Makeup air unit size calculator

How the makeup air load is calculated

A makeup air unit replaces the air a hood throws out, so the airflow follows the exhaust and the heat follows the winter temperature rise. The page takes the exhaust CFM, the share the unit covers, and the rise from the design outdoor temperature to the supply temperature. It returns the heat the unit puts into the air, the burner input at the efficiency from the data sheet, the gas flow, and a check of the supply temperature against the room. For 3,000 CFM from 0 °F to 65 °F the heat is 210,600 BTU/h.

Makeup airflow

makeup CFM = exhaust CFM x share / 100IMC 508.1 asks for makeup air from all sources approximately equal to the exhaust of all exhaust systems in the building. At 100 percent the unit carries all 3,000 CFM. With 50 percent it carries 1,500 CFM and transfer air or another supply makes up the other 1,500 CFM.

Temperature rise

delta T = supply temperature - outdoor temperatureThe default rise is 65.0 °F, from 0 °F outdoors to 65 °F supply. A supply at or below the outdoor temperature needs no heat, and the page reads zero.

Heat to the air

Q = 1.08 x density ratio x CFM x delta T (BTU/h)This is the sensible heat equation with the textbook factor of 1.08, corrected for elevation. 3,000 CFM times 65.0 °F times 1.08 gives 210,600 BTU/h, which you can check with a calculator. The unit's heating capacity has to cover this at the design day.

Elevation

density ratio = (1 - 6.8754e-6 x elevation)^5.2559Air at 5,280 ft has a density ratio of 0.823, so the same airflow needs 173,402 BTU/h in place of 210,600 BTU/h. The ratio comes from the standard atmosphere in the ASHRAE Handbook, the one the other pages of this site use.

Heat in kW

kW = BTU/h / 3,412One kilowatt-hour is 3,412 BTU, so 210,600 BTU/h is 61.7 kW. The figure is the heat to the air. An electric coil would draw that power, and a gas unit gets there through the burner input below.

Burner input and gas flow

input = Q / (efficiency / 100) and cubic feet per hour = input / heating valueEfficiency comes from the unit's data sheet. At 100 percent the input equals the heat, 210,600 BTU/h, and at 85 percent it is 247,765 BTU/h. With gas at 1,000 BTU per cubic foot the default case burns 210.6 cubic feet per hour, and 247.8 at 85 percent.

Supply against the room

|supply - room| compared with 10 FAs the Washington State Building Code Council quotes it, 2015 IMC 508.1.1 limits the difference between makeup air and the air in the conditioned space to 10 °F, except where the added heating and cooling loads of the makeup air do not exceed the capacity of the HVAC system. A 65 °F supply in a 70 °F room is 5.0 °F off and passes. At a 55 °F supply the difference is 15.0 °F, which is over by 5.0 °F.

Worked examples

The default case is 3,000 CFM of makeup air from 0 °F to 65 °F at sea level, and you can reproduce it with a calculator. The rise is 65.0 °F, and 1.08 times 3,000 times 65.0 is 210,600 BTU/h. That is 61.7 kW of heat in the air. The room is at 70 °F, so the supply is 5.0 °F off, inside the 10 °F limit.

The table below gives the heat for 1,000 CFM at a 65 °F supply and sea level as the design temperature falls. Scale it by the airflow. 3,000 CFM at 0 °F is three times the 1,000 CFM figure of 70,200 BTU/h.

Seven cases below run through the same function the form uses. Row two shows the input when the burner is 85 percent efficient, 247,765 BTU/h. In the third, the unit covers half the exhaust. A cold climate comes fourth, with 8,000 CFM at -20 °F and 734,400 BTU/h. At 5,280 ft the sixth case repeats the default, and thinner air takes 173,402 BTU/h. Last comes a 55 °F supply, over the room limit by 5.0 °F.

Standards and data sources

• IMC Section 508.1 Makeup air, as amended by Minn. R. 1346.0508 - the rule that the makeup air supplied to the building from all sources is approximately equal to the exhaust air of all exhaust systems, and that the two systems are electrically interlocked. Minnesota's 508.1.1 sets makeup air at not less than 50 °F at the supply diffuser. That number is Minnesota's. The 2015 IMC rule, as the Washington council quotes it, is the 10 °F one below.
• Washington State Building Code Council, Interpretation 18-08 (2015 IMC and Energy Code) - its quotation of 2015 IMC 508.1.1, that the temperature differential between makeup air and the air in the conditioned space shall not exceed 10 °F except where the added heating and cooling loads of the makeup air do not exceed the capacity of the HVAC system. That page also recommends transfer air first, and a dedicated makeup air unit where there is no transfer or conditioned supply air to use.
• CaptiveAire toolbox, Sensible Load / CFM - the sensible heat equation, load in BTU/h equal to a factor times CFM times the temperature difference, with the textbook 1.08 factor corrected for altitude. CaptiveAire's page has no worked example, so the 210,600 BTU/h of the default case is the product worked by hand.
• ASHRAE Handbook - Fundamentals, Ch. 1 Psychrometrics, standard atmosphere (Eq. 3) - the pressure and density of air against elevation, which scales the heat. At 5,280 ft the density ratio is 0.823.
• U.S. Energy Information Administration, British thermal units (Btu) - the conversion of 1 kilowatt-hour to 3,412 BTU, which turns the heat to the air into kW.

Limitations of this calculator

• Winter heating only. Cooling or dehumidifying the makeup air in summer is outside this page, and so is the humidity of the outdoor air. A unit that also cools needs its own load work.
• The 1.08 factor is for standard air at 70 °F. Air at 0 °F is denser, so a unit rated by CFM at the outdoor temperature moves about 15.2 percent more mass than standard air does. The page reads the CFM at about 70 °F at the site elevation and leaves that difference out.
• This is not a model selection. The page returns a load, and the capacity, the temperature rise range and the turndown come from the manufacturer's data for the unit you pick. It does not tell a direct-fired unit from an indirect-fired one, and the efficiency is whatever the data sheet says.
• No source for a typical burner efficiency was read for this page, so none is built in. The field takes 50 to 100 percent and opens on 100, which makes the input equal the heat to the air. Enter the data sheet figure for the real input.
• The 10 °F limit is the text of 2015 IMC 508.1.1 as quoted by the Washington State Building Code Council. Other editions and local amendments differ, and Minnesota uses 50 °F at the diffuser. Check the code the job is permitted under.
• The page does not size the hood exhaust. That figure comes from the hood type, the appliances and the listing of the hood, and the commercial kitchen hood CFM calculator covers that side.
• A gas burner is rated for sea level, and manufacturers reduce the rated input above 2,000 ft. The page does not derate the input. Compare the burner input with the unit's derated rating at the site, as in the furnace altitude derate calculator.
• Building pressure is not calculated. The code asks for makeup air approximately equal to the exhaust, and the balance of the space with doors, other exhaust fans and transfer paths is a field and design matter.
• This is a planning estimate. A licensed contractor or engineer, the equipment manufacturer's instructions and the authority having jurisdiction decide the design, and the page makes no claim of code compliance.

Common makeup air sizing mistakes

1. Heating to the room temperature. The heat follows the supply temperature, not the room temperature. Taking 70 °F as the target gives 226,800 BTU/h for the default case, 16,200 BTU/h more than the 210,600 BTU/h at a 65 °F supply. Pick the supply temperature on purpose, and check it against the room.
2. Sizing on a mild day. A design temperature of 30 °F gives 113,400 BTU/h for 3,000 CFM, and the same hood at 0 °F needs 210,600 BTU/h. Choosing a unit for the mild day leaves it 97,200 BTU/h short on the cold one. Use the winter design temperature for the site.
3. Reading the burner input as the heat to the air. The input is larger than the heat the air takes, by the losses of the burner. At 85 percent efficiency, 210,600 BTU/h of heat needs 247,765 BTU/h of input and 247.8 cubic feet per hour of gas. A gas line sized to the heat alone comes up short.
4. Using the sea-level factor at a high site. At 5,280 ft the density ratio is 0.823. The sea-level 1.08 overstates the heat the air takes by 21.5 percent there, 210,600 BTU/h against 173,402 BTU/h. Burner input ratings fall with elevation as well, so look at the derated rating before the heat.
5. Sizing the unit for all the exhaust when transfer air is available. The Washington interpretation recommends using transfer air and conditioned air from the kitchen before adding a dedicated unit. If the unit covers half the exhaust, 1,500 CFM, the heat drops from 210,600 BTU/h to 105,300 BTU/h. The code still asks for the total from all sources to be approximately equal to the exhaust.
6. Counting a window or door as makeup air. A county plan-review worksheet on the 2012 IMC says windows and door openings shall not be used for the purpose of providing makeup air. Makeup air is mechanical or gravity means of sufficient capacity, interlocked with the exhaust so that it runs whenever the exhaust does.

Common questions about makeup air units

Related calculators

• Commercial kitchen hood CFM calculator How much a hood exhausts, which sets the makeup air to replace.
• Range hood CFM calculator The home case, where the question is whether makeup air is required at all.
• HVAC design temperatures The winter design temperature to enter as the outdoor temperature.
• Furnace altitude derate calculator The input reduction for a gas burner at a high site.
• All tools Every public BuildSolver calculator in one list.

Used on the same job

• Gas pipe sizing calculator Size the gas line for the burner input and the cubic feet per hour from this page.
• Combustion air calculator The air a fuel-burning appliance inside the building needs for combustion.
• Fan law calculator What a change in fan speed does to the airflow of the unit.