How Much do Duct Sizes Matter

Crissie

I am back looking for advice again. I had a Manual D and S done for my home. Planning to replace all the ductwork. There were some area with limited space for the main duct runs. The engineer sized return ducts at 3x10" and 3x14". He sized two of the branch supply runs at 4" round, saying this is needed to create the velocity to deliver the CFM for the room. Again, it's because the main supply is limited in size.
One of my contractors looked at this and wanted to change everything, saying that using oval returns would have less friction than a 3x14" rectangle and using 6" round supplies would be better than 4", even if it slightly oversized.
What do you all think? Is the contractor wrong, should I insist on the Manual D design even though it requires custom duct sizes?

Big Ed_4

Size matters too small you can over heat a furnace heat exchanger or freeze up an AC system . Too tight you will get noise , too big you will not get enough throw to condition the space . Heating systems are different than a AC system .

EBEBRATT-Ed

I don't think much of 4" round, you can't get much air through those, but because you have an engineer it's the engineers job to size it and the contractors job to put it in.

If the contractor wan't to change something put the two of them in the ring with boxing gloves and tell them you want a system that will work and have them work it out

Crissie

EBEBRATT-Ed said:

I don't think much of 4" round, you can't get much air through those, but because you have an engineer it's the engineers job to size it and the contractors job to put it in.

If the contractor want to change something put the two of them in the ring with boxing gloves and tell them you want a system that will work and have them work it out

Egos. Contractor wants to own it, even though he didn't do the design and doesn't seem able to. The Engineer doesn't care, he delivered on his commitment. Ugh. The 4" branch was because the main duct feeding it was small. Higher velocity needed to deliver the CFM.

Zman

Post your plans and equipment spec and get some feedback.
I know some brilliant engineers and some absolute idiot engineers. The same goes for contractors. The trick is figuring out who's who.

unclejohn

Need more info. What is the system size 2ton 3 ton. What BTU for furnace.

Crissie

Zman said:

Post your plans and equipment spec and get some feedback.
I know some brilliant engineers and some absolute idiot engineers. The same goes for contractors. The trick is figuring out who's who.

Here is a near final draft of the plan. The 4" wide returns on main level need to be changed to 3". Amana 60K modulating furnace and Amana 2 ton heat pump. There are space restrictions in the ceiling that is crossing from the Powder to the Hall, that is why the ducts are so small. No other way to run those ducts but with all the 90 degree turns. Would love everyone's input!

Crissie

Just looking for feedback on above design. Does anything seem out of whack?

HVACNUT

How about a Unico or Space Pak system?

Zman

The engineer is not very impressive:

1. How will Bed 3 and Den be heated?
   
2. The 4" round for the small rooms like the 1/2 bath and maybe the master bath make sense, they have low airflow needs and are close to the trunk. The others should be bigger, 5" might be a good trade-off for areas with longer runs in the ~ 50 CFM range.
   
3. Why isn't the master bath register near the window?
   
4. How will the lower level return air be provided? I would guess there will be a louvered door and/or a register at the mech room?
   
5. Why did he/she decide to put 3 runs of return air to the same spot in the Entry/Family room? It would be way easier to install 1 large grill near the 1/2 bath.
   
6. The Entry/Family SA/RA is out of balance by 81 CFM
   
7. He/she appears to have designed the supply air and return air ducts using the same static. The return air ducts should be larger than the supply air.
   

Crissie

Zman said:

The engineer is not very impressive:

1. How will Bed 3 and Den be heated? Bed 3 will have mini split. Den doesn't have a load.
   
2. The 4" round for the small rooms like the 1/2 bath and maybe the master bath make sense, they have low airflow needs and are close to the trunk. The others should be bigger, 5" might be a good trade-off for areas with longer runs in the ~ 50 CFM range. Ok
   
3. Why isn't the master bath register near the window? No room, there is a tub there.
   
4. How will the lower level return air be provided? I would guess there will be a louvered door and/or a register at the mech room? Yes. The lower level is like a basement, questionable musty air, so I intentionally did not want returns there.
   
5. Why did he/she decide to put 3 runs of return air to the same spot in the Entry/Family room? It would be way easier to install 1 large grill near the 1/2 bath. The Family room currently has returns in that location. We can't get access to the wall by the 1/2 bath because of basement joists.
   
6. The Entry/Family SA/RA is out of balance by 81 CFM wow.
   
7. He/she appears to have designed the supply air and return air ducts using the same static. The return air ducts should be larger than the supply air. I thought the returns were larger. Let me look. You have a keen eye, thanks for all the feedback!
   

Zman

Eliminating the return air at the lower level will not fix the problem. If you do not provide return air, you will end up with inadequate supply air or the air will find its own path back to a different return. You cannot pressurize a space with a residential furnace, what goes in must come out. Finding the source of the musty air or installing an HRV to exchange outside air would be a better plan.

Crissie

Zman said:

Eliminating the return air at the lower level will not fix the problem. If you do not provide return air, you will end up with inadequate supply air or the air will find its own path back to a different return. You cannot pressurize a space with a residential furnace, what goes in must come out. Finding the source of the musty air or installing an HRV to exchange outside air would be a better plan.

Would I be better off not having supply ducts in that lower level? I was going to put a mini split, only added the supplies to assist with heat. I am putting in either an HRV or ERV. The musty odor is coming from the existing ductwork and furnace, right by a bathroom. Until I tear out ductwork I won't be able to figure out precisely what the issue is to be able to remedy it.

Zman

What is causing the musty smell? Why do you want to trap it in the basement?
If it is caused by a lack of air movement, why not ventilate it? If it is caused by mold, why not find the source and treat it.

Crissie

@Zman The musty odor is coming from the existing ductwork and furnace. Until I tear out these I can't precisely figure out what is going on. The musty smell is carried through my house from the furnace or AC running. I think it is mold in the ductwork. My whole effort here is trying to get to the bottom of it to remedy, but I still don't like the idea of returns in that lower level. I would never put a return in a basement. I am planning to ventilate that area.

neilc

wait a minute,
musty smell ?
do you mean like, dirty socks ?
there is phenomena where ac coils will grow a bacteria and cause, badness, dirty sock syndrome.
do you smell it downstairs when the system is, has been, off ?
are you sure it's the system, and not something else from the house?
(apologies if this is redundant)

Zman

If you don't want the air from your basement to circulate through the rest of the house, don't put supply or return registers in the basement. Air that passes through the supply duct will eventually end up in the return side of the furnace and will circulate through the building. The air doesn't know or care what you want it to do.

Crissie

neilc said:

wait a minute,
musty smell ?
do you mean like, dirty socks ?
there is phenomena where ac coils will grow a bacteria and cause, badness, dirty sock syndrome.
do you smell it downstairs when the system is, has been, off ?
are you sure it's the system, and not something else from the house?
(apologies if this is redundant)

@neilc Yes, that is how it smells. I thought perhaps it was my coil so had it replaced 2 yrs ago and added an UV light. There is a light smell of it even when the system is off. I put tape over all the registers. The ductwork had so many huge holes and someone ran a supply into the upstairs return branch, it could be any number of things. The furnace sits in a tight corner against a bathroom, so perhaps it is something in the bath, although if I close the door to the bathroom then walk in it after a couple of hours it does not smell. I think it is the ducts. They are falling apart and not adequate so either way they need to go.

Crissie

@Zman So if I run a ductless mini split in that lower level, are you saying that air will eventually make it's way into a forced air system even if there are no supply or return ducts down there? Or, are you saying that even only supply ducts would create the airflow to make it into the forced air system? Thanks so much!

Zman

If you use ductless mini splits on the entire lower level, you will not be mechanically forcing the air from downstairs to the other floors. The plan you posted does show forced air going into the eating/kitchen and workout room. That air will find its way upstairs through door undercuts and other paths.

I am not much of a fan of ignoring problems and putting band-aids on things.

Crissie

@Zman I am not trying to ignore it. I can't to the bottom of it until the existing ductwork and furnace is ripped out. The bathroom is brand new, put in 8 yrs ago, new shower, vanity, tile. Perhaps it is a slow plumbing vent leak. I found one of those already, it took 2 yrs and 10+ plumbers to find it. Or maybe it is because that level is on a concrete slab and doesn't have a vapor barrier. If I get everything torn out and still smell it, I will open up walls, but this entire area was gutted just 8 yrs ago except for the walls surrounding the furnace.
Anyway, got your point. Thanks!