IRC M1503.3/4 and kitchen range exhaust vibration isolation

[rossn]

I wasn't sure if this was better placed in the IRC or mechanical code section, but thought folks in this section may have more specific input.

I have a single family dwelling with a range exhaust system going in above an island. It will be a 15' or so run of 10" galvanized duct run with a roof mounted blower. Given a prior bad experience with vibration from a wall mounted ductless mini-split, I want to do whatever is possible to isolate the ducting from the roof top blower. The attic is technically inaccessible.

IRC 1503.3 & .4 call for smooth walled, metallic ductwork. Would most inspectors allow or reject a rubber or neoprene section just ahead of the roof top blower? Options linked, below. I can see potential fire hazard of such a situation, so if someone knows of a better solution (it will be in an attic, zone: Denver), I'm all over that, as well.

Two styles:
1. Fantech's Fast Clamps. Neoprene lined steel clamps... they call for 1/4" gap between the fan and duct work (so, 1/4" of neoprene exposed) to reduce vibration. I'm not sure how much vibration these will reduce.

2. Nordfab's QF Vibration Isolator. Basically a rubber section with a metal coupler at each end. Max temp 194F, but I assume typical residential range exhaust will be lower than this.

 

[rossn]

Another option being a more traditional 'canvas' like this, then wrapped loosely in fiberglass duct wrap.

 

[Gregg Harris]

​

​

iii. Silicon​

Fabric made from silicon has a special silicon rubber layer that features exceptional resistance to low and high temperatures.

This type of fabric will not burn and releases negligible fumes when subjected to open flame.

Flexible duct connectors with silicon fabric are highly suitable for projects where high temperature is of primary interest in both outdoor and indoor HVAC system installations.

 

[rossn]

​

​

iii. SView attachment 15259ilicon​

Fabric made from silicon has a special silicon rubber layer that features exceptional resistance to low and high temperatures.

This type of fabric will not burn and releases negligible fumes when subjected to open flame.

Flexible duct connectors with silicon fabric are highly suitable for projects where high temperature is of primary interest in both outdoor and indoor HVAC system installations.

Gregg - perfect! I had no idea about flex/canvas made of silicone rubber. I bet that will do the trick and be acceptable to the inspector, so thanks so much for the recommendation! Since this blower 'pulls' instead of pushes, I do wonder how a junction like that will work (turbulence-wise) just ahead of the blower (the only place I can practically install it). I'm going to start searching for a pre-fabbed one, but if you know of any companies making one, I'm all ears.

Insulation-wise, I suppose I can loosely wrap duct-wrap around it without too much vibration transmitted.

Many thanks!!

 

[klarenbeek]

Would most inspectors allow or reject a rubber or neoprene section just ahead of the roof top blower?

Because the code specifically calls for metal duct with a smooth interior finish, this is something you would want prior approval from the AHJ before using. They could accept it as an alternate method. It's something I would consider but would want to see the specs on first, and probably see the install before it was insulated. The requirement for smooth interior finish is so there aren't places for grease to collect and build up, which might be an issue with a flex connector of any kind.
Its always better to check first instead of beg for forgiveness.

Also, the flex connector will stop vibration through the duct, but not through the roof structure where the fan is mounted. If the outdoor condenser with the mini split was the vibration issue, the fan should have way less vibration (almost none) unless its out of balance.

 

[ICE]

Duct wrap could stiffen the canvas. There's a few reasons why I would not want a fan on a roof.

 

[rossn]

@Gregg Harris - the silicone idea sounded very good. Upon further checking, silicone flex connector material tends to top out as rated around 500F and it appears that (at least the ones I've found specs on), are not very resistant to oil and grease! I know they coat some fire blankets in silicone, though. I did find another 'super high temp' flex duct connector material made by Elgen that is rated at 1800F, but I'm still not sure that will block a fire and am waiting to hear back from Elgen on oil/grease resistance. It seems Neoprene is kind for oil/grease.

@klarenbeek - yes, would definitely run anything like this by my inspector, first. Definitely concerned about anything non-smooth. For this application, where the fan has no spring or similar vibration isolators (mounted directly to the curb/roof), I would not envision the flexible duct connector being installed with ripples in the fabric section, rather just a tiny bit of slack. Agreed on any vibration potentially going through the roof structure as well; however, I'd expect any issues to first or greater manifest themselves in the ducting, due to low mass. As to the mini-split, we will probably end up moving it to the ground. It's only at certain loads and speeds... it must just hit the resonant frequency for the structure. They actually replaced the compressor (separate issue), and it's actually worse now. I see a lot of comments by HVAC contractors stating they will no longer hang them on a wall for this reason.

@ICE - The duct wrap could do that, though I think I would wrap it quite loosely, after sealing off the adjoining duct wrap. I know they even make some flex duct connector material that is insulated with fiberglass embedded. If you have seen a better way of doing this, I am all ears. I've heard 1000-1200 CFM internally mounted range exhaust blowers run, and I feel like I'm on the tarmac next to a 747 taking off!

At the end of the day, I've been looking for a solution on this for most of a day, and unless someone else has some additional ideas, if the high temp product doesn't come back oil resistant, I think I'll have to bail on the idea and hook it up rigid and test (though grease build-up over time isn't something you can check for). Given the area is inaccessible, sure would be nice to have some vibration management. That might be an untapped business opportunity for someone.

I appreciate everyone's help.

 

[Gregg Harris]

@Gregg Harris - the silicone idea sounded very good. Upon further checking, silicone flex connector material tends to top out as rated around 500F and it appears that (at least the ones I've found specs on), are not very resistant to oil and grease! I know they coat some fire blankets in silicone, though. I did find another 'super high temp' flex duct connector material made by Elgen that is rated at 1800F, but I'm still not sure that will block a fire and am waiting to hear back from Elgen on oil/grease resistance. It seems Neoprene is kind for oil/grease.

@klarenbeek - yes, would definitely run anything like this by my inspector, first. Definitely concerned about anything non-smooth. For this application, where the fan has no spring or similar vibration isolators (mounted directly to the curb/roof), I would not envision the flexible duct connector being installed with ripples in the fabric section, rather just a tiny bit of slack. Agreed on any vibration potentially going through the roof structure as well; however, I'd expect any issues to first or greater manifest themselves in the ducting, due to low mass. As to the mini-split, we will probably end up moving it to the ground. It's only at certain loads and speeds... it must just hit the resonant frequency for the structure. They actually replaced the compressor (separate issue), and it's actually worse now. I see a lot of comments by HVAC contractors stating they will no longer hang them on a wall for this reason.

@ICE - The duct wrap could do that, though I think I would wrap it quite loosely, after sealing off the adjoining duct wrap. I know they even make some flex duct connector material that is insulated with fiberglass embedded. If you have seen a better way of doing this, I am all ears. I've heard 1000-1200 CFM internally mounted range exhaust blowers run, and I feel like I'm on the tarmac next to a 747 taking off!

At the end of the day, I've been looking for a solution on this for most of a day, and unless someone else has some additional ideas, if the high temp product doesn't come back oil resistant, I think I'll have to bail on the idea and hook it up rigid and test (though grease build-up over time isn't something you can check for). Given the area is inaccessible, sure would be nice to have some vibration management. That might be an untapped business opportunity for someone.

I appreciate everyone's help.

Your exhaust temps at that connector with make up air is probably going to be around 100 degrees F if over 400 cfm or more. without makeup 140 ish.

A good quality exhaust fan direct drive properly installed will have negligible vibration.