How to tell if an attic fan is working

This is the test that resolves most cases, and it costs nothing. Attic ventilator thermostats are adjustable across a wide range, and a unit set near the top of it will stay off through weather that feels blisteringly hot from below.

1. Find the control — usually a small box on the fan housing or nearby framing, with a dial marked in degrees.
2. Note where it is set before you touch it, so you can put it back.
3. Turn it down slowly until the fan starts. It should come on within a few degrees of the current attic temperature.
4. Turn it back up to a sensible working figure. What that is, and the humidistat that often sits beside it, are in thermostat and humidistat settings.

If the fan starts, your diagnosis is finished. Many fans marked “broken” by a home inspector are doing exactly what their set point instructs.

A running motor is not proof of airflow. Shutters seize, blades loosen on shafts, and a fan can hum without moving air.

• From outside, hold a strip of tissue at the gable vent or roof cap while the fan runs. It should be pushed firmly away, not fluttering.
• At the soffits, the tissue should be drawn inward. If nothing moves at the soffits while air leaves the vent, the fan is getting its air from somewhere else — and that somewhere is usually your house. That failure and its arithmetic are in attic intake versus exhaust.
• At the shutter, all the louvers should be open. One stuck blade is a permanent restriction.

The performance question is not whether the fan spins but whether it changes anything. Put one thermometer in the attic, away from direct sun, and use the outdoor temperature as the reference.

A ventilated attic on a sunny afternoon typically runs well above outdoor temperature, and a working powered ventilator with adequate intake narrows that gap while it runs. What you are looking for is movement in the gap when the fan cycles on, not a specific number — roof color, insulation and sun angle make the absolute figures meaningless between houses.

If the gap does not move at all when the fan runs, the likely causes in order are: not enough intake, an exhaust short circuit (a powered gable fan fighting a ridge vent), or a fan too small for the attic.

1. Check the breaker. Attic circuits are frequently shared with lighting, so a tripped breaker may not be obvious from below.
2. Look for a disconnect switch near the unit. Roofers, insulation crews and pest-control visits all turn these off and forget to turn them back on.
3. Listen for a hum with no rotation. That is the classic failed start capacitor or seized bearing. Switch it off: a stalled motor draws heavily and heats up.
4. On a solar unit, check the panel. Shade from a tree that has grown, or a layer of dust and pollen, will stop it in the exact weather you want it in.

Attic ventilators live in the hottest, dustiest part of the house and their motors are consumables. A humming motor that will not start, a bearing rumble that survives cleaning, or a housing corroded at the mounting points all point the same way.

Two questions before spending:

• Does the intake support the fan you have? If not, replacing like with like buys you nothing, and a smaller fan matched to the soffits may ventilate better.
• Is a powered attic ventilator the right machine at all? Improved passive venting, air sealing at the ceiling, or a whole-house fan may serve the house better — the comparison is here.

If the answer is a replacement, the mounting decision comes first: gable versus roof mount, and the current field is in attic fans, ranked.