How to determine the max capacity of existing duct work.

thfurnitureguy_4

How to determine the max capacity of existing duct work.
House is 3900 sq ft in South Eastern PA
The existing AC is 3.5 ton. (Rough calculation says good for about 2200 sq feet)
Runs about 80% of the time on a hot day. Upstairs hot down stairs comfortable.
Former owner should have installed larger unit or 2 units?
With out tearing up walls and installing more duct work, how can I determine the maximum capacity for the existing duct system?
I would like to replace the current system, with the largest available for the existing duct system.

Brewbeer

"Upstairs hot down stairs comfortable."
This is my in-law's house. Cheap builder grade forced air furnace installation, retrofitted for central AC a few years later. If you want it comfortable upstairs, is't cold downstairs. If you want it comfortable downstairs, it's too hot to sleep. Heat is ok, but cools badly. A better set-up would be separate furnaces for each floor.

HVACNUT

A Manual J heat gain calculation should have been done. Then equipment and ductwork gets sized based on that. Sounds like everything was guesswork.

thfurnitureguy_4

All true I am sure.
The situation is the system is a dead r22 unit. I could fix the leak, charge it and it will go for a few years.
I can also replace it with the same 3.5 ton and expect similar performance, which is what my contractor is recommending. He says we can add some flow control to help balance out up and down stairs.
He is advising against a bigger unit because of the existing duct size, he is concerned it will freeze up. Other than eyeballing the system he has done no calculations of the ducts, he does installations not designs.

I would like to install a bigger unit because just by square footage of house it looks like I am undersized.
I am told 3.5 ton cools about 2200 sq ft
So 3900sq ft needs like 6 tons?
-How can I determine the largest unit that my ducts can handle?
All the supply ducts add up to 297 sq inches.
The system was better than no AC before and I am not looking for perfection. Can I get some improvement with a larger system and how can I determine what my ducts can handle?

ChrisJ

> @thfurnitureguy_4 said:
> All true I am sure.
> The situation is the system is a dead r22 unit. I could fix the leak, charge it and it will go for a few years.
> I can also replace it with the same 3.5 ton and expect similar performance, which is what my contractor is recommending. He says we can add some flow control to help balance out up and down stairs.
> He is advising against a bigger unit because of the existing duct size, he is concerned it will freeze up. Other than eyeballing the system he has done no calculations of the ducts, he does installations not designs.
>
> I would like to install a bigger unit because just by square footage of house it looks like I am undersized.
> I am told 3.5 ton cools about 2200 sq ft
> So 3900sq ft needs like 6 tons?
> -How can I determine the largest unit that my ducts can handle?
> All the supply ducts add up to 297 sq inches.
> The system was better than no AC before and I am not looking for perfection. Can I get some improvement with a larger system and how can I determine what my ducts can handle?


I have a 1600sqft house I installed a 3 ton 2 stage system into. All of my supply ducts add up to 385sq inches.

I highly doubt you can go bigger.... What's your current static pressure?

The only way to know what size system you need is to do a manual J as has been said. There's no such thing as going by square footage. That doesn't work.

DZoro

Bigger is not necessarily the correct answer. You stated the ac runs 80% of the time on a hot day. That leads me to believe that it is approx. 20% too large already. A even larger system will only reduce it's run time and virtually eliminate any chance of dehumidification.
2 systems or a 2 zone system is where you should be looking. This will help with the 20% of run time you are missing, we want to see longer runs and that will help with distribution to the second floor. Your contractor is probably on the right track.
What size is your return air ducts?
Are there return air ducts throughout the upper floor? Are they at the ceiling on the upper floor?
D

thfurnitureguy_4

Thanks for the come backs
Returns are 24x24 at the unit. The trunk splits at the plenum into 1 -24x18 and 1 -8x8. They run to every room and can be manually set to suck from the floor or ceiling. Return ducting to the rooms is 8 and 9” galvanized. Larger rooms downstairs have 2 returns and all the upstairs rooms have 2.
Static pressure not sure how to measure but it pushes water up a tube about 3/32” at the plenum.
Not sure what to do. When the system was running you could shut all the down stairs registers and you could barely feel cold air in the up stairs vents.

HVACNUT

I dont know if something is available online but what you want is called a Ductulator. Physically, it's a slide rule to calculate duct sizes to allowable CFM at a specific static pressure which is needed for a 'properly' sized system. Find a new contractor who will perform the manual J.
Maybe you do need bigger. Maybe not.
The new system should have an ECM blower motor so it will be more forgiving to the ductwork but you dont want a 5 ton drive motor plowing through 3.5 ton ductwork.
If you're not aware, 12K BTU= 1 Ton. 1 Ton= 400 CFM @ .2 sp

EBEBRATT-Ed

You need to move a minimum of 1400cfm for 3.5 tons. You stated 24 x 18 duct. That can't possibly be right. Thinking you meant 24 x 8 which is good for 1100 cfm. The 8 x 8 is good for 200. So the total is 1300 cfm so your ductwork is maxed out now for 3.5 tons

The right way is to do a heat gain and start from scratch.

DZoro

Sounds like a distribution/duct flow issue. Work on the air flow more so than larger A/C.
Work with your contractor on the duct system first step. During this step normally will provide the heat gain calcs.

ChrisJ

EBEBRATT-Ed said:

You need to move a minimum of 1400cfm for 3.5 tons. You stated 24 x 18 duct. That can't possibly be right. Thinking you meant 24 x 8 which is good for 1100 cfm. The 8 x 8 is good for 200. So the total is 1300 cfm so your ductwork is maxed out now for 3.5 tons

The right way is to do a heat gain and start from scratch.

Not necessarily.
My 3 ton system has a 25x20 return at the largest point. I think the smallest part of it is something like 25x12 but that's after a 9" run to a bedroom.

pecmsg

1st a manual "J" Load / Loss calculation needs to be performed
Then a Manual "S" equipment selection
Then a Manual "D" Duct size is done.

Bigger is not better! Mostly makes it worse.

Here is a rough chart for duct size and CFM's.
Warning: It is not a replacement for a Manual "D" just for general capacity's

thfurnitureguy_4

Thanks guys we are forming a plan