Installation of dead vent + fan - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 04-01-10, 09:19 AM Thread Starter
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patchesj

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Re: Installation of dead vent + fan

Room will be 20x14x8
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Old 04-01-10, 09:22 AM Thread Starter
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patchesj

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Re: Installation of dead vent + fan

500 CFM is 30000CFH. Room is 2240CF so 13x per hour? Seems steep to me.
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Old 04-01-10, 09:25 AM
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Bryan Pape

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Re: Installation of dead vent + fan

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

Bryan

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
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Old 04-01-10, 04:54 PM
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SGA

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Re: Installation of dead vent + fan

Quote:
patchesj wrote: View Post
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:
1. Negligible net room envelope heat transfer (walls, ceiling, floor) since room is entirely below grade and conditioned space above.
2. Lights are off.
3. Occupant heat gain (sensible) = 225 Btu/hr per person (per ASHRAE) x 7 people = 1575 Btu/hr.
4. Projector heat gain = 175 Watts = 597 Btu/hr.
5. Troom = 74F.
6. 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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Old 04-02-10, 08:48 AM
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Greg

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Re: Installation of dead vent + fan

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

-Greg

Don't worry... nothing new here, I've already made that mistake. Trust me.
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Old 05-16-10, 06:03 PM Thread Starter
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patchesj

Join Date: Jun 2009
Posts: 372
Re: Installation of dead vent + fan

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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