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Old 09-06-2019, 10:37 AM   #1
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Best way to determine duct size for AC/Heat Pump


A couple of years ago, my AC went out and we hired a local guy to install a new unit for us. We had a 1600 sq. ft. loft with skylights and an open floorplan.

We were using a 3.5 Ton unit, but he suggested that we upgrade to the 5 Ton unit. Which we did.

It has been really loud ever since and I'm afraid the ductwork is the wrong size. I posted once to this forum and was given some advice about ductwork but I cannot find the thread.

I have another HVAC guy coming today and I am going to ask him if our ductwork is adequate for the size of our new unit.

I would like to have some calculations handy to double-check his work since I have repeatedly been told that most HVAC guys use a "rule of thumb" calculation that may/may not be sufficient.

Can someone point me to the best way to calculate duct sizes for a 5 Ton 2000 CFM Trane AC in a horizontal configuration?

I have approximately 5 open rooms (half walls with 15 ft. ceilings) and my ductwork is all exposed. I have only 6 vents in the entire place and it is really loud. Mostly at the vents.

In fact, when working near one of the vents, I noticed dust was spewing out of the side where the vent was attached to the duct. When I closed the gap by pushing on it, it got quieter.

I'm wondering if I just need to tape down the vents to the ducts to help with some of the noise.

I hate to be so distrustful, but we have had problems with HVAC techs here in Georgia in the past. I'd like to have my bases covered and know a little more than the average homeowner when the guy arrives.

Thanks in advance for any suggestions on how to determine adequate duct sizing for my 5 Ton, 2000 CFM 60000BTU AC/Heat Pump located in a 1600 sq. ft. loft with an open layout and 15 ft. ceilings with 5 rooms and 4 skylights (one per room except for bedroom).
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Old 09-06-2019, 10:57 AM   #2
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Re: Best way to determine duct size for AC/Heat Pump


I’d say your first problem is the oversized 5 ton unit.
In no way should a larger tonnage unit ever be considered an upgrade of any kind.
Manual J for equipment sizing.
Manual D coupled with a room by room manual J for duct design.
If you size the duct for 5 tons worth of airflow, it’ll be oversized when your unit does next time and you put a smaller unit back in.
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Old 09-06-2019, 01:25 PM   #3
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Re: Best way to determine duct size for AC/Heat Pump


Pay someone to do a heat load calc and duct design for you. Depending on your local laws, sometimes HVAC designers are allowed and are far cheaper then an professional engineer. You'll be looking at a few hundred. If you have accurate existing drawings of the house and duct work, you'll be able to get some deals. You can even try posting it here. One of us might have the time to help you out.

We will need to know the nearest town or major airport to use the local climate.

The main trunk would have to be 20" for flex, or 16x16 square metal or similar to match industry standard. (Ie. 14x20, 12x22, 10x28) Larger to be ultra quiet. (note that this is inside dimensions. Insulation takes up more space.)

Cheers!

Last edited by supers05; 09-06-2019 at 01:32 PM.
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Old 09-06-2019, 01:35 PM   #4
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Re: Best way to determine duct size for AC/Heat Pump


i would have the right size put in rather than alter ducts for a 5 ton.

you may still need duct alterations.
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Old 09-06-2019, 01:51 PM   #5
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Re: Best way to determine duct size for AC/Heat Pump


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Originally Posted by user_12345a View Post
i would have the right size put in rather than alter ducts for a 5 ton.



you may still need duct alterations.
I agree, but what is the right size? That'll have to be determined first.

Cheers!
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Old 09-06-2019, 02:18 PM   #6
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Re: Best way to determine duct size for AC/Heat Pump


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Originally Posted by supers05 View Post
I agree, but what is the right size? That'll have to be determined first.

Cheers!
Exactly. No reason to redo the ducts for an oversized unit when you’ll have to modify them again when the oversized unit is replaced.
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Old 09-06-2019, 03:12 PM   #7
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Re: Best way to determine duct size for AC/Heat Pump


I suspect the original 3.5 ton was much more appropriate than the 5 ton. Could probably use a 3 ton also. Still, a proper load calculation should be done.
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Old 09-07-2019, 10:41 PM   #8
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Re: Best way to determine duct size for AC/Heat Pump


OP, how did the original system heat cool your home? Other than noise, does the new 5 ton cool you better on those hot Atlanta days and nights?

Just picking your brain here! The experts have given you the correct information on how to acquire the correct size unit and duct work, but, here comes my opinion as a home owner.

If the original ran all the time and never cooled during the hotter days and the 5 ton cooled but cycled quickly and left more humidity, you may want to consider a compromise (in between) regardless of what the manual j suggests.

Food for thought.
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Old 09-07-2019, 10:49 PM   #9
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Re: Best way to determine duct size for AC/Heat Pump


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If the original ran all the time and never cooled during the hotter days
it's not fair to assume the original was delivering rated capacity.

i think truly undersized equipment is extremely rare. Sure, up-sizing can cover up capacity problems.
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Old 09-07-2019, 11:06 PM   #10
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Re: Best way to determine duct size for AC/Heat Pump


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Originally Posted by user_12345a View Post
it's not fair to assume the original was delivering rated capacity.

i think truly undersized equipment is extremely rare. Sure, up-sizing can cover up capacity problems.
Let me say that I think, no I know, that the systems designed by a manual j are correct based on their temp design range. For me in Texas, to design a system with a top end temp design of say 95, which mine was, would never cut it on the higher days, 97-100. So we had no problem at all with removing the 3 ton units and installing 4 tons for the extra capacity.

Are we paying more for it monthly yes, but last month here in Houston our electric bill was less than 150.00. Downstairs set at 75 and upstairs (no one there) set at 80. 1800 up and 1800 down.

As a consumer, its where we want to be. We want to be able to turn the systems on and have them cool. Its a tough position for an installer: Do I install based on the manual j or what the consumer wants.
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Old 09-07-2019, 11:33 PM   #11
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Re: Best way to determine duct size for AC/Heat Pump


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Originally Posted by digitalplumber View Post
Let me say that I think, no I know, that the systems designed by a manual j are correct based on their temp design range. For me in Texas, to design a system with a top end temp design of say 95, which mine was, would never cut it on the higher days, 97-100. So we had no problem at all with removing the 3 ton units and installing 4 tons for the extra capacity.

Are we paying more for it monthly yes, but last month here in Houston our electric bill was less than 150.00. Downstairs set at 75 and upstairs (no one there) set at 80. 1800 up and 1800 down.

As a consumer, its where we want to be. We want to be able to turn the systems on and have them cool. Its a tough position for an installer: Do I install based on the manual j or what the consumer wants.
This type of thing can be figured on the manual J when the installer talks to the customer.
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Old 09-07-2019, 11:40 PM   #12
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Re: Best way to determine duct size for AC/Heat Pump


Exactly, why I brought it up. For us, the home was already built.

Thanks!
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Old 09-07-2019, 11:40 PM   #13
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Re: Best way to determine duct size for AC/Heat Pump


Quote:
Originally Posted by digitalplumber View Post
Let me say that I think, no I know, that the systems designed by a manual j are correct based on their temp design range. For me in Texas, to design a system with a top end temp design of say 95, which mine was, would never cut it on the higher days, 97-100. So we had no problem at all with removing the 3 ton units and installing 4 tons for the extra capacity.

Are we paying more for it monthly yes, but last month here in Houston our electric bill was less than 150.00. Downstairs set at 75 and upstairs (no one there) set at 80. 1800 up and 1800 down.

As a consumer, its where we want to be. We want to be able to turn the systems on and have them cool. Its a tough position for an installer: Do I install based on the manual j or what the consumer wants.
A decent designer will take desired indoor temperature into account as well as capacity loss as it gets hotter outside. Along with duct loss.

Even if a unit is undersized for real, it's better to reduce capacity required than upsize.



I'm betting your systems are in an unconditioned attic being in texas and if the roof deck was insulated and attic not vented but part of the house, you have been happy with your 3 ton.
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