# Return Size



## bluebird5 (Dec 13, 2010)

HVAC contractor has installed 16x20 return grill for 2 ton unit. I read 200 sq inches per return. Is this big enough or do I need to make him change to a 20x20?

A hole he has cut in the floor that hooks to the 16 inch flex duct for the return is 10x16. Is this large enough? 

I just want to make sure this is done right before sheetrock. It is easy to change now, but will be a pita after the house is finished only to find out it is too small. 

Please advise thank you


----------



## GettingBy (Aug 19, 2010)

FWIW 

16x20 rectangular return = 320 sq. inches in area
20x20? square = 400
16 inch round flex duct = 201 sq. in.
10x16 rect = 160


----------



## bluebird5 (Dec 13, 2010)

GettingBy said:


> FWIW
> 
> 16x20 rectangular return = 320 sq. inches in area
> 20x20? square = 400
> ...



So what are you getting at? The HVAC man said that a 16 inch flex was equivalent to a 14 inch hard pipe is this accurate or is he just making stuff up to not have to change the filter grill size and cut that hole in the floor bigger?


----------



## beenthere (Mar 5, 2009)

He is correct. A 14" hard pipe is 1050 CFM at a .1" pressure drop per 100' of TEL, a 16" flex is also 1050 CFM at a .1" pressure drop per 100' of TEL.

A 16x20 filter grille is good for 512 CFM at 300 FPM velocity(300 FPM is recommended for air filters). A 1016 filter grille would be good for 256 CFM at 300 FPM.

Air filters will still do a good job up to 500 FPM. after that, filter grilles become loud/noisy.

The 2 returns he has cut are okay for a 2 ton unit.


----------



## bluebird5 (Dec 13, 2010)

so basically it is a bit small at 512 cfm at 300fpm. If a 2 ton pulls 800 then it is quite a bit too small right? 
Especially when the filter gets dirty.


----------



## GettingBy (Aug 19, 2010)

FYI, a 14" round pipe area is PI x (14^2)/4 = 154 sq. in.


----------



## beenthere (Mar 5, 2009)

bluebird5 said:


> so basically it is a bit small at 512 cfm at 300fpm. If a 2 ton pulls 800 then it is quite a bit too small right?
> Especially when the filter gets dirty.


Doesn't the floor return and wall return go to the same unit.


----------



## beenthere (Mar 5, 2009)

GettingBy said:


> FYI, a 14" round pipe area is PI x (14^2)/4 = 154 sq. in.


Or, R^2 times pie.


----------



## bluebird5 (Dec 13, 2010)

beenthere said:


> Doesn't the floor return and wall return go to the same unit.


Yes its a universal stack head. The air goes through the filter intot eh stack head and then through the hole in the floor and into a 16" flex


----------



## beenthere (Mar 5, 2009)

bluebird5 said:


> Yes its a universal stack head. The air goes through the filter intot eh stack head and then through the hole in the floor and into a 16" flex


Ah, okay, I thought they were 2 separate returns.

Yeah, too small.


----------



## m1911 (Feb 24, 2009)

beenthere said:


> Or, R^2 times *pie*.


blueberry or apple?



:laughing::laughing::laughing:


----------



## GettingBy (Aug 19, 2010)

m1911 said:


> blueberry or apple?
> 
> 
> 
> :laughing::laughing::laughing:


This 
http://en.wikipedia.org/wiki/Pi_(film)
is a really disturbing movie. 
The guy uses a supercomputer to analyze pi so that he can be like God.

And he is, for a while, but he can't stand it so he puts a drill bit into his head to remove the part of his brain that is giving him trouble.

I can't believe this thread found a problem with the contractor the OP hired. Ha! Good job!


----------



## bluebird5 (Dec 13, 2010)

beenthere said:


> Ah, okay, I thought they were 2 separate returns.
> 
> Yeah, too small.


I am having him change it to a 20x20. Is this sufficient? This guy told me 16x20 was fine and would flow 1000 cfm and the unit was only gonnna flow 800 cfm, but everything I read says 1 square inch will flow 2 cfm. so 400 sq. inch minimum. 

Good contractors are soo hard to find. I really liked this guy on first impression, but I was not happy with the install. They are all about production and not quality. I really should not have to tell him to make the return bigger. He should autimatically have his installers change it, but I know he won't. I will not be using this guy again.


----------



## GettingBy (Aug 19, 2010)

You maybe should question more stuff he does, either by ignorance or to knowingly cut corners. 
It's the 'hidden cost' of hiring guys like this but how could you know?

For my own info, if you got more than one bid where did his bid land?


----------



## bluebird5 (Dec 13, 2010)

GettingBy said:


> You maybe should question more stuff he does, either by ignorance or to knowingly cut corners.
> It's the 'hidden cost' of hiring guys like this but how could you know?
> 
> For my own info, if you got more than one bid where did his bid land?


The guy does 100 installs a year. He has lots of repeat business he says. I found him on rheem's website. 

He was about in the middle. 

I had 6 people out and they were all squirrley.

1st guy was gonna cut the holes in the block with a chisel
2nd guy wanted to install the hvac before plumbing and electric 
3rd guy just gave price and left and he was the most expensive didn't say much
4th guy gave a price range of the bid instead of a price
5th guy wanted to do some scratch and dent and I didn't like his vent placment
6th guy was this guy he had good knowledge of the equipment, but I found out he is strictly production and not quality. 

No one did a manual j or d. I just got tired of looking for people and made a decision. I hate wasting people's time. None of these were craiglist guys. They all had storefronts or were on manufacturers website .

I just think the hvac trade is hacked up.


----------



## beenthere (Mar 5, 2009)

A 20x20 filter grille is good for 640 CFM at 300 FPM. 800 at 400 FPM. You would need a 20x25 for 800 FPM at 300 FPM.

The figures I posted are generic. So depending on brand and style, air flow could be +or-10%.


----------



## flashheatingand (May 3, 2008)

bluebird5 said:


> The guy does 100 installs a year. He has lots of repeat business he says. I found him on rheem's website.
> 
> He was about in the middle.
> 
> ...


Perhaps that would be nice. But, kinda like a customer who wants a free quotes. That stuff takes time and/or cost money. Nothing is free. Besides, whats to say the person who does the man J/D knows what they are doing? 

Bottom line is one needs to have faith in the contractor. Good ones make things right should something go amiss.


----------



## GettingBy (Aug 19, 2010)

bluebird5 said:


> I had 6 people out and they were all squirrley.


If 
your experience is typical for your area and it's not something you are doing or some problem with your house 
then 
the typical HO has less than 1 chance in 6 of getting competence + integrity.

Bad odds.


----------



## GettingBy (Aug 19, 2010)

It might be worthwhile to hire an ASHRAE consulting engineer for a large upfront sum who then 
writes up a procedure and parts list for what you need, and 
you fax the thing to several other HVAC places.

That way they are all bidding on the same job and they are just installing, just following a recipe, and not designing.


----------



## flashheatingand (May 3, 2008)

The AShrae guy may not be aware of low pitched roof that provides minimal access for properly sized returns (example) or the pony wall below or whatever obstacles that are inherent within the project.

Eventually somebody has to do the work, and be held liable. Classic example of "If I am cooking the dinner, I buy the groceries".


----------



## griz (Nov 26, 2009)

Mechanical Engineers design HVAC systems all the time.

Have him visit the site.


----------



## beenthere (Mar 5, 2009)

flashheatingand said:


> Perhaps that would be nice. But, kinda like a customer who wants a free quotes. That stuff takes time and/or cost money. Nothing is free. Besides, whats to say the person who does the man J/D knows what they are doing?
> 
> Bottom line is one needs to have faith in the contractor. Good ones make things right should something go amiss.


There are those contractors that do Manual J calcs as a dog and pony show. But on average, if a person finds a contractor that does a Manual J load calc. Even if it has errors in it, it will be far more accurate then another contractors guess.


----------



## flashheatingand (May 3, 2008)

Just my opinion: Too many variables to say one size fits all. Is the house occupied throughout the day or primarily evenings? What temp does the home owner like the house to be? How big is the existing ductwork? 

I am not discrediting man J. If we are dealing with new construction, there is no reason not to do man J/D. Just, if the customer isn't happy with the results, we can't say "according to man J..." If "my guess" is wrong, it's on me. 

We are like referees. If nobody recognizes us, then we have done our job well.


----------



## GettingBy (Aug 19, 2010)

flashheatingand said:


> we can't say "according to man J..."


Using it shows Due Diligence. 
And you may meet the reasonable person standard. This mythical person is not always right but did act reasonably.

"In law, a reasonable person (historically reasonable man) is a composite of a relevant community's judgment as to how a typical member of said community should behave in situations that might pose a threat of harm (through action or inaction) to the public"

The more variables you take into account the more accurate the final answer should be but not all variables carry equal weight in getting to the final answer. 
Manual J uses a half dozen variables to get within 10%?

BTW, Lindeburg writes books for the PE exam in many engineering fields and the Mech book probably has formulas and worked-out problems for just about everything. For some reason my local library has several of his books on the shelves.

HVAC stuff is worse than elec. eng. All of the graphs have curved lines, all of the formulas have exponents, air is compressible, human engineering is involved, etc..


----------



## bluebird5 (Dec 13, 2010)

I have been doing some reading on the matter if return grill size. Is this accurate? 


300 fpm is the maximum allowable velocity allowed on a disposible filter. 

Therefore, a 2 ton unit at approx. 400cfm per ton. So 800cfm/300fpm= 2.67square ft of free filter area x 144= 384 square inches. 

A 20x20 filter is 400 square inches, but has a 1 inch rim all the way around the filter to hold the media. Therefore a 20x20 filter is only 18x18 free area because you loose 1 inch per side. 18x18=324 and the minimum size I need is 384 so we are already short 60sq inches before loading the filter with dust. 

A 20x25 filter is 18x23 = 414 

800cfm / 2.25sq ft(20x20 filter size) = 355fpm (55 above the max)
800cfm/ 2.875sq ft (20x25 filter size)= 278fpm (22 below the max)


Is 300fpm the maximum velicity for all disposable filters? 

The a/c man changed the grill to a 20x20. Should I leave it alone or insist on a 20x25 if that is indeed the correct size? 

He also says a pleated filter has twice the surface area which makes a bit os sense, so does that mean that a pleated filter will get me below the 300 fpm?


----------



## beenthere (Mar 5, 2009)

300 FPM isn't the max. 500 FPM is the max without excessive noise.

Your 20x20 will give you roughly 375 FPM at 800 CFM. 

What brand is your filter grille.


----------



## bluebird5 (Dec 13, 2010)

beenthere said:


> 300 FPM isn't the max. 500 FPM is the max without excessive noise.
> 
> Your 20x20 will give you roughly 375 FPM at 800 CFM.
> 
> What brand is your filter grille.


IDK it has not been purchased yet. 

I thought 300 fpm was the max from the filter companies to achieve merv rating. 

I am thinking about using 4" filters so that I can get all that extra surface area. Would this give me the lowest possible fpm? 

Can I get a filter grill that uses 4" filters?


----------



## flashheatingand (May 3, 2008)

Where is the furnace located? Is it hard to get to? Why not a filter box at the furnace?


----------



## bluebird5 (Dec 13, 2010)

flashheatingand said:


> Where is the furnace located? Is it hard to get to? Why not a filter box at the furnace?


The furnace is in the crawlspace. Thats kinda what I am thinking now. 

If I use this honeywell 20x20x4 in the filter grill, would that be equivalent to the filter box?

here is a link of the filter although I would get a merv 8 I think instead of this 11 https://airfilterbuy.com/pla/product/furnace-filter-brands-honeywell-filters-honeywell-return-grille-filters-honeywell-20x20x5-return-grille-fc40r1003/20x20x5-honeywell-merv-11-aftermarket-return-grille-filter-fc40r1003/?utm_source=google&utm_medium=pla&utm_campaign=google%20pla&gclid=COTg3_TH6sMCFUUQ7AoduhYAdw


----------



## txgencon (Jan 4, 2011)

The last house I built had a two ton system. The guy who designed the system is friend who is a mechanical engineer who teaches at a HVAC/refrigeration school. His design spec'd a total of 410 sq in of returns and a 17.5 x 28 Merv 10 filter at the unit (in the attic). That's 490 sq in for the filter but, as someone has stated, there are losses with the frame and inherent restrictions in the filtering process. It seems to be working pretty well. It's absolutely silent. 

All the returns were via stud bays. The limiting factor is the smallest cross sectional area anywhere in the duct sustem. In my case, the 410 sq in was accomplished with: 

Master bedroom - 3.5 x 20.5 = 71.75 sq in
Dining Room = 3.5 x 28.75 = 101 sq in
Living Room = 3.5 x 27 = 95 sq in
Middle Bedroom = 3.5 x 19 = 66 sq in
Front Bedroom = 4.875 x 15.5 = 76 sq in

Basically, his design matched the amount of return to the amount of supply. 

In the past, I've had HVAC companies put in less than half the amount of return as supply and there always seemed to be wind noise associated with some of the returns.


----------



## beenthere (Mar 5, 2009)

bluebird5 said:


> The furnace is in the crawlspace. Thats kinda what I am thinking now.
> 
> If I use this honeywell 20x20x4 in the filter grill, would that be equivalent to the filter box?
> 
> here is a link of the filter although I would get a merv 8 I think instead of this 11 https://airfilterbuy.com/pla/product/furnace-filter-brands-honeywell-filters-honeywell-return-grille-filters-honeywell-20x20x5-return-grille-fc40r1003/20x20x5-honeywell-merv-11-aftermarket-return-grille-filter-fc40r1003/?utm_source=google&utm_medium=pla&utm_campaign=google%20pla&gclid=COTg3_TH6sMCFUUQ7AoduhYAdw


It will work as good as having a filter box at the furnace.


----------

