Counting Every Exhaust Device in the House

Why this matters

Every exhaust fan in a house gets installed by someone thinking about one room. The kitchen designer sizes the hood for the cooktop. The bath remodeler picks a fan for the bathroom. The appliance installer hooks up the dryer. Nobody adds them up, and the house does not care which room a fan is in: every one of them pulls on the same skin. A house with six modest fans can carry an exhaust budget as big as one large pro-style hood, and on a Saturday morning several of them run at once. The total is what decides whether the house can feed its fans through its leaks or starts pulling air down the nearest vent.

What counts as an exhaust device

Anything that moves air from inside the boundary to outside it:

• Bath and toilet fans, including the one in the basement bath that nobody remembers.
• The clothes dryer. A vented dryer is a fan, and a big one by household standards. See related: The Clothes Dryer as the Fan Nobody Counts.
• The range hood or downdraft cooktop vent, when it ducts outdoors. A recirculating hood moves no air out of the house and is not on the list.
• A central vacuum whose power unit exhausts outdoors or into the attached garage. If its exhaust is piped back into the house it removes nothing.
• A continuous exhaust ventilation fan, the kind that runs all day to meet a ventilation requirement.
• A whole-house fan, which belongs on the list in its own category because it is meant to run only with windows open.
• Hidden fans. A furnace or air handler with supply duct leakage outside the boundary acts as an exhaust fan while it runs, and a fireplace draws house air up its flue while it burns. Neither has a label. See related: Duct Leakage Outside the Envelope.

Supply-only fans and balanced ventilation (an ERV or HRV with equal supply and exhaust) are not exhaust. A supply fan offsets some exhaust; a balanced system is neutral to house pressure when it is working as designed.

Worked example: filling in the inventory

A two-story, 2,600 square foot house with a basement, a natural-draft gas water heater in the basement, and a blower door result of 1,800 CFM50, meaning 1,800 cfm of leakage when held 50 Pa below outdoors. The shop is quoting a basement bath and wants to know whether adding one more fan matters.

  EXHAUST INVENTORY
  2-story, 2,600 sq ft, 1,800 CFM50

  device            where        rated   typical run
  ----------------  -----------  ------  -----------
  bath fan          2nd fl main    80    20 min
  bath fan          2nd fl hall    80    20 min
  bath fan          1st fl powder  50    short
  clothes dryer     laundry       150 *  60 min
  range hood        kitchen       400    10 to 30 min
  central vacuum    garage unit   110    30 min
  ----------------  -----------  ------
  subtotal, all running at once   870 cfm

  whole-house fan   hall ceiling  3,000  windows open
                                         only, not in
                                         the subtotal
  * illustrative: dryers rarely carry a cfm label
  rated flows from fan labels, in cfm

Check the total. 80 plus 80 plus 50 is 210 for the bath fans. Add 150 for the dryer, 400 for the hood and 110 for the vacuum: 210 plus 150 is 360, plus 400 is 760, plus 110 is 870 cfm.

A realistic worst moment. All six rarely run together. A Saturday morning with the hood on high, the dryer running and someone in the main bath with the fan on is common: 400 plus 150 plus 80, or 630 cfm.

What that does to the house. Using the depressurization estimate laid out in Kitchen Hood Makeup Air, pressure equals 50 times exhaust over CFM50, raised to about 1.54. 630 over 1,800 is 0.35, raised to 1.54 is about 0.199, and 50 times that is about 10 Pa below outdoors.

Why the total matters more than any one fan. Run the same three devices one at a time:

Added up, the three separate figures make about 6.4 Pa. Together they make about 10. The relationship between flow and pressure is curved, so the second and third fans push the house further than they would alone. A tech who checks each fan by itself and finds each one harmless has not checked the house.

All six at once. 870 over 1,800 is about 0.48, raised to 1.54 is about 0.33, times 50 is about 16 Pa.

The question the shop asked. Adding an 80 cfm basement bath fan to the Saturday-morning case makes it 710 cfm: 710 over 1,800 is about 0.39, raised to 1.54 is about 0.24, and 50 times 0.24 is about 12 Pa, up from 10. Not a dramatic change on its own. The real finding is that this house already reaches about 10 Pa on an ordinary morning with a natural-draft water heater in the basement, which is well past the worst-case limits combustion safety standards set for natural-draft appliances, which Kitchen Hood Makeup Air cites. That is the conversation to have, not the extra fan.

These are one-zone estimates that ignore stack and wind and assume rated flows. They tell you whether to worry. A qualified combustion-safety tech measures the real worst case.

Where the numbers come from

Labels first. Bath fans and range hoods carry a rated flow on the housing or in the manual, usually behind the grille or the filter. Central vacuum power units list airflow on the unit's label. Whole-house fans list it on the motor housing or the box.

The dryer is the usual blank. Most dryer manuals do not state an exhaust flow. Use an illustrative figure, mark it as an estimate, and if the dryer is the line that decides the answer, measure it.

Measure when the answer is close. An exhaust fan flow meter or a powered flow hood held over the grille gives delivered flow. A hood and a dryer can be measured at the outdoor termination with a vane anemometer and the opening's free area, taken as an approximation. Record measured figures in the same column with an M beside them, so the next person can tell which lines were read and which were assumed.

Walk the whole house. Basement baths, a laundry fan, a garage-side central vacuum unit, a fan in a finished attic room and the downdraft vent in a kitchen island are the ones that get missed. Count the switches and timers, not the rooms you were called to.

Rated flow is not delivered flow

The cfm on a fan label (for many fans, a rating certified through the Home Ventilating Institute program) is measured at a stated static pressure. A fan pushing through a long run of crushed flex duct, an undersized duct or a stuck backdraft damper delivers less, sometimes a fraction of its rating. Hoods usually come closer to their rating than bath fans because their ducts are short and large. A clogged dryer vent cuts the dryer's flow too.

That cuts both ways. A budget built from labels overstates exhaust from poorly ducted fans, so the real depressurization may be lower. But the day somebody fixes the ductwork, cleans the dryer vent or replaces a tired fan with a good one, the house's exhaust goes up to the rating, and a house that tested fine last year may not. Note on the inventory whether a figure is a label rating or a measured flow.

What the code looks at, and what it does not

The model code's makeup air requirement for kitchen exhaust looks at the hood alone, by its own rated capacity. Kitchen Hood Makeup Air covers the rule, its threshold, the section numbers by edition, and the caution that it is binding only as adopted. The point for this card is narrower: that rule does not add the dryer, the bath fans and the central vacuum to the hood. A 350 cfm hood can sit below the trigger while the house's total exhaust is more than twice that. The budget is a building-science check the shop does because the code does not.

What running several at once does

The house goes more negative everywhere. All the fans pull on the same volume, so the pressure drops across the whole house, not just in the rooms with fans.

The replacement air comes from wherever the holes are. Rim joists, attic bypasses, the attached garage, the fireplace, and any vent open to a room: a natural-draft water heater's draft hood, a furnace with a draft diverter. See related: How a House Pulls Flue Gas Back Down a Vent, and The Attached Garage as a Pressure Neighbor.

The neutral pressure plane rises. More of the house sits below it and pulls in, which in winter adds to the stack effect at the basement. See Stack Effect and the Neutral Pressure Plane.

Interior doors matter. A closed laundry or mechanical room door with the dryer inside it puts most of the dryer's pull on that one room.

Fans fight each other. In a tight house, a weak bath fan running at the same time as a hood may move almost nothing, because the house is already negative and its little fan cannot push against it. The customer reports a fan that "doesn't work on Saturdays."

Where the decision forks

• Leaky house, no natural-draft appliances. The budget is a comfort and humidity question. Big exhaust pulls in unconditioned air and, in humid weather, moisture. Route makeup-air or ventilation design to HVAC if the customer feels drafts or sees humidity climb.
• Tight house or recently tightened, natural-draft appliance present. The budget is a safety question. Before adding exhaust, and after any job that tightened the house, the shop flags it and routes a worst-case depressurization and spillage test to a qualified combustion-safety tech, and confirms working CO alarms are installed outside sleeping areas. See How to Screen for Spillage After You Add an Exhaust Fan.
• All sealed-combustion or electric appliances. No vent to backdraft, but the budget still matters for radon and garage air in houses that have either. See Radon and Soil Gas Entry Paths.

The fix is not always makeup air. Replacing the last natural-draft appliance with a sealed-combustion or electric one removes the safety half of the problem. Choosing a smaller, well-placed hood shrinks the biggest line on the list. See Kitchen Hood Makeup Air and Combustion Safety After a Partial Conversion.

References

• See related: Kitchen Hood Makeup Air (depressurization estimate, makeup air rule by edition)
• See related: The Clothes Dryer as the Fan Nobody Counts; The House as One Pressure System
• See related: Combustion Safety After a Partial Conversion; How Draft Actually Works
• Home Ventilating Institute, certified ratings program for residential fans (rated airflow at stated static pressure)