Exhaust Fan Makeup Air After Tightening an Older Home

Make an inventory of exhaust equipment

List the kitchen hood, bathroom fans, dryer, whole-house fan, and other exhaust equipment. Record models, rated flows where available, operating modes, and discharge locations. Distinguish a recirculating range hood from equipment that actually exhausts outside.

Note which devices commonly run together, such as the dryer while dinner is cooking. The inventory helps a qualified reviewer define relevant operating conditions; do not deliberately create a worst-case test around combustion appliances yourself.

Describe what changed in the house

Provide the air-sealing scope, changed attic or crawl-space boundaries, any blower-door records, and equipment alterations. Identify an attached garage, foundation spaces, and rooms with natural-draft appliances. The attic natural-draft furnace guide addresses that specific location.

Tightening is a reason to reassess the arrangement, not proof that the ventilation system is now unsafe. The reviewer needs the actual building and appliance configuration.

Separate three different air functions

Source exhaust removes air at a bathroom, kitchen, or dryer. Whole-house ventilation supplies an overall fresh-air strategy. Combustion air and venting serve fuel-burning equipment. Ask the reviewer to identify how each function is provided and how the systems interact.

PNNL’s pre-retrofit guidance discusses depressurization concerns and makeup air. One inlet should not be assumed to satisfy all three functions merely because outside air can pass through it.

Use a replacement-air review table

Swipe horizontally to read all columns on a small screen.

Have the qualified assessor record findings and limitations, including equipment that could not be inspected.

Do not use a flow threshold as a safety certificate

PNNL’s kitchen guidance discusses makeup-air requirements and how adopted code editions and appliance configurations matter. It also notes that backdrafting can occur with flows below commonly cited thresholds. This guide does not use 400 cfm as a universal exemption or safe-flow boundary.

Ask the reviewer to identify locally adopted requirements and the actual performance assessment. Rated airflow is not necessarily the measured installed flow through a particular duct system.

Keep required exhaust and combustion paths intact

Do not disable a bathroom fan, block an appliance opening, or seal a dryer route to reduce drafts. Have an appropriate professional define the correction and controls. A CO alarm or suspected poisoning is a separate immediate concern: leave for fresh air and seek emergency help rather than testing fan combinations.

The attached-garage air-sealing guide addresses that shared boundary. Air entering through a garage should not be accepted casually as the replacement-air plan.

Ask for design and commissioning evidence

Request the makeup-air path, controls or interlocks where required, equipment details, and relevant airflow or pressure verification. Ask how the arrangement operates in cold weather and under normal simultaneous use. Have necessary permits or authority checks identified before installation.

The winter-window guide addresses one humidity symptom; it does not size this system. The bathroom duct guide addresses a local route that also needs to remain properly configured.

Retain an operating brief for the household

Have the contractor explain ordinary controls, maintenance, and conditions requiring service. Record any deferred equipment issue and the responsible party. A system that depends on remembering to crack an unspecified window every time a fan runs is not a documented design from this article.

The seasonal humidity guide helps connect ventilation operation with moisture goals. Keep the combustion-safety and fresh-air conclusions distinct from whether one room’s humidity number decreased.

Questions homeowners ask

Continue with the next project question

• Winter Window Condensation After Air Sealing — Wet windows after air sealing need a surface and humidity diagnosis. Locate the moisture, compare cold-weather conditions, and check sources and ventilation.
• Where to Put Home Humidity Loggers After Insulation — Place humidity loggers to answer a specific question. Separate room reference readings from cold-surface or event locations, and keep time, temperature, and use notes.
• Indoor Humidity Targets: Why Roof Assembly and Season Matter — A general RH range is not proof a roof is dry. Set a seasonal operating brief using room temperature, cold surfaces, roof design, moisture sources, and ventilation.

Technical sources and reading

These references support the material and building-science principles discussed above. Check the current product instructions and locally adopted requirements for your specific project. The checklists are planning tools, not site inspection findings.

• PNNL: pre-retrofit assessment of ventilation systems
• PNNL: kitchen exhaust retrofit duct guide
• CDC: carbon monoxide poisoning basics