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Exhaust fan with 6 inch duct.

Greg1707
Senior Member

I purchased a Nutone exhaust fan today that specifies a six inch duct. I noticed the square hole in the side of the metal box was the same size as fans with four inch ducts. The only difference was the plastic adapter that connects between the square hole and the round duct was six inches in diameter instead of the usual four inches. So instead of the four inch square to four inch round there is a four inch square to six inch round.

I wonder how much more air can flow through a four inch square hole if there is a six inch pipe on the other side rather than a four inch pipe?
Can anyone do the math?

infinity
Moderator
drcampbell
Senior Member

It can be quite significant for long runs, and “long” is a lot shorter than you might expect when the fan is capable of developing only a little bit or pressure. Using a 6-inch diameter duct reduces the velocity by more than half that in a 4-inch, and reduces the duct friction by a factor of six.

The actual calculations can be found in any good fluid mechanics textbook, the standard handbooks for mechanical & chemical engineers, ASHRAE Fundamentals. and Crane’s Technical Paper No. 410 – Flow of Fluids. (1942)

jumper
Senior Member
drcampbell
Senior Member
JFletcher
Senior Member

I purchased a Nutone exhaust fan today that specifies a six inch duct. I noticed the square hole in the side of the metal box was the same size as fans with four inch ducts. The only difference was the plastic adapter that connects between the square hole and the round duct was six inches in diameter instead of the usual four inches. So instead of the four inch square to four inch round there is a four inch square to six inch round.

I wonder how much more air can flow through a four inch square hole if there is a six inch pipe on the other side rather than a four inch pipe?
Can anyone do the math?

The math depends on duct length, number of turns, corrigated or smooth pipe, and elevation changes. Having 5′ of 4″ smooth pipe on the fan exhaust til it goes outside will result in a lot less backpressure than 55′ of 6″ that has 3 90s and 15′ of elevation rise.

The fan outlet being 4″ vs 6″ will create some obstruction and backpressure, tho if it’s the only obstruction, the fan is designed for it.

If you have unacceptable performance, the only options really are redoing the ductwork or installing a booster fan in the run. A larger bathroom fan will likely just make more noise if the ductwork is overly restrictive.

I purchased a Nutone exhaust fan today that specifies a six inch duct. I noticed the square hole in the side of the metal box was the same size as fans…

Step away from that booster fan: Your home’s ductwork may have bigger issues

If an HVAC technician recommends a duct booster fan when you have major issues like these going on, it’s a cop out

The Holmes Group

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    I was driving the other day when I heard an ad on the radio about duct booster fans. I couldn’t help but laugh. On a recent job we did the homeowners was having trouble with their HVAC so they called us in. They’d had an HVAC technician look at it. What was their solution? A duct booster fan. Did it work? No.

    A duct booster fan — most people just call them booster fans — is a device that you can attach to your HVAC system’s ductwork. It’s supposed to increase airflow to rooms in your home that are far away from heating and cooling systems.

    Step away from that booster fan: Your home’s ductwork may have bigger issues Back to video

    That’s why most technicians or homeowners will install them on long stretches of duct. So if you have a room in your house that’s always cold in winter and hot in summer, the idea is that a booster fan can push air to that room.

    The problem is when these devices are used to solve issues that an HVAC pro should fix. For example, let’s say your home’s ductwork or furnace is undersized, or your furnace needs to be replaced or maybe flex line was used instead of proper HVAC ducts.

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    If an HVAC technician recommends a duct booster fan when you have major issues like these going on, it’s a cop out. Some people try to save money this way — it’s cheaper than a new furnace. But that’s a mistake.

    An HVAC pro should examine the entire system and find the deficiency. There could be a disconnection somewhere in your ductwork. Or maybe the seams aren’t properly sealed. It might be that it has nothing to do with your HVAC and actually with your insulation. The point is a booster fan isn’t a one-cure-fits-all solution.

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    Duct booster fans are consumer products — at least in my experience. I don’t remember an HVAC professional using or recommending a duct booster fan on any job I’ve worked on. The only reason you would want to install a booster fan is because the furnace is not blowing air to a specified area. But if you have a new furnace, do you need to install a booster fan? No.

    A booster fan is a good solution if you have a heating run that’s too long. Or if the exhaust venting for the dryer needs to make a long stretch to the exterior. A booster fan will give you that extra push so the air can get to where it needs to go.

    But if you want even distribution of airflow throughout your entire home, then having ductwork with even distribution is what helps. That’s basic and it makes sense.

    Ducts are like the veins of your house. They feed warm and cool air to all the areas in your home. You can’t expect proper airflow to every room if there’s no way of it getting there efficiently.

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    I’ve seen too many homes with 80% of the ductwork on just one side of the house. And then you wonder why some rooms are always cold. The problem isn’t airflow. It’s the ductwork. And if that’s the problem, not even 10 booster fans will make a difference — just a lot of noise.

    Booster fans are loud. They can sound like a jet engine is running in your basement. Some homeowners learn to cope with the noise. They’ll install the fan 20 feet from vents to minimize the sound from travelling. Other people can’t stand it.

    Some people love booster fans. Others think they’re a waste of money. I think you have to be smart.

    Booster fans are inexpensive. You can buy one for something like $60 or $70. I’ve even seen some for $25. Others can cost as much as $300. But like I’ve said a million times, you get what you pay for.

    I can understand why some homeowners opt for a booster fan. When you compare $10,000 to replace ductwork to $60 for a booster fan, it’s no surprise which one most homeowners are going to choose. Not everyone has $10,000 lying around, especially not for something that’s going to be hidden in the walls. But if a booster fan can’t fix the problem, I don’t care if it costs $2. It’s a waste of money.

    Bottom line: The systems in your home should do their jobs. No one said doing it right was easy or cheap. I’ll be the first one to tell you it’s not. That’s why there’s so much crap everywhere. But doing it right is worth it. Why live uncomfortably if you don’t have to?

    Catch Mike Holmes in his new series Holmes Makes It RightTuesdays at 9 p.m. on HGTV. For more information, visit hgtv.ca. For more information on home renovations, visit makeitright.ca.

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    If an HVAC technician recommends a duct booster fan when you have major issues like these going on, it’s a cop out