Calculate CFM for 4 Ton Unit

[MagnumVP]

After having my AC system serviced, the technician told me that my intake (20x25 with a 16" Duct) does not have the proper CFM intake for a 4 ton unit. He said that a 20x25 is 1000 CFM and a 4 ton unit requires 1600 CFM. He rattled off some numbers for Ext. Static Pressure which is currently .85.

He told me that all the numbers are suede because of the improper intake. He mentioned that we need to add another 14x25 intake to compensate for the CFM issue.

Is this correct? We have been using this unit for 11 years without issue.

[amd]

We have been using this unit for 11 years without issue.


Your system probably isn't pushing enough air for a 4 ton unit unless the air handler or furnace has a blower designed for 5 tons of cooling.

Just because it worked for 11 years doesn't mean it's running at rated capacity or efficiency.

0.85" WC is through the roof - it shouldn't be much above 0.5".

He's right.

[Chris_Worthington]

Did he actually check the airflow?

Sounds like it would be noisy, but depending upon the length of,, maybe you're good on air??

Contingent upon the heat source (If available) it is a pretty simple formula to be sure of the airflow...

"If" no heat source is available, then ask him what your "Coil Bypass Factor" is, that will most certainly trip him up...

Other then that, might want to seek out a second opinion,,,,, IMHO :cheers:

[MagnumVP]

Thank you for the confirmation.

I wasn't questioning the number of years without issue, but more the numbers of CFM's required for a 4 ton unit and the current intake we have.

It doesn't sound any noisier than normal.

He also said that the SEER is 10 (unit is 11 years old) and that the coolant is half of full. But since the CFM are off and the intake is too small it could be resulting is the numbers being off. The only way to be sure is to add the second intake and see if the coolant numbers and Ext. Static Pressure change.

If we add another intake should it reduce the Ext. Static Pressure? Will it also increase efficiency to the unit?

What if the ducts to the rooms only need 1000 CFM and adding more intake only adds intake but not output.

Here are the numbers of his test;

Indoor RH 36%
Crawl/Bsml RH 85.6%
Ext. Static Pressure .85

[youngtech]

Did he actually check the airflow?

Sounds like it would be noisy, but depending upon the length of,, maybe you're good on air??

Contingent upon the heat source (If available) it is a pretty simple formula to be sure of the airflow...

"If" no heat source is available, then ask him what your "Coil Bypass Factor" is, that will most certainly trip him up...

Other then that, might want to seek out a second opinion,,,,, IMHO :cheers:

coil bypass factor?? The esp is .85, enough said, doesnt static have to be in range to determine cfm

[dan sw fl]

4-ton = 1,400 CFM
@ 300 Feet per Min for Filter Area
Minimum Filter Area should be 4.7 Square Feet ( 670 Square Inch)

20"x25" = 500 Square Inch or about 3.5 Square Feet.

Need 170 (670 - 500) square inch (1.2 Sq. Feet) more of filter area.

[chuckcrj]

Your tech is ahead of the average, sounds like efficiency would increase and noise would decrease if you follow his recomendation.

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.

How many supply registers does your house have?

[MagnumVP]

We only have the one return. It's 20x25 with a 16" duct attached.

[chuckcrj]

We only have the one return. It's 20x25 with a 16" duct attached.

How many supplies?

[udarrell]

After having my AC system serviced, the technician told me that my intake (20x25 with a 16" Duct) does not have the proper CFM intake for a 4 ton unit. (NO)

He said that a 20x25 is 1000 CFM and a 4 ton unit requires 1600 CFM. He rattled off some numbers for Ext. Static Pressure which is currently .85. (TOO HIGH)

He told me that all the numbers are suede because of the improper intake. He mentioned that we need to add another 14x25 intake to compensate for the CFM issue.

Is this correct? We have been using this unit for 11 years without issue.
Is that a filter grille rack or just a return air grille?

What size is the furnace filter rack area?

I like a 20" duct RA metal duct for 1600-cfm for around 700-fpm velocity & 0.02" friction rate per 50-feet with one 90.

A mere RA grille figure 90 to 95% (Ak) free-air-area.

If it's a return Air Filter Grille that Ak varies with the kind of filter used... could be around .65% Ak, or a different percent.

There4fore, it takes a lot larger physical area for a RA Filter Grille.

You can practice figuring different filter's Ak & physical area requirements...

Yes, when the blower can more easily pull RA, the supply air duct velocity & static & CFM will all increase, but the RA - minus is added to the SA static which helps reduce it.

Result is more airflow to & from the rooms, & a more fully heat-loaded evaporator coil with a higher BTUH performance; that is our objective.

[beenthere]

Your 20X25 grille is allowing more then 1000 CFM through it. How much, can't say from here.

Adding more return will help, but as posted earlier, your supply static will increase some then.

Checking charge by SH and SC would tell if you really need to add more return.

[MagnumVP]

It is a 20x25 filtered grated supply. From the pictures attached you can see the supply coming from the left side ceiling and attaching directly to the unit.

You mention that it will increase my "static". I'm not familiar with "static" can you explain what it's measuring?

[Chris_Worthington]

coil bypass factor?? The esp is .85, enough said, doesnt static have to be in range to determine cfm

The CBF is a psychometric thing.....

Static has nothing to do with a known BTU output divided by TD X 1.08 :cheers:

[btuhack]

Whenever I see returns directly connected to the furnace~no return plenum, I begin to think in " levels of bad".

Safe to say that corrections could be made that would improve noise, efficiency, filtration and life of equipment- all benefits to the home owner.

[chuckcrj]

It is a 20x25 filtered grated supply. From the pictures attached you can see the supply coming from the left side ceiling and attaching directly to the unit.

You mention that it will increase my "static". I'm not familiar with "static" can you explain what it's measuring?
Supplies are the registers where the air comes out.

[youngtech]

The CBF is a psychometric thing.....

Static has nothing to do with a known BTU output divided by TD X 1.08 :cheers:

Wouldnt TD change with static change?

[Chris_Worthington]

Wouldnt TD change with static change?

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?

[MagnumVP]

That also was something that the tech mentioned that having the return directly attached was an issue with a "box". I'm assuming he was referring to a return plenum. In 1999/2000 when this house was built/completed that wasn't required or at least enforced by code enforcement.

The technician mentioned that he will install a 20x25 return and attach it to the other side of the furnace which would give me plenty of supply.:cheers:

A lot of the issues come from 1999 codes and enforcements (or lack there of). In 1999 they didn't require duct testing or sealing of the furnace. Now everything needs to be tested and sealed.

He said that if he were to test 10 homes in my neighborhood (of track homes) that 8/10 would fail since they were all built before 2005 when the codes changed.

[amd]

Capacity in heating mode isn't fixed = lower airflow = hotter heat exchanger -> less heat transfer from combustion gasses.


He also said that the SEER is 10 (unit is 11 years old) and that the coolant is half of full. But since the CFM are off and the intake is too small it could be resulting is the numbers being off. The only way to be sure is to add the second intake and see if the coolant numbers and Ext. Static Pressure change.


Based on that, he knows what he's doing - you don't need a second opinion.

0.85 WC is too high - normal noise is simply what you're used to.

Insufficient airflow reduces capacity, efficiency, and throws refrigerant/pressure readings off.

[martyinlincoln]

Sounds like you got a good tech who is trying to get your system performing like it's supposed to be.

[MagnumVP]

Insufficient airflow reduces capacity, efficiency, and throws refrigerant/pressure readings off.

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:

[youngtech]

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?

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

[Dr.phil]

I'd sure like to see access like that a little more often. At least you've got that going for you.

[MagnumVP]

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.

[MagnumVP]

I'd sure like to see access like that a little more often. At least you've got that going for you.

That is one thing that is nice about it, it easy to get to and work on. Saves on labor cost in the long run.

[btuhack]

Youngtech, follow the link:
http://htalk-ef.com/articles/Cooling_with_Dehumidification.pdf

[Chris_Worthington]

Youngtech, follow the link:
http://htalk-ef.com/articles/Cooling_with_Dehumidification.pdf

:D

[ampulman]

Adding more return will help, but as posted earlier, your supply static will increase some then.



Is this true for both PSC and VS blowers?

[beenthere]

Is this true for both PSC and VS blowers?


Yes, if the VS blower wasn't able to deliver set CFM.

[crayx4]

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?



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.

[chuckcrj]

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?

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.

[crayx4]

Thanks.

So a fairly low total ESP with filters in place can mean not enough air flow?




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.

[chuckcrj]

Thanks.

So a fairly low total ESP with filters in place can mean not enough air flow?

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.

[hvacvegas]

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.

[udarrell]

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.

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.

[MagnumVP]

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?

[hvacvegas]

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?

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.

[MagnumVP]

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.

[hvacvegas]

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.

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.

[MagnumVP]

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.

[kinghomes]

inspectors are looking for safety issues. It is not their job to insure systems are properly engineered. Engineering an H VAC system is is a tough, time consuming and difficult job, but it is not the job of a municipal inspector.

[MagnumVP]

inspectors are looking for safety issues. It is not their job to insure systems are properly engineered. Engineering an H VAC system is is a tough, time consuming and difficult job, but it is not the job of a municipal inspector.

That is what the technician said. The inspector's job to make sure the thing won't catch fire and cause a hazard not to check for efficiency.

[kinghomes]

someone with no experience can do something correctly by learning. Reading books and watching and reviewing the work of others is one way. If so inclined, it is even possible to do things correctly by using you own intelligence. But, if you don't know what right is you might not even know if you have done it wrong.

[hvacvegas]

inspectors are looking for safety issues. It is not their job to insure systems are properly engineered. Engineering an H VAC system is is a tough, time consuming and difficult job, but it is not the job of a municipal inspector.

Lucky you.


Most of my new home inspectors count the number of screws I use, if my tape/silicone/mastic is UL approved, duct sizing, register location in relation to approved plans.

[hvacvegas]

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.


Most likely, if it was the same lazy inspector, which is probably was.

[udarrell]

Where split-systems loose the most efficiency & delivered CFM & BTUH is due to the poor design & lack of sealing &, where needed, insulation of the duct systems.

Therefore, in respect to residential duct systems & air flows; IMO, manual D should be followed, & code enforced.

On existing systems M-D enforcement would allow for how much of it is possible to achieve; however, all elements of M-D should be applied to the extent possible.

IMO, there will not be much improvement in the sizing & design of duct systems & equipment tonnage selection, until we make manual J & D a part of an enforced code; irrespective of who performs them.

In the near future Home Energy Efficiency Audits ought to become an initial part of the Energy Efficiency Code; as that should be the first part of a properly sequenced "Best Practices" process.

You may not like it, however, we must meet higher standards of work & system performance.