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Old 02-09-2013, 12:48 PM   #1
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Will the size of my ducts have to be changed?


I am in the early stages of hiring someone to install an a/c - heating unit in my two story house. I currently only have forced heating ducts in my house. My 1965 house was not built with an air conditioner. Only a heating unit. Overtime I have changed about 30% of the ducts using a thicker gauge duct and sealing all ducts with silver backed adhesive tape and topping that with cloth tape with a pasty glue on all joints meaning it will never leak or come apart. I used to be an insulator so I have some experience in doing this. I am thinking the size ducts I will actually need will probably be dependant on the size unit I buy for my house. My neighbor just had a 4 ton unit installed in his attic and he has 1400 sq ft. I have 1950 sq ft and I am thinking 5 ton just because of increased sq footage. I still have a lot to learn. My ducting goes from a 24" plenum attached above the 1st floor heating unit to 8" horizontal ducts reducing to 6" ducts for 1st floor rooms. There are two 8" ducts coming out of plenum and one is used for 1st floor rooms farther away from unit and the other 8" duct is sending air to the vertical reducer headed upstairs which reduces to a 6" duct for the two rooms upstairs. The current heater I have says its input is 100,000 btu/hr and the bonnet capacity is 80,000 btu/hr. I am guessing this would have helped determine size of the duct back then. Is it possible to do the size layout I need by myself,maybe getting it verified by someone in this field and then get a good installer to finish the job? I am retired, remodeled most of my house over the last 25 years so I think this is something I could do. I have been in my attic many times insulating ,running new ducts,etc.... I would appreciate any input. Thanks

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Old 02-09-2013, 01:01 PM   #2
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Will the size of my ducts have to be changed?


Doubt your neighbor needs 4 tons, and doubt you need 5 tons.

Might want to do a load calc and find out what size you really need, can save you a lot of money.

http://www.hvaccomputer.com/gtarget1...FQSqnQodVmgAsQ

49 dollar fee to use for 60 days as I recall.

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Old 02-09-2013, 04:24 PM   #3
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Will the size of my ducts have to be changed?


Quote:
Originally Posted by beenthere View Post
Doubt your neighbor needs 4 tons, and doubt you need 5 tons.
Might want to do a load calc...
Pawl... don't start multiple threads to discuss the same issues.

Mostly the same people are reading them all and they will mostly the same advice will be given... but details will tend to be lost by dividing the flow.
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Old 02-09-2013, 08:16 PM   #4
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Will the size of my ducts have to be changed?


Your neighbor would have probably been better off in the 2 to 2.5 ton range. Please make sure the contractor you hire performas a load calculation. You say you've added insualtion and replaced windows? That will help quite a bit in reducing the tonnage needed.
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Old 02-10-2013, 11:37 AM   #5
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Will the size of my ducts have to be changed?


Pawl, as another homeowner, let me tell you my experience. My home was built in 98 and soon after we moved in. It was an energy star rated home by the local utility, which was suppose to mean it had a lot of sealing, double pane windows and insulation, etc. I only say this because I know yours is not and to point this out. I live in Houston Texas.

I have 3600 square feet with 1800 up and down. The load calc done on this home said it needed 2 3 ton units. (unofficial rule of thumb is 400 sq feet per ton) While the 3 ton downstairs was ok, it certianally was not for the upstairs. So we argued and fought with the builder and the AC company and after letting them do balancing and checking over and over, with no good results, we gave them a choice: replace the units with 4 tons, or go to court.

They replaced both units with 4 ton units. Yes my electric bill went up some, but we accomplished the comfort levels and operations that we wanted from the units, especially upstairs.

So my point to you is as one homeowner to another is this: you are in a very old home, in contrast to mine which has a lot of insulation and energy efficient stuff. If you want comfort on the days when it gets tough outside, you probably will need to go bigger than what the load calc suggests. I base this on the old age of your home and poor insulation and sealing in walls and attic.

So based on the 400 per square foot unofficial theory and poor insulation and sealing, 5 tons could be what you want. Would the experts here agree, probably not because they prefer to build based on a load cals. But I have seen more and more where some experts have said that sometimes they need to re-evaluate the load calc and listen more to the customer.

In my case, the smaller 3 ton unit upstairs would run and run on extremely hot days. I know that a unit needs to run a ceertian amount of time in order to remove humidity. Cycling too fast is not good. Once we put the 4 ton in, not only would it cycle on the challenging days, but it felt more comfortable.

One other thing that you need to consider for comfort that also assisted us was the addition of more return airs, especially upstairs. This assisted in comfort, in my opinion, but adding these alone would not have resolved the long run times on hot days.

I am not an expert, there a lot of great ones here for those opinions! I can only tell you that from a homeowners standpoint, you may have to be ready to put your foot down on what gets installed. I think load calcs are great on new built tight and sealed homes.

JMO.
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Old 02-10-2013, 12:18 PM   #6
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Will the size of my ducts have to be changed?


Load calc doesn't have ability to account for heat rising from first floor to second floor. Your first floor unit probably should have stayed a 3 ton, as the upstairs cool air also falls down to the first floor. If you had kept the first floor 3 tons. It would probably have a much better humidity level.

The second floor probably would have been fine with a 3.5 ton. Splitting hairs? maybe, but often a 1/2 ton increase in size will reduce temp by 6 to 8 degrees, and keep the humidity in much better check.

Your also forgetting your location compared to other posters. Your area is a hotter area.
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Old 02-10-2013, 01:59 PM   #7
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Will the size of my ducts have to be changed?


Quote:
Originally Posted by beenthere
Load calc doesn't have ability to account for heat rising from first floor to second floor. Your first floor unit probably should have stayed a 3 ton, as the upstairs cool air also falls down to the first floor. If you had kept the first floor 3 tons. It would probably have a much better humidity level.

The second floor probably would have been fine with a 3.5 ton. Splitting hairs? maybe, but often a 1/2 ton increase in size will reduce temp by 6 to 8 degrees, and keep the humidity in much better check.

Your also forgetting your location compared to other posters. Your area is a hotter area.
Thanks and you are probably right about the 3 .5 .

Thanks for the info on heat calc, see after many years I have learned again how wrong they were in what they told me!

Thanks
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Old 02-11-2013, 01:53 PM   #8
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Will the size of my ducts have to be changed?


I am going to be talking about a lot of things on here because I value everyone's opinion so be patient. I am all over the place trying to get questions answered. I am looking into the suggestion for a do it yourself $49 load calculation as we speak. I feel I can do the load calculations myself and maybe a more thorough job because its my house and I have more time.

I have popped a piece of drywall out on the 2nd floor where the original ducting in the 1st floor attic changes in size from 8" round to rectangular (5'h x 14"w x 3"d) to fit into the vertical wall going up and comes back out in the 2nd floor attic 6" round in diameter. And my hunch was right --->>>. After cutting away the drywall I could see the original installer did not seal all joints properly so heat has been leaking out of this rectangular ducting in the wall since the house was built in 1965.

I needed to open the wall anyway because I am going to have to run return air ducts from the 2nd floor ceilings to the 1st floor attic.There are no return air ducts because the house has never had air conditioning. I will just position the return air ducting in between the wall studs right next to the existing rectangular duct.

Is it true that each room should have the ceiling register where air is coming out of positioned in each room closer to the area where the heat is the highest? Say for example near a window that brings in more heat from the sun as opposed to an interior wall in the same room. I just figured the middle of the room.

After determining where to put the ceiling registers where or how far away does the return air vents go in the same room. On the 2nd floor would I include return air ducts in the bathroom and the small area at the top of the stairs with the walkway to each bedroom?
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Old 02-11-2013, 04:35 PM   #9
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Will the size of my ducts have to be changed?


Bathrooms don't get returns. Don't want all the odor spread through out the house.

Return in the ceiling of the second floor landing will help a lot in cooling season.

The supplies should throw the air toward the highest heat gain/loss area. Returns should not be in any direct line of throw from the supply register. So if the supply is in the middle of the room. The return should then be near the inside wall.
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Old 02-12-2013, 12:28 PM   #10
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Will the size of my ducts have to be changed?


1.) Does the area at the top of the stairs need a return or supply duct? The square footage of floor area on the 2nd floor landing is 51 sq.ft

2.) Do you feel that using rigid duct is the best way to go? Original duct is rigid,pretty dirty inside and 48 years old so I am going to change it.

3.) Just for my education today is their a rule of thumb when talking about return ducts. Are they the same size as supply or 75%,50% or 25%.

4.) I am going to run new ducting so I can move supply registers closer to heat gain/loss (window). Are there ceiling register configurations you feel are better than others? It sounds like a rectangular one placed in the ceiling dispursing air four ways is the way to go.

5.) Upstairs there is a total of 381sq.ft in both bedrooms plus 40 sq.ft in bathroom. Is it worthwhile or even possible to try and isolate the 2nd floor when using only one ac/heating unit for the whole house?

6.) Is it possible to put something in the duct system that gives me a choice to direct air only to first floor? I rarely go upstairs. Most of my time I spend downstairs. I saw this clip on the internet where this family would shut down certain areas of the house but it was causing a whistle in the floor registers from too much pressure in one zone area so they placed a weighted damper in the supply duct routing the additional pressure back into the return duct.
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Old 02-12-2013, 01:59 PM   #11
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Will the size of my ducts have to be changed?


hope this helps...

1. supply us with a drawing, including windows and all here can help you locate the supplies and returns for you.

2. I believe that the best choice for duct work is rigid steel. Note that old ducts can be cleaned, so dirt is not the issue. Air leaks (sealing), izing and rust are issues that could be corrected with new ductwork.

3. Standards for returns are tied to the size of the supply ductwork size and air handler sizes. Supply ductwork sizes are tied to the room size and calculated hot and cold expectations based on windows, insulation, air tightness, air handler size, throw of air from the register, noise and orientation of the room to the sun. static pressure within the system is the form of measurement you need to test, along with FPM air flow at the air filter. There is lots and lots of math and science that goes into design of the ductwork. In order to get it right you have to have a real pro design it or do the homework information gathering on the internet. Dont mean to scare you away from doing your own work, it is possible!!!!! (I did it !!)

4. drawings of the floor plan would help the forum tell you the best placement of supplies and returns!! even a napkin drawing would be good!!

5. zoning is what you are thinking of.... yes, zoning is good and possible, but it has some drawbacks. its my understanding that a correctly built zone system can do what you want, but with anything, that extra ablility to control things comes at a cost. Time, money, complexity and maintiance. But it can be done!!!!!!!!!

6. it is possible... but the supply to return pressure bleed off sure seems like a hack job to me.. I think what you are looking for is a single hvac unit and two duct (supply and return) systems tied to the single unit, with the ability to switch between the two, given the need. This is great and would be really neat if it was all automatic and all designed to provide the correct air flow into the two zones in all circumstances. given the proper controls and sizing everything is possible.. but cost and complexity enter into the idea... as well as effency.. as I would guess there is no insulation and air sealing between your floors so keeping the heat or cool between the floors might be tough. but it could be done...

again, this is just one guys opinions and self taught knowledge.
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Old 02-13-2013, 11:57 AM   #12
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Will the size of my ducts have to be changed?


I did the $49 residential load calculation myself. Under the Whole House totals it showed The Total Heat Loss BTUH was 39,758 & Total Heat Gain BTUH was 17,631. The Latent Heat Gain BTUH was 1,495 and Sensible Gain BTUH was 16,136. Below the Total Heat Gain column it listed (1.5 tons). The square footage totaled 1626.
Maybe someone can give an explanation of these numbers to better educate me. Thanks
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Old 02-13-2013, 12:09 PM   #13
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Will the size of my ducts have to be changed?


Quote:
Originally Posted by Pawl View Post
I did the $49 residential load calculation myself.
Maybe someone can give an explanation of these numbers to better educate me.
They're saying that your 1626SF house in mild California...
doesn't much help to maintain a comfortable temperature.

Some additional reading:

http://www.engineeringtoolbox.com/la...oad-d_245.html
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Old 02-13-2013, 12:28 PM   #14
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Will the size of my ducts have to be changed?


These are the Design Conditions that the program used for my area in Cypress,Calif.
Indoor: Summer Temperature: 75 - Winter Temperature: 72 - Relative Humidity: 50
Outdoor: Summer Temperature: 80- Winter Temperature: 43 - Summer grains of moisture: 84 - Daily Temperature Range: Medium

1.) Under Total Heat Gain (BTUH) they listed 19,238 (1.5 tons) - Are they saying I need a 1.5 ton A/C unit under these Design Conditions listed at the top to maintain those temperatures?

Last edited by Pawl; 02-13-2013 at 12:34 PM.
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Old 02-13-2013, 04:36 PM   #15
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Will the size of my ducts have to be changed?


Quote:
Originally Posted by Pawl View Post
These are the Design Conditions that the program used for my area in Cypress,Calif.
Indoor: Summer Temperature: 75 - Winter Temperature: 72 - Relative Humidity: 50
Outdoor: Summer Temperature: 80- Winter Temperature: 43 - Summer grains of moisture: 84 - Daily Temperature Range: Medium

1.) Under Total Heat Gain (BTUH) they listed 19,238 (1.5 tons) - Are they saying I need a 1.5 ton A/C unit under these Design Conditions listed at the top to maintain those temperatures?
Yes, that is what they are saying.

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