Has anyone looked at the low velocity consequences as a result of marrying these statements pulled from the resource documents…“Use 8-inch round smooth metal ducts” and “ASHRAE and other groups recommend a minimum of 100 cfm.” and “HVI recommends 100-250 cfm airflow rate”?

Just saying if authoritative voices are offering up resources there is a need to address minimum recommended velocities and cross check recommended duct sizes with recommended air flow rates.

Not sure if the residential research is looking at this but 500 fpm for horizontal ducts is recommended minimums for commercial kitchen duct work (operative word being minimums) …seems to be an absence of recommendations for velocities in vertical ducts…

Glad to see this forum up and running…

Cheers,

Robert Bean

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Replies to This Discussion

I'm not familiar with any recommended minimum velocity for residential range hood exhaust ductwork. I suspect that long-term grease deposition in the ducts is minimal for most residential applications, given the intermittent use of the cooktop (and the even more intermittent use of range hoods in most homes)...though I'll have to get out the camera snake and visit a serious cook's house to investigate.  

Interestingly the 500 fpm was a drastic reduction from a previous requirement of 1500 fpm established by NFPA following an ASHRAE research project which looked at the impact of velocity on grease deposits ...100 cfm in an 8" duct is less than 300 fpm - well below the minimum...

The comparisons between residential and commercial use is valid and as you alluded to...looking at the use in a home of a serious cook would be useful...and if 300 fpm is still good in a horizontal duct is it good in a vertical duct? What happens at the discharge hood under low velocity with grease and moisture laden air?  Is there sufficient throw to disperse the pollutants or do they accumulate on siding or roofing materials? Does it matter if its a vertical discharge or a horizontal discharge?

Inquiring minds want to know...

Below is a copy/paste from a side bar with Rick Karg...joining as soon as he gets approved...

  • Rick Karg

    Robert, regarding your low velocity consequences posting: I don't think the velocity is an issue. Velocity is primarily limited because of noise; the fans make more noise than the high velocity in the duct would, making the velocity superfluous.

  • I don't seem to be able to respond to your post, I imagine because I have not been invited to be a part of the kitchen ventilation group.

  • 1:15 PM
  • Me

    Hi Rick, I had to request to join....the velocity issue is related to pollutant suspension...the ASHRAE research project discovered the high velocity actually made deposition worse thus the reduction from 1500 to 500 fpm...but the 500 was set as the minimum....what happens to pollutant suspension if the velocity is lower?

Bob:

Good questions. I will defer to HVAC & grease experts on this.

Is the main safety issue grease fires in the exhaust duct & hood? If so, do residential hoods & stoves get hot enough to be a problem, or is it just for "commercial/professional" grades stoves in homes?  What is the typical emission rate of grease in commercial vs. residential cooking settings?

In the ROCIS best practice doc, I recommended the 8 in duct based on noise & flow considerations discussed in LBNL studies.

Also, I supsect that a bigger issue re: air flow and grease depostion is how to ensure frequent cleaning of the grease filters, esp. in low income, rental, & elderly populations.

Hi Tom,

All good comments and questions that need to be explored for residential projects.

We have large residential system (1200 cfm) designed for 800 to 1000 fpm with single inline fan located 30 to 40 feet away from the range hood...two silencers placed before and after the fan...two 45 bends plus straight lengths.....from witnessing the owners use the range the sound and extraction performance with a deep sump hood are exceptional ...

I would consider the less than 500 fpm minimum - if there were research to support the values.  


One of the challenges we have is marrying multi speed hood inserts with variable speed exhaust fans....if anyone has an elegant solution do tell...manufacturers offering packaged systems I hope you are paying attention! 

RB 

Seems like high velocity and low noise are at odds with each other. And one of the biggest impediments to people using range hoods in their homes is noise. Wouldn't requiring higher velocity exacerbate this problem? Sorry forgive my ignorance but what is the issue with velocities <500 ft/min? The concern is the wall cap not spitting out the pollutants sufficiently far away enough from the exterior wall?

At first glance this seems like a solution to a problem that may not be very common in residential buildings, and the "solution" would seem to worsen the noise issue -- which is a very common impediment to adequate ventilation in residential kitchens. I believe commercial hoods also require 100% capture efficiency? I don't think anyone is proposing that as a requirement for residential hoods. I feel like the commercial requirements should inform the residential ones, but they needn't (and shouldn't) be identical.

Hi Chris,

Today there is no research that I can find to support less than 500 fpm.

As noted above we have much higher velocity systems without noise issue due to locating the fan away from the hood, minimizing fittings and using silencers.

What I am doing here is drawing attention to the recommend flows and ducts sizes where velocity has not been considered and suggesting that it may or may not be a trivial concern...

Hoping someone from LBNL might join the dialogue to hear their thoughts...

Cheers,

RB 

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