ductwork design

[nnmmbj]

Hello everyone, I am not sure I am posting this in the proper place, but here it goes.

We are installing duct work in my mom's house. Due to a financial bind we are doing it ourselves. We had installed duct work in our own home that was designed by a heating in cooling company years ago so we used the same basis for the design of my mom's. We are not contractors and we really were short on cash. This is the design:

We have a 12" round metal supply duct that travels from her new basement, through the old basement, up to through a chase to the attic. It then stretches from a T in two directions. One direction is short about 10' and then it L's for another 5 feet. The long side off of the T is about 30' then it L's for another 5 feet. The total length for the supply 12" pipe is about 100 feet.

The supply travels the same path, but it is 14" round metal duct. Total length of about 100 feet.

The supply and the return both have about 4 90 degree elbows.

The return trunk has 7 returns, one in each room, except the living room has a vaulted ceiling so we put in 2 returns. The returns have grates that are 6"X12" tied to 8" flex duct.

The supply trunk supplies (11) 4" X12" grates with 6" duct.

The house square footage is 2100.

The heating and cooling in the past was 4 tons.

We were hoping someone could tell us if we have enough returns is it based upon grate size or the flex duct size?

Will the duct work be large enough for a four ton unit?

Thank you in advance and I pray that someone understands this post.

[NCHeat]

where do you live that you have 4 TONS for 2100 sq. ft. What size is the basement duct? Sounds a little small, but not terrible

[Gib's Son]

Guarantee you will have airflow issues with those size/ length of runs @ 1600 cfm.

At 0.45 PD/ 100 feet my guess is you will be at 0.9 to 1.15 inches of static (min) without even adding a filter. (12 inch duct at 1600 cfm)

[udarrell]

Clipped...
This is the design:
We have a 12" round metal supply duct that travels from her new basement, through the old basement, up to through a chase to the attic. It then stretches from a T in two directions. One direction is short about 10' and then it L's for another 5 feet. The long side off of the T is about 30' then it L's for another 5 feet. The total length for the supply 12" pipe is about 100 feet.

The supply travels the same path, but it is 14" round metal duct. Total length of about 100 feet.

The supply and the return both have about 4 90 degree elbows.

The return trunk has 7 returns, one in each room, except the living room has a vaulted ceiling so we put in 2 returns. The returns have grates that are 6"X12" tied to 8" flex duct.

The supply trunk supplies (11) 4" X12" grates with 6" duct.

The house square footage is 2100.

The heating and cooling in the past was 4 tons.

We were hoping someone could tell us if we have enough returns is it based upon grate size or the flex duct size?

Will the duct work be large enough for a four ton unit? NO!

The house square footage is 2100.

If you mean the total conditioned area is 2100-sq.ft., is half on the 1st floor & half on the 2nd floor or 1050sf?

Then you would probably be using 2 to 2.5-Ton, at the most, of cooling for the upper floor area. Where do you live?

Looking at the longest Supply Air & Return Air runs & all the register fixtures, transitions & turns, it appears a Manual D would be advisable so you know what you're doing.

You need to find a local contractor that could do the engineer/design for you, so the equipment performs optimally... It will cost less in the mid & long-term to do it RIGHT... take pride in doing work you can be proud of because you did it the right way...

[nnmmbj]

Thank you all for the replies.

the house is simply a ranch style home. The original basement was only about 6.5 feet tall and very small. My mom had a basement added on years ago that is actually attached to the back of the house. The roof of that basement is actually the patio. You cannot put a furnace in the old basement, it will not fit. We have to cut the old one up just to get it out of there. The furnace was put in and then the floor.

the new basement is large enough. The duct work travels underground into the old basement then up a chase to the attic. I was wrong in my measurement the duct work only stretches a total length of 70 feet. That includes traveling underground to the old basement, up through the first floor and into the attic.

the contractor we had out at the house did not use any calculations. The second contractor refused to come out and give a quote, because we did the duct work. The third contractor is coming out tomorrow to give a quote for the furnace, but I wanted to be armed with the knowledge of our mistakes and if he doesn't point them out I will be disappointed.

I truly appreciate all of the guidance. Would it help if I drew out the floor plan?

Do the supply CFM base upon the duct work size or the grill size? No heating in basement or attic space. We are located in central Indiana. We were told by first contractor to go with 4 ton and we only needed one return.

Thank you!

[udarrell]

Guess I thought all that length of duct lead me to wonder if there were two floors, -sorry.

Indianapolis, is 90-F dry bulb & 74 wet bulb at 47.62% Relative Humidity (RH).
four ton seems a bit much especially as there is NOT the duct system sizing to support that tonnage.

The above 90DB, 74WB, means there is 102.21 grs. of moisture per lb of air.
The indoor air at 75F DB & 63F WB is at 51.76 RH,but only has 67.82 grs. moisture per lb of air.
That is why reducing air infiltration to a minimum is so critically important.

We have some 2-Ton systems cooling 2400-sf homes here in SW WI, & there isn't much difference in the summer design.

Never install more tonnage than the duct system airflow will support, as it is a wasteful use of energy.

There appears to be a fair humidity load that requires less tonnage & longer runtimes to handle.

Were I you, for a lot of reasons I'd have an energy efficiency audit performed on your home & do the retrofitting so a manual J heatload calc would show a tonnage down to around 3-Ton or less.

[mofotech]

If your going to do it,...do it right the first time. Or you'll be crying all the way to the bank later.

[joepierson]

11 6 inch supply lines from one 12 inch duct doesn't make sense, you probably need two 12 inch supplies, one 12 inch line at that length can do 600 cfm at correct velocities if your lucky

[wahoo]

Hire a pro to do it right. This sounds like a DIY job from He......to me. I know you're trying to save your mom some money, but if done wrong, it'll just cost her money, month after month and year after year!

[marvin]

air flow is probably the least understood aspect of this trade.
get a pro to design & lay it out for you & preferably to install it.
your mom will thank you for a job well done.
given the size of the old furnaces vs the new ones the new
may fit in the old basement just fine. if height isnt enough you may be able
to hang it horizontal.

[genduct]

Guarantee you will have airflow issues with those size/ length of runs @ 1600 cfm.

At 0.45 PD/ 100 feet my guess is you will be at 0.9 to 1.15 inches of static (min) without even adding a filter. (12 inch duct at 1600 cfm)

Unless you really wanted a high velocity system, you have got a turkey that won't deliver with Thanksgiving right around the corner.
No good, Pilgrim. You should have asked for guidance BEFORE you decided to copy the duct system for what was probably a 2Ton unit

[nnmmbj]

I appreciate the frank discussion. You are absolutely correct in that I should have contacted you guys before leaping. However, to be fair on my end, I was going under the assumption that it would only be at most a 3 ton system. I am sure that I still did a lot of wrong things, but at this point I just would like to do what I can to correct the situation.


I corrected the length of my original post to be around 70-75' of length of ductwork for each the supply and the return. Would that change your calculations?

I also had another contractor out and he did the manual J calculations and stated the house needed an 80,000 BTU furnace, is that a 2.5 ton? Will the duct work be ok now? The contractor stated the duct work is too small on his quote, so we are waiting to hear why?

I can add more returns and supplies but I would be hard pressed to change the main trunk lines.

Thank you very much for responding.

[genduct]

We can't see what you are doing and you are not skilled enough to sketch out what you are doing.
You may use my profile contact info, if you care to