HVAC silencer boxes & ducted design

your fresh air requirements don't change regardless of the approach used. the mini-split reduces the amount of cfm used for the temperature control. if you have little issue with humidity and no need for full filtering, then the mini-split will work as most do not manage humidity (like an AC unit, then can condense moisture but not add it) or have HEPA filtration/UV etc options. you do still need fresh air and depending on the energy levels of people in the room, you need 25-35% of your air volume replaced 6-10x per hour. so if you have 1000ft3, then 8x = 8000ft3. 25% of that is 2000ft3 per hour of fresh air. figure a 50cfm ERV unit will meet that need @ 50fpm velocity with a 1ft2 area duct.

Brilliant...thanks Glenn! :D

gullfo wrote:

your fresh air requirements don't change regardless of the approach used. the mini-split reduces the amount of cfm used for the temperature control. if you have little issue with humidity and no need for full filtering, then the mini-split will work as most do not manage humidity (like an AC unit, then can condense moisture but not add it) or have HEPA filtration/UV etc options. you do still need fresh air and depending on the energy levels of people in the room, you need 25-35% of your air volume replaced 6-10x per hour. so if you have 1000ft3, then 8x = 8000ft3. 25% of that is 2000ft3 per hour of fresh air. figure a 50cfm ERV unit will meet that need @ 50fpm velocity with a 1ft2 area duct.

I've always calculated the amount of air based on the total number of people that will use the space rather than the volume of the space. At least 15cfm per person, or in the UK the standard is 10L/sec per person. Here's a quote from Rod Gervais: "I have stated before and will state again...... calculations based on air exchanges per hour just do not make sense..... Tke a room 10 x 14 with an 8' ceiling - based on your calculations that room would have 6 exchanges oer hour for a total of X cubic feeet of air per hour (the actual number is not relevant) Now - if we decide to make the ceiling 18' high - we suddenly need more than twice as much air? Why? We won't fit one more person in the room than we could before - and they are the ony thing we need to worry about dying if they don't get enough air........... Fresh air supply should be provided based on the maximum number of people who will be in the room........ and then that number should be multiplied by a factor you feel is reasonable for the purpose. As an example of this - the Internation Mechanical Code sets a minimum cfm per person in a music room of 15 cfm. They have different rates for dfferent occupancies - but this is what they set for music rooms." More on the topic here: https://www.gearslutz.com/board/studio- ... -hour.html

thanks! agreed a minimum per person can be useful when you know the number or using ASHRAE /ICCSAFE minimum numbers for computation. reference: https://codes.iccsafe.org/content/IMC20 ... entilation

Thanks guys I must say that "per person" makes sense given that the fresh air supply is all about keeping people awake and ultimately alive. I'd had seen Rod's calculation basis (and that quote) in my study of his book but the room volume approach seemed to dominate posts I was seeing - so I just launched in to that as a starting point. Andrew

i tend to use the volume approach for CR and isolation spaces - for sizing of equipment since i know they're not going to have crazy ceiling height :-) but use the person estimation for large live rooms (e.g. 20 piece orchestra, 5 piece high energy band, etc) to assess versus volume figures. in most of my cases the volume calcs come out just slightly larger than per person so i have a margin of error. if the volume calcs are more than 20% larger i go with the people calc unless i have some other parameter indicating the larger number is more likely to be safer.

Hope it's OK to ask here as opposed to a whole new thread. Search has failed me on this topic. So, nitty gritty on silencer box construction -- what's the preferred way to attach duct liner to the box? Some kind of spray glue or perhaps those fasteners that have a nail with a large plastic washer? It looks like, when lining metal duct (just for example, I don't plan on this), it's recommended to use a weld on fastener like this. Although I have no idea how one would actually weld one through the liner, inside a duct. But anyway, I would assume the nail-on would work OK with MDF. Spray glue seems more risky as it could start to fail years later and you'd never know. The other question is, what about seams? Do you just make cuts as precisely as possible for an interference fit or is some kind of caulk involved? Any other recommendations on assembling these? It sure seems cutting up this expensive liner to fit tightly inside a 3D space would be pretty challenging.

glue can leave some noxious residue so roof shingle nails or wide head nails with the rubber washers are a good choice.

Thanks. How about seams? Just make them tight? I've seen people talk about rounded corners and the like. Not sure how one would do that with duct liner. Maybe working a bead of caulk but that seems like a messy combination.

i generally use duct board because i can form the interior pathway, tape it, and drop it into the box with the dividers.

Hi, Where I live (the Netherlands) duct liner and or duct board are not available. I was thinking to buy rockfon suspended ceiling tiles to line my silencers as they have tested/known acoustical properties, they are easily found and they come with a fleece on the front and the back. I my mind, the front and back fleeces prevent fibers to come loose and pollute the air, just like it this is the case with duct board/duct liner. Also, since they are made from mineral wool, they are inherently fire resistant and can handle high humidity. What do you guys think?

I see no problem with that acoustically etc. But isn't Rockfon really expensive? And... surely duct liner must be used in the Netherlands. The names of fibre products seem to change all the time so Google.....

It is not so expensive, as it only takes a few m2 of area you can pick up rest material from craigslist (marktplaats.nl in the Netherlands). I spent hours googling for duct liner/duct board, with all large insulation material suppliers (Knauf, Rockwool, Isover) and could not find a suitable equivalent. My assumption is that in general, it is not common practice to insulate the inside of ductwork rather than the outside, so there is not a very large market for it. But then again, I am far from an expert :)

there is a difference between duct board and duct liner - the duct liner is often a secondary insulation/noise damping that is added to hard ducts. duct board is like a hard sheet with the liner already attached. you use a razor knife to cut and duct tape (the real stuff) to hold it together - commonly you cut a V shape in the liner so you can easily fold it - and you now have a duct with liner :-) if you fold the duct board back over on itself (hard parts touching, liner exposed) you can create the interior baffles to line the box, notch the top and bottom sheets so the baffles sit snuggly in them and keep them stable. then drop that assembly into the mass box - plywood/drywall, etc. typically i do this for in-room plenums as part of distributing the air in the room.

Thanks Glenn! This bit of information about how to use duct board cleared things up nicely for me. Really appreciate the thinking you've put into this topic. I'm much more confident about tackling proper ventilation in my next build. Stuart

In a four silencer box system, with an inline fan removing stale air, would it be feasible to place the fan in the box located on the inner leaf? That is, at the end of the pathway just before the stale air exits the box and moves through a sleeve and on to Flexi duct. I'm thinking that my inner leaf box will protrude into the room and my outer leaf box can sit in the cavity between leafs, sleeved through the outer leaf with a grill or hood over the external opening. Same approach on the passive intake side (minus the fan). Ease of access to the fan would seem to be best achieved by having it in the internal box. Any thoughts? Or am I completely off track here? Stuart

you could incur noise from the fan + its vibrations. best bet have the fan on the input of the supply exterior box and output of the return exterior box - easy access and decoupled.

Just not sure about access if I had to replace a fan located at the output of the return external box, if that box was between the leafs. I could put the external box on the outside of the structure. Assuming it was well sealed and painted, housing a fan in there should be ok? Thanks Glenn.

the fan doesn't need to be in the box, just at the entry/exit points to force an air flow. so it should work.

Got it, thanks Glenn.