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Calculator · Outdoor air load
Outdoor air load calculator
Outdoor air has to be cooled and dried in summer and heated in winter before it does anything for the room, and that load rides on top of the envelope. Take 1,000 CFM at 95 °F and 75 °F wet-bulb into a 75 °F room at 50 percent RH. It carries 4,500 lb/h of dry air, and the enthalpy difference of 10.18 BTU/lb makes the load 45,805 BTU/h, 3.82 tons. Of that, 22,160 BTU/h is sensible and 23,645 BTU/h is latent. The field formulas, 1.08 and 4840, give 44,840 BTU/h, 2.1 percent below it, and this page shows both. Those constants read CFM as a design CFM, air at about 70 °F at this elevation, and the form opens on that basis. A switch reads the CFM as measured in the outdoor air. To mix outdoor air with return air first, use the mixed air temperature calculator. For how much outdoor air a dwelling needs, use the ASHRAE 62.2 ventilation calculator, and for the full state of one air stream, the psychrometric calculator.
- Cooling reads hot, humid outdoor air against the room. Heating reads cold outdoor air and adds the moisture needed to hold a room RH when one is given.
- Type the outdoor air CFM, the outdoor dry-bulb with a wet-bulb or an RH, the room dry-bulb and RH, and the elevation.
- In summer the result shows the total, sensible and latent load in BTU/h and tons; in winter it shows the heating load, with humidification beside it only when moisture has to be added. The direction is in words, and the 1.08 and 4840 figures beside it.
Example result for the default inputs
1,000 CFM of outdoor air to a 75 °F room, summer cooling, sensible and latent.
Total cooling load
45,805 BTU/h
Sensible cooling load
22,160 BTU/h
Latent cooling load
23,645 BTU/h
| Outdoor air | 1,000 CFM at 95 °F dry-bulb, 39.8% RH, 75 °F wet-bulb |
|---|---|
| CFM basis | Design CFM, air at about 70 °F at this elevation, 0.075 lb/ft3 scaled to it |
| Room air | 75 °F dry-bulb, 50% RH, 62.6 °F wet-bulb |
| Dry air in the outdoor stream | 4,500 lb/h |
| Outdoor humidity ratio | 0.01401 lb water / lb dry air |
| Room humidity ratio | 0.00921 lb water / lb dry air |
| Outdoor enthalpy | 38.26 BTU/lb dry air |
| Room enthalpy | 28.08 BTU/lb dry air |
| Sensible load | 22,160 BTU/h, adds to the cooling load |
| Latent load | 23,645 BTU/h, adds to the dehumidification load |
| Total load | 45,805 BTU/h, adds to the cooling load |
| Total load in tons | 3.82 tons |
| Field formula, sensible (1.08 x CFM x dT) | 21,600 BTU/h |
| Field formula, latent (4840 x CFM x dW) | 23,240 BTU/h |
| Field formula, total | 44,840 BTU/h |
| Field formula against the exact figure | 2.1% lower, comparing sizes |
- This is the load of the outdoor air stream alone, at the airflow entered. Walls, windows, people and equipment are other loads, and a coil sees this one only after any mixing or heat recovery.
- The CFM is read as a design CFM, air at about 70 °F at this elevation, so the dry air is 60 times the CFM times 0.075 lb/ft3, scaled by the density ratio at this elevation. A field formula reads CFM the same way.
- The field formulas, 1.08 and 4840 with the density at this elevation, give 44,840 BTU/h for the total. That is 2.1% below the exact figure. Both read CFM as a design CFM, so the difference is the moist-air terms of the exact method.
- Heat or energy recovery, fan heat and duct gain are not in these figures. The page applies no effectiveness for an energy recovery ventilator.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the outdoor air load is calculated
Outdoor air has to be brought to the room state, and the energy that takes is the change in enthalpy of the dry air that flows. Each CFM is first turned into pounds of dry air, with the specific volume of the outdoor air for a measured CFM or a fixed design density for a design CFM. That mass times the enthalpy difference is the total load. The sensible part is the dry-bulb change and the latent part is the moisture change, and the two add up to the total exactly. Two field formulas, 1.08 and 4840, are shown beside the exact figures so the gap between them is visible.
- Dry air in the outdoor stream
m = 60 x CFM x 0.075 x density ratio (design CFM), m = 60 x CFM / v (CFM as measured)Which one applies depends on how the CFM was stated. A design CFM is read as air at about 70 °F at this elevation. That is 0.075 lb/ft3, the density of standard air at sea level, scaled by the density ratio, and it is not an SCFM. The default case, 1,000 CFM, carries 4,500 lb/h of dry air on that basis. A CFM read in the outdoor air uses the specific volume from the psychrometric engine for that dry-bulb, humidity and elevation, v = 14.29 at 95 °F and 75 °F wet-bulb, and carries 4,197 lb/h. The load comes out 7.2 percent apart on the two bases, 45,805 against 42,724 BTU/h. Opening on the design basis, the page runs the ASHRAE examples on the measured one, and the choice belongs to whoever knows how the CFM was given.- Enthalpy of moist air
h = 0.240 t + W (1061 + 0.444 t) BTU per lb of dry air, t in °FASHRAE writes this as h = 1.006 t + W (2501 + 1.805 t) in SI, Eq. 32 of the 2001 chapter. Outdoor air in the default case is at 38.26 BTU/lb with a humidity ratio of 0.01401. Room air at 75 °F and 50 percent RH is at 28.08 BTU/lb with 0.00921.- Total load
Q_t = m (h_out - h_in)The difference is 10.18 BTU/lb, so 4,500 lb/h gives 45,805 BTU/h, 3.82 tons at 12,000 BTU/h per ton. ASHRAE's cooling and dehumidification equation adds the heat carried off in the condensate, which this page leaves out. That term is 0.7 percent of the book's Example 3 total.- Sensible load
Q_s = m (0.240 + 0.444 W_out) (t_out - t_in)This is ASHRAE's sensible heating or cooling equation, Eq. 42, where the humidity ratio stays unchanged and the heat is m times the enthalpy change. The default case gives 22,160 BTU/h. With no room RH entered it is the only figure the page shows.- Latent load
Q_l = m (W_out - W_in) (1061 + 0.444 t_in)The moisture that comes in with the outdoor air, priced at the room dry-bulb. In the default case that is 23,645 BTU/h, 51.6 percent of the total. The split is a convention of this page, and the total does not depend on it.- Heating
Q_s = m (0.240 + 0.444 W_out) (t_in - t_out), Q_l = m (W_in - W_out) (1061 + 0.444 t_in)Cold outdoor air is heated to the room dry-bulb, and the latent part is the moisture to add to hold the room RH. For 1,000 CFM at 10 °F and 50 percent RH to a 70 °F room at 30 percent RH, that is 64,878 BTU/h sensible and 19,557 BTU/h of humidification. With no room RH only the 64,878 BTU/h sensible figure is shown. ASHRAE Example 2 is this case without the humidity.- Field formulas
Q_s = 1.08 x CFM x dT, Q_l = 4840 x CFM x dW (dW in lb of water per lb of dry air)CaptiveAire prints the same two, with 0.68 x CFM x grains per pound as the latent form. The project's infiltration module notes that the 1.08 rolls in a sea-level air density of 0.075 lb/ft3. For the default case they give 21,600 BTU/h sensible, 23,240 BTU/h latent and 44,840 BTU/h total. That is 2.1 percent below the exact total on the design basis and 5.0 percent above it on the measured basis. The field formulas read CFM the design way, so part of the second gap is the basis and not accuracy.- Elevation
density ratio from the ASHRAE standard atmosphere, applied to the field formulasPressure sets the humidity ratio and the specific volume, so the exact load follows it directly. At 5,280 ft the air is 82.3 percent as dense as at sea level. The default case carries 3,705 lb/h there against 4,500 lb/h. Sensible comes to 18,381 BTU/h there, latent 28,031 BTU/h and the total 46,412 BTU/h.
Worked examples from the ASHRAE Handbook
Rows one and two of the second table below are Examples 3 and 2 of the 2001 ASHRAE Fundamentals chapter on psychrometrics, converted from SI. Example 3 takes 10,594 CFM at 86 °F and 50 percent RH to saturated air at 50 °F. ASHRAE prints 197 kW, about 672,000 BTU/h, and this page gives 678,000 BTU/h without the condensate term. Example 2 heats 21,189 CFM of saturated air from 35.6 °F to 104 °F. Its printed figure is 492 kW, about 1,679,000 BTU/h, against 1,674,000 BTU/h here. Both are chart readings.
Next come the field formulas, set against the exact total on both CFM bases for five summer outdoor states and the default room. One fixed set of constants cannot follow the outdoor air, so the gap moves with the outdoor state. At 5,280 ft the air is 82.3 percent as dense as at sea level, and the exact load follows it.
| Outdoor air | Exact, design CFM | Exact, CFM as measured | Field formulas | Field against design | Field against measured |
|---|---|---|---|---|---|
| 85 °F, 68 °F wet-bulb | 18,415 BTU/h | 17,583 BTU/h | 18,074 BTU/h | -1.9% | +2.8% |
| 90 °F, 73 °F wet-bulb | 37,611 BTU/h | 35,430 BTU/h | 36,848 BTU/h | -2.0% | +4.0% |
| 95 °F, 75 °F wet-bulb | 45,805 BTU/h | 42,724 BTU/h | 44,840 BTU/h | -2.1% | +5.0% |
| 100 °F, 72 °F wet-bulb | 33,135 BTU/h | 30,808 BTU/h | 32,522 BTU/h | -1.9% | +5.6% |
| 105 °F, 73 °F wet-bulb | 36,973 BTU/h | 34,089 BTU/h | 36,305 BTU/h | -1.8% | +6.5% |
Eight cases follow, all run through the same function the form uses. Case three is the default. A humid day and a hot, dry day come fourth and fifth, and the dry one can turn the latent part into a credit. Winter fills the sixth and seventh, with and without a room RH. The last is the default at 5,280 ft. A credit is a part where the outdoor air helps the room, and the page names it in words.
| Case | Setup | Result |
|---|---|---|
| ASHRAE Example 3 states in IP units, CFM as measured | 10,594 as measured CFM outdoor at 86 °F, 50% RH to a room at 50 °F, 100% RH | Total cooling load 677,678 BTU/h, sensible cooling load 400,579 BTU/h, latent cooling load 277,099 BTU/h, field formulas 701,757 BTU/h |
| ASHRAE Example 2, heating at constant humidity, CFM as measured | 21,189 as measured CFM outdoor at 35.6 °F, 100% RH to a room at 104 °F | Sensible heating load 1,673,673 BTU/h, field formulas 1,565,274 BTU/h |
| Default summer case | 1,000 design CFM outdoor at 95 °F, 75 °F wet-bulb to a room at 75 °F, 50% RH | Total cooling load 45,805 BTU/h, sensible cooling load 22,160 BTU/h, latent cooling load 23,645 BTU/h, field formulas 44,840 BTU/h |
| Humid summer day | 1,000 design CFM outdoor at 90 °F, 78 °F wet-bulb to a room at 75 °F, 50% RH | Total cooling load 59,443 BTU/h, sensible cooling load 16,736 BTU/h, latent cooling load 42,707 BTU/h, field formulas 58,175 BTU/h |
| Hot, dry summer day | 1,000 design CFM outdoor at 110 °F, 70 °F wet-bulb to a room at 75 °F, 50% RH | Total cooling load 24,929 BTU/h, sensible cooling load 38,255 BTU/h, latent credit 13,326 BTU/h, field formulas 24,702 BTU/h |
| Winter, room at 30 percent RH | 1,000 design CFM outdoor at 10 °F, 50% RH to a room at 70 °F, 30% RH | Sensible heating load 64,878 BTU/h, humidification 19,557 BTU/h, field formulas 64,800 BTU/h |
| Winter, sensible only | 1,000 design CFM outdoor at 10 °F, 50% RH to a room at 70 °F | Sensible heating load 64,878 BTU/h, field formulas 64,800 BTU/h |
| Default summer case at 5,280 ft | 1,000 design CFM outdoor at 95 °F, 75 °F wet-bulb to a room at 75 °F, 50% RH, 5,280 ft | Total cooling load 46,412 BTU/h, sensible cooling load 18,381 BTU/h, latent cooling load 28,031 BTU/h, field formulas 45,335 BTU/h |
Standards and data sources
- 2001 ASHRAE Fundamentals Handbook (SI), Chapter 6 Psychrometrics, Typical Air-Conditioning Processes - the enthalpy equation, the sensible heating or cooling equation and two worked examples. Example 3 takes 10,594 CFM at 86 °F and 50 percent RH to saturated air at 50 °F, and the book prints 197 kW, about 672,000 BTU/h with the condensate term. This page gives 678,000 BTU/h without it, and the tests allow 2 percent. Example 2 heats 21,189 CFM of saturated 35.6 °F air to 104 °F and the book prints 492 kW, about 1,679,000 BTU/h. This page gives 1,674,000 BTU/h. Later editions carry the same chapter as Chapter 1.
- ASHRAE Handbook - Fundamentals (2021), Ch. 1 Psychrometrics - the humidity ratio, specific volume, enthalpy and wet-bulb equations (Eq. 22, 26, 32, 35 and 37) and the standard atmosphere that the psychrometric engine behind this page uses for the state of the outdoor air and of the room.
- CaptiveAire, Outdoor Air Load calculator - the field formulas as printed there, sensible load as 1.08 x CFM x the temperature difference, and latent load as 0.68 x CFM x the humidity ratio difference in grains per pound or 4840 x CFM x the difference in pounds per pound. It gives no total formula and no altitude note.
Limitations of this calculator
- One steady state. The figures are for one outdoor condition and one room condition, an instant and not a season, so a peak load and an annual energy figure are different numbers.
- Only the outdoor air stream. Envelope, people, lights and equipment are other loads. What a coil sees depends on how the outdoor air mixes with return air first, which the mixed air temperature calculator handles.
- No heat or energy recovery. A recovery unit changes the state of the air before it enters the space, and the page does not model one.
- No fan heat, duct gain or duct leakage. Outdoor air that travels through a hot attic or past a fan arrives at a different state than the one entered.
- The latent part is split off by a convention. Air is cooled at the outdoor humidity ratio and then dried at the room dry-bulb, and the total does not depend on it. The condensate's own enthalpy is left out, small against the total.
- A latent figure in heating is a humidifier load. With no humidifier it is not a load on the heating equipment, and the room runs at a lower RH.
- The field formulas read CFM as a design CFM at one fixed density. They ran 2.1 percent below the design-basis total in the default case and 5.0 percent above the measured-basis total, and the gap moves with the outdoor state and the elevation. Neither basis is the more accurate one, and the right one is the one the CFM was stated in.
- Humidity comes from the project's psychrometric engine, which uses the Magnus-Tetens saturation curve, within about 0.5 °F of Hyland-Wexler in the HVAC range. Outdoor dry-bulb runs from -40 to 125 °F, relative humidity from 1 percent, airflow to 200,000 CFM and elevation from -500 to 12,000 ft. An outdoor dew point above 90 °F is refused as a typo, a choice of this page and not a limit from a standard.
- This is a planning estimate. A licensed contractor, the equipment manufacturer's instructions and the authority having jurisdiction decide the design, and the page makes no claim of code compliance.
Common outdoor air load mistakes
- Leaving out the latent load. Sensible alone is 22,160 BTU/h in the default case, and the total is 45,805 BTU/h. The latent part is 51.6 percent of it. A system sized on the dry-bulb difference alone is short on a humid day.
- Using 1.08 at elevation with no density correction. At 5,280 ft the air is 82.3 percent as dense as at sea level. The plain 1.08 x CFM x dT overstates the sensible load there by 17.5 percent, with the CFM read as a design CFM, because the same CFM carries less dry air.
- Taking the system CFM for the outdoor air CFM. A 10,000 CFM unit that draws 20 percent outdoor air brings in 2,000 CFM. The default conditions put 91,609 BTU/h on that stream, and 458,046 BTU/h if the whole system airflow is entered by mistake.
- A wet-bulb that is a few degrees off. The wet-bulb sets the moisture in the air, and the latent load follows it. Reading 80 °F in place of 75 °F, a 5 °F error, moves the default total from 45,805 to 68,690 BTU/h, 50.0 percent. Read it with a clean wick and with air moving over it.
- Taking a credit for mild outdoor air. Outdoor air at 60 °F and 30 percent RH is cooler and drier than a 75 °F, 50 percent room, and the page shows a credit of 45,571 BTU/h, 16,298 sensible and 29,274 latent. That credit exists only while the weather holds. A design load has to cover the hot hours.
- Counting winter humidification as a heating load. 1,000 CFM at 10 °F to a 70 °F room is 64,878 BTU/h of heat. Holding 30 percent RH adds 19,557 BTU/h more, and that energy goes to a humidifier. Without one, the room settles at a lower RH and the heating equipment never sees it.
Common questions about outdoor air load
How do you calculate the outdoor air load?
Turn the CFM into pounds of dry air per hour on the basis it was stated in, then multiply by the enthalpy difference between the outdoor air and the room. A design CFM, the default basis, takes 60 x CFM x 0.075 x the density ratio at the elevation. For 1,000 CFM at 95 °F and 75 °F wet-bulb to a 75 °F room at 50 percent RH, that is 4,500 lb/h times 10.18 BTU/lb, or 45,805 BTU/h. On the measured basis the mass comes from the specific volume of the outdoor air, 14.29 ft3 per lb of dry air here, which gives 4,197 lb/h and 42,724 BTU/h. In tons the design figure is 3.82.
What are the 1.08 and 4840 formulas, and how far off are they?
They are the field forms, sensible load as 1.08 x CFM x dT and latent load as 4840 x CFM x the humidity ratio difference. CaptiveAire prints both. For the default case they give 44,840 BTU/h against 45,805 BTU/h on the design basis, 2.1 percent below it, and 42,724 BTU/h on the measured basis, 5.0 percent above that. A field formula reads CFM the design way, so against a measured CFM the gap says little about accuracy. Use them for a quick check.
Should I enter the CFM as a design CFM or as measured?
Enter the basis the CFM was stated in. Use design CFM when the figure comes from a schedule or a calculation that already assumes air at about 70 °F, 0.075 lb/ft3 at sea level, and measured when it was read in the outdoor air at its own density. A design CFM is not an SCFM, since the page scales it for elevation. For 1,000 CFM from 10 °F to a 70 °F room, the sensible load is 64,878 BTU/h on the design basis and 72,999 BTU/h on the measured basis. Cold air is denser than that, so a measured CFM at that temperature carries more dry air.
What is the difference between sensible and latent outdoor air load?
Sensible is the heat of changing the dry-bulb temperature, 22,160 BTU/h in the default case. Latent is the heat tied to the moisture in the air, 23,645 BTU/h there. Together they make the total of 45,805 BTU/h, and in the default case the latent part is 51.6 percent of it.
How many tons of cooling does outdoor air add?
Divide the BTU/h by 12,000. The default case, 1,000 CFM at 95 °F and 75 °F wet-bulb, is 45,805 BTU/h, which is 3.82 tons. Load scales with CFM, so 2,000 CFM of the same air is 91,609 BTU/h.
How is the outdoor air load calculated in winter?
Heat the cold air to the room dry-bulb, and add the moisture needed to hold the room RH if a humidifier runs. 1,000 CFM at 10 °F and 50 percent RH to a 70 °F room is 64,878 BTU/h of heat. With the room at 30 percent RH, the humidification is 19,557 BTU/h more. The page shows it apart from the heat and never takes moisture the outdoor air brings in off the heating.
Does elevation change the outdoor air load?
Yes. Thinner air carries less mass for the same CFM. At 5,280 ft the default case has 3,705 lb/h of dry air against 4,500 lb/h at sea level, and the sensible load falls from 22,160 to 18,381 BTU/h. Humidity ratio at the same wet-bulb is 0.01813 there against 0.01401, so the latent part moves from 23,645 to 28,031 BTU/h and the total reads 46,412 against 45,805. The field formulas take a density ratio for this, 82.3 percent at that elevation.
Why does the page say credit and not show a negative number?
A load that comes out below zero means the outdoor air is helping, and the page says so in words. Outdoor air at 60 °F and 30 percent RH gives a credit of 45,571 BTU/h against a 75 °F, 50 percent room. It holds only while the weather does, so a design load does not count on it. Plan for the hot hours.
Is this the load on the coil?
Only when all the air through the coil is outdoor air. With a mixing box, the coil sees the mixed state, and the outdoor air load is the part of the total that the outdoor air adds. The mixed air temperature calculator finds the mixed state, and a measured airflow settles how much outdoor air there is.
How close is this to the ASHRAE examples?
For Example 3 the page gives 678,000 BTU/h without the condensate term, within 0.1 percent of the book's figures for the same terms and 0.8 percent from the printed 197 kW. Example 2, the heating case, comes out at 1,674,000 BTU/h against 1,679,000, 0.3 percent apart. The book values are chart readings.
Is there a limit on how many times I can run this?
No. This calculator runs in your browser with no daily limit and no signup.
Used on the same job
Use the outdoor air CFM and a real wet-bulb
The load follows the CFM of the outdoor air itself. A unit that draws a fifth of its airflow from outside carries a fifth of the load. The wet-bulb sets the moisture in the air, so read it with a clean wick and moving air. You can also ask the assistant to talk through the numbers for a job.
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