Noisy/Ineffective Kitchen Exhaust

I bought a home with a CRIS30SS600 vent hood for the gas range. I should have spotted these issues but it is what it is.

The hood is really loud and also not very effective at venting. Its a 3 speed motor rated for 335, 450, and 560 CFM. After some inspection, I noticed the builder transitioned right after the hood from 6" to 4" piping. So the 4" is def undersized and I dont have access to replace all of it with 6" pipe.

Any thing I can do to fix this? Get a lower CFM motor? Replace as much of the 4" duct with 6"?

Thanks.

 

Is sound your main problem/concern?

What does you vent sound like? If there is a metallic rattling sound you can run the fan and press your finger against various spots until you find what's rattling.

6" is even a small exhaust duct. I think my hood called for 10". The 4" exhaust ducting is definetely hurting the vent performance. Increasing the duct size will help airflow but it won't do much for the noise. So, if you want quiet I think you need to start with a quiet vent hood.

If you really want to keep that hood I can't think of an inexpensive way to make it quieter since the blower/motor are contained within the hood. That makes finding a simple, bolt in quiet replacement extremely difficult to find. You could install a blower somewhere in the duct which would allow you to choose a quieter blower and it would be positioned further away. But by the time you enlarge the duct, run power and install the blower (and the fan controls on your current hood probably wouldn't work) that's a LOT of expense and work just to preserve the shell of your existing hood.

 

First check that the outside louvers or whatever they are using works OK.

Check that the metal filter is clean.
Remove the charcoal filters and try running it with them out. If it is quieter then replace the filters.
The noise may just be the way it is. You could remove the unit and run it without being attached to the vent to see if that is the case.

 

Thanks for responses.

Priority is really both performance and less noise. Its definitely not meeting spec in either category. Manufacturer (which you can take with a grain of salt), rates the motor at 45dba at low speed.. a quick google search tells me thats quiet conversation, near libray-volume levels. Im definitely not near that; you can't have a conversation at normal volume with the exhaust on.

Some things to note:

• Motor seems fine, no rattles/banging/clinking noises; its strictly wind noise (+poor exhausting performance) -- makes me think its resistance from the undersized duct.
• I only have access to 4ft of the ducting immediately after the the hood. After that, the remaining ~20 ft of run is buried in the ceiling and inaccessible (new construction in a condo building) without some distruction
• I think I know where the exhaust outlet is; I will have to check its opening fine later tonight
• filter/catches are clean and clear (running blower with out them in didn't improve sound / flow

Two things I am considering, but not sure they will be effective:

1. The builder attached a 6" to 4" transition right ontop of the exhaust. This has to be creating a tone of back pressure. I can easily move the transition about 4 feet higher up the line..
2. install a lower speed motor.. pricey but it might resolve some of the issues here. I am thinking that even at low speed (~400CFM), the duct sizing is creating too much turbulence/back pressure that is preventing effective venting. While a lower speed motor will move less air, it won't create the turbulence you get at higher speeds and as a result, might exhaust better. At the least, this will reduce noise, but really, I need better performance too to make it worth the $$$.

Not sure either of those moves will actually make a difference.

Thanks for feedback.

 

Am I missing something here?
I thought this was a multispeed fan and it has a 400 CFM setting.

 

No, moving the duct reducer will not noticeably improve airflow or reduce noise.

You can go to a lower RPM motor but it won't be much different than running your current one at a low setting. A better quality motor could be quieter but not a huge difference. The biggie is probably the blower itself. It would take a whole new blower, likely physically larger in size (and expensive) to generate more airflow while working at lower rpm. It would probably be easier & cheaper to replace the hood. But... you've still got that 4" duct that will always kill your vent's performance.

---
When I built my house I made my own downdraft vent which exhausted through 4" duct. I designed it from the beginning to work with 4" duct so I picked a high pressure squirrel cage blower and a quality motor. The special blower was capable of generating higher pressure which maintained adequate airflow through the small duct. The high quality blower & motor were quiet to begin with and I installed it between floors and enclosed to further reduce noise. It can be done but it was totally custom and expensive even with me doing the fabrication.

When I remodeled the kitchen I abandoned the downdraft and put in a off the shelf island vent hood and 10" ducting to exit straight up and out the roof. Total cost was about half what my custom system was.

 

thanks, guys.

It is multispeed ; 400cfm is the lowest and its still loud and doesn't perform great. If I up the fan speed, performance seems to get worse (which I attribute to the fact you are trying to shove more air down a duct that already couldn't handle it, creating lots of turbulence and poor flow).

I do have some space where I might be able to hide an inline motor (prob not enough space for a squirel cage set up). Do you think putting and inline motor in could help? If so, is there a brand/type/spec you might recommend (assuming im stuck with a the 4" duct)?

 

I doubt an inline fan would offer any improvement or reduce the blower noise.

Cristal SS spec sheet - pdf

 

Thanks for feedback.

I checked and the external louver/flap is opening when the blower is on. I also pulled off the reducer and found it also has the two "half moon" black flow preventer flaps. So, one issue I noticed is, the two flaps were blocked from opening all the way, so they would only get to about 45 degrees open. I removed the damper flaps and things are very slightly better.

It is clear though, when I run the fan without the 4" duct work connected, it moves air and is fairly quiet. The reducer they use does it in a step (not sloped or gradual). I would think this makes things marginally worse since the air is slaming into a flat wall that bridges the 6" opening to the 4" opening. I will pick up a sloped/gradual reducer that might help a little.

It is pretty obvious that when the reducer and undersize 4" duct it on, you get a lot of turbulence and backwards flow of the air, practically eliminating an upward flow.

I wonder if a weak inline motor, maybe 100-200cfm, would preform better than this set up cause at least you will eliminate the turbulence.

 

I have had 4 range hoods including one downdraft. My current one is a Z-Line 36 inch island hood. All of the OTR hoods I have owned have been very noisy but efficient at removing smoke and odors. Anything over the lowest exhaust setting is irritatingly noisy. The downdraft was very quiet but almost useless as a exhaust fan. Even at it's highest exhaust setting it only removed smoke and odors from the two back burners.

I am convinced that there is no way to get a quiet but efficient OTR exhaust given their current design. I don't think we have ever used ours at anything other than the lowest setting. Sheet metal bodies and exhaust ducts without any noise dampening insulation, and a high volume exhaust motor drawing a high volume of air through a restrictive mesh filter. At least in my case the noise levels are mostly caused by the motor/fan. I have no way of measuring operating noise but I am also convinced the the noise levels touted by range hood companies are mostly BS and they are developed under lab conditions.

I'm surprised that nobody seem to offer a hood with a remote fan located at the point of exhaust.

 

yeah, I recently did a kitchen remodel on another property and I installed a Best D49M36SB with a ~1,000 cfm externally mounted blower. The thing is great; it exhausts well from any burner. No smells. No grease. Lowest two settings, noise is negligible, highest two it can get loud but tolerable. I used 10" or 12" ducting for this set up (funny to think the builder at my apt used 4" -- really annoys me).

While my current OTR isn't quiet at low speeds, its not bad -- what makes it intolerable is the back pressure caused by the 4" duct. I just wish they size the duct properly, or if they were limited to 4", chose a lower end exhaust that at least would operate with the 4".

thanks