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Humidifier size calculator
A whole-house humidifier is sized by the water it has to add to the air that leaks in. AHRI Guideline F gives the leakage by class, 0.5 air change per hour for a tight house, 1 for average and 1.5 for loose, and the FGIA chart sets the indoor RH that keeps the glass clear. For a 2,000 sq ft house with 8 ft ceilings, average construction, 20 °F and 70% RH outside and 70 °F inside, the balance at the chart's 35% comes to 13.4 GPD, which is 4.65 lb of water an hour or 27.4 grains per pound of dry air. The chart allows 40% RH on days just above 20 °F, and a humidifier has to carry that day too, so the page reports the peak, 16.0 GPD. Aprilaire's sheet lists 13.4 GPD for the same size and class at the AHRI conditions. A rated GPD is a different matter, because an evaporative rating assumes 120 °F plenum air and the blower on all day. This page gives the load and leaves the model to the maker's data. For the summer side, use the dehumidifier size calculator. For humidity ratio and grains at any reading, use the psychrometric calculator.
- Enter the heated floor area and the average ceiling height, then pick tight, average or loose. AHRI Guideline F gives 0.5, 1 and 1.5 air changes per hour for those three.
- Enter the outdoor temperature and humidity of a cold design day and the indoor temperature, and read the peak load, which the page takes between that temperature and 40 °F. Leave the indoor RH blank to hold the highest value in the FGIA chart for that outdoor temperature, or type your own.
- The result gives the gallons per day and the pounds of water an hour to add. Compare it with a humidifier's rated GPD only after checking the 120 °F plenum and full-day blower that an evaporative rating assumes.
Example result for the default inputs
2,000 sq ft at 8 ft, average construction. Outdoor 20 °F at 70% RH, indoor 70 °F, sea level.
Humidifier output needed (peak)
16.0 gallons per day
Water to evaporate
5.57 lb per hour
Indoor RH used
40 %
| Peak load | 16.0 GPD just above 20 °F outdoors, the most between 20 and 40 °F |
|---|---|
| Load at your design temperature | 13.4 GPD at 20 °F, 35% RH |
| House volume | 16,000 cu ft |
| Air leakage (AHRI Guideline F) | 1 air change per hour, 266.7 CFM |
| Indoor RH | 40%, the FGIA chart for just above 20 °F outdoors |
| Indoor humidity ratio | 0.00619 lb water per lb dry air |
| Outdoor humidity ratio | 0.00150 lb water per lb dry air at just above 20 °F outdoors |
| Moisture to add | 32.9 grains per lb dry air |
| Dry air leaking in | 1,187 lb per hour |
| Water to evaporate | 5.57 lb per hour, 0.668 gallons per hour |
| Basis of an evaporative rating | 120 °F plenum air and the blower on all day (Aprilaire) |
- The FGIA chart lowers the target RH as it gets colder, so the load does not keep rising. At 20 °F outdoors the target is 35% and the load is 13.4 GPD. Between 20 and 40 °F the most is 16.0 GPD just above 20 °F outdoors, at 40%. A humidifier has to carry the peak, so the result is the peak. The calculation holds the outdoor RH at 70% for every temperature.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the humidifier output is calculated
A whole-house humidifier has to put back the water that the leaking air loses. Outside air comes in cold and carries very little water, the house warms it, and every pound of it has to be brought up to the indoor humidity before it stops pulling water out of the walls, the wood and the people. This page turns the volume of the house and an AHRI air change rate into pounds of dry air per hour, multiplies by the humidity ratio difference, and converts the pounds of water per hour to gallons per day. The indoor RH is the highest value that the FGIA chart lists for the outdoor temperature. Worked numbers in this section are for the default house at its 20 °F design day. The form gives the largest load from the design temperature up to 40 °F, which for the default house is 16.0 GPD. What comes out is the load. Choosing the humidifier model is a separate step with the maker's data.
- Leakage
CFM = ACH x floor area x ceiling height / 60AHRI Guideline F sets 0.5 air changes per hour for a tight house, 1 for average and 1.5 for loose. The default 2,000 sq ft house at 8 ft is 16,000 cu ft, and at 1 air change per hour that is 266.7 CFM of outside air coming in.- Indoor RH
RH = highest value in the FGIA chart for the outdoor temperatureFGIA lists 40% at 20 to 40 °F, 35% at 10 to 20 °F, 30% at 0 to 10 °F, 25% at -10 to 0 °F, 20% at -20 to -10 °F, 15% below -20 °F, for 70 °F indoor air. At a boundary temperature two rows apply and the page takes the lower one. That makes it 35% at 20 °F outdoors, the same indoor humidity the AHRI table uses.- Humidity ratios
W = 0.621945 x p_w / (14.696 - p_w)Pounds of water per pound of dry air, indoors at the target and outdoors, from the same psychrometric code as the psychrometric and dew point calculators (ASHRAE Handbook Fundamentals, Chapter 1, with Magnus-Tetens saturation pressure over ice below 32 °F) at sea level pressure. For the default case W is 0.00541 indoors and 0.00150 outdoors.- Moisture to add
grains per lb = (W_in - W_out) x 7,000A pound is 7,000 grains, so the default difference is 27.4 grains for every pound of dry air that comes in. The page shows it because humidifier and psychrometric charts often work in grains.- Dry air per hour
lb dry air per hour = ACH x volume / v_inHere v is the specific volume of the indoor air in cu ft per lb of dry air, so 1 / v is the dry air density. The calculator reads ACH as a volume of indoor air, which for the default case is 1,188 lb of dry air an hour.- Water per hour
lb water per hour = lb dry air per hour x (W_in - W_out)The default house needs 4.65 lb of water an hour, or 0.558 gallons. When the outdoor air carries as much water per pound as the target, the load is zero and the page does not credit it as drying.- Gallons per day
GPD = lb water per hour x 24 / 8.34Water weighs 8.34 lb per gallon, so 24 hours divided by that gives 2.88 GPD for every lb per hour, the 2.88 that GeneralAire prints. At 20 °F the default is 13.4 GPD. It is a daily total at full-day operation, which is how a rating is quoted.- Reading it against a rating
rated GPD >= load, at the rating's own basisAn evaporative rating assumes 120 °F plenum air and the blower on all day, so compare it with this load only after checking the furnace run time and the supply temperature. A model with a rating above 16.0 GPD on that basis covers the default house at its peak.
Worked examples from published humidifier data
The default case is the AHRI Guideline F condition: 8 ft ceilings, 70 °F and 35% RH indoors, 20 °F and 70% RH outdoors. Aprilaire's sheet prints the GPD needed for 1,000 to 6,000 sq ft in each class. The table below sets those 24 printed cells next to this page's load at the same conditions. Every cell agrees within 0.15 GPD, and the largest gap is 0.12 GPD. Aprilaire's sheet does not print its own conditions, so the match is to the AHRI conditions that Honeywell's guide states.
| Sq ft | Tight, printed | Tight, page | Average, printed | Average, page | Loose, printed | Loose, page |
|---|---|---|---|---|---|---|
| 1,000 | 3.3 | 3.3 | 6.7 | 6.7 | 10.0 | 10.0 |
| 1,500 | 5.0 | 5.0 | 10.0 | 10.0 | 15.0 | 15.1 |
| 2,000 | 6.7 | 6.7 | 13.4 | 13.4 | 20.0 | 20.1 |
| 2,500 | 8.3 | 8.4 | 16.7 | 16.7 | 25.0 | 25.1 |
| 3,000 | 10.0 | 10.0 | 20.0 | 20.1 | 30.1 | 30.1 |
| 4,000 | 13.4 | 13.4 | 26.7 | 26.8 | 40.1 | 40.1 |
| 5,000 | 16.7 | 16.7 | 33.4 | 33.5 | 50.1 | 50.2 |
| 6,000 | 20.1 | 20.1 | 40.1 | 40.1 | 60.1 | 60.2 |
The indoor RH comes from the FGIA chart, which lists a lower value as the outdoor air gets colder. Below, the default 2,000 sq ft house runs at the coldest point of each row of the chart, which is the highest load in that row. Across the rows that load runs from 7.4 to 16.0 GPD. Inside a row, colder air takes more water. At each step the target RH drops 5 points and the load falls, so the highest load sits just above a step: 16.0 GPD just above 20 °F, where the chart allows 40% RH. The calculator sizes for the largest load between your design temperature and 40 °F, so a design day of -20 °F gives 16.0 GPD, the peak just above 20 °F, and shows the load at -20 °F on its own line. Holding 45% at 20 °F, above the chart's 35%, takes 18.7 GPD.
| Outdoor, °F (FGIA row) | Highest indoor RH | Run at | Load, GPD |
|---|---|---|---|
| 20 to 40 | 40% | just above 20 °F | 16.0 |
| 10 to 20 | 35% | just above 10 °F | 15.4 |
| 0 to 10 | 30% | just above 0 °F | 14.0 |
| -10 to 0 | 25% | just above -10 °F | 12.1 |
| -20 to -10 | 20% | -20 °F | 9.9 |
| below -20 | 15% | -24 °F | 7.4 |
Eight cases below run through the same function the form uses. The tight and loose houses give 8.0 and 24.0 GPD against 16.0 GPD for average, and a larger house with 9 ft ceilings shows how the volume works with the peak day. As a second check, GeneralAire's manual prints a chart factor for each of four sample homes at 0.5 air changes per hour and 0 °F outdoors. This page reads 4 to 6 percent above those factors, which puts its first sample home at 14.9 GPD against the chart's 14.3.
| Case | Setup | Result |
|---|---|---|
| AHRI conditions, average house | 2,000 sq ft, 8 ft ceilings, average, 20 °F outdoors at 70% RH | 16.0 GPD, 5.57 lb/h, 40% RH, peak just above 20 °F |
| Same house, tight | 2,000 sq ft, 8 ft ceilings, tight, 20 °F outdoors at 70% RH | 8.0 GPD, 2.79 lb/h, 40% RH, peak just above 20 °F |
| Same house, loose | 2,000 sq ft, 8 ft ceilings, loose, 20 °F outdoors at 70% RH | 24.0 GPD, 8.36 lb/h, 40% RH, peak just above 20 °F |
| Same house at 0 °F outdoors | 2,000 sq ft, 8 ft ceilings, average, 0 °F outdoors at 70% RH | 16.0 GPD, 5.57 lb/h, 40% RH, peak just above 20 °F |
| Same house at -20 °F outdoors | 2,000 sq ft, 8 ft ceilings, average, -20 °F outdoors at 70% RH | 16.0 GPD, 5.57 lb/h, 40% RH, peak just above 20 °F |
| Same house at 40 °F outdoors | 2,000 sq ft, 8 ft ceilings, average, 40 °F outdoors at 70% RH | 8.8 GPD, 3.05 lb/h, 40% RH |
| Same house held at 45% RH | 2,000 sq ft, 8 ft ceilings, average, 20 °F outdoors at 70% RH | 18.7 GPD, 6.49 lb/h, 45% RH |
| Larger house, 10 °F outdoors | 3,500 sq ft, 9 ft ceilings, average, 10 °F outdoors at 65% RH | 32.3 GPD, 11.22 lb/h, 40% RH, peak just above 20 °F |
Standards and data sources
- AHRI Guideline F, Selection, Installation and Servicing of Residential Humidifiers, as printed in the Honeywell TrueSTEAM Humidification Reference Guide - the air change rates of 0.5, 1 and 1.5 per hour for tight, average and loose construction, the definitions of those three, and the conditions of its GPD table, 8 ft ceilings, 70 °F and 35% RH indoors and 20 °F and 70% RH outdoors. AHRI's own document could not be downloaded when this page was written, so it is read here as Honeywell reproduces it. Honeywell's guide also states the basis of a pad humidifier's GPD rating.
- Aprilaire, Model 720 Series Humidifier specification sheet (form 387, 06.24) - the table of GPD needed by building tightness for 1,000 to 6,000 sq ft with 8 ft ceilings, which this page reproduces to within 0.12 GPD in all 24 cells at the AHRI conditions. Aprilaire's sheet also gives the 2 gallons per day that a family of four adds, and the 120 °F plenum and full-day blower behind an evaporative rating.
- Fenestration and Glazing Industry Alliance (FGIA), Understanding Indoor Condensation - the recommended indoor RH by outdoor temperature at 70 °F indoors, from 40% at 20 to 40 °F to 15% below -20 °F, and its caution that condensation ratings do not predict how a given home behaves. Its Fahrenheit column is attributed there to Home Energy Resource of Minnesota.
- GeneralAire, Basic Humidification 101 training manual - a second method to check against. Its chart uses 0.5 air changes per hour and outdoor air at 0 °F and 50% RH, and multiplies lb per hour by 2.88 for gallons per day. It adds 5% per open flue fireplace and 25% for very old homes with single pane windows, which this page does not apply.
- ASHRAE Handbook Fundamentals, Chapter 1 Psychrometrics - the humidity ratio and specific volume equations behind the psychrometric code this page shares with the psychrometric, dew point and dehumidifier calculators.
- USGS Water Science School, Water Density - the weight of water behind 8.34 lb per gallon, which turns pounds of water per hour into gallons.
Limitations of this calculator
- The load is leakage only. Air change rates are AHRI's class values for tight, average and loose houses, each one steady all day. A blower door test, an older house with a tight addition or a home with a new air barrier is better described by a measured rate than by a class.
- The result is a moisture load and the page does not choose a humidifier. An evaporative rating assumes 120 °F plenum air and the blower on all day, so a furnace that runs a few hours a day or a heat pump with cooler supply air delivers less than the rating. Honeywell's guide gives an 18 GPD pad unit on a furnace that runs 8 hours a day that may deliver 5.8 GPD. Steam units do not depend on furnace heat, and their maker's data decides the model.
- Published sizing tables are not identical. For 1,000 sq ft, Honeywell's guide prints two sets, 2.2, 4.5, 7.2 and 2.3, 6.6, 10.7 GPD for tight, average and loose, and the current Aprilaire sheet prints 3.3, 6.7 and 10.0. This page does not reproduce any one table. It computes the moisture balance and agrees with the current Aprilaire sheet to within 0.12 GPD.
- Moisture from people, cooking and bathing is not credited. Aprilaire's sheet says a family of four adds about 2 gallons per day, so leaving it out puts the result on the high side by about that much for such a household.
- Open flue fireplaces and single pane glass are not applied. GeneralAire adds 5% per fireplace and 25% for very old homes with single pane windows, and a 25% increase takes the default 16.0 GPD peak to 20.0 GPD. Use your own judgment on those two.
- The indoor RH follows the FGIA chart, which is written for 70 °F indoor air, names no window type and ends at 40 °F outdoors. FGIA itself says its condensation ratings do not predict how a home behaves, so glass that fogs below the chart's RH needs a lower setting. At a boundary temperature the page takes the lower row.
- One outdoor condition is used for the whole day, at sea level pressure. Mechanical ventilation, exhaust fans and elevation are not modeled, and the outdoor humidity range starts at 40% so the shared psychrometric code can solve the outdoor air down to -24 °F.
- This is a preliminary load for estimating. A licensed contractor, the manufacturer's selection data and the authority having jurisdiction decide the equipment and the installation.
Common humidifier sizing mistakes
- Buying by the nameplate GPD. A pad humidifier's rating is a full-day figure at 120 °F plenum air with the blower on, and Honeywell's example is an 18 GPD unit that may deliver 5.8 GPD on a furnace running 8 hours. Compare the peak load of 16.0 GPD for the default house with what the model delivers on your furnace, not with the number on the box.
- Holding the same RH on the coldest night. The FGIA chart drops the highest RH from 40% at 40 °F to 25% at 0 °F and 20% at -20 °F. Holding 45% on a 20 °F day takes 18.7 GPD against 13.4 GPD at the chart's 35%, and the glass is then past the line in the chart. The chart's own targets give 11.3 GPD at 0 °F and 9.9 GPD at -20 °F, but the calculator sizes for the largest load from the design temperature up to 40 °F, which for the default house is 16.0 GPD just above 20 °F outdoors.
- Picking loose to be safe. Loose is a house built before 1930 with little insulation and no weather stripping, and it carries 1.5 air changes per hour against 0.5 for tight. For the default house at 20 °F that is 20.1 GPD against 6.7 GPD, 3.0 times as much. Choose the class that fits the house, and use a measured air change rate when you have one.
- Sizing by floor area alone. The load follows the volume of air that leaks in. With 8 ft ceilings the default 2,000 sq ft house needs 13.4 GPD, and with 10 ft ceilings it needs 16.7 GPD. A table in square feet assumes 8 ft, as Aprilaire's does.
- Using the summer dehumidifier number. A dehumidifier is sized for a humid day in pints removed, and a humidifier for a cold day in gallons added. The two use different outdoor air, and the humidity ratio difference has the opposite sign. For the summer side, use the dehumidifier size calculator.
- Leaving out a fireplace or old glass. GeneralAire adds 5% for each open flue fireplace and 25% for very old homes with single pane windows. This page applies neither. If both apply, a 25% increase moves the default 16.0 GPD peak to 20.0 GPD, and the glass will also fog at a lower RH than the chart.
Common questions about humidifier size
How many gallons per day does a whole-house humidifier need?
It depends on the volume of the house, how tight it is and how cold it is outside. For the default 2,000 sq ft house with 8 ft ceilings, average construction at 1 air change per hour, 20 °F and 70% RH outdoors and 70 °F and 35% RH indoors, the balance on this page comes to 13.4 GPD at 20 °F. The chart allows 40% RH just above 20 °F, where the load is 16.0 GPD, and the calculator reports that peak. Aprilaire's table lists 13.4 GPD for the same size and class at the AHRI conditions.
Where do the tight, average and loose air change rates come from?
AHRI Guideline F estimates infiltration at 0.5 air change per hour for tight construction, 1 for average and 1.5 for loose. Honeywell's reference guide prints those with the definitions, and Aprilaire's sheet cites the same Guideline. At the AHRI conditions this page lands within 0.12 GPD of every one of the 24 cells in Aprilaire's table.
What indoor humidity should I hold in winter?
The highest RH that the FGIA chart lists falls as it gets colder outside, 40% at 20 to 40 °F, 35% at 10 to 20 °F, 30% at 0 to 10 °F, 25% at -10 to 0 °F, 20% at -20 to -10 °F, 15% below -20 °F, for 70 °F indoor air. Above those values condensation can form on the glass. The chart names no window type and FGIA says it does not predict how a given home behaves, so check the glass of the house.
Does a colder day always need more humidifier output?
Not across a step of the chart. Within a row of the chart, colder air carries less water and the load rises. At each step the highest RH drops 5 points and the load falls. The default house needs 13.4 GPD at 20 °F and 35%, which is 27.4 grains per pound. It needs 16.0 GPD just above 20 °F and 40%, 11.3 GPD at 0 °F and 25% (23.2 grains) and 9.9 GPD at -20 °F and 20% (20.3 grains). A humidifier has to carry the worst day, so the result is the largest load from your design temperature up to 40 °F, and the page shows the load at your own temperature on a separate line. Hold the higher RH on the cold days and the load, and the condensation risk, goes up.
How do I choose between tight, average and loose?
Use AHRI's descriptions. Tight is well insulated with vapor retarders, tight storm doors, weather-stripped windows and a dampered fireplace. Average has loose storm doors and windows. Loose is generally built before 1930 with little or no insulation and no weather stripping. For the default house at 20 °F the three give 6.7, 13.4 and 20.1 GPD, so tight to loose is 3.0 times the load. A blower door test gives a measured rate that is better than a class.
Is a humidifier's rated GPD what it delivers?
Not always. A pad humidifier's rating assumes 120 °F plenum air and the blower active all day, according to Aprilaire's sheet. Honeywell's guide gives the example of an 18 GPD pad unit on a furnace that runs 8 hours a day, which may deliver 5.8 GPD. Compare the load to what the model delivers on your furnace, using the maker's data.
Does the load include cooking, showers and breathing?
No. Aprilaire's sheet says a family of four adds about 2 gallons of humidity per day through everyday activities. This page leaves that out, which makes the result a little high for a busy household. The size of the credit is the household's own, so the page does not guess it.
How is this different from the dehumidifier size calculator?
A dehumidifier takes water out of humid summer air and is sized in pints per day. Humidifier sizing puts water into dry winter air, in gallons per day, with the target RH held down by the glass. Both use the same psychrometric code. The dehumidifier size calculator covers the summer side.
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
Find the load, then read the rating
The gallons per day here are the water the humidifier has to add on the worst day between the temperature you entered and 40 °F. Take the maker's data for the model, check the output at the plenum temperature and the furnace run time of the house, and look for a rating at or above the load on that basis. For the moisture side of a humid summer, use the dehumidifier size calculator, and for the dew point of the glass, the dew point calculator. You can also ask the assistant to go over a humidifier selection.
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