Kitchen Range Exhaust System - A few install questions (vibration, support, internal fasters...)

[Rossn]

I'm installing a residential range hood exhaust system.

It includes a (used) 54" Wolf island range hood, 10" ducting, and a Broan 335 rooftop mounted 1200 CFM variable speed blower. The ducting run (including 2' below ceiling level) is about 15' long, and consists of 10" rigid duct, 3 long-radius 5-gore elbows (2 at 90, one slightly less), and a Fantech LD10 duct silencer.

A diagram is shown below, with a few pics of the space.

I have a few install questions...

Q1: Vibration management - is there any value in using dampened hangers for the suspended duct, given the two ends (blower and hood) are attached to the structure? I assume there isn't a canvas that meets M1503. Note: I've had mechanical vibration issues before, and appreciate their potential severity.

Q2: Duct supports - I assume for the shown space, it will be better for vibration to support with hangers, versus supported from underneath. I will be wrapping the duct in fiberglass duct wrap (likely R8). What's the best way to suspend the wrapped duct (both vibration and thermal considered)? I'm planning on one hanger at each end of the duct silencer, and one at the elbow nearest the range hood. Is that acceptable?

Q3: Internal fasteners - the attic will basically be inaccessible. Where the ducting attaches to the roof mounted fan, I'd like to fasten the duct to the fan, using internal fasteners. This is to allow potential replacement of the fan on the roof, and by removing the fan motor, the fasteners could be removed. Are typical screws acceptable for this use and meet code, or should I consider pop rivets installed from the inside?

Thanks in advance!

Note: Duct silencer shown is near final location, perhaps a couple inches suspended.

 

[Breplum]

Kudos for choosing the silencer !
I've never used dampening hangers for duct, just equipment itself. So long as you use adequate strap (steel duct strap is better than 'plumber's tape', that should be fine since the strap helps diminish vibration harmonics.
Variable speed is recommended.
Q3: internally mounted screws sound fine.

 

[Sponsor]

 

[Rossn]

Kudos for choosing the silencer !

Thanks! I went back and forth on it for grease reasons, but do see range hood mfg's selling them and the inspector approved. Residential range exhaust systems are offensively noisy!

I've never used dampening hangers for duct, just equipment itself. So long as you use adequate strap (steel duct strap is better than 'plumber's tape', that should be fine since the strap helps diminish vibration harmonics.

I might have heightened concern because I have a mitsubishi mini spit compressor on a solid brick wall that sounds like a helicopter/diesel outside at the right speed. Still working through that with the contractor.

I see various metallic duct straps available at supplyhouse. 3/4", 1", 1-1/2" widths and 22, 26, 28ga. What's ideal for this, and is there a case for screwing the strapping to the end of the duct silencer (has about a 2" face) or just lassoing the insulation?

Variable speed is recommended.

Yep, the fan+hood (and makeup air system) are both infinitely variable speed.

Q3: internally mounted screws sound fine.

Ok, thanks - wasn't sure if that passed the sniff test for range exhaust!

 

[Rossn]

I see various metallic duct straps available at supplyhouse. 3/4", 1", 1-1/2" widths and 22, 26, 28ga. What's ideal for this, and is there a case for screwing the strapping to the end of the duct silencer (has about a 2" face) or just lassoing the insulation?

I found a mepacademy.com article referencing a SMACNA chart, shown below. Also a NC mechanical code requirement of strapping at least 1" and an equal or thicker gauge to the duct. Now, that silencer is 30 lbs (much heavier than ductwork and 14" OD), so I assume I should go with the 1", 22 gauge, if that sounds sufficient for its weight. I suppose it could also be doubled up.

 

[Fitter30]

3/16 duct hanging cable. Don't think vibrations travel up them.

 

[Fitter30]

How will these fans controlled?

 

[Rossn]

3/16 duct hanging cable. Don't think vibrations travel up them.

Thanks - I'll check it out. Sounds like this is not a rod but a cable.

 

[Rossn]

How will these fans controlled?

The roof top exhaust blower will be powered through the range hood, which as a pot style infinitely variable control.

The makeup air system uses the Fantech FMAC controller that a 3rd party developed for their gen 1 makeup air systems, also used in their Gen 2. It uses CTs to measure current flowing to the exhaust fan, and you set the ceiling and floor set points, and then fine tune fan output with two pots for amount of air flow. It then ramps between the set points proportional to the current range you selected. When fans are running, it sends a constant 24v to the heater, which uses an SCR control to a setpoint. Actually, a very well thought out control system.

 

[Fitter30]

The roof top exhaust blower will be powered through the range hood, which as a pot style infinitely variable control.

The makeup air system uses the Fantech FMAC controller that a 3rd party developed for their gen 1 makeup air systems, also used in their Gen 2. It uses CTs to measure current flowing to the exhaust fan, and you set the ceiling and floor set points, and then fine tune fan output with two pots for amount of air flow. It then ramps between the set points proportional to the current range you selected. When fans are running, it sends a constant 24v to the heater, which uses an SCR control to a setpoint. Actually, a very well thought out control system.

Fan air flow aren't linear. Looks like new construction building should be tight. HRVor ERV should be used and bathroom exhaust fans might make a difference.
Ever walk.lnto a office building where the entrance doors or blowing open or take noth hands to open them. That's building pressure.

 

[Rossn]

Fan air flow aren't linear.

Fair enough. It's just trying to set a range based on current draw.

Looks like new construction building should be tight.

Really old re-construction. Most of the home will be reasonably tight.

HRVor ERV should be used and bathroom exhaust fans might make a difference.

Lunos spot HRVs will be used. There's no ducting on those; they work in pairs push-pull.

Ever walk.lnto a office building where the entrance doors or blowing open or take noth hands to open them. That's building pressure.

Hopefully I'll be able to adjust the makeup air controls to be neutral or slightly positive.

 

[Rossn]

Scratch this post... the duct work I bought from the supply house some time back isn't the 26 gauge I had on the receipt. I put a mic on it, instead of calipers, and read 0.014" +/- 0.001". I don't know what that is... maybe thick 30 gauge. I'll get new duct that's actually 26 gauge.

 

[Fitter30]

28 is easier to work with . 26 is more of a commercial grade. A 4" or 6" grinder with a thin disk is a easy way to cut round pipe. Ear, eye protection and gloves needed. Hand crippers for making the ends to slip into the raw end. Three self tapper screws per joint seal with aluminum tape or duct mastic.

 

[Rossn]

28 is easier to work with . 26 is more of a commercial grade. A 4" or 6" grinder with a thin disk is a easy way to cut round pipe. Ear, eye protection and gloves needed. Hand crippers for making the ends to slip into the raw end. Three self tapper screws per joint seal with aluminum tape or duct mastic.

Thanks. Most of the supply houses here I only see 26 and 30, vs 28. I'm not quite sure what I have already, but it feels a bit flimsy.

Have grinders that should work and use PPE.

I think I actually have both mastic and aluminum tape with a squeegee. Is one better for this application?

I've also got to figure out, esp. given the silencer, if it should have slope or be flat (condensation). I don't expect much with R8.

 

[Rossn]

28 is easier to work with . 26 is more of a commercial grade. A 4" or 6" grinder with a thin disk is a easy way to cut round pipe. Ear, eye protection and gloves needed. Hand crippers for making the ends to slip into the raw end. Three self tapper screws per joint seal with aluminum tape or duct mastic.

Fitter30...

For galvanized ducting - what screw size is preferred? #8x1/2"?

By self-tapping, you're referring to the first one, not the second (self-drilling) one?

Edit: I removed a question about duct sealants, as I spoke to specialist at Hercules, and he guided me to their in-house MTS100 product as the best offering they have for longevity. They're a large distributor in my region, and he seemed to be the guy who specializes in sealants for their company, so I'll go that way.

 

[Fitter30]

Fitter30...

For galvanized ducting - what screw size is preferred? #8x1/2"?

By self-tapping, you're referring to the first one, not the second (self-drilling) one?

View attachment 103829

View attachment 103828

Edit: I removed a question about duct sealants, as I spoke to specialist at Hercules, and he guided me to their in-house MTS100 product as the best offering they have for longevity. They're a large distributor in my region, and he seemed to be the guy who specializes in sealants for their company, so I'll go that way.

Drill screw. 1.5" cheap chip brush foe the mastic. Just leave it in the container till your done then pitch it.

 

[Rossn]

@Fitter30 - went to install the ductwork, and ran into a real hitch. The flange on the bottom of the roof-top blower does not mate up correctly with 10" ducting. It's more like 10-1/4"-ish, but maybe not quite round. I called and talked to Broan tech support, who originally said it should be 10", then came back after pulling a blower from stock and said it's sized to accommodate larger than 10", and that the bulging ring behind factory crimp should just fit into it, then screw and tape the gap. Sounds way wonky.

With inaccessible attic, I also planned on being able to pull the blower from above and disconnect from the ducting with the screws joining duct to blower inserted from the interior outwards. But, tape would prevent removal.

I can tell you are a very resourceful guy. Do you have any ideas to offer? The best thought I can come up with is to take a piece of 10" duct with factory crimp, and cut it to extend maybe 3" beyond the blower, then fill the gap with silicone(??).

The riveted sheet metal isn't quite centered, and is a little oblong... measuring 10-1/16" to 10-5/16" across, depending on where you measure:

Here I've inserted the male end of a section of duct, and you can see the 3/8" clearance on one side, when pulled to the opposite:

 

[Fitter30]

Use your crimpers make it a little smaller, screw it and use some duct sealer to seal the seam. Or cut a strip of metal 32" long pop rivet to the 10" to make it bigger ,screw it and use duct sealer.

 

[Rossn]

Use your crimpers make it a little smaller, screw it and use some duct sealer to seal the seam. Or cut a strip of metal 32" long pop rivet to the 10" to make it bigger ,screw it and use duct sealer.

Ok, thanks. I thought I had successfully avoided buying a tool that would be used once (crimpers... instead, bought factory crimped sections), but I'll either do that or the metal strip spacer. Either way, I think I have to make a 6" section first, where I can remove it from above, but would have to get creative on the sealing. I don't think duct sealant on the interior of that one seam is allowable.

 

[Fitter30]

Ok, thanks. I thought I had successfully avoided buying a tool that would be used once (crimpers... instead, bought factory crimped sections), but I'll either do that or the metal strip spacer. Either way, I think I have to make a 6" section first, where I can remove it from above, but would have to get creative on the sealing. I don't think duct sealant on the interior of that one seam is allowable.

Build the inserted pipe a larger diameter sealer will be on the outer edge. Everything up to the fan wheel is in a negative.