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Hi pros,

My system is about 3 years old. It has been working fine since it was installed 3 years ago. Last week I received a special for a tune-up on my Air conditioner, so I decided to get it tune-up since I never had it done before. Unfortunately, the news that the tech gave me was a shock. The tech is a nice and honest man, but before I spend this much cash. I want to ask the expert opinions.

Here is my duct design based on his drawing. My current system is a carrier 4 ton A/C in the attic. He said it is suppose to push 1600CFM total. This is his drawing

Return Duct:
One 14"=950CFM

Supply ducts:

1 7" = 150CFM x 2 = 300 CFM
3 6" = 100CFM x 3 = 300 CFM
2 8" = 210 CFM x3 = 630 CFM
2 5" = 60CFM x 2= 120 CFM

He said that my ductwork is way undersized my for current system and I will have alot of problems with the furnace and A/C if the ductwork is not size properly. I went up there with him and the ductwork is what he said it is, he also show me how he calculated with his duct calculator. It makes sense to me with his drawing, but my question is my furnace a/c worked fine and never had any problem before, and is it really neccessary to spend the money?

2. I have a 14" return too. I have a 2 ton system!

If those supply ducts are flex, he's being too generous!

3. First let me state that I do believe the tech is on the right track. However, there are things happening in the system which you cannot see. Chief among them would be high static pressure. That is the pressure that the blower creates to move the air through the ducts. The blower is a stupid machine and just turns at one speed (except for variable speed units that are basically even MORE stupid but that's another story) and thus has a fixed amount of air to deliver. So if you were to picture a balloon uninflated, the begin to inflate it with the blower. As the balloon fills with air, it gets bigger and bigger and finally it will bust! Well, your ducts can't get bigger so the pressure created by the blower just goes up. That increased pressure is what slows the airflow. It's call static pressure and can easily be measured. Before you jump off the cliff, ask the tech for the static pressure figures.

Adding to his ductolator calculations, I'd proceed to having a full load analysis of the house done to determine if it's the ducts that are under sized or the mechanical equipment that is over sized. He's delivered you some bad news but he's not creating a falsehood. There is a problem but he's taking for granted that the equipment is properly sized. So first a load analysis, then a duct design to a lower static (remember to get the current static) and based on those two design calculations, you can begin to formulate a plan as to what you need or want to do.

Your system is not going to blow up or be dangerous but it is likely far from an ideal system. The actual airflow cannot be determined by a ductolator but I do think I'd want some more data before I'd say the system is okay or need remedial work. Stick with the guy, he's on the right track to do the right thing for you but I think he needs to bring more info to the table before recommending costly modifications. I suspect a static test will reveal a high number and that will seal his prediction as accurate. From there a good load analysis is a must as simply increasing the size of the branch run-outs isn't likely the total solution.

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Both Bald and Skipped are correct. If your ducts are flex he's being overly generous on the numbers. The other thing is the actual R/A grill opening size. With the system only being 3 years old one hopes you had a load calculation done and you actually need a 4 ton system. If a 4 ton was determined to be necessary for your house then yes the ductwork is undersized.

I know it's always worked before you got your tune up, but the big question is "did it work correctly, and optimally?" The answer is no. The tech you have is defiantly on the right path and is one of the few that are trying to create an optimum system for you.

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Thank you everyone for your input, the tech did write the system current static pressure on his invoice. He noted that Manufacture required Static Pressure= .5 WC", Current system static pressure= 1.4 wc ". He also noted that no warranty on system if damaged. As far as system sized out properly, They did do a heat load calculation when they installed the system, but no one ever show me the duct and how it was calculated. It was two different installers that did the job. I was assuming that they are both legit, so I wasn't questioning them. The new tech also did a new duct map with the figures, he goes room by room and measure out the square foot. He show me how the new duct should be. even thought I am ignorant in hvac, but it makes sense to me. I think he is right and that he is not bull****ting me. He seem very honest and not pushy. He answered all my questions properly. When I watch him do the tune-up, I can see that he is an experience tech. I guest I have to bite the bullet and go with him. He also gave me a price, but I am not going to shop around since I am confident with his analysis. Thanks everyone

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He's doing it right. Good job!!

7. jackd1283

You are using the computer of a Pro Tech Member here. I can only assume one of three things. You are authorized to use his computer, you now own his computer and using it for personal use or you are the Pro Member masquerading as a Home Owner. The other possibility is that he used your computer to log onto his account.

I sent an email to the Pro Member asking for an explanation for this account name and for "homeownerneedshelp" account which was recently opened.

Can you furnish any further explanation please?

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9. A 1.4" static is way too high, need to get it down to or, close to 0.5".

It might pay you to have a Home Energy Efficiency Audit & after the retrofit work is completed do a load calc., this might call for less cooling tonnage.

The supply air diffusers may also be too restrictive along with not near enough Return Air filtering area.

It should be sized so a media filter (not a 1" deep pleated filter) has only 300-fpm of air velocity going through it.

If you only had one filter area for 4-Ton, Hart&Cooley Engineering says use a 36X30 filter that's 1080-sq.ins. but, 783.360sq.ins. or 5.440-sq.ft of 'free-air-area' or (Ak) for 1633-CFM (1633-cfm/300-fpm= 5.44-sf | 1600-cfm/300-fpm= 5.33-sf of AK 'free-area') I'd use two separate oversized RA filter areas.

If your 4-Ton is oversized slow the blower CFM down to 350-cfm per ton or, 1400-cfm.

If it has a TXV metering device it will throttle the refrigerant flow to keep the superheat at the right level; it should perform similar to a 3.5-Ton unit.

What you are trying to do is deliver less airflow volume at a lower velocity & static pressure; the Return duct needs to be at least 22-inches or two 14-inchers with two oversized RA filters.

Might be able to save on doing a lot of the other duct retrofit if those things get the static down close enough to 0.5-inches WC.

If you live in a high humidity climate it will be helpful toward controlling indoor humidity. What type climate do you live in?
Last edited by udarrell; 03-29-2012 at 02:12 PM. Reason: Emphasis...

11. Note to self: Use a tunneling service if I need to hide from Dad for some reason.

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I created that username yesterday but was unable to activate, so I created another username. The reason I post this question here is I want to know the answer to it. Sorry, I am unable to tell you the reason. Anyhow, thanks everyone for your answers.

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DANGER WILL ROBINSON.......DANGER WILL ROBINSON.......DANGER...WILL...ROBINSON

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