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Calculate CFM for 4 Ton Unit

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Discussion starter · #21 ·
amd said:
Insufficient airflow reduces capacity, efficiency, and throws refrigerant/pressure readings off.
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He initially came out to test for leaks and to figure out why the refrigerant was half of what it was. Then he saw the "mess" in the attic with the return and told me that the refrigerant is probably normal since we have supply issues along with all the other issues combined over the last 11 years of code changes.

I would like to thank EVERYONE for your input and feedback. I'm not a "HVAC guy" nor to do I pretend to play one on TV, so this forum is a blessing to find. I'm rather handy (services my truck transmission this morning before he arrived) and can do most things mechanical but have never touch a HVAC so I couldn't even begin to troubleshoot any issues or know where to start.

I feel much better with the information that the technician gave me and I feel confident that what he was telling me was correct and should be done. I'm referring to this forum as my second opinion.

I know this is a VERY general question, but roughly what would the cost be to install a second return with the "box" if the return duct is 40 feet in length (based on where he wants to install it). I'm aware this changes from state to state and labor cost, but a ballpark figure would be nice to have an idea of what to expect. I'm in CA.

THANK YOU!!!!:payattention:
 
#22 ·
Chris_Worthington said:
Of course, but determining CFM'S by with a known BTU output divided by TD X 1.08 has nothing to do with static :D

Can I get by with lower then 350-400 CFM'S per ton?? Sometimes, sure why not?
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Guess i dont understand, i do know cfm is directly affected by ESP. And as long as evap is able to pick up enough heat at lower than 350cfm then i suppose it would be alright, probably a bigger coil helps with that, I enjoy trying to learn from you all and just so you know i really m trying to better myself and thats why i pick at your brain Not arguing
 
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Discussion starter · #24 ·
For me it's about the cooling portion of the system. I hardly never use the furnace for a whole house heater. We have a pellet stove during the winter that heats the house for us. On those REALLY cold nights the heater will turn on but that is only about a dozen times a year.
 
#30 ·
I am interested in your comment: how about if supply static is low instead?
How do I deduce the breakdown if I only have the figure my Carrier Infinity stat shows?

chuckcrj said:
It would be nice if you knew the breakdown of supply and return static. If the supply static is already high, adding return will make it higher.
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#31 ·
crayx4 said:
I am interested in your comment: how about if supply static is low instead?
How do I deduce the breakdown if I only have the figure my Carrier Infinity stat shows?
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If for example supply static is .5" and return static is .6" for a total of 1.1" then adding returns will not lower the total very much. As you decrease the resistance of the return side you are adding airflow causing the restricted supply side to get even higher. So if both supply and return are the same and the total is high you need to fix both sides. This is why I asked the OP for both readings.

If supply is low and return is high then there is a good chance adding return air will bring the total down significantly.

The Infinity stat is only going to give you the total ESP. If you want the breakdown you will need to measure it with a manometer.
 
#34 ·
Last time I did a central return, we used 16" r/a with a 25x25, for a 2.5 ton system.

You've got 16" for a 4 ton.

With your current setup, the return is undersized.
Possibily the supply also. Since he didn't mention the supply to you, it's probably fine.
 
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#35 · (Edited)
chuckcrj said:
If for example supply static is .5" and return static is .6" for a total of 1.1" then adding returns will not lower the total very much. As you decrease the resistance of the return side you are adding airflow causing the restricted supply side to get even higher. So if both supply and return are the same and the total is high you need to fix both sides. This is why I asked the OP for both readings.

If supply is low and return is high then there is a good chance adding return air will bring the total down significantly.

The Infinity stat is only going to give you the total ESP. If you want the breakdown you will need to measure it with a manometer.
============================
Low is better! High ESP means airflow restriction.

Its best to measure the ESP with filters in and blower running at full cooling CFM. Or heat cfm if that is higher.
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chuckcrj & hvacvegas, - two excellent posts.

Also, if the supply is too low, - that can be a signal that there is gross duct air leakage.

If you need to lower the supply, U could run more branch runs.

Sometimes the supply will be okay when the return ducting & filter areas are enlarged enough.

If the SA needs more velocity for throw, then lowering the RA static could help.

Rooms that need more air may need another supply run.
 
Discussion starter · #36 · (Edited)
*****UPDATE*****

The technician came out today to give me his cost of what it would take to repair or replace the unit.

$$$$ to repair the unit which includes
Fix duct wok
Add a second return
Duct testing
Add Secondary overflow
Add Plenum boxes to box returns

OR

$$,$$$ to replace the entire system to a 17 SEER 4 ton unit. This price also includes the repairs listed above.

Now my question. Since all the technician is doing is fixing my ducts (which they mentioned is to add a coupling to where the previous AC ducts are joined together instead of using a continuous run) and adding a second return, how is that $,$$$K?

Can I add the return myself and fix the ducts with a coupling from my local hardware store? In the past I added a second duct to a bedroom to get more air flow. How much more tough can it be (I'm aware it's more complicated....I'm just looking for a cost justification)? The most complicated part is coupling it to the furnace with a plenum box. Can I just copy what they have done on the other return and go from there?
 
#37 ·
MagnumVP said:
The technician came out today to give me his cost of what it would take to repair or replace the unit.

repair the unit which includes
Fix duct wok
Add a second return
Duct testing
Add Secondary overflow
Add Plenum boxes to box returns

OR

some type of a ammount to replace the entire system to a 17 SEER 4 ton unit. This price also includes the repairs listed above.

Now my question. Since all the technician is doing is fixing my ducts (which they mentioned is to add a coupling to where the previous AC ducts are joined together instead of using a continuous run) and adding a second return, how is that a chunk of change?

Can I add the return myself and fix the ducts with a coupling from my local hardware store? In the past I added a second duct to a bedroom to get more air flow. How much more tough can it be (I'm aware it's more complicated....I'm just looking for a cost justification)? The most complicated part is coupling it to the furnace with a plenum box. Can I just copy what they have done on the other return and go from there?
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Please remove the pricing from your post.


No, you can't install the return yourself.
Someone who had some type of experience in the industry didn't do it right. How can someone with no experience do it correctly?

Plus, you won't find what you need at a bLowes, or a Home Desperate.
 
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Discussion starter · #38 ·
Perhaps I should clarify. The system was installed correctly for 2000 standards and code enforcement. The return didn't need a plenum in 2000 and even with today standards of 2011 (local code) the CFM's don't need to be tested based on the system.

The only thing they check is that the system ducts don't leak beyond a certain percentage. They don't need to prove that a 4 ton unit requiring 1400-1600 CFM's have the proper return or supply.

You have a 2000 Sq foot home and a 4 ton unit, the ducts leak 10%....you're good. System is passed.
 
#39 ·
MagnumVP said:
Perhaps I should clarify. The system was installed correctly for 2000 standards and code enforcement. The return didn't need a plenum in 2000 and even with today standards of 2011 (local code) the CFM's don't need to be tested based on the system.

The only thing they check is that the system ducts don't leak beyond a certain percentage. They don't need to prove that a 4 ton unit requiring 1400-1600 CFM's have the proper return or supply.

You have a 2000 Sq foot home and a 4 ton unit, the ducts leak 10%....you're good. System is passed.
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Other than the 2000 IMC code that states that return air is required to have 6sq inches of return per 1k btu. So, it wasn't up to the 2000 standard.

A contractor doesn't have to "prove" anything. It's the inspectors job to "find" the problems.

I've never seen an inspector check for leakage %, unless it was a commercial building spec.

Actually, an inspector IS SUPPOSED to make sure that a system has proper return.

Square footage is only one part of tonnage requirement. The state of Ohio, and Kentucky require a load calculation with any new home permit, and most states also require it.

IMC:
"SECTION 312 HEATING AND COOLING LOAD CALCULATIONS

312.1 Load calculations. Heating and cooling system design loads for the purpose of sizing systems, appliances and equipment shall be determined in accordance with the procedures described in the ASHRAE/ACCA Standard 183. Alternatively, design loads shall be determined by an approved equivalent computation procedure, using the design parameters specified in Chapter 3 of the International Energy Conservation Code."

"603.2 Duct sizing. Ducts installed within a single dwelling unit shall be sized in accordance with ACCA Manual D or other approved methods. Ducts installed within all other buildings shall be sized in accordance with the ASHRAE Handbook of Fundamentals or other equivalent computation procedure."


So, yes, they do.
 
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Discussion starter · #40 ·
If that is the case, then the inspector missed every single home in my neighborhood which is about 120 homes. Almost every home on my street has the same 4 ton unit with a 25x20 return and a 16" duct.

If the home is 2200 Sq' or more they put a 5 ton unit in with a 25x25 return and 18" duct. At the time the model homes went from 1880 to 2200 Sq' and a model that was 2800 which was a two story where they placed two 25x20 with 16" ducts.

I think it's going to come down to local code enforcement and how they differ from location to location.

In my current area, each new residential building requires a duct system check and certificate before completion.
 
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