Whole-House Ventilation Strategies for Non-HVAC Trades

Why this matters

An electrician gets a service call for "a bathroom fan that won't shut off." A painter finds a humming box in the basement with four ducts and unplugs it so the crew can hear. A handyman swaps a strange wall timer for a plain switch because the homeowner asked. In a newer or tightened house, each of those can be the removal of the only thing replacing the air the leaks no longer bring in. None of these trades is expected to design ventilation. Every one of them is expected to recognize it and leave it working.

Three ways to ventilate a house on purpose

A whole-house ventilation system moves a steady, modest amount of outdoor air through the house all the time, or on a timed schedule, to dilute moisture, odors and indoor pollutants. It is different from spot ventilation, the bath fan you flip on during a shower or the range hood during cooking, which removes a burst of pollution at its source. Some systems do both jobs with one fan.

There are only three arrangements, and they differ in what they do to house pressure.

 EXHAUST-ONLY       SUPPLY-ONLY        BALANCED
 +-----------+      +-----------+      +-----------+
 |  fan  ->  |      |  <-  fan  |      | out-> HRV |
 |           |      |           |      | <-in  ERV |
 | house  -  |      | house  +  |      | house  0  |
 |<- leaks   |      |   leaks ->|      |  leaks    |
 +-----------+      +-----------+      +-----------+
 air in at          air out at         fan moves
 random leaks       random leaks       both ways
 (garage, crawl)    (top of house)     no net push

Exhaust-only. One or more fans blow house air outdoors, usually a bath fan rated for continuous duty, sometimes a central exhaust fan ducted to several bathrooms. Nothing is ducted in. The house goes slightly negative and makeup air comes in through whatever leaks exist. It is the cheapest system and the most common in new production houses.

Supply-only. A fan blows outdoor air in. The most common version for an existing ducted house is an outdoor air duct run to the furnace or air handler return, with a motorized damper and a controller that opens it and runs the blower for part of each hour; this is often called central fan integrated supply. A dedicated supply fan does the same thing without the air handler. The house goes slightly positive and house air leaves through the leaks.

Balanced. A ventilator with two fans moves roughly equal air in and out through separate ducts, so the house pressure barely changes. A heat recovery ventilator (HRV) passes the two streams through a core that transfers heat between them; an energy recovery ventilator (ERV) transfers moisture as well. The equipment, filters and commissioning belong to the HVAC side. See related: Indoor Air Quality - ERV and HRV Reference.

How hard each one actually pushes the house

The pressure a continuous ventilation fan creates is small, and it is worth knowing how small, because the other trades often blame "that fan" for problems a range hood or dryer causes.

Air leakage through a house follows a power law: flow rises with pressure raised to an exponent, and for whole houses that exponent is typically near 0.65. If you know the blower door flow at 50 pascals, you can estimate the pressure a smaller flow creates by scaling down along that curve: pressure equals 50 Pa times the fan flow divided by CFM50, raised to the power 1 divided by 0.65, which is about 1.54. That assumes the fan is the only thing moving air, with no stack or wind, so treat the answers as the fan's own contribution, not the reading you will see on a gauge on a real day.

Worked example. A 2,000 sq ft two-story house with 8 ft ceilings has 16,000 cubic feet of volume. Its energy certificate on the electrical panel shows 3.0 ACH50, so CFM50 is 3.0 times 16,000 divided by 60, which is 800 CFM50. It has an exhaust-only system: a bath fan running continuously at 50 CFM.

• 50 divided by 800 is 0.0625. Raised to the 1.54 power that is about 0.014. Times 50 Pa is about 0.7 Pa negative.
• The same 50 CFM fan in a leaky house at 2,400 CFM50 (9 ACH50 on the same volume): 50 divided by 2,400 is 0.021, raised to 1.54 is about 0.0026, times 50 is about 0.13 Pa. The tight house feels the same fan more than five times as hard (0.7 divided by 0.13 is 5.4), which is the whole story of why tightening matters.
• A supply-only system at the same 50 CFM in the tight house: about 0.7 Pa positive, same curve, opposite sign.
• A balanced ventilator moving 50 CFM each way: close to zero, off only by however unbalanced the two fans are.
• For scale, a 300 CFM range hood running in the same tight house: 300 divided by 800 is 0.375, raised to 1.54 is about 0.22, times 50 is about 11 Pa negative, roughly sixteen times what the ventilation fan does.

Check one result in reverse: at 0.7 Pa, 800 CFM50 times (0.7 divided by 50) to the 0.65 power is 800 times about 0.062, which is about 50 CFM. The curve closes.

The lesson for a non-HVAC trade is not the decimals. It is the ranking: a continuous ventilation fan moves the house by around a pascal; a big hood, a dryer and a couple of bath fans together can move a tight house by ten or more. See related: Counting Every Exhaust Device in the House, which owns that budget.

What each strategy does to moisture and makeup air

Small pressures still decide which way air leaks through the walls, all day, every day, and that has consequences that depend on climate.

Exhaust-only pulls makeup air through the leaks. That air comes from wherever the holes are: the attached garage, the crawlspace, the rim joist, wall cavities. In a hot, humid climate, outdoor air pulled through a wall cavity toward a cold, air-conditioned interior surface can deposit moisture behind it, the classic case being vinyl wallcovering on an exterior wall. In a house with an attached garage, it is one more reason garage air reaches the house. See related: The Attached Garage as a Pressure Neighbor.

Supply-only pushes house air out through the leaks. In a cold climate in winter, that is warm, humid indoor air pushed into the upper walls and the attic, where it meets cold sheathing. Stack effect already pushes outward at the top of the house in winter; a supply system adds to it. Cold-climate designers use supply-only with care for that reason.

Balanced does neither on its own. The leaks still move air with stack and wind, but the ventilation system adds no push. That is part of why balanced systems are favored in very tight, cold-climate houses.

Combustion appliances. Half a pascal of continuous depressurization is rarely what tips an atmospheric water heater into spilling, but it adds to everything else. If you are adding or changing an exhaust-only fan in a house with an atmospheric appliance, the spillage screen applies the same as for any new exhaust. See related: How to Screen for Spillage After You Add an Exhaust Fan.

How to recognize a ventilation system in the field

The systems are often unlabeled, quiet and installed by someone who is long gone. These are the tells:

• A bath fan with no off position, or a wall control with modes like continuous low, boost and timer. The fan runs all the time at low speed, often barely audible.
• A timer or controller marked for ventilation, sometimes in a hall or utility room, that runs a fan for a set number of minutes each hour.
• A small insulated duct from outdoors to the return plenum, often with a motorized damper and a wire to a controller near the air handler.
• A box with four duct connections in the basement, attic or utility closet, two to outdoors and two into the house. That is an HRV or ERV.
• Paperwork. An energy certificate on the panel, a ventilation label, or the builder's manual. A tested result under 5 ACH50 is a strong hint the house was built to need mechanical ventilation; see related: Why Air Sealing Changes What a House Needs for Ventilation.

The IRC in the 2012 through 2021 editions, as adopted, requires the whole-house system to have controls that allow manual override, in substance; check the section in your edition. "Override" means the occupant can turn it off temporarily. It does not mean the system is optional, and it does not mean the next trade should make "off" the permanent state.

The call that comes in as a broken switch

Take the electrician's call from the opener. The customer moved into a 2019 house in the fall and says the upstairs bath fan "never stops." The tech finds a fan running quietly with a multi-mode wall control and a builder's label on the panel showing a passed blower door test.

The tempting fix is ten minutes: replace the control with a plain switch, invoice, done. Here is what that does. This house's only designed ventilation is that fan. With it off, the occupants' moisture, cooking and off-gassing have nowhere to go but the leaks, which were built small on purpose. The customer will not notice in October. They will notice window condensation and a stale smell in January, and they will not connect it to the switch.

The right call takes the same ten minutes and a different conversation:

• Confirm what the fan is: continuous-duty rating on the label, the control's modes, the duct reaching outdoors.
• Tell the customer what it does, in one sentence: "This fan is how the house gets fresh air; it is supposed to run all the time, quietly."
• If the complaint is noise, the fix is a fan or duct problem for the HVAC contractor (a fan running on a crushed duct is loud and moves little), not a switch.
• Leave a label at the control: "Whole-house ventilation, runs continuously, do not disconnect."
• Write on the ticket what you found and that you left it running.

The same reasoning covers the painter and the humming HRV. Unplugging it for a day of work is fine if you plug it back in, confirm both fans are running and air is moving at a house-side grille, and say so. Leaving it unplugged because it was noisy removes the system without changing anything anyone will notice for weeks, which is exactly why it tends to stay off.

What changes the answer

A fan that was never a ventilation fan. Some older bath fans run all the time because of a failed switch or a wiring fault. A standard fan with no continuous-duty rating, on a plain toggle that does not turn it off, in a leaky older house with no ventilation design, is a repair. The tells above are what separate the two.

A system that is genuinely failing. A ventilator with a seized motor, a supply damper stuck open in winter, or a continuous fan venting into the attic instead of outdoors is a real fault. Report it and route it to the HVAC contractor; do not disable it as the fix. The rates the system should deliver come from the design, and the sizing method is covered in Ventilation Fans + CFM Sizing Reference.

References

• International Residential Code, whole-house mechanical ventilation (M1507.3 in the 2012 and 2015 editions, M1505.4 in 2018 and 2021), and R303.4 for when it is required, as adopted by your jurisdiction
• See related: The House as One Pressure System; Counting Every Exhaust Device in the House
• See related: Indoor Air Quality - ERV and HRV Reference; Ventilation Fans + CFM Sizing Reference
• See related: Why Air Sealing Changes What a House Needs for Ventilation; How to Screen for Spillage After You Add an Exhaust Fan