What to Do When Your Commercial Exhaust Hood Stops Working

A commercial kitchen exhaust hood is the lungs of your restaurant. When it stops pulling air, the kitchen fills with blinding smoke, carbon monoxide builds up to lethal levels, and the ambient temperature skyrockets to 120°F+.

If your exhaust fan fails during a Friday night dinner rush, your kitchen will be shut down by the fire marshal or health inspector within minutes. Here is exactly what you need to do to troubleshoot the failure before calling for emergency hot work repair.

Immediate Action: Ensure Safety

1. Shut Down the Cooking Line: If the exhaust hood is completely dead, turn off all fryers, ranges, and charbroilers immediately. You cannot legally or safely cook without proper ventilation.
2. Check the Fire Suppression System: Ensure the Ansul system has not accidentally tripped. If the system deployed, it automatically kills power to the equipment and the exhaust fans.

Troubleshooting the Exhaust Fan

1. Check the Breakers and Disconnects

The most common and easiest fix is a tripped electrical breaker. Go to your main electrical panel and check the breakers labeled “Exhaust Fan” and “Make-Up Air” (MUA). If they are tripped, reset them. Next, safely access the roof (if your building allows) and check the service disconnect switch mounted directly next to the mushroom exhaust fan. Sometimes roofers or other contractors accidentally bump the disconnect to the “OFF” position.

2. The Snapped V-Belt

If the motor is humming but the fan blade isn’t spinning, you have a broken belt. Inside the exhaust fan housing on the roof, a rubber V-belt connects the electric motor to the fan blade pulley. In hot, grease-heavy environments, these belts stretch, crack, and eventually snap.

• The Fix: This is why smart facility managers always keep two spare belts zipped-tied inside the exhaust fan housing. A technician can replace a snapped belt in 10 minutes, saving your dinner rush.

3. Grease-Locked Bearings

If the exhaust fan makes a horrific grinding noise and then shuts off, the fan shaft bearings have likely seized. Kitchen exhaust air is laden with vaporized grease. If the fan is not properly cleaned and the bearings are not regularly lubricated, the grease will harden like concrete, locking the bearings in place and burning out the electric motor.

The Make-Up Air (MUA) Unit Problem

Sometimes the exhaust fan on the roof is running perfectly, but the kitchen still fills with smoke. Why? Because your Make-Up Air unit is broken. For every cubic foot of air the exhaust hood sucks out of the kitchen, it must pump a cubic foot of fresh air back in. If the MUA unit on the roof fails (due to a broken belt, blown fuse, or frozen coil), the kitchen becomes a massive vacuum. The exhaust fan will literally choke and become unable to pull smoke up the duct.

Preventative Action

Exhaust failures are entirely preventable. If your restaurant relies on heavy cooking, you must have your exhaust belts replaced and your bearings greased every 6 months.

If your hood is down, contact the HP Mechanical Kitchen Equipment Team. We specialize in high-volume restaurant ventilation systems across Portland and Vancouver, providing priority 24/7 emergency dispatch.