Going from 4 ton to 2 ton, will oversized ducts be a problem?

adamfre

Good afternoon all!
I spent most of last fall getting my steam heat straightened out, but now that we are (hopefully) coming into spring it is time for me to straighten out my AC.
My house is an old, built in 1870. 3 floors, about 5000 sq ft. My existing AC is a very old, 4 ton unit, installed in my crawl space and only cooling the ground floor. 2nd and 3rd floors have no AC installed. My plans for the 2nd floor will be retrofitting with a Unico high velocity system of appropriate size, 3rd floor we are still waffling on (it has 3 rooms but has not been used as living space for years).
I would like to replace the 1st floor AC with a conventional 2 ton system (1st floor requires 22,773 BTUH). My question is, will I have problems with installing a 2 ton system into ductwork previously designed for the 4 ton system?

Thanks!

GW

Yes and no, it all depends on what your duct sizes are.

Dropping tonnage is common but dropping two tons is a leap.

The returns aren't the issue, it's the supplies. If too big you want have enough air at the furthest outlets.

Gary

EBEBRATT-Ed

It could become a distribution problem. If you adjust the dampers inside the supply diffusers you may get noise.

If you have decent dampers in the branch duct takeoffs you should be able to balance the air flow.

The answer is yes, it can be made to work but you may have to spend some time balancing and making adjustments

adamfre

Yes, @GW I was concerned about that. The dining, kitchen, and butlers pantry are the furthest rooms away, ones I'm most worried about cooling. @EBEBRATT-Ed I had thought about using dampers to balance everything. They are not currently installed however I could add them as I'm installing everything else...

Jack

I think step one here is to do a pressure test on your duct system and see if it can deliver the air it is supposed to. You will likely have a significant sealing job ahead of you and then you have to evaluate the size.
While Unico and Spacepak are nice systems they cannot touch the efficiency of the mini-split heat pumps. The mini-splits have some small air handlers that do a nice job and are concealed.

ChrisJ

I'd be curious what the current system was sized for.

Seems like a lot of guys size for 0.1" per 100ft.

I sized all of my stuff for 0.05" per 100ft and my ductwork is a whole lot bigger than what's typical in residential.

Point being, the system may not only work, it may work a lot better and quieter. You may need do some tweaking like add dampers if they're not already there etc.

JUGHNE

It is possible that someone put a 4 ton unit ln and maybe only put in 3 tons or less of ductwork......and it seemed to work.
Happens a lot....especially on the return side.

ChrisJ

@adamfre

What size runs are there to the rooms and how many?
What style diffusers\registers and where are they located?

adamfre

@ChrisJ the registers are 14" x 7", fed with 6" duct. 9 registers total.

EBEBRATT-Ed

Rule of thumb 6" round will carry 100cfm 100 x 9=900cfm.

2 tons of cooling needs 400-450 cfm/ton so the branch ducts look spot on. What size is your trunk duct and does the returns match the supplies in quantity and size?

ChrisJ

adamfre said:

@ChrisJ the registers are 14" x 7", fed with 6" duct. 9 registers total.

How many 6" ducts? 9?
According to my chart that's only good for 765 CFM or 2 tons tops assuming they're hard pipe.

See what I mean by my sizing being bigger than others?
That's for 0.06" per 100'.

adamfre

@EBEBRATT-Ed, I'll have to crawl under there to measure the main trunk. It steps down in size a few times, however I've never measured it. The return is just one big duct which jogs over then up the wall, returning air from a central location under the stairs. Interesting side note; replacing the air handler is going to be a HUGE pain, there is only one way to get back to it to replace it, and it will require me to dis-assemble the main trunk and pull it out in order to clear the only path to the front of the house, where it is suspended in the crawl. Yay me lol

@ChrisJ its qty 9, 6" round ducts, sorry I wasn't clear earlier.

EBEBRATT-Ed

Rule of thumb 6" 100cfm, 7" 150, 8" 200, 10"400, 12" 650?,

had it memorized for years. Too bad I couldn't remember the "neutral pressure adjuster" on the other thread haha brain is slipping

adamfre

I appreciate all of yall's help regarding this. This site was tremendous help when I had to re-build my steam heat, here's hoping the AC goes as good as the heat did!

ChrisJ

EBEBRATT-Ed said:

Rule of thumb 6" 100cfm, 7" 150, 8" 200, 10"400, 12" 650?,

had it memorized for years. Too bad I couldn't remember the "neutral pressure adjuster" on the other thread haha brain is slipping

Those must all be at 0.1" which as I understand it, isn't recommended anymore. At least, that's what the books I found said.

Does it actually matter? I have no clue.

Here are the charts I used.

ChrisJ

@adamfre So as usual, your system was grossly undersized but for once it plays in your favor as it's perfect for the new equipment.

How are your returns?

adamfre

@ChrisJ It's a single, 16" round duct that shoots off about 10 feet, 90's up the back of a closet and travels about 9 feet, then terminates in a filter box / register in the wall.

ChrisJ

adamfre said:

@ChrisJ It's a single, 16" round duct that shoots off about 10 feet, 90's up the back of a closet and travels about 9 feet, then terminates in a filter box / register in the wall.

What size filter grill?

adamfre

24" x 24"

ChrisJ

adamfre said:

24" x 24"

That's going to run around 300 fpm face velocity, also perfect.

adamfre

Nice!

ChrisJ

At 4 ton it was running 600FPM which is about 200FPM higher than recommended for a filter grill.

300-450 is recommended.

GW

hey Chris what math are you using? 800 / 4 = 200 is what i get. In my opinion, the returns are the least of the worries; bigger is fine (just more metal than needed----more possible heat loss)

ChrisJ

> @GW said:
> hey Chris what math are you using? 800 / 4 = 200 is what i get. In my opinion, the returns are the least of the worries; bigger is fine (just more metal than needed----more possible heat loss)

I used engineering data from Hart & Cooley's filter grills. I guess the slots restrict it.

GW

ahh yes, I guess it can go either way, we're both right

ChrisJ

GW said:

ahh yes, I guess it can go either way, we're both right

I did 808sqin for my 3 ton system.

EBEBRATT-Ed

Not a big forced air guy but around here I usually find the duct work undersized and am thrilled if I fid a job that was sized at .1

I have seen a few jobs that were properly sized and it make scorched air livable.

most size the ductwork base on the heat load or gain but what if your heat loss is 80k and the furnace installed is 95k output? Then the ducts are screaming and everything is undersized