Is this return too close to the supplies?

[jonblack]

I am in the process of getting my HVAC system designed. If you care to read more information please check out my thread here:

http://hvac-talk.com/vbb/showthread....would-you-make

One of my returns is sized to deliver 1200 CFM, and it will be a 16" rigid pipe set behind a 24" by 24" return grille.
The supplies in the photo are 10" (14" by 14" by 10" Speedi-Boot) and are sized to each deliver 325 CFM at 0.06" WC/100ft.

Are these supplies too close to the return grille?

Please let me know if I can provide more info to help you make a determination.

Thank you for your assistance.

jonblack

[jonblack]

Just to clarify there will be more than one return. There will actually be four returns. One return will be in the hallway and there will be a return in each of the two offices. The return in question cannot go any higher due to small limitations as there is a 16" stuctural I-beam behind the partition wall as shown in the photo below.



Thank you
jonblack

[SBKold]

Looks ok to me. The supply air should throw across the room and will fall since cold air is less dense.

[penderway]

It does look fine. When finished, you could verify by measuring the air temperature of the room and compare it to what is going into your return. With your particular configuration, return air that is cooler than the room air would indicate the supply air is not mixing well before being returned.

The goal is to supply cool dry air to the room, mix it well with the existing room air and return it to the air handler to be conditioned. If this is done without anyone feeling or hearing it, then consider it perfect.

[jonblack]

I guess I am just worried that the return would suck in the air from the two 10" supplies that are closest to the return grille. I know about the blowing out a match versus sucking out a match analogy, but still looking for some peace of mind.

jonblack

[jonblack]

I also thought about turning the return air grille upside down so it pulls air from above the return more readily than from below the return. Plus I think it will be more visually appealing since you won't be able to see inside the return grille if it is upside down.

jonblack

[qwerty hvac]

I also thought about turning the return air grille upside down so it pulls air from above the return more readily than from below the return. Plus I think it will be more visually appealing since you won't be able to see inside the return grille if it is upside down.

jonblack

This is what I would do. You can also balance those two grills closet to the return down and have more CFMs coming out of the others if it becomes a problem.


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[penderway]

After re-reading your post, I calculate the velocity of your supplies at <300 FPM. Your may want to discuss the "throw" of the supply registers with your designer.

[penderway]

I guess I am just worried that the return would suck in the air from the two 10" supplies that are closest to the return grille. I know about the blowing out a match versus sucking out a match analogy, but still looking for some peace of mind.

jonblack

I never heard that analogy before, but I love it. Too many people put too much emphasis on the location of returns when it's the supplies that they should be worried about.

I see your concern, and as I mentioned, the throw (or blowing out) does need to be addressed.

[penderway]

I also thought about turning the return air grille upside down so it pulls air from above the return more readily than from below the return. Plus I think it will be more visually appealing since you won't be able to see inside the return grille if it is upside down.

jonblack

Turn it so you can't see in it, but not for any other reason. Address the supplies to ensure good mixing and to prevent short circuiting the air.

[jonblack]

After re-reading your post, I calculate the velocity of your supplies at <300 FPM. Your may want to discuss the "throw" of the supply registers with your designer.

I am not an expert so I may be missing something, but my ductulator (McQuay DuctSizer) shows 325 CFM at .06" giving 585.8 FPM in a 10.1" diameter pipe.

I also used the Velocity Calculator on the page linked in your signature and got 596 FPM for a 10" pipe at 325 CFM.

Can you give me details at how you arrive a <300 FPM?


jonblack

[penderway]

In short, duct sizing and register sizing are different.

I just did some approximation not knowing the engineering data on your registers, 14 x 14 = 15.3" round equivalent = <300 FPM@325CFM

I would need the make and model of your grille to give you exact numbers, but that is what you hired your designer for. Discuss it with him, i'm sure he can explain it to you.

[jonblack]

Thanks for the reply and for the explaining the difference between duct velocity and supply velocity. That is a facet of this subject that I have not considered in the few weeks I have been learning.

I will double check what supply registers can be used with that size register box. I will also look at the Hart & Cooley engineering data and make sure to select a register that is going to give us the throw we want. I could always go with a different size supply box if that meant a more optimal register could be used.

I appreciate you brainstorming with me. Every little bit of knowledge helps.

jonblack

[jonblack]

penderway

Can you suggest a sidewall grille make/model that would be suitable for my application? The problem I have run across, at least with the people that I have dealt with, is that most installers don't have any engineering data to back up their product recommendations. They just want to slap something in and go.

Hart & Cooley has so many model numbers that it is hard to find a supply diffuser that is suitable for my application.

Is there another diffuser manufacturer that provides engineering data that I should consider?

Thank you
jonblack

Thank you
jonblack

[penderway]

The answers you seek are in the ASHRAE Handbook - HVAC Systems & Equipment, and/or the ACCA Manual T, and also Hart & Cooley's Engineering Data (not sure if other manufactures data is readily available). Yes there is a lot to know, and a lot of important choices to make, therein lies the work.

Most installation contractors do what they believe will "work" based on rules of thumb, trial & error from past experience and suggestions from peers. Any of which may achieve satisfactory results or be good advice, but is not always comprehensive in scope or detail. I mean, how much can a person really explain in a few paragraphs?

Not all contractors do the engineering. They aren't required, nor should they be expected to. If you find one who does, great. But in lieu of DIY, you may want to find an engineer.

I sincerely hope your project turns out well, and I don't believe I can give any better advice than this.

[jonblack]

penderway

Thank you for your help and advice.

jonblack

[btuhack]

I didn't read the comments from the other post, just looked at the pretty pictures. I would tell any potential customer that wanted what you're proposing to have very low expectations. The opposite side of the room will likely be a very different temperature than the wall with the grills.

If it were me (and i had a proportionate budget of the floor finish guy) , I'd run a couple pieces of exposed spiral at least 2/3 of the distance and distribute above the floor with 4 grills, leaving the wall return.

The small details don't usually matter that much if the overall concept is poor.

[jonblack]

btuhack

Thanks for your reply.

I am guessing you saw the pictures of the floor from the other thread. If you would design and install the HVAC system for the budget of the floor finish guy I would hire you in a heartbeat as my wife and I did the floor! The expenses we have are chemicals, tools, and labor in the floor...so you wouldn't be getting paid much! At this point we are looking at spending between 5 and 10 times what the floor cost.

I am pretty sure that the wall registers are not ideal, but that is all I can afford at this time. We also talked about (in the other thread) not being about to run spiral duct out into the room because there is a 16" beam hanging down, unless I were to run the spiral duct lower than the beam, which is always a possibility.

I really just need to get the 5-ton system I have installed and working and if I need to add a system up by the windows next year then hopefully I will have the budget to do it.

I'm still brainstorming in an attempt to get it "right", considering my budget limitation, so your input is helpful.

jonblack