Translate This PageHere’s my ventilation plan. Just for the live room – the CR is simpler given there are more places to “hide” registers and it’s a smaller room volume, hence smaller silencers.
There are 2 questions at the end.
I must say – this is the hard bit. I thought I had it sorted from a velocity and CSA drop down perspective but once I looked at the static pressure, it forced me back into another round of tweaking to get into a safe static pressure operating range for the fan (and a change of fan).
So a word of warning to anyone in design phase. The experienced builders here will tell you to do the HVAC plan early. DO NOT ignore this advice! Juggling conflicting objectives of:
• Silencer box size – to fit in the space
• Silencer box weight
• Static Pressure capacity of the fan
• Duct size – to fit in the gap; and
• Size of CSA drop downs
is seriously brain bending stuff – every part of my system has changed en route.
Anyway here’s my plan….
(Apologies up front for my switching between metric and imperial – they changed over here half way through my schooling so I have “measurement schizophrenia”..)
My outer silencers will be hung on the wall separating the studio from the workshop, with duct pipes running to and from the external world from there. They will be standard silencers – while the inner silencers will be the dual flow ones.
Here is the “big picture” plan. (Ignore the half-finished door spaces in the model. Also, there’s obviously tons of fluffy in the ceiling gap but I’ve left that out for clarity.)
First a shot for context with roof removed.
Silencer/duct sizing calculations are as follows:
Room volume = 3,428 cu ft
CFM at 8 changes per hour = 457 CFM
Fresh air requirement
@ 10 CFM with 8 persons = 80 CFM
@ 20% = 457 x 0.2 = 91 CFM
@ 30% = 457 x 0.3 = 137 CFM
I’ve based my design on the “10 CFM fresh air per person” measure but stress tested it a bit and worked out that I can run at up to 27% of room CFM within the static pressure capacity of my chosen filter with a velocity still less than 200 fpm at the register exit. (While I feel the “per person” measure is already a stress test as I’ve allowed for 8 people and it would never get above 6 – this is life and death stuff so I’ve given myself more capacity and invested in a CO2 meter to be safe.) 27% room CFM is just under 16 CFM fresh air per person for 8 people.
My intended fan is a Fantech 125mm Ecowatt unit.
Here’s the velocity calculations and duct CSA measurements - working back from the inner silencer register:
(All calculations are from the inside of the 25mm of duct liner)
Registers
190mm x 160mm (CSA = 0.33 sq. ft) but with 2 channels it’s effectively doubled = 0.66 sq. ft.
Velocity = 80/0.66 = 121 fpm
Inside the Inner silencer
140mm x 110mm (CSA = 0.155 sq. ft) but with 2 channels it’s effectively doubled = 0.33 sq. ft
Velocity = 80/0.33 = 242 fpm
Inner-Outer silencer duct
125mm dia = (CSA = 0.132 sq. ft)
Velocity = 80/0.132 = 605 fpm
Outer silencer
178mm x 200mm CSA = (0.39 sq. ft)
Velocity = 80/0.39 = 205 fpm
Outer duct
125mm dia = (CSA = 0.132 sq. ft)
Velocity = 80/0.132 = 605 fpm
CSA Step ups:
Into the outer silencer – ~ 3 times
Into the inner silencer – ~2.5 times
Construction details
• The silencer body will be 1 layer of 16mm mdf > green glue > 1 layer of 18mm mdf. This is generates mass/m2 in line with my wall mass
• The top of the inner silencers will sit around 40mm from the outer ceiling
• I’m running the long 5” duct run on the return side inside some 6” flexible duct – to help keep it in shape (I’m pretty sure that is one of Glenn’s ideas)
• I’m using 12mm Neoprene blocks under the silencer between the silencer and the support beams and some leftover insertion rubber on top of the beams where the internal duct (from the return inner silencer to the outer silencer) will rest
• The fan will be located on the supply side to keep pressure on the inner doors
• Louvres/filters will be located at the terminal ends of the 125mm duct runs to and from the outside world.
• The spigots I’m using have 90mm of sleeve which will go through the outer wall nicely and present enough exposed sleeve to attach the flexible duct
• For duct liner I’m using Bradford Supertel https://www.bradfordinsulation.com.au/c ... t/supertel and will follow Starlight’s stapling approach.
I’ve picked up some Sashco Big Stretch for filling penetrations around wall holes for spigots which I’ll also use for electrical and AC penetration filling.
These internal dimensions mean that:
The outer silencers have outside dimensions of 1.29m x 0.56m x 0.33, weighing in at 64 kg
The Inner silencers have outside dimensions of 2.07m x 0.47m x 0.23, weighing 78 kg
Static pressure
Using the engineering toolbox, I’ve calculated that the inner silencers would have an equivalent round duct diameter of 5.33 inches and the outer ones, 8.18 inches.
Using the “Duct Diameter x 60 equation” for converting 90 degree bends into an equivalent length of straight duct gives
5.33 x 60 x 9 (90 deg turns) = 240 2ft equivalent of straight duct for each side of each INNER silencer
and
8.18 x 60 x 10 (90 deg turns) for each outer silencer = 409 ft of straight duct for each OUTER leaf silencer.
For the inner silencer static pressure, I’ve halved the fresh air flow to 40 as only half goes through each side.
Adding the static pressure from 40 feet of 5” duct across the internal and external runs (0.0544 wg) and allowing 0.1 .wg for louvres/filters gives me a total static pressure of 0.357 + 0.0544 + 0.1 = 0.36 .wg = 89 Pa
My fan specs allow up to 261 Pa. As mentioned above, that gives me capacity to run the fan to generate fresh air at up to 27% of room CFM.
1. What have I stuffed up? My results seem pretty consistent with similar builds but HVAC keeps throwing curve balls at me every few weeks so I’m far from over confident
2. I’ve used a 3” (75mm) overlap (see below) on the baffles inside my silencers per what I understand of the meaning of “X+3” in Gregwor’s model. Perhaps I’m looking in the wrong places but there seems to be quite a bit of variation in the overlap of baffles across the various silencers I’ve seen. The largest is in Rod’s book (p155 - 2nd Ed.) where he shows a diagram with 17 ¾” long baffles and a 10 ¾” gap to the other wall implying a 7”overlap. Others seem to have used minimal overlap.
Any rules here?
