1/8 inch per foot (1 percent). Some air handler instructions ask for 1/4 inch per foot (Goodman, Aspen), and where the unit's instructions ask for more, they govern (IMC 304.1, IRC M1401.1). |
| Condensate pump, rated flow at its lift | at least 1.57 gallons per hour |
| Water condensed | 13.09 lb per hour |
| Entering air humidity ratio | 0.01113 lb water per lb dry air |
| Leaving air humidity ratio | 0.00861 lb water per lb dry air |
| Entering air dew point | 60.3 °F |
| Dry air through the coil | 5,200 lb per hour |
For estimation purposes only. Consult a licensed engineer for final designs.
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How condensate and the drain are calculated
Water off a coil is the moisture the air carried in minus the moisture it leaves with, times the pounds of dry air going through. The drain size and slope come straight from the mechanical code, not from the water rate.
- Water condensed
m_w = 60 x CFM / v x (W_entering - W_leaving), lb per hourW is the humidity ratio, pounds of water per pound of dry air, from each dry-bulb and wet-bulb pair. v is the specific volume of the air the CFM was measured in. When W_leaving is as high as W_entering, the result is 0.- Gallons
gal/h = m_w / 8.34, gal/day = gal/h x hours of cooling8.34 lb per gallon is water at 50 to 60 °F, figured from the NIST gallon and pound and the USGS density table.- Drain size, one unit
3/4 inch minimum, never smaller than the pan connectionIMC 307.2.2 and IRC M1411.3.2. The line also may not get smaller anywhere between the pan and the point of disposal.- Drain size, shared line
IMC Table 307.2.2 by total tons on the lineApplies where drains from more than one unit are manifolded together. The IRC sends a shared drain to an approved method instead of a table.- Slope
fall = run x 1/8 inch per footIMC 307.2.1 and IRC M1411.3 set a minimum of one-eighth unit vertical in 12 horizontal, a 1 percent slope, in the direction of discharge. Some air handler instructions ask for 1/4 inch per foot, and the unit's instructions govern where they ask for more (IMC 304.1, IRC M1401.1).
Water off four coils, and the code table
Each row goes through the same function as the form, airflow measured in the air entering the coil, 12 hours of cooling a day.
| Readings | Entering dew point | Gal per hour | Gal per day |
|---|---|---|---|
| Worked example, 1,200 CFM, 80/67 to 55/54 | 60.3 °F | 1.57 | 18.83 |
| Humid return, 1,200 CFM, 78/68 to 55/54 | 63.1 °F | 2.32 | 27.84 |
| Five-ton air handler, 2,000 CFM, 78/65 to 55/53 | 57.8 °F | 2.20 | 26.43 |
| Drier return, 2,000 CFM, 76/64 to 54/53 | 57.2 °F | 1.73 | 20.80 |
The humid return has the same airflow and the same leaving air as the worked example. Its entering dew point is 2.8 °F higher, and that alone makes 1.5 times the water. The moisture in the return air sets the water rate far more than the size of the unit.
Shared drains, 2021 IMC Table 307.2.2. A drain for one unit is 3/4 inch minimum and never smaller than the pan connection.
| Total capacity on the drain | Minimum pipe diameter |
|---|---|
| Up to 20 tons | 3/4 inch |
| Over 20 to 40 tons | 1 inch |
| Over 40 to 90 tons | 1-1/4 inch |
| Over 90 to 125 tons | 1-1/2 inch |
| Over 125 to 250 tons | 2 inch |
At 1/8 inch per foot, a 30 ft horizontal run needs at least 3.75 inches of fall between the trap outlet and the end of the run, and 7.50 inches at the 1/4 inch per foot some air handler instructions ask for. Plan the hangers before the ceiling closes.
Codes and data sources
- 2021 International Mechanical Code, Sections 307.2.1, 307.2.2, Table 307.2.2 and 307.3 - condensate slope, minimum drain size, shared-drain sizing by tons and the shutoff rule for pumps in attics and crawl spaces.
- 2021 International Residential Code, Sections M1411.3, M1411.3.2 and M1411.4 - the same slope and 3/4 inch minimum for one- and two-family dwellings, and the pump shutoff rule.
- ASHRAE Handbook, Fundamentals, Psychrometrics chapter - humidity ratio, specific volume and the water balance across a cooling coil. The test suite reruns the chapter's worked coil example.
- Goodman/Daikin AMST air handler installation manual IO-4011A, and Aspen AAM installation manual IO-123073 - drain pitch of 1/4 inch per foot asked for by the manufacturers, above the 1/8 inch code minimum.
- NIST Special Publication 811, Appendix B.8, and USGS Water Science School, Water Density - the gallon, the pound and the density of water behind 8.34 lb per gallon.
Limitations of this calculator
- The water rate is only as good as the airflow and the wet-bulb readings. It scales one for one with CFM.
- Readings give the rate at that moment. A hotter, more humid day makes more water, so size a pump from readings or design conditions near your worst case.
- Code text is the 2021 IMC and IRC. Your jurisdiction may have adopted another edition or local amendments, so check the adopted code.
- Shared drains above 250 tons are beyond IMC Table 307.2.2 and are not sized here.
- Auxiliary pans, secondary drains, traps and float switches have their own code sections (IMC 307.2.3 and 307.2.4, IRC M1411.3.1) and the equipment manufacturer's instructions. This page does not size them.
- Condensate from condensing furnaces and boilers is a different stream and is not calculated here.
- The page flags readings that work out below 250 CFM per ton, a sensible heat ratio below 0.3, or leaving air below 40 °F. Those limits are this calculator's assumptions for a comfort cooling coil, not code limits. Readings inside them can still be wrong.
Common condensate drain mistakes
- Reducing the line after the trap. The code says the drain may not decrease in size from the pan connection to the point of disposal. A 3/4 inch pan outlet bushed down to 1/2 inch tubing is a violation and a clog waiting to happen.
- Running the line flat. A line that looks level sags between hangers and holds water. Support it often enough to keep at least 1/8 inch per foot of fall the whole way, or 1/4 inch where the air handler instructions ask for it.
- Picking a pump by its open-flow number. Pump flow drops as lift goes up. Compare the calculated gallons per hour with the pump's rating at the height and run you actually have.
- Leaving out the safety shutoff. In an attic or crawl space the pump has to stop the equipment when it fails (IMC 307.3, IRC M1411.4). Wire the pump's switch into the control circuit, not to an alarm alone.
- Using one hot afternoon's reading for the year. Water rate swings with indoor humidity. A reading on a dry day can make a weak pump look fine.
Common questions about AC condensate
How much condensate does an AC produce per hour?
It depends on airflow and on how much moisture the coil pulls out, so it changes from house to house and day to day. On the worked example, 1,200 CFM going from 80 °F dry-bulb and 67 °F wet-bulb to 55 °F and 54 °F, the coil condenses 13.1 lb of water an hour, about 1.57 gallons, or 18.83 gallons over 12 hours of cooling.
What size condensate drain line do I need?
For one unit, the IMC and IRC both set 3/4 inch as the minimum, and the line may not be smaller than the drain pan connection. Where several units drain into one manifolded line, IMC Table 307.2.2 sizes it by total tons, 3/4 inch up to 20 tons, 1 inch over 20 to 40, 1-1/4 inch over 40 to 90, 1-1/2 inch over 90 to 125, 2 inch over 125 to 250.
What slope does a condensate line need?
At least 1/8 inch per foot, a 1 percent slope, toward the point of disposal (IMC 307.2.1, IRC M1411.3). A 20 ft horizontal run needs at least 2.50 inches of fall. Some air handler instructions, Goodman and Aspen among them, ask for 1/4 inch per foot, 5.00 inches over the same run, and the unit's instructions govern where they ask for more (IMC 304.1, IRC M1401.1).
How do I size a condensate pump?
Pick a pump whose rated flow at your lift and run is at least the gallons per hour this calculator gives at your worst-case conditions. How much margin to add above that is your call, and the page does not add one. In an attic or crawl space the pump must also shut the equipment down when it fails.
Why does the calculator show zero condensate?
Because the leaving air holds as much moisture per pound as the entering air. The coil is running dry, with its surface above the entering dew point. That is normal with very dry return air. If water is still showing up in the pan, recheck the wet-bulb readings.
Is there a rule for gallons of condensate per ton?
Rules like that float around, but the amount of water depends on indoor humidity and airflow, not on tons alone. In the table on this page two coils at the same 1,200 CFM make 1.57 and 2.32 gallons an hour. Measure, or use your design indoor condition, and calculate.
How is this different from the dehumidifier size calculator?
The dehumidifier calculator estimates how much moisture a space takes on per day, in pints. This page takes the readings across a coil and returns the water that coil is removing, plus the drain it needs.
Used on the same job
Get the cooling load in chat
For a new install you have no readings yet. In chat the assistant runs the free whole-house Manual J load, which gives the cooling tons and the sensible heat ratio, and it can solve the air state at any dry-bulb and wet-bulb you give it. The air leaving the coil comes from the manufacturer's data, not from the chat. Take it from there and run the water rate on this page.
Open the chat