AVS Forum Special Member
Join Date: Aug 2006
Location: Austin, TX
Mentioned: 19 Post(s)
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The overall design relied heavily on the Rod Gervais book, Home Recording Studios - Build It Like the Pros. Additional inspiration came from this forum, as well. The theater consists of a Main Room ("MR") and a Lobby. The MR will be built as a room-in-room with a small attic space which will house all of the HVAC equipment suspended from the rafters. The Lobby outside the MR includes a bathroom, a small snack area and a Rack Space or AV closet ("AV"). Cooling will be provided by a Mitsubishi minisplit system with a combination of a ducted air handler as well as two wall mounted ductless units. The system will also include additional dehumidification as well as a fresh air source. Submittal sheets for most of the components will be attached below.Lobby
The Lobby will have a dedicated 9,000Btu minisplit wall unit near the entry door to provide cooling (or heating). At one end of the Lobby will be a small bathroom with a dedicated Panasonic ERV (FV-04EV1). This is an all-in-one 40cfm Energy Recovery Ventilator ("ERV"). It's not super efficient, but it is pretty quiet and it only needs to vent the bathroom while it is in use. There is also the option to run the bathroom ERV unit continuously at 20-40cfm if the Lobby needs additional fresh air. At this point, I'm not planning on sharing the MR fresh air system with the Lobby.Main Room
The Lobby will also house a small drink refrigerator and a popcorn machine in the Snack Area and will have a dedicated vent hood with a makeup air supply to match the vent hood exhaust flow, although this has not yet been specified. The plan is to connect the vent hood to a timer. A motorized damper will also be connected to the timer which will open a duct to the outside to allow makeup air to enter the Lobby near the popcorn machine while the vent hood is in use. Without the makeup air, the vent hood effectiveness would gradually diminish as the vent hood depressurizes the Lobby. At some point the negative pressure in the Lobby would match the pressure differential achieved by the vent hood and no more exhaust flow would occur. I have no idea how long this would take and whether makeup air is really needed or if this is just overkill. Adding the makeup air ducting with a motorized damper would be relatively inexpensive, but I'm not sure what the routing might entail....
Near the entrance to the MR will be the AV closet which will house equipment racks. I have room for two racks and a small entrance door, but I have not decided whether I want a door or a simple false wall or some other design. At the far end of the AV closet will be an electrical sub-panel and the area around the panel will be relatively open. Above the racks, inside the AV closet, I plan to install a second 9,000Btu ductless minisplit wall unit. I'm still not sure whether I need 9,000 Btu or if 6,000Btu would be sufficient (I've never done this before). If you look at the attached spreadsheet, I'm anticipating around 7,000Btu of heat generation during peak use, but this is highly dependent on how much heat my amplifiers produce during normal listening vs. reference vs. play time...I just don't have a good feel for this. For now, I've plugged in a 10% power usage to determine my Btu output. The amplifier capacity was determined by the watts needed to play at reference plus 3dB of headroom based on Titan mains, HTM12s for surrounds and Concentric 8s for Atmos. I've allotted 6 channels for subwoofers at 2,000 Watts per channel to play 6-10dB hot over reference.
The spreadsheet has been pretty handy for determining duct size and air handler sizing as well as providing a double check for grille sizing. For example, I had originally planned in a 12,000Btu air handler, but the fan capacity would be maxed out meeting the room air exchange requirements. Also, at the highest airflow, the noise produced would also be increased, requiring more extensive duct silencing. Lastly, the smaller unit did not have any reserve capacity to overcome airflow resistance introduced by ducting and filtration. Moving up to the next larger air handler, 18,000Btu, would then overtax the planned outdoor unit, so I had to increase the size of the outdoor unit as well. While normally you can install 130-150% greater capacity of the indoor units with a minisplit system, there was a small footnote that said you should only install indoor units up to 100% of rated outdoor capacity when you are using a ducted air handler. Sooo, I was forced to increase the outdoor unit Btu capacity yet again, especially when you take into account derating factors such as outside air temperatures and piping lengths. All things considered, my total cooling derating is 94%.Discussion
One aspect of the Gervais design that was new to me was the need for a barometric pressure relief valve. Basically, if we have truly built an airtight room, and we are constantly supplying it with fresh air from the outside, we will eventually reach a point where the positive pressure inside the theater exceeds the fresh air fan's ability to force more air into the room. Thus we need some sort of pressure relief system to vent stale air out of the room as new fresh air is brought into the room.
As I researched systems that utilize Barometric Pressure Relief Dampers ("BPRD"), I soon realized that there really aren't any BPRDs sized for home use. Furthermore, most commercial BPRDs operate in a higher differential pressure range than I expect to have inside my theater. As a result, I decided I would just use a common backdraft damper, with the hope that the opening pressure is sufficiently low as to not impede fresh air circulation into the MR. Since the Stale Air vent creates a direct connection between the MR and the outside, I felt it would be necessary to introduce some sort of muffler to minimize sound escaping outside and defeating the whole purpose of the room-in-room.
On the other hand, the Fresh Air vent enters the return air plenum close to the air handler and does not create a direct path between the room and the outside. I've elected to use a passive Fresh Air system to take advantage of the relatively negative air pressure near the mouth of the air handler on the return side. My hope is that the pressure differential will be sufficient to entrain an adequate volume of fresh air into the room to meet the calculated fresh air volume. Ever the optimist, I'm planning an air flow regulator to limit the fresh air flow to the maximum needed for a full house in the MR. The CAR-II-LP is a clever passive device that continuously varies its orifice size to maintain a constant air flow over a reasonable range of differential pressures. The Fresh Air circuit will include a large air filter for minimal flow resistance. A motorized damper will also be incorporated to prevent back-flow when the system is not operating (this connection is shown as a>>a in the schematic). In the event the passive system fails to work as expected, I can always add a small inline fan to ensure positive fresh air flow.
I've sized all of the grilles to achieve airflow velocities of less than 200fpm, and many are closer to 100fpm. Grille placement is shown in the drawing below. 8"x48" Linear bar grilles are placed up front with a 15° bar angle to direct the airflow away from the screen and toward the seats. 16"x25" filter intake grilles are placed in the rear of the MR in outboard positions. The remaining two grilles in the rear are 14"x14" and one serves the dehumidifier and the other is for the Stale Air / BPRD system. I plan to maximize duct size to minimize velocity within the ducts to limit aerodynamic noise and I plan to line all of the ducts with insulation to absorb as much noise as possible. In addition, both the supply and return ducts will incorporate 90° bends to try to attenuate HVAC-based noise transmission into the MR as well as to prevent theater sound from escaping the MR into the attic space and thus flanking into the rest of the house.
Lastly, I plan to use a single wired controller in the MR to control all of the minisplit units. I also plan to have a humidistat in the MR for controlling the MR humidity.
All of this is new to me, and I may have overlooked something incredibly basic. Regardless, I am really looking forward to your comments and constructive criticism on this design. I plan to install most of this system early during the construction so I can take advantage of a comfortable room while I complete the interior. Therefore I thought it would be prudent to start this discussion early. My thinking right now is to do the majority of the mechanical and electrical installation and have a technician come out and perform the system checkout, evacuation and charging. I'm not sure how this impacts the equipment warranty, and this is one area I still need to investigate.
Thanks in advance for all your time and assistance,