Q: Our 10-year-old multispeed kitchen exhaust fan drips greasy water back down onto the stove. Three of the highest fan speeds no longer work. The lights in the hood have also stopped working.

It’s in a custom wood surround with a custom internal stainless steel frame, so buying a different model will become an installation nightmare. I’m worried the fan is a lemon and don’t want a new one to fail like this one.

I can tell you that years ago the highest speeds of the fan were so loud that you couldn’t hear in the kitchen so I almost always used the lowest speed.

What’s going on, and how would you solve this problem? Did I inadvertently cause the problem?

A: I have a very similar kitchen stove exhaust fan to this one. In fact, I had one at my last home that I thought was so powerful it would suck an infant up into the air. It sounded like a jet engine. The interesting thing is that after years of use, I never had one drop of greasy water drip from my hood.

When you cook on a stove, your high school chemistry and physics teachers are grinning somewhere. As you may have learned in class all those years ago, the process creates vast amounts of water vapor as you heat food and boil water. When you heat up or fry meat, you also melt grease. Some of it gets converted to vapor, and tiny droplets of the grease can hitch a ride on some that vapor as it floats up from your stove.

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It’s important for this water vapor to be piped out of your house as rapidly as possible before it changes back to water. This is why exhaust fan manufacturers have very explicit requirements for both the minimum diameter and maximum length of the exhaust pipe that connects your fan to the outdoors.

This is why most high-quality exhaust fans have high-speed fans. The fan is trying to blast the water vapor outdoors in just a few seconds, rather than allowing it to meander through the exhaust pipe.

The dripping greasy water problem is exacerbated by cool or cold weather, so it’s a good idea to insulate both kitchen and bathroom exhaust pipes so they stay as warm as possible.

Remember in your physics class when your teacher discussed condensation? If you live in a humid climate, you undoubtedly experience this in the summer. Think about that cold can of beer or soda you take from the refrigerator and place on your patio table. It’s dry when you take it from the refrigerator but on a humid day within a minute a fog on the cold metal. Minutes later, a small puddle of water has developed at the base of the can.

Now imagine the surface area of your kitchen fan’s exhaust pipe. It could be 50 or 100 times the surface area of a single can of soda, and could certainly produce enough greasy water to drip down from the vent hood if it’s not properly ventilated away.

It’s also important to match the kitchen exhaust fan speed to the amount of food being cooked on the stove. If you’ve got all the burners going, pots of water boiling, skillets frying, etc., you should have the fan on high speed. If there’s just one small pot simmering, then the low speed is usually fine. Match the fan speed to the amount of water vapor being created.

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The writer told me she does not allow the fan to run after the burners are turned off. That’s a mistake. Some high-quality exhaust fans come with a built-in timer that keeps the fan running for five, 10, or even 15 minutes after you’re finished cooking.

This is done so any light condensation on the inside of the exhaust pipe can evaporate and be blown outside before it has a chance to drip back down to the cooking surface.

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Tim Carter has worked as a home-improvement professional for more than 30 years. To submit a question or to learn more, visit AsktheBuilder.com.