Ducting Question

Old 10-04-02, 07:54 AM
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Need Information on Ductwork Layout

I'm going to be installing a forced air propane furnace in an existing large, two story house, and need general information on laying out, sizing and installing supply and return ductwork. There is no ductwork, so I'll be starting from scratch.

I've read everything I can find, which hasn't been much! I saw a reference here to a post by Lynn Comstock, but haven't been able to locate it. Could anyone recommend a book or website that covers some of the general rules?

I have a few options for running branches for the upstairs. All 1st floor rooms but one have dropped ceilings, and there's plenty of space for ductwork between, so I'll have a good bit of flexibility once I get that far.

Is it better to run two large branches off the extended plenum, then run small lines from them, or should I try to run more smaller branches with less duct running from each?

I've read that both 90 degree turns and flexible duct inhibit airflow. Does the same thing apply to semi-rigid? Using semi-rigid duct instead of tees or elbows could eliminate a couple 90 degree turns. Can metal ductwork and semi-rigid be used together?

Could someone recommend a distributor online that shows or lists most of the elbows, etc. available? Are Y's, 45 degree and flexible elbows available in most round duct sizes?

Should the cfm of a branch equal the combined cfm of any lines running from it?

Should the extended plenum's cfm equal that of combined branches?

Should the return air inlets be equal to supply outlets? If so, must this balance be achieved for each room individually?

Any help or information will really be appreciated. Thanks.

Last edited by JudyRoyal; 10-05-02 at 07:30 AM.
Old 10-04-02, 05:37 PM
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duct layout

what you need is a ductalator. this will tell you what volume corresponds to what size pipe, and what bends in pipe will count as added length. different opinions are out there on trunk line vs. "spider" duct systems. i personally think that if the individual duct runs are less than 20 ft, a spider system would be fine as long as volume dampers were installed at the takeoff point. cfm (airvolume) is additive, and the sum of the branches should equal the capacity of the trunk. this will help maintain static pressure, velocity, and "throw" from the register. your air return duct is sized at a different static pressure, so it is calclated differently. basically, the bigger the return duct the better, this will reduce velocity, static pressure and most importantly noise. most manufacturers make comparable quality duct and fittings, be sure to seal all joints and seams, as well as insulating well. if using flex, stretch it out before using, and avoid sharp turns, sags, etc. if a sharp turn (into a boot, etc.) is necessary, use a metal elbow to reduce crimping.
Old 10-05-02, 07:29 AM
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ductwork calculations

hvac4u, thank you!

I did a search for "ductalator" and found some good information (I think!) One article seemed to be saying that "Manual J" guidelines are more correct than the results of using a ductalator. One reason mentioned in the article is that a ductalor doesn't take into consideration the differences in flexible and rigid ducts. And I found a short form version of Manual J online at


I just hope I can understand it!

I think a "spider" system is out of the question. While it would probably work for the first floor, runs to the 6 upstairs rooms would be 'way too long--in places over 40', unless I run vertical ducts up outside walls--and this would require tearing out walls in places.

One thing I should make clear: I absolutely won't be installing the furnace myself, and possibly won't be doing the ductwork. But I've had 3 hvac contractors out here, and each has very different ideas about furnace size and how to run the ductwork. None want to show me a design layout. All say that doing a design layout would take too much time, and that they just "know" proper sizes, etc.

I've been doing as much reading as possible (including over 20 hours on this board) so I've learned at least a little--enough to know than NONE of these contractors (all of which are generally considered some of the best in this area) are doing things right.

All 3 say that duct tape is sufficient, because the basement gets some heat. Sealing with mastic takes more time, and isn't really necessary.

One said we'd only need one vertical run to the upstairs, with a large horizontal run (probably 12" round, when I asked) to supply registers in 5 bedrooms and a bath. Because of limited access to the upstairs, this horizontal run would have to run parallel to the extended plenum in the basement. (backtracking)

Another didn't seem to think that running vertical ducts in outside walls would be a problem...

Another said I wouldn't really need cold air returns upstairs, as air would flow down the stairs to the first floor return registers.

ALL plan on just covering space between joists for cold air returns.

I don't know much about ductwork, but I DO know that the longer the run, the lower the pressure, and the less air will be delivered. Bends and flexible pipe inhibit air flow. I also know that when a 6" supply line is taken off a branch duct, the branch duct has to be reduced correspondingly to maintain proper pressure in the duct beyond the tee. Heated air only travels about 8-10 foot from a register, and larger rooms need multiple registers. Rooms on the West (wind) side of the house will most likely need more heat than those on the protected side. Cold air returns are necessary both to aid in air distribution and to return partially heated air to the furnace. Neither supply or return ducts should be run adjacent to unheated areas. Both need to be sealed, and in some cases, insulated.

That's about it. As you can see, I only have a very limited general understanding of how air flows through a heating system. Unfortunately, I keep getting the uneasy feeling that I know about as much as the "professionals" that have been out to look at what I need done. Either that, or they simply don't care.

This is a huge old house, and if furnace(s) aren't sized properly and ductwork run correctly and efficiently, I simply won't be able to afford to heat it. I also know that putting ductwork in an old home like this is a lot tougher than in new construction

I am perfectly willing to pay someone to do it right--but I want to learn enough to KNOW that it's being done as well as possible, and that corners aren't being cut. But I'm certainly not going to pay someone to do an inferior job. I hope to learn enough to either lay it out myself, or at least recognize a good efficient layout.

I'd like to know, for example, if a 12" vertical duct (10') will supply four 6" runs totaling 42' (2 having 90 degree bends)

If I can't find a contractor I feel confident with, I just MAY give it a try myself, but really would rather not.

Thanks for your input. I need all the help I can get!
Old 10-05-02, 11:26 AM
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Lightbulb In My Professional Opinion

Hello: Judy Royal. Welcome to our Heating and Cooling forum and the Do-It-Yourself Web Site.

All the help you can get? Here's more.

Have you thought about "Zoned Heating?" By this I mean two individual and seperate heating & cooling units. One unit for each level. There are many advantages to have two units.

Forced air heating units containing both heating and air conditioning {Dual Packs} are available in horizontal and vertical units.

An attic installation of a combo heating and air conditioning unit should not pose any problems since you stated having the room and space available.

Depending upon the space available in the attic for the upper level unit, will determine the type of unit needed. Roof mount units are also available.

The new self contained all in one unit heating & A/C combination units {Dual Packs} can be totally suspended from the attic rafters. This method of installation reduces vibration and noise.

Replacing the existing lower level unit to provide heat only to the lower level will provide better heating abilities, use less fuel and help save energy costs.

Best sources of information on both of these types of units would be directly from the manufacturer and local heating and cooling dealers in your area.

Installing both the units and there venting systems, must meet all local codes and manufacturers requirements to insure safety, proper operations and to maintain the warranty.

And there you have it...."My Entire Two Cents Worth."...

Regards & Good Luck, Forum Moderator.
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Old 10-06-02, 11:48 AM
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2 heating units


Thanks for your suggestions, and for answering some of my questions.

I guess I didn't explain the house layout well enough:

At one time the house (10 rooms, total) was divided into apartments. There's pretty good insulation between dividing interior walls. Each unit had living room and kitchen on the first floor, and bedrooms upstairs. (One bedroom for each apartment overlaps the 1st floor of the other apartment.)

I rarely use the rooms that made up the back apartment, and I'm at least thinking about renting it at some time in the future. For this reason, I will most likely want to install two furnaces with separate ductwork to each "apartment". Each heating unit, probably in the basement, will have to supply heat to rooms on both floors.

There is also a full third floor, with an attic above that. I'd eventually like to add a 3rd apartment on the 3rd floor, so wouldn't want to run ductwork there, even if it is otherwise feasible.

Also, I'm not planning on central air conditioning. This old house stays unbelievably cool in summer, probably because of the 3rd floor, and some large trees that shade the house in the afternoon. We had two weeks of temps in the mid-nineties this summer, and by simply opening windows at night and closing them in the morning, the house stayed pretty comfortable.

I'm most concerned about properly sizing the furnaces, and making sure supply and return ducts are designed, run, sealed and insulated properly. As I said in my previous post, the information I'm finding here and other places on the Net conflicts with what local contractors are telling me, and what I'm finding here makes a lot more sense.

Knowing the house gives me at least one advantage over the contractors who have looked at it. I know where ductwork CAN be run from the basement to the 2nd floor without destroying the looks of the house.

There are at least 4 places where large vertical ducts could be put for supply and return runs to the 2nd floor. There are dropped ceilings (12-16") in the first floor rooms above which branch ducts could be run for rooms upstairs.

I can see several possible ways that ducts COULD be run, and I'm trying to learn as much as I can about how they SHOULD be run.

I kow it's impossible for you to answer any of my specific questions without having some idea what the house is like. I have a website, and will upload some diagrams of the house, and link them to a post here. Then, if you, or someone else on this board, wouldn't mind, maybe you could tell me if I'm approaching this right.

Thanks again.

Last edited by JudyRoyal; 10-06-02 at 12:07 PM.

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