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HVAC design

post #1 of 17
4/24/12 at 10:47pm
Thread Starter 
Has anyone taken an active roll in designing their HVAC system? Or do most people just give general requirements to HVAC contractor and hope for the best? If you gave requirements, how did you test that they were met?

For walls I see people get pretty specific on how the walls are built to achieve low noise floor in room. I am looking for similar level of detail for HVAC in my room, or how to get there.

Some general questions:

  1. Is there a particular model or line of AHU that are noted for their quietness?
  2. With variable speed fans, can you set them to run low rpm for lower noise?
  3. Is it better to run the theater HVAC in a soffit inside of the isolation envelope (around the room), or external and drop the vents into the room?
  4. Is dual zone better than dedicated unit?
  5. Are dead vents quieter than normal forced air?
  6. For fresh air, are ERV units recommended or are they noisy? Are the make up air intakes enough?
  7. Recommendations are to oversize ducts, but how do I tell what size they should start at?
  8. How do people here maintain the recommended humidity ranges?
  9. Has anyone actually bought a system capable of the -30F ambient operating temperature that's been recommended here?
  10. Why is flex duct recommended over conventional square metal ducts with duct liner?
  11. Is it better to make 90 degree turns with a plenum, or just bend the flex duct? Wouldn't bending the flexduct cause turbulence which adds to the noise?
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post #2 of 17
4/25/12 at 4:04am
Those are all great questions.

1. Can't help on this one, but I believe at least some of the manufacturers publish the dB ratings.
2. Setting a variable speed fan to a certain speed defeats the purpose of a variable speed fan. As resistance increases the fan speed ramps up; I don't know if you can actually override that.
3. I would say that you want to limit the number of penetrations into your aquarium.
4. Consensus seems to be that a dedicated unit is the way to go. The requirements of the theater are usually completely different than any other room. That being said, it would also depend on what the other zone was, and what the cooling capacity for each space is.
5. A dead vent is quieter than no dead vent. I'm not sure what you mean by "normal forced air".
6. If you are required to provide fresh air, the ERVs I've seen are extremely quiet. Trying to cool a theater with only an ERV is a bad idea. Put 8 people in a room and try to cool it with humid 90 degree summer air.
7. I'm not sure that it is recommended to oversize the ducts. They should be sized such that the velocity is 250FPM at the face of the grill. There is some confusion about sizing of ducts, but I have found 600FPM in flex to be fine. You size the ducts in accordance with Manual D. First, determine how much cooling you need in CFM (use Manual J to calculate this), then size the ducts.
8. I didn't worry about humidity, but depending on climate it may be a larger concern. AC will obviously take care of too much humidity.
9. Can't comment on that one.
10. There may be other reasons, but if you use only rigid duct, you have rigidly connected the diffuser to the air handler, which will transmit noise from the unit into the room more efficiently. There is also a perforated flex that is supposed to help deaden sound, but I haven't used it. I would use lined duct for the trunks and take off with flex.
11. Depends on the situation. With less friction the blower should run at a lower speed, but at some point it becomes moot.
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post #3 of 17
4/25/12 at 4:09am
1, Trane
3, Inside
4, yes
5, better at keeping theater sound in the theater
10, Metal duct is fine if it is lined with acoustic duct liner and there are beaks along the path to mitigate vibrational transfer of fan noise, like the last 10 feet or so.
Flex is a lot cheaper alternative.
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post #4 of 17
4/25/12 at 5:19am
There is also the Lennox Signature Series, which we have that is very quiet. A lot of noise will come from the duct work, including the vent sizing, not the unit itself. If your HVAC contractor does the proper Manual D & Manual J, I should see no problems.
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post #5 of 17
4/25/12 at 8:46am
This is a great idea for a thread!

I suspect you are already aware of the items that I have to offer, but I'll post them anyway in case someone else finds it useful.

2) Are you asking bout the AHU fan speed? The unit that we are installing is variable speed, and my understanding is that it will initially turn on at the lower speed. There are several reasons for that, but reduced noise is one of the benefits. Also, I don't see why they couldn't wire it to turn on the blower at low speed when the theater is in use regardless of whether the AC is on or not. This would help circulate air and should delay when the AC is actually called for. (Most thermostats have an option to either run the blower intermittently or constantly whether the AC is on or not)

4) I know this isn't the answer you are looking for, but I think it depends. My take on this so far is it's easier to size a dedicated unit for a theater. However, if you put a theater on a dedicated zone, you should be able configure that zone such that you get the same performance. There are lots of pros and cons, but don't forget that a dedicated unit will require space in your yard. I'm a bit biased at this point, and prefer a zoned approach, but that may change once I actually get moved in and see how "real world" experience stacks up against what I've read.

7) +1 to Mr. Tim. Determine the cfm required to cool the space, and then choose duct and register sizes to ensure velocity is below 250 fpm. Don't forget that you can run, for example, a 6" duct to your theater, and then upsize to a larger duct for the last part of the run. It doesn't have to be the same size for the entire run. Hopefully someone will chime in and offer some advice on how to determine the cfm that's necessary for a given number of people and the size of the unit.

8) Two options, either with a dedicated dehumidifier or use the HVAC. Dennis posted in another thread that the AC is inefficient at dehumidifying the air. However, when I discussed this with my HVAC contractor early on, he said that the cost of running a central dehumidifier would be greater than the costs necessary to run a properly sized variable speed HVAC (AHU and compressor). Obviously he may have been pushing his higher end products, and may have had ulterior motives, but he passed all of my other "tests" so I took his word for it.

9) Our HVAC contractor was very specific in stating that he did not like square ducts. His reasoning was that air doesn't flow in a square (I'm paraphrasing). I don't think that's strictly true, but I think he has a point with regard to turbulence and laminar flow which will affect noise. I also vaguely remember from my fluid dynamics classes that square cross section pipes/ducts have reduced flow efficiency compared to circular cross section ducts. But it's been a long time ago, so I may be mistaken there.

10) When the air is forced to turn abruptly, either in a duct or plenum, it will result in turbulence. The plenum might have an advantage due to a larger volume which will reduce the air velocity.
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post #6 of 17
4/25/12 at 11:49pm
1. I have a dual-stage Trane. Runs on 1st stage (lower fan speed) 90% of the time. Very quiet.

2. As long as your room air quality remains satisfactory, reducing fan speed with a variable speed controller should reduce both fan noise and regenerated noise from air movement as a result of lower FPM.

3. From a sound isolation standpoint if done properly this should be a wash. If the 'soffit' is outside the envelope make sure it remains decoupled from outer framing and also that there is no gap between dd/gg and outer soffit. In other words, the mass that is used for soffit should be directly connected to dd layer and not connected with flex ducting.

11. If your plenum is made from heavy mass (eg MDF, DD, OSB) then a lined 90 degree bend is better at low frequency isolation. Any sharp turns or obstructions will introduce regenerated noise from the turbulent air movement. Duct liner or flex insulation is most effective at absorbing the higher frequencies.
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post #7 of 17
4/26/12 at 6:29am
J_P_A, I was wondering yesterday that if you used oval ductwork vs square or round, if it would cut down on the turbulance?
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post #8 of 17
4/26/12 at 8:52am
Quote:
Originally Posted by gregzoll View Post

J_P_A, I was wondering yesterday that if you used oval ductwork vs square or round, if it would cut down on the turbulance?

Given a certain FPM, rectangular ducts will generate more noise than round. However, rectangular ducts are are better at attenuating any noise travelling down the duct. Round ducts are better at preventing noise from escaping (flanking) through the duct sidewalls.
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post #9 of 17
4/27/12 at 1:19am
1- York seems to be popular brand in the mid high end line. I would say Trane would be next.

2- Depends on the unit it self and the T-stats you use.

3- I don't know how your room is designed. So can't really say one way or the other.

4- Zoning will require a little more duct work. You might need dumps. If your current duct work is not sized for Zoning it might add more noise.

5- Dead vent are a sound buffer. Good for keeping thing quite if designed correctly, you do not want to lower the the FMP of your return or conditioned air. ( keep this in mind when buying a new unit )

6- ERV will add unwanted noise. Try bringing filterd fresh air through though the furnace if you can. depedning on your duct system you migh be adding more external static pressure. ( somthing you do not want. )

7- do not over size your duct work ! Follow The schedule D and J in the Hvac manual.

8- April-Air makes some nice stand alone units. You might not need one.

9- What is this number for ? My interpretation is -30F to accommodate X-amount of people. If your not planning 200 seat theater then you don't need a unit like that. room sizing, number of people, etc. is covered in manual D and J. ( i.e. your home and entertainment room will never heat otr cool correctly. ) Again, not sure what this number means.

10- easy to install.

11 You should not have any turbulence in any duct if installed correctly. They do make a flex that has a different material on it than what you seen in the big box stores, and it's low noise. Very expensive stuff about the same as sheet metal duct. I can't think of the name I only installed once .

I don't know the Sqft of your theater room or your home. If you want to use your current duct work in your home to condition the air in the theater room ( which I assume is an addition ) You will need some minor to major modifications in duct work and a different size unit. There are many other factors in play here, to many to mention and beyond the scope of this forum.

What I did to my home. I removed all my existing duct work and started like a new home build. Metal Duct work, custom bends made bends not squared in the turns, but smooth bends.( Your Hvac contractor might have access to the equipment that does these bends, some don't ) My home is two stories and about 2500 sqft. I installed- 2- York Gas furnaces and one 4 ton AC condenser. The 2nd furnace is only to heat downstairs; when needed as well as the theater\\ entertainment room. on separate duct system also controlled from a single T-stat with sensors. Fresh air is also provided though the outside that is filtered and controlled from the T-stat for auto turn-on. I did not install another humidifier on the 2nd furnace for many reasons. The metal duct work is wrapped with standard R-8 only thats it. I also installed returns in every bedroom and living space, along with to main returns upstairs and down. It's better than having one or two central location for the return air. That alone will bring down noise like you would not believe. My home very quiet even when both furnaces are running; the ducts do not make any noise at all and are sized correctly as well as to the fan curves. only a slight noise of the fans can be heard.

Ask your contractor about a separate unit as a well seperate duct system, might be less expensive.
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post #10 of 17
4/28/12 at 3:35pm
Thread Starter 
Thanks for the responses so far. I posted a lot of questions, so there is a lot to respond to for followup/clarification. I'll keep the same format.

1. I was looking for models that are good, not so much brands. Trane makes a line of Hyperion AHU that are advertised as "ultra quiet", but that's a lot like "reduced fat". It's all relative. I haven't found any noise specs. The Hyperion XL has variable speed fan and when paired up with their XL20i heat dual compressor heat pump, it will supposedly run lower speed and quieter when it can. But I thought an axial fan would be quieter, so I might try to get something engineered from their commercial side. The commercial side has acoustic modeling software (TAP).

2. Trane (Honeywell) makes a really cool thermostat seen here: http://www.youtube.com/watch?feature...&v=9VQ3Fs8PQGU

3. Less holes in the aquarium makes sense. I just thought since the ducting itself can have noise associated with it, it might make more sense to keep that outside of the isolation envelope. I also wondered if a mini-split located inside of a soundproof soffit would make any sense. Mits has some 19dB mini splits.

4. With dual zone, the variable speed fan shouldn't require dumps. For the same money I could also trick out a single AHU better than two dedicated systems - i.e. add filtering, ERV, nice tstats, etc. But I was told the dampers themselves, when closed to only supply air to the HT, would cause noise from the air bouncing off them. But I'm not sure how much that really matters.

5. It's new construction, so if I used a dead vent I would size the exterior of the theater to handle the heat load of the interior. I'm just wondering if dead vents are quieter than running dedicated supply & return ducting around the inside of the HT (from the AHU). In other words, when designing a room from scratch, are dead vents a better approach than a conventional "ultra quiet" central forced air HVAC system?

6. For ERV, I meant one attached to the furnace. Like Trane FreshEffects. My understanding is it's built into the return ducting to exhaust a portion of the stale air, and replaces with fresh air.

7. My plans call for over sized ducts, and similar has been quoted here as de facto AVS standard, similar to GG & DD for walls. I think larger ducts reduce static pressure for a given CFM, which should reduce noise, and that's where the recommendation comes from. It may also be to build in some breathing room since the guys on the ground often make a lot of compromises during install.

8. The comfortlink II thermostat has controls for humidity. The AHU has options for hydronic heating. The ERV is also suppose to help humidify in winter (by retaining the moisture present in the fresh air). I think I will only address high humidity, which the AC system can do on it's own fairly well without needing a dedicated humidifier/dehumidifier.

9. The requirement for -30F has also become an AVS de facto HVAC recommended standard.
This goes back to my original question, are people here actually following the recommendations/designs they receive? Not once have I ever seen anyone question, why -30F? That's a significant upgrade to be able to hit that number. So much work goes into wall design - clips, glue, and even discussion about which model screws to use. When it comes to HVAC, it's basically a NC noise requirement, with a few commercial standards for air quality, some best practice advice, and general acknowledgement that it's unlikely your typical residential HVAC guy will be able to achieve those requirements.

10. Everyone likes flex because it's easy to install. I'm just not sure it makes sense in the context of HT. After you've done special framing, clips, channel, 2 layers of drywall, and got smelly green glue all over everything... does 'ease of installation' for the HVAC lines really matter?
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post #11 of 17
4/29/12 at 6:13pm
Quote:
Originally Posted by rabident View Post

3. Less holes in the aquarium makes sense. I just thought since the ducting itself can have noise associated with it, it might make more sense to keep that outside of the isolation envelope.

The noise added by the air moving through ducts (also known as regenerated noise) is higher frequency noise (1-4KHz) at lower decibels. If your soffit is made of some mass and absorption (say MDF and duct liner) then those noises would only be noticeable at the register. Therefore having the soffit inside or outside the shell would make no difference.

Quote:
Originally Posted by rabident View Post

4. But I was told the dampers themselves, when closed to only supply air to the HT, would cause noise from the air bouncing off them. But I'm not sure how much that really matters.

That can certainly matter. Any form of turbulence (elbows, dampers, etc) will cause noise in your ducting. If you place those dampers as far away from the registers as possible then the absorption in your ducting can minimize their impact.

Quote:
Originally Posted by rabident View Post

5. It's new construction, so if I used a dead vent I would size the exterior of the theater to handle the heat load of the interior. I'm just wondering if dead vents are quieter than running dedicated supply & return ducting around the inside of the HT (from the AHU). In other words, when designing a room from scratch, are dead vents a better approach than a conventional "ultra quiet" central forced air HVAC system?

Here again you have to separate regenerated noise from air movement from other noise (LFE getting out of the room or house noise getting into the room). The advantage of a dead vent (not connected to central AHU) is that noise will not be able to use the ducting to travel between the room and the rest of the house. However a dead vent has a dedicated fan that is normally right next to the room so care has to be taken that the fan noise does not become an issue inside the room.

Quote:
Originally Posted by rabident View Post

10. Everyone likes flex because it's easy to install. I'm just not sure it makes sense in the context of HT. After you've done special framing, clips, channel, 2 layers of drywall, and got smelly green glue all over everything... does 'ease of installation' for the HVAC lines really matter?

I'm with you on this one. Flex duct being the first line of defence after poking a hole through the DD/GG layer doesn't make a lot of sense to me. I prefer the mass introduced by a soffit (MDF, OSB etc) along with some 90 degree bends as the first line of defence against low frequency leakage. Adding some flex as an additional decoupling and high frequency tool to finish it off. At least that's the plan in my current build thread
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post #12 of 17
4/29/12 at 7:47pm
Quote:
Originally Posted by 235 View Post

The noise added by the air moving through ducts (also known as regenerated noise) is higher frequency noise (1-4KHz) at lower decibels. If your soffit is made of some mass and absorption (say MDF and duct liner) then those noises would only be noticeable at the register. Therefore having the soffit inside or outside the shell would make no difference.

So, duct board within a soffit doesn't need to be treated any further (i.e. DD/GG)? Do you line the drywall soffit with MDF and then ductboard within that? Or is it sufficient to have a drywall "shell" lined with ductboard?
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post #13 of 17
4/30/12 at 6:22pm
Quote:
Originally Posted by rs691919 View Post

So, duct board within a soffit doesn't need to be treated any further (i.e. DD/GG)? Do you line the drywall soffit with MDF and then ductboard within that? Or is it sufficient to have a drywall "shell" lined with ductboard?

To clarify, I was suggesting a soffit can be placed either inside or outside the sound envelope. Either one works assuming the soffit is designed the same way. The soffit encasing the hvac needs to be framed with some mass (DD/GG or MDF, OSB etc) and then lined with some absorption. I hope my previous post wasn't confusing .
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post #14 of 17
5/1/12 at 4:37am
Quote:
Originally Posted by 235 View Post

To clarify, I was suggesting a soffit can be placed either inside or outside the sound envelope. Either one works assuming the soffit is designed the same way. The soffit encasing the hvac needs to be framed with some mass (DD/GG or MDF, OSB etc) and then lined with some absorption. I hope my previous post wasn't confusing .

No, I'm just easily confused!
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post #15 of 17
5/2/12 at 12:04am
Thread Starter 
I also read that the soffit frame should be mounted to the channel. Maybe that's obvious since the only other option would be the studs which we all know is bad. But I guess something to plan for. Know where your channels are and build your soffit in a way that they can be attached.
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post #16 of 17
5/2/12 at 7:40am
You could use OSB for your first layer instead of drywall. Then you can put screws wherever is convenient.
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post #17 of 17
5/2/12 at 8:57am
Can I ask a question also?

So to be as effective from allowing noise to exit the room the whole soffit should be doubled layered with gg of some sort to prevent sound from leaking through the flex say where it leaves the envelope? Not just at the registers in the room? I say this since flex doesn't prevent sound leakage alone along with absorptive material surrounding the flex?
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