HVAC Sizing - what do I need to make sure I ask?

Not sure about your question but one thing to consider is airspeed and the noise associated with it. I recently did some research looking for the proper sized diffusors to use as my two supplies. Take a look at my build thread if your interested, a discussion between myself and Mr. Tim. Page 2.

FWIW, I am going with two 6" supplies and 2 dead vent returns. Good luck

I'm not sure what to ask either, but I decided to engineer mine all by myself - which is to say, I'm guessing. I'm guessing the same things you guys are, pretty much. Dave's room is a little larger than the one you're building (mine is almost identical volume to yours). I was planning on two cold supplies, one hot return, and one dead vent (return to adjacent space). I haven't really figured the sizing of ducts yet, but was anticipating 6". The rule is supposed to be that 4 is appropriate for most HTs (I think that is stated at Ted's website), but I'm not sure how that has been determined. I have made myself paranoid enough about vent noise (and cheap enough not to hire someone) that I'm planning to build giant lined boots inside my soffits. - I imagine you've seen the pictures Big has posted from the Black Cat (and elsewhere?) - that's the idea I'm working on. The feature/function of systems that I haven't seen this current generation of HT builders deal with is throw - or (in non-precise-only-marginally-correct terms) the distance the jet or plume of air is cast out form the diffuser. I'm concerned that over-sizing ducts and building larger register boots will lead to such a low and diffuse (quiet) flow, that air mixing in the room will be inefficient and sub-optimal. Hopefully, shooting for 6 exchanges per hour can make up for any shortcomings in throw (still guessing).

Here's the specs I'm trying to shoot for - 4 to 6 air exchanges per hour (that's one every 12 to 15 minutes), and air speeds at vents not faster than 250 fpm. I think if you're HVAC design meets those two goals, you're good in general. Noisy vents can still happen depending on diffuser (grille) design.

Like most aspects of HT design - this gets very technical very fast. So, I'll be crossing my fingers.

Sizing the diffusers is relatively easy. The problem is that you need two other pieces of information prior to sizing the diffusers:
-you need to know the cooling load of the space
-you need to know the static pressure of the existing system

On the cooling load: where you are geographically and how your room is constructed will make a big difference. A theater in Michigan that is completely below grade will require less cooling than a theater in Florida that is above grade. You're in Georgia, so expect the load would be higher.

On the static pressure: how the other ducts have been sized will effect how you size the addition. Consider a spigot on the outside of your house. You connect a wye fitting, then two garden hoses to that wye. On one garden hose there is nothing on the open end. On the other you put a nozzle. Open both, how much water comes out of the nozzle? Now make one hose 100 feet and the other 25 feet. What kind of nozzle (diffuser) your existing hose (duct) is using will make a difference when you connect your new hose. Similarly, a 3/8" garden hose may supply enough water for the flower pots, but if you're using a 3/4" farm hose to do the job, you are going to need a even bigger hose to water the vegetable garden. You may be able to mitigate the issue by closing the wye (dampers), if you have them.

Barring a complete analysis of the existing system, I would calculate the theater and an existing room. Then correlate the diffuser and duct size in the existing room to the new work.

That being said, a good guess usually works. Typically everything is oversized, which is fine if you have the space. The cost is the throw of the diffuser which will take it a bit longer to circulate the cool air throughout the room.

1 CFM/sf plus 15 CFM per person is a rule of thumb. It could be just right or completely off base for your room. A competent HVAC contractor will calculate the required CFM (a single room is 15 mins of work with the Manual J software). Once you have the CFM you can plug that into an online duct size calculator for 0.5" WC (another guess) to get the supply duct size. If you want two supplies, simply calculate the duct size for half the required CFM.

You take the same CFM number and refer to the manufacturer's engineering data for a particular diffuser to properly size it for the FPM or NC you are looking for.

HTH,
Tim
Edited by Mr.Tim - 7/6/12 at 2:35pm

My HVAC guys said don't dual zone for HT because the dampers add noise when one zone is closed off and only the HT is getting air. They said I could do a dedicated system, but felt I would be wasting my money. Do 1 zone, put the thermostat in the HT, and leave the door open when not using the HT. Put separate returns in any room that will have its door shut, including the projector booth. Good advice or flawed advice?

It all depends on the design and the existing system. If you are zoning a 1.5 ton system I wouldn't be too worried. If you intend to zone a 5 ton system into a single room, I think you would have a problem.

A bypass damper will alleviate some of the problem.

I think your contractor raises valid issues that should be addressed, but I also think their suggestion is a tremendous compromise. You'll spend maybe 8 hours a week in the theater and 160 hours in the rest of the home and you're going to let a thermostat in the theater control the whole zone?

In a perfect world there are returns in every room. In practice it rarely happens. In some cases they undercut the doors like they're supposed to when there's no return the the room; more often they don't.

In short, need more info. I would try to divide the house into smaller AHUs (ie 1 for forst floor and one for second floor) so you are zoning off a smaller unit. I would also make sure they calculate the cooling load for the entire house properly.

Tim

Can you explain how your supplies are routed into the HT, building a joist muffler may be necessary. Also, a quick note why I am going with two dead vent returns. My target CFM pull is 450 - 500. Instead of running one in-line fan rated at 500 CFM at full speed, I would rather run two of those same fans at half speed, which is much more silent. You also have to consider the flex duct you are probably adding to the supplies and the added resistance. I am still contemplating where I want to transition from rigid to flex so that I can keep the noise to a minimum, but also keeping in mind performance. Location is also key as Mr. Tim stated. I live in MD, which gets pretty hot and humid, but the basement is entirely below grade and stays pretty cool. Good luck, and keep posting your progress.

Hey Tim

Mine would be 1 unit per level, so decision is basement level.


Blipscyz,

I assume my situation matches yours? If not, I will get out of your thread.

My builder spec'd 2 dedicated units for the basement. General HT advice is 1 unit with 2nd zone. HVAC company says 2 units in basement is overkill, and advise against 2nd zone due to noise. So I have 3 options, with 3 different recommendations I was leaning towards the 2 dedicated units because it's cleanest in theory, but I have space issues locating the 4th condenser & AHU which would force compromises in noise.

Aside from a mini-split solution I've never seen a unit less than 1.5 tons. That size unit is probably overkill for dedicated solution for a single room. They make a ducted mini-split, which if you have a budget for 2 AHUs may be an excellent solution.

What size unit do they specify for the basement? Again it all comes down to somebody performing a cooling load analysis.

If you zoned the theater off of the rest of the basement and added a bypass damper I don't think you would have an issue.. but then again I don't know the size of the unit you propose zoning from.

Don't forget that all that equipment requires cooling and can produce a lot of BTUs. Even if it's not in the theater itself, you probably want to furnish cool air to the equipment whenever the theater requires it. The cooling load can be calculated from the sum of the power consumed by the equipment.

Tim