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post #1 of 16 Old 12-14-2014, 11:31 PM - Thread Starter
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Help with Ventilation Plan

I have completed framing for the HT and will move onto wiring this weekend but I still have not been able to lock down my ventilation plan. The finished dimensions will be 18’8” X 12’ X 8’and will have seating for 7, Normal use will be my wife, two kids, and I. I have read and searched but I am still not sure what I should do.

While finishing the rest of the basement I had a supply line ran into the theater room, because the main trunk would be covered. This line is tied to the thermostat on the 1st floor (now I am not sure this line will do me any good) I did this before I had researched anything about the HT. Now after reading this forum, I’m not sure what to do with it. One post I found had described the same thing I have but now I can’t find it. I thought I marked it to save it.
If I remember correctly this person used the supply to provide conditioned air into the room and used a dead vent to pull air out. I believe this is what I should do but I am still unsure how to go about it.
My thought is:
1> Remove the hard duct back as far as I can go and replace with flex duct.
2> Snake the flex duct to the front of the room so air will flow to the rear.
a. This is where I start to get lost; I can’t just put in a vent I need to build a oversized plenum to slow the air down?
3> Place dead vent in storage area and snake flex duct to rear of room.
a. Split dead vent to help pull warm air out of AV closet.

I still have access to the return if needed, but it will be a tight fit. My guess is that since it’s not on its own zone this might not do me any good.
Please help me with any suggestions and if you know of a build thread that will show me good examples please point me in the right direction.
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post #2 of 16 Old 12-15-2014, 03:18 AM
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The first issue is having the theater tied into the first floor.

Cooling is going to be what you need; unlikely you will need much heat even though the first floor will be calling for it all winter.

You could put a motorized damper on the supply and a separate thermostat. What size is the supply? Was the air flow calculated (CFMs)?

With 7 people you are probably in the realm of a separate system (eg mnisplit). At the very least I would make accommodations for it.

Where is your equipment?

Tim
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post #3 of 16 Old 12-15-2014, 08:36 AM - Thread Starter
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Quote:
Originally Posted by Mr.Tim View Post
The first issue is having the theater tied into the first floor.

Cooling is going to be what you need; unlikely you will need much heat even though the first floor will be calling for it all winter.

You could put a motorized damper on the supply and a separate thermostat. What size is the supply? Was the air flow calculated (CFMs)?

With 7 people you are probably in the realm of a separate system (eg mnisplit). At the very least I would make accommodations for it.

Where is your equipment?

Tim
I do not know if it was calculated, the person who did the install ran a 7 inch line and said it should be good for that size of room. I would be able to put a damper inline, but would not be at the trunk. It would be at about the middle of the supply.

One thought I had was to seal that supply off and just exchange the air in the theater with air from other parts of the basement.

I have also looked into a mini split, but the problem I have is that the theater is in the front of the house and I would prefer not to have the outside unit in a place that can be tampered with. I was also wondering if cooling would work during the winter months if it was outside. Could the outdoor section be put into the garage? If the room is sealed does the unit replace the air in the room or would I still need a vent to bring in fresh are and pull out bad air.

The equipment is going to be in in the closet at the bottom of the plan. It will be facing out into the room though.
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post #4 of 16 Old 12-15-2014, 09:59 AM
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Can't really do anything but guess without some hard numbers.. but I would say 7" would be ok for 3 people with no equipment in the room. That's because I had 7" in my theater and I designed it that way.

I had 6 people and after an hour the temp was pretty high. Bearable but uncomfortable. That was with the A/C running nonstop. My basement was on a separate zone so I could do that without freezing the first floor.

I fixed it by cooling the theater to 65 before the movie.

But there is a lot that goes into the calculation. What my 7" produced versus what yours will is unknown. Your fan could be a different size, the trunks different size, the delta-T across the coils different etc. I was basically dumping 20 feet straight down from the attic into the theater.

Somebody else did an air exchange with the rest of the basement. I don't remember the thread.. But they used 100w lightbulbs to simulate people and (IIRC) some sort of pressure differential controller in addition to the temp controller. I think there was a problem with all the air flow creating an issue with negative pressure. It is a pretty well-known thread, so I am sure somebody will drop in and let you know which one it was. In short, it was not a simple process.


The inverter for the minisplit can be installed inside. If you have the room that would probably be the best solution. Put the inverter in a ventilated boiler room or whatever. Then it doesn't matter what the temp outside is. I know they make a cold air kit for regular CAC systems.. they may also make ones for the minisplits but I'm not sure.

Tim
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post #5 of 16 Old 12-15-2014, 08:27 PM
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You may be ok simply severing the supply feed to the theater and then using a straightforward dead vent setup to swap the theater air with that of the remainder of the basement. Looks like you air volume is greater outside the theater and if it maintains a comfortable temp year round, just size your CFM of the intake/exhaust fans for the 4x/hour air turnover by volume.

If your HVAC feed is tied to the upstairs unit, the theater will get heat in the winter when you need cooling. If you install a mini split, you will still need to swap the air in the room with a fresh air source (I feel) even though the room will not be totally sealed. I had several mini splits installed in my old house and loved them, but they are expensive and take up space and may really not be needed in the end for a small space that is not sealed up. I would consider having a plan on how and where to add one though if you determine it is needed.

I persevirated on this topic for sometime and then ultimately decided to just do the deadvent ONLY approach. Full disclosure - I am not done so do not know the final result. I will say though that working in this now sealed room with 500W of halogen light and the fantech FG6XL fans both on full blast (no dimmer installed yet) - it is fine for the longest stretch that I have worked in there. If you are interested, you can check out my build as I went overkill on my deadvent box builds and think I showed enough pics that you could extract the basics.
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post #6 of 16 Old 12-15-2014, 11:15 PM - Thread Starter
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Quote:
Originally Posted by Mr.Tim View Post
Can't really do anything but guess without some hard numbers.. but I would say 7" would be ok for 3 people with no equipment in the room. That's because I had 7" in my theater and I designed it that way.

I had 6 people and after an hour the temp was pretty high. Bearable but uncomfortable. That was with the A/C running nonstop. My basement was on a separate zone so I could do that without freezing the first floor.

I fixed it by cooling the theater to 65 before the movie.

But there is a lot that goes into the calculation. What my 7" produced versus what yours will is unknown. Your fan could be a different size, the trunks different size, the delta-T across the coils different etc. I was basically dumping 20 feet straight down from the attic into the theater.

Somebody else did an air exchange with the rest of the basement. I don't remember the thread.. But they used 100w lightbulbs to simulate people and (IIRC) some sort of pressure differential controller in addition to the temp controller. I think there was a problem with all the air flow creating an issue with negative pressure. It is a pretty well-known thread, so I am sure somebody will drop in and let you know which one it was. In short, it was not a simple process.


The inverter for the minisplit can be installed inside. If you have the room that would probably be the best solution. Put the inverter in a ventilated boiler room or whatever. Then it doesn't matter what the temp outside is. I know they make a cold air kit for regular CAC systems.. they may also make ones for the minisplits but I'm not sure.

Tim
I have the space in the garage, I can mount it high on the wall since the ceiling is about 15' up. Wife might not like that option though. I will have to look for that thread you mentioned if someone does not post a link to it. I don't recall seeing it while I have been looking. I'll try to search some different phrases.
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post #7 of 16 Old 12-15-2014, 11:25 PM - Thread Starter
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Quote:
Originally Posted by PApilgrim View Post
You may be ok simply severing the supply feed to the theater and then using a straightforward dead vent setup to swap the theater air with that of the remainder of the basement. Looks like you air volume is greater outside the theater and if it maintains a comfortable temp year round, just size your CFM of the intake/exhaust fans for the 4x/hour air turnover by volume.

If your HVAC feed is tied to the upstairs unit, the theater will get heat in the winter when you need cooling. If you install a mini split, you will still need to swap the air in the room with a fresh air source (I feel) even though the room will not be totally sealed. I had several mini splits installed in my old house and loved them, but they are expensive and take up space and may really not be needed in the end for a small space that is not sealed up. I would consider having a plan on how and where to add one though if you determine it is needed.

I persevirated on this topic for sometime and then ultimately decided to just do the deadvent ONLY approach. Full disclosure - I am not done so do not know the final result. I will say though that working in this now sealed room with 500W of halogen light and the fantech FG6XL fans both on full blast (no dimmer installed yet) - it is fine for the longest stretch that I have worked in there. If you are interested, you can check out my build as I went overkill on my deadvent box builds and think I showed enough pics that you could extract the basics.
Its diffidently sounding like I need to seal off that supply and take it out of the equation. My basement is a little on the cool side during winter months and just right during summer. I mostly close the supply vents in the rest of the basement and only open when its a little to cold. Haven't had to use them during the summer yet. From your plan it looks like you had two openings on each line, but in the build it looks like just one. what did you go with? I may attempt something similar.

How do I go about finding this out "just size your CFM of the intake/exhaust fans for the 4x/hour air turnover by volume" I'm sure I will be able to find something at some point but asking might be a little quicker return.
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post #8 of 16 Old 12-16-2014, 03:12 AM
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I had one supply and one return. When I calculated the CFM that was all I needed to meet the noise criteria I was looking to achieve.

Your room is 1786 cubic feet * 4 = 7144 cubic feet per hour. 7144/60 = 119 cubic feet per minute (CFM)

I do believe the 4x per hour is simply the recommended air exchange to keep the room air from getting stale, not so much a cooling calculation.

Tim
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post #9 of 16 Old 12-16-2014, 03:15 AM
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Black Cat Theater was the thread I was thinking about.

His testing begins at post 32.

Tim
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post #10 of 16 Old 12-16-2014, 05:43 AM
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The black cat had a design flaw, the intake and exhaust for the air exchange were too close together and at opposite ends of a hall, he ended up warming the hall and not getting the benefit of the larger volume of air in the basement.

He also had too much air going through too small of ducts and it produced a lot of noise.

I would shoot for more exchanges per hour than 4 maybe as much as 8-10 or so. Keep air speed at the vents less than 250 ft per min.

I would also worry that if you have a group over you might deplete the cooling capacity of the outer room if it is not connected to your HVAC.

The cooling air loses it's effectiveness the less the difference in temperature between the theater and the outer room.
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post #11 of 16 Old 12-16-2014, 08:23 AM
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As a follow up I did a project with supply tapped off the main duct and a powered dead vent return. It wasn't my preferred solution but the physical limitations dictated it. You need to be sure there is a real return connected to the space where you want to dump your dead vent, other wise you won't have an effective push/pull air circulation pathway. If your first floor and basement share an air handler and you can't zone the setup, during the winter when the upstairs is calling for heat and your theater is already hot you will have a problem. One solution is to turn down the thermostat a couple of degrees upstairs and set your fan to run continuously. It will basically be functioning like an air exchange system using your whole house. The excess heat in the theater will help heat the rest of the house.

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post #12 of 16 Old 12-16-2014, 10:19 AM
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Great advice above too. Think through the flow path you want the air traveling in the room. I decided to have the fresh air inlet be in the front of the room in front of the screen in my false ceiling to wash back over the seating and then the return will be in the rear of the right side soffit to pull hot ceiling air in the vacinity of the projector exhaust.

My theater is less than 25% of the total available basement air volume not even counting the fact that hot air is continuously rising up my open basement access stairway from the 1st floor. Unless doing a Star Wars 1-6 (and soon 7) marathon with door shut and a heavy mexican lunch prior to the start - I feel I should be in the ballpark!

Certainly more turns/hour would be better assuming you can keep the setup quiet. I sized my vents such that the max velocity, even with the fans on full theoretical cfm @ something like 250CFM, to have a speed of under 200fpm.

That formula is fluids 101:

Q=VA where Q is the volumetric flow (cfm or ft^3/min of fan), V=velocity of air (ft/min or fpm) at a vent opening with a cross sectional area of A (ft^2).

So, a 250cfm fan would need an area of about 1.25ft^2 if you desire a max vent velocity of say 200fpm.
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post #13 of 16 Old 12-16-2014, 04:05 PM - Thread Starter
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Thanks for the help, I will review this week and see if I can make a plan to have you all Check for me.
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post #14 of 16 Old 12-17-2014, 03:09 AM
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Quote:
Originally Posted by PApilgrim View Post
So, a 250cfm fan would need an area of about 1.25ft^2 if you desire a max vent velocity of say 200fpm.
Also keep in mind that is free area. Most registers will have a data sheet showing you the Ak. If not, assume 50% (therefore a register having an area of 2.5 sf would be required in the above scenario).

Tim
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post #15 of 16 Old Yesterday, 04:43 PM - Thread Starter
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Well from what I have gathered from you guys and some additional reading I think I have an idea to be looked over.


The total area is 1952 cf from both the theater and AV closet. So I would be looking for a fan with 192 cfm for 6 exchanges per hour or 325 cfm for 1o times per hour. I found one on Amazon with 392cfm which is closer to 12 exchanges per hour.


I would like to run 2 fans one for supply and one for return. I will dump into the hall outside the theater which is close to the basement return. I plan to pull from the main room which is about 3 times the size of the theater. The hall connects the main room to the theater.


The plenum would be 1.96. However I do not think this is correct based off Mr Tim’s last post, and I am still confused on what that number means. If I need a longer skinner vent how do figure that out? I would need to be 10” by XX by 10”


Please let me know what I need to change.
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Free air has to do with the grill. So that would be A x B. The second concern would be duct size, but there are plenty of online duct calculators for that. Last is plenum size, which was addressed in the Black Cat thread I think.

I got my linear grille from atlanta supply. THey have a spec sheet on it that tells you the free air.

Also note that two fans in a push-pull configuration does not increase CFM beyond what the fans are rated for. eg If you have a 200cfm fan exhausting and a 200cfm pushing air into the room, the maximum air you will be moving is 200cfm.

Fans have to be in parallel to increase CFM.. eg Two fans at 200 cfm pushing and two fans at 200cfm pulling would give you 400cfm.

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