Gas and Oil Home Heating Furnaces - Adding A/C ducts

Our house was built in 1950 and all the HAVAC duct work is sheetmetal covered with vinyl backed insulation. I need to add A/C ducts to my bathroom and dressing room. How do I add on to the existing duct work?:confused:

I have been waiting for someone else to tackle this seemingly simple question. Here goes:

There is a design issue and a mechanical issue.

The parts can be purchased and assemled simply enough for most people to do. Howerver, they need to be correctly sized and sealed.

Duct Size By RULES OF THUMB:

Once you have somehow selected the furnace and air conditioner needed you can size the ductwork. Figure 400 cu. ft. per min for the AC per ton of cooling. (Example: a 2.5 to AC will need to move 1000 cfm for best results with a wet coil.) Divide up the air proportionately to the square footage served.

(Example: for a 1000 sq. ft. home each sq. ft will get 1 cfm...and a 140 sq ft bedroom will get 140 cfm.) Now size the ducts and branches so that the velocity is 600 to 900 ft. per minute for sheet metal and 600 ft per min. Maximum for flex duct.

(Example: a 7" dia. round flex duct has an area of 38.5 sq. in. {or .267 sq.ft.} and 140 cfm will pass through it with a velocity of 539 ft. per min.) This is rule of thumb information and experienced people would increase the airflow to key rooms (+10 to 20% for master BR and Family room and +30% to the kitchen). Then decrease the air to closets and bathrooms 10 to 30%. Finally if the proportion of windows is more in any room, that room will need more air to compensate. Trial and error learning can make this method work pretty good on smaller and uncomplicated homes.

Design like the Pros: you can buy Manuals C, J, S, D and T for residential design. Go to http://www.acca.org/redesign/catalog/Category.asp?cid=3 to order these reference manuals. They are well written for layman or pro to use.

(Sadly most AC contactors use the rule of thumb methods because the public does not know how complex good design can be and they are unwilling to pay for a time-consuming real design. Thus AC and heating discomfort is very common, especially in bigger and more complex homes. A quality contactor will guarantee comfort satisfaction in writing or Money back...IF you can find one.)

Hate to step on anyone's toes here but NO advise is better than BAD advise.

I waited several days before providing my information. So improve on it.

The question is VERY general. Room size and heat load are unknown. Physical layout and other constraints are unknown. The construction and location of the existing ducts are unknown.

How can such a question be answered without referring to some basic principles? And these must be simplified for a non-professional. As I said a lot of ductwork is installed by rules of thumb rather than science. Some contractor run 6 or 7 inch ducts everywhere.

I personally believe that the miserable comfort in homes and offices proves my position. Comfort is the exception rather than the rule. Hot and cold rooms are accepted in home and offices because comfort is not a part of the CONTRACT. So people end up with machines and ductwork but not comfort.

Even the professional engineers tend to distribute air in commercial buildings in a uniform way (the same cfm per square foot) even though the heat load is never distributed uniformly. Thus conference rooms, copier rooms and lobbies (for example) are rarely comfortable.

SEE the following information provided by the EPA.
http://www.epa.gov/iaq/largebldgs/i-beam_html/ch8-budg.htm#n2.22
And select Table 8.2 IFMA Survey: Top 5 Complaints of Corporate Tenants (1991)

Guess what the top 2 complaints are? #1 is TOO HOT. #2 is TOO COLD. What does that say about comfort? These are buildings have ENGINEERED comfort systems no less. # 4 on the list is IAQ (indoor air quality). Our industry seems to make people miserable rather than comfortable. Is that what people really want when they buy air conditioning and pay the power bill? It is certainly what they are accustomed to getting.

Thank you very much for your information.:)

Rule of thumbs are an inexperienced lazy excuse for not doing the right thing.

Beginning technicians start learning the wrongs rather than the rights, consumers who have no clue, believe in it because its cheaper and easier, and you sound like you have it together. Then when the consumer hires a contractor gets sold a bill of goods because he will rarely believe the guy who knows his stuff. Why? Because it cost more to do the right thing and since they heard it here, from a friend, neighbor, neighbors friend who knows the guy down the street who met the beginning technician above. So the guy who tries to do the right thing is walking away shaking his head because he’s just been deemed a rip-off. Well why not? Six inexperienced guys know the wrong way, one guy knows the right way and is more expensive. He has to be wrong!

You would do a better job of helping this guy by asking him for the answer to the unknown’s you mentioned then providing a proper solutions.

:) Well,I guess I was not really clear about what I was asking. What I meant was, how do I tap into the existing sheetmetal ductwork and what tools are required. I know I can buy the vents and the circular ducts, but how do I cut a hole in the existing sheetmetal and attach the new ductwork to it and then run it to my bathroom/dressing room which is not very big. I think approx. 10'x10'. Sorry for the confusion.

Drills to make initial starting hole. Aviation Snips to enlarge and cut to size needed. Caulking gun and caulking to seal the seams so they are airtight.

A hammer and the corner of a sharp chisel can make the initial hole if you are careful not to beat up the duct.

Your information is very helpful and I may have other questions in the future. Thank you again.

Deborah