# Installation of dead vent + fan



## patchesj (Jun 17, 2009)

My question is not so much the cosntruction of the actual dead vent, but how does one "attach" this to a room-in-room theater? I will have 2 dead vents (one intake, one exhaust) on opposite corners of the room. The plan is for 2 inline fans on either dead vent ~350CFM each. The dead vent boxes will be installed outside of the theater room (one under a stairwell then other in the HVAC/mechanical room). The theater is going to be insolated double walls with 2x5/8 drywall/green glue on the inside and single 5/8 on the outside of the outer wall. In short, I need to connect the dead vent enclosure to the inside ot the theater room wall, but keep it as sealed as possible. It seems to me that I would want to extend the MDF from the dead vent all the way to the back side of the double 5/8 wall. Of course, then I'm allowing those wall vibrations to "move" across the MDF to the secondary rooms. What is the best way to do this (other than putting the dead vent enclosure inside the theater.)?

Also, is single layer 3/4 MDF OK or should I look at 1/2x2MDF to build the dead vent?

Thanks


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## bpape (Sep 14, 2006)

3/4" should be fine. Don't forget to do the bends. You can attach it to the inner walls but outside the room. For farther away, you can use something like PAC Intl DC-04 clips

Bryan


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## patchesj (Jun 17, 2009)

I was planning to go out of the room horizontal near the ceiling, down 90 degrees to floor (~6 feet), 90 away from room, ~1 foot, 90 back towards ceiling, ~3' to fan, up 3' more, 90 degrees to left, exit dead vent into 5' flex to register on wall of main open area.

Is that enough length/bends?

What about the CFM for the blowers? Anyone recommend something in that size range that is quiet/reliable?

Thanks.


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## bpape (Sep 14, 2006)

That's enough bends and borderline on length but will probably work OK. I usually recommend a total of 15' from exit of isolation to the vent.

Bryan


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## sga2 (Nov 14, 2009)

patchesj said:


> What about the CFM for the blowers?


That depends, mostly, upon how much heat load will be in the room and the expected temperature of the room from which you will be drawing the supply air. I can give a recommendation if you answer a few questions:

How many people will the theater accomodate when full? 

Can you provide a list, with power requirements, of the equipment (projector, amps, etc.) that will be located in the room? You can ignore the small stuff (BD and CD players, small network routers/switches, cable/satellite box) but be sure to include the pre/pro, amps, subwoofers, and projector since these all can produce significant heat. *Be sure to include any forseeable upgrades.*

Does this room have any surfaces (walls, floor, roof) that will be exposed to outside or any other unconditioned space (attic, garage)? If so, any idea of the insulation R-value for each condition?

Any windows? If so, what size?

Any idea of the expected temperature of the adjacent space? 

What temperature range do you want to maintain in the theater room?


Some side notes:

I would recommend using a wall-mounted speed controller on the fan so you can adjust fan speed (i.e., noise) down when you don't need maximum air movement. 

Why are you using a fan on the "supply" vent? Could you get by with a fan on the return/exhaust vent only (let the supply vent draw air via "gravity") to reduce cost and, most importantly, noise.

sga2


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## patchesj (Jun 17, 2009)

sga2 said:


> That depends, mostly, upon how much heat load will be in the room and the expected temperature of the room from which you will be drawing the supply air. I can give a recommendation if you answer a few questions:
> How many people will the theater accomodate when full?


Max seating will be 6-7. Most of the time 3-4.



> Can you provide a list, with power requirements, of the equipment (projector, amps, etc.) that will be located in the room? You can ignore the small stuff (BD and CD players, small network routers/switches, cable/satellite box) but be sure to include the pre/pro, amps, subwoofers, and projector since these all can produce significant heat. *Be sure to include any forseeable upgrades.*


Projector is Panasonic AE2000, almost everything else will be outside the room. Potentially an Oppo 83 and pre/pro in the room, but I haven't decided yet. If they are in the room, they'll all be flushed into the front false wall and the "exhaust" vent for the room will be back there effectively draing the heat out at/near the source.



> Does this room have any surfaces (walls, floor, roof) that will be exposed to outside or any other unconditioned space (attic, garage)? If so, any idea of the insulation R-value for each condition?


Floor is basement concrete. 2 other walls are exterior concrete, but all walls/ceiling will be at least R13 fiberglass + 2x5/8 drywall.



> Any windows? If so, what size?


No windows



> Any idea of the expected temperature of the adjacent space?


Basement is usually 68-70 in summer (cooler in winter)



> What temperature range do you want to maintain in the theater room?


no warmer than 74



> Some side notes:
> 
> I would recommend using a wall-mounted speed controller on the fan so you can adjust fan speed (i.e., noise) down when you don't need maximum air movement.
> 
> ...


I've been under the impression that "balanced" airflow into the space is desired. I could certainly use a single fan, but I would think this would be better on the supply side.


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## sga2 (Nov 14, 2009)

I would recommend designing the system to deliver 500cfm through the room at your conditions (7 people; 240W projector; negligible heat load from other equipment, lighting, or walls; air supplied from adjacent room at 68-70F). This assumes you are near sea level elevation (increase airflow 4% for every 1000ft).

As far as 1 vs. 2 fans… Only 1 fan is needed to move air through the room. A case can be made for 2 fans where room pressurization is a concern; however, often those concerns can be mitigated or eliminated by good design of intake and exhaust ducts. I like 1-fan solution whenever possible because more fans = more opportunity for noise and vibration issues, higher cost, and more duct and electrical work.


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## glaufman (Nov 25, 2007)

How do you arrive at the 500cfm number?


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## bpape (Sep 14, 2006)

If we knew the dimensions of the room, we could calculate the cubic footage and divide by the 500cfm and determine how many times per hour the air will be exchanged. As it is right now, at 500cfm, that will move 30000 cubic feet of air per hour. If the room was say 12x20x8, that's 1920 cubic feet. So the air would exchange approx 15.5 times per hour. That's a pretty high exchange rate but I'm purely guessing at the room volume. You will also likely need to upgrade to a higher flow unit than would otherwise be required due to the restrictions in the air pathways for sound control.

Bryan


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## patchesj (Jun 17, 2009)

I know I was figuring 50CFM per person, based on previous answers to some of my house IAQ questions elsewhere. 

500CFM would equal an 8" round duct required (maybe 10" if you consider flex+elbows, etc). It would also require something along the lines of a 10"x10" or larger register. Pretty close to what I had figured on using before, but was upsizing to keep airflow noises down.


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## patchesj (Jun 17, 2009)

Room will be 20x14x8


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## patchesj (Jun 17, 2009)

500 CFM is 30000CFH. Room is 2240CF so 13x per hour? Seems steep to me.


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## bpape (Sep 14, 2006)

Yes. You want high flow, low velocity register vents.

Bryan


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## sga2 (Nov 14, 2009)

patchesj said:


> 500 CFM is 30000CFH. Room is 2240CF so 13x per hour? Seems steep to me.


The airflow is based on a simplified steady state heat balance calc factoring the actual heat load in the room. Air-changes-per-hour rules of thumb are good for quick preliminary sizing; however, it is best practice to always calculate the actual heat load whenever possible. The equation I used is:

CFM = qsens / (1.1 x delta-T)​where,

CFM = Airflow, cubic feet per minute.
qsens = Sum of space sensible heat gains, Btu/hr.
1.1 = Heat capacity of air at 50%RH (density x specific heat x unit conversion factors)
delta-T = Temperature rise, degrees F, from point of entry to point of exit = Troom - Tsupply​
Assumptions based on your previous input:


Negligible net room envelope heat transfer (walls, ceiling, floor) since room is entirely below grade and conditioned space above.
Lights are off.
Occupant heat gain (sensible) = 225 Btu/hr per person (per ASHRAE) x 7 people = 1575 Btu/hr.
Projector heat gain = 175 Watts = 597 Btu/hr.
Troom = 74F.
Tsupply (from adjacent space) = 70F.
Therefore, 

CFM = qsens / (1.1 x delta-T)
CFM = (1575 + 597) / (1.1 x (74-70))
*CFM = 494 (round up to 500)*​A few things to consider:

1. This is just an estimate and the results are only as good as the assumptions. If the adjacent room is really 65F, you will need less air. If you have 10 people in the room, you will need more air. For a "proper" HVAC design, we would use more robust calculations but that will require much more info on the construction details and, personally, I think the extra effort would not be justified in this case. Once you've made the commitment to build a "dead vent" system with fans, acoustically treated ducts, etc., the extra cost to go from 350 cfm to 500 cfm will be minimal.

2. This equation assumes the room is at a _steady state_ condition and does not take transients into account. In reality, it takes several hours for all of the mass in the room (walls, floor, furniture, etc.) to warm up so it is quite possible that 3 people watching a 2-hour movie may not need any airflow at all (at least, not simply to maintain desired temperature). However, 7 people sitting through a Lord of the Rings Extended Edition Trilogy marathon might appreciate the full 500 cfm by the time Frodo sails into the sunset. 

3. Panasonic PT-AE2000 max power consumption is listed as 240W in the manual but we find that most A/V equipment draws much less power than nameplate. Since the lamp alone is listed as 165W, I assumed ~175W for the entire unit to include the cooling fan and misc electronics.

4. Placement of the registers has significant effect on performance. For example, if your return grille were located near the projector, you could get by with less air since much of its heat will be removed before it can mix with the room air. 

7. Since it is reasonable to expect that the room will normally need less than full airflow, you should consider a speed controller to adjust the fan airflow based on need.

I hope this helps.

sga2


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## glaufman (Nov 25, 2007)

Helps me a lot! Thanks! Not that I'm building or designing anything now, but I've often wondered...


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## patchesj (Jun 17, 2009)

I've reconsidered putting the amps in the room (behind the false front wall). Following the calculations provided here I'll be looking at 6400 btu/hr or ~1000CFM. That's a substantial amount of airflow. I just realized I still have a server rack AC unit (Ice Qube IQ6000T) that is 6000BTU cooling if I remember correctly. Just need to think about integrating this into my dead vent plan for cooling the space. I guess the other option is to just use it to circulate the room air and provide a small amount of additional basement air to keep CO2 down.


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