Can you install a whole-house fan yourself?

The answer to the question depends almost entirely on which generation of whole-house fan you buy.

• Traditional joist-cut fans. A large fan mounted directly in the ceiling, with a shutter. The opening is wide enough to require cutting and heading off at least one ceiling joist. That is structural carpentry, and it is the version that earned this job its reputation.
• Ducted fans. The motor and blade live in the attic, connected by flexible duct to a grille in the ceiling. Grilles are sized to drop into the space between standard joists, so nothing structural is cut. This is what almost everything sold today is.

The ducted design also solves the two complaints about the old ones: the motor is no longer directly above the grille, which is most of the noise, and the insulated damper closes against the ceiling instead of a flimsy louver. The buying side of that is in whole-house fans, ranked.

1. Attic access and headroom. You need to get the unit in and work beside it. A hatch that takes a 16-inch box and a roof with two feet of clearance over the joists is the minimum.
2. A ceiling location on the right floor. Central hallway, top floor, away from the bathroom extract and from bedrooms’ heads of beds. Check what is above — ducts, wiring, plumbing and recessed light housings all occupy joist bays.
3. Attic exhaust area. This is the one people skip. The air has to leave the attic as fast as the fan delivers it, through existing roof and gable vents. Undersized exhaust pressurizes the attic and the fan moves far less than its rating — the same net free area arithmetic as attic intake and exhaust, applied in the other direction.
4. Power. A switched connection to an existing lighting circuit is within reach of a competent DIYer in many jurisdictions; a new dedicated circuit at the panel is not. A 4,280 CFM unit draws 408 W on high, so the load itself is modest.
5. Permits and rules. Requirements vary by jurisdiction and this is a ceiling penetration with an electrical connection. Check locally before you cut anything; an insurer will.

1. Mark the grille from below, between two joists, and confirm from the attic that the bay is clear.
2. Cut the ceiling opening to the template. On a ducted unit this is drywall only.
3. Hang the fan housing from joists or rafters with the supplied straps, far enough from the grille that the duct has a gentle run. Keep it clear of insulation and stored items.
4. Connect the duct without kinks or sharp bends. Every bend costs airflow, and the published CFM assumes a sensible duct run.
5. Fit the grille and damper. An insulated, gasketed damper is what stops the fan being a hole in your insulation for nine months of the year.
6. Wire it to a switch or the supplied control. This is the step to hand over if you are not confident.
7. Test on every speed with a window open, and check the damper closes fully when it stops.

A whole-house fan exhausts indoor air into the attic, and an equal volume has to enter the house from outside. That means open windows, every time it runs. Running one with the house shut up pulls air down chimneys and flues and achieves nothing.

Open roughly the same window area as the fan moves air through, on the coolest side of the house, and run it when outdoor air is actually cooler than indoor air. The same principle governs window fans on exhaust, at a smaller scale.

Published figures for a 4,280 CFM unit show what the speed control is worth:

Low speed delivers 55% of the airflow for 18% of the power — three times the efficiency per watt. Overnight, low is almost always the right setting, and it is also the quiet one. What that costs across a season is in running cost for a whole-house fan.

• The fan needs a new circuit at the panel.
• You are buying a traditional joist-cut unit, which means altering a ceiling structure.
• The attic is a spray-foam or unvented roof assembly. Exhausting house air into a sealed attic is a different building problem and needs specific design.
• There is knob-and-tube wiring, vermiculite, or no safe footing in the attic. None of those are DIY conditions.
• You cannot confirm the attic’s exhaust area. An installer measures it as a matter of course, and it is the difference between the rated airflow and half of it.