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The Ductopus or the Centipede - Which HVAC Duct System Is Better?

 

duct system trunk and branch hvac design endcapI've been thinking lately that I might be in danger of losing all my friends in the HVAC business because I've been a bit heavy on showing examples of what they get wrong but not so good at showing what they do well. If you read my article Why Won't the HVAC Industry Do Things Right?, you may recall that in answering the question I didn't place all the blame on HVAC contractors. There's plenty to go around.

Anyway, today I'm going to show you an example of something that I liked when I saw it in a home being built. The photo above shows part of a trunk-and-branch duct system. What you see is the end of a trunk line, which carries the conditioned air from the air handler out to the branches, which distribute it to the rooms in the house. Here's why I like this:

  • The trunk line is made of rigid sheet metal, not flex duct.
  • They've kept all the branches a good 6" from the end of the trunk, which helps get better air flow into the branches.
  • The branches aren't jammed up against each other where they come off the trunk.

The house that I saw this in had some problems (HVAC, insulation, drainage plane...), but I'd much rather see a trunk-and-branch system like this than a radial system the way it's usually installed. Last week I wrote an article asking if flex duct should be banned from green building programs and concluded that I think it's OK when used as shown here - for relatively short branches in a trunk-and-branch system.

Ideally, I'd like to see whatever system gets installed go through a full HVAC design and commissioning process. Every house should have it. ENERGY STAR Version 3 requires it. Building codes require it, too, even if it doesn't get enforced in most places.

But if we go by the 12-step program mantra of progress, not perfection, every time we see a system like the one in the photo above, we should remember that this is progress...because if it looks like that, it doesn't look the what you see below, which we affectionately term the ductopus. And I'm sorry, that system wasn't designed, and it's not going to perform well either.

ducts radial system ductopus ugly

Comments

Allison, 
 
First of all...I like your blog... 
 
It reminds me of the "parallel universe" in Seinfeld where all the friends are polite. 
 
I am an Architect from North Texas (no,I am not pretending to be an Architect :-) 
 
 
 
I have recently noticed some radial duct sytems(doctopus) in production homes. 
 
Can you talk a little about why radial duct designs do not perform well. 
 
 
 
How about starting a blog about Duct Design Basics for Dummies (Architects). 
 
Posted @ Thursday, April 21, 2011 5:14 AM by John Brooks
John: It's not that radial duct systems can't perform well. They can. The main problem here is that they're usually just thrown together once the installers arrive on the jobsite with little to no planning ahead of time. They've got a starting point (the air handler) and several endpoints (the vents), and their job is to connect them with the bags of flex duct in the back of their truck.  
 
The main problem from an airflow perspective is that the takeoffs in the photo above are too close together, the ducts are all crammed in there with kinks and sharp turns, and they use up just about all the available area on the plenum. The air from the blower is not likely to be distributed according to the size of the duct in this case. It's going to be about location of the takeoff and the static pressure down each line. 
 
Good idea about the Duct Design blog! Maybe I'll do that.
Posted @ Thursday, April 21, 2011 5:31 AM by Allison Bailes
We much prefer the trunk and branch or extended plenum design (as featured in my home).  
 
And you certainly aren't making Moncrief mad... you're helping us by educating consumers on the right and wrong (which also happens to be the cheap and pervasive) way of doing things in the residential industry.
Posted @ Thursday, April 21, 2011 5:58 AM by Phil
Phil: I think most HVAC contractors who try to do things right (and Moncrief is definitely in that category) stay away from the radial system. As I said above, it's not that it can't work - it's usually not done well enough to work and is often a sign of cost-cutting on the part of the installer. 
 
Glad to do my part in educating people about good HVAC.
Posted @ Thursday, April 21, 2011 6:06 AM by Allison Bailes
Allison, Is the best duct No Duct?...You mentioned in another blog that your next house may employ ductless mini-splits. 
 
Are you thinking of providing a "head" for each bedroom? 
 
If not... how would you provide conditioned air to bedrooms with doors closed? 
 
Posted @ Thursday, April 21, 2011 8:20 AM by John Brooks
Manual D gives examples of radial systems, and they look nothing like the typical installs found in residential.  
 
John: I recently specified a mini-split for a 4 bedroom Passive House project with peak loads of less than a ton. Putting a head in each room would require a much larger heat pump than necessary (one that supports five ports). Some folks say oversizing isn't an issue with inverter compressors, but this is incorrect. The high SEER rating assumes the unit is properly loaded. The SEER (and the operating range) goes down if the unit is oversized. Moreover, the cost would be cost prohibitive. And finally, the smallest heads available would have been grossly oversized for peak bedroom loads.  
 
We went with a ducted mini-split (Mr. Slim SEZ/SUZ), but this approach has it's own challenges because available static is only 0.20 IWC. I helped the mechanical contractor design an extremely low static duct system.
Posted @ Thursday, April 21, 2011 2:36 PM by David Butler
David, thanks for the feedback. 
 
Would it be difficult for you to elaborate on the features of an "extremely low static duct system"? 
 
Posted @ Thursday, April 21, 2011 4:23 PM by John Brooks
John, note that a straight run of metal duct with same cross-section as supply outlet will have almost no friction loss. It's all about takeoffs and fittings.  
 
The key is to keep velocity low since friction rates drop dramatically with velocity. Fortunately, Manual D 3rd Ed. includes a new Appendix 4 that shows how to convert Appendix 3 equivalent lengths for less-than-standard velocities.** Right-D makes this adjustment automatically. 
 
** Appendix 3 EL's are based on 900 FPM, which would be insane in this application. 
 
In the Passive House project, the AHU is hung from the laundry room ceiling at one end of a hall. The nominal ceiling which defines the pressure boundary is at the 9' level, with dropped 8' ceilings in hallway, closets and bathrooms. It helps if house is designed with duct layout in mind.  
 
The supply trunk, essentially an extended plenum, runs down the hall with "reduction take-offs" to each room (Manual D Appendix 3 Group 3). Each supply take-off serves as the grille boot with no further transition required. Right angle and end-entry boots are a killer. All supply diffusers are located above door headers, thus minimizing run-out lengths.  
 
BTW, avoid putting supplies in closets and other rooms that have virtually no load. This steals precious air from rooms that need it. 
 
On the return side, I specified a very short but fat return plenum that connects to a single 20x30 filter grille in the ceiling. The 20x30 filter is 3x the area of the return opening. 4x would be even better if you have space. I specified Filtrete 1500 (purple) media filter. Throw away the cheesy filter that comes with the unit. I specified jump ducts for rooms with more than 40 CFM design airflow. (Manual D includes a handy table that shows the door undercut required for various door widths and design airflows.)
Posted @ Thursday, April 21, 2011 7:21 PM by David Butler
A question for Allison and/or David - What are your thoughts on high velocity/small diameter duct systems? 
 
My experience has been that when properly designed (Manual J) and installed, they are VERY effective, and you can run the small diameter flex ducts through much tighter spaces.
Posted @ Friday, April 22, 2011 6:38 PM by Howard Vics
Howard, the Unico and Spacepak high velocity systems are great for AC retrofit in homes with no ducts and no way to install regular ducts. But I would never spec one if I have a choice. The blower uses MUCH more energy than a conventional blower. In fact, their SEER ratings are lower than 13 because of this. I recall reading a document on Unico's website that claims a higher *delivered* SEER due to tighter ducts and less loss to unconditioned space due to less duct surface area. However, if you compare apples-to-apples (e.g., compare with convention system with tight ducts installed inside the envelope), you'll definitely take a performance hit due to the higher fan power.
Posted @ Friday, April 22, 2011 6:57 PM by David Butler
Two comments: 
 
First to Allison; Your ductopus picture is also a nightmare for proper chimney installation as well. Luckily (I think) it's in a garage. And the good ducts are in an unconditioned attic in a hot climate - but that's another issue. 8-) 
 
And to John Brooks - I would argue that the best duct system is almost no ducts at all. If you build a great building envelope, distribution become a non-issue. I would also argue that since loads are so low, proper duct design no longer matters. Specifically, my own home and my neighbors (that I retrofitted) have minimal HVAC distribution, dramatically undersize AC systems, and yet they remain comfortable all year long. And my neighbors envelope isn't really that impressive. Just dense-packed full cut 2x6's. My own house is foam board R-28 walls with some cavity insulation, and various roof r-values, though all with continuous foam board. 
 
I believe that decades of poor building envelopes have caused us all to focus a lot on proper distribution. I say build a good envelope and distribution concerns go away.
Posted @ Friday, April 22, 2011 9:20 PM by Robert Susz
Hi Allison, 
 
 
 
We install predominatnly radial designs with small surface area typical 2000 square foot house might have only 6 ducts. Grill velocity at 500-700fpm,with noise less than 25decibels. Our systems typically have total external static pressures in the .25IWC range, for 1.5 ton systems Watt draw at about 160W, duct leakege below 20CFM25, and less that 3 degrees temperature stratification ceiling to floor. And annual heating cooling cost of less that 20 cents a square foot for 5600 HDD, on existing homes. I prefer radial design because we have better access to balance the system, and remove the temptation for the homeowners to adjust airflow at the registers. I think the point is that poor design is poor desing. In our neck of the woods trunk and branch systems are favored and they have pathetic performance based on the way that they are installed, they must have went to the same class as the ductopus installers. The goals are great performance. Low static, low energy consumption, balanced system that effectively mixes the house is whats needed, I will propably keep on doing that with a radial design.
Posted @ Friday, April 22, 2011 9:26 PM by Gavin Healy
Robert, I'm usually on your side of this issue, constantly having to reassure HVAC contractors who don't believe a design airflow of 0.20 or 0.25 CFM/SF can possibly work. I have to convince them to quit worrying about register throws and forget about perimeter outlets. Old habits are hard to break. 
 
However, I respectfully disagree that distribution issues go away with a good envelope. 
 
I notice you live in a heating dominated climate. In a small high performance home with an open floor plan, I can accept that unbalanced airflow (in heating mode) is less of an issue for many folks. In my own home, I keep the supply vent to the master bedroom shut all winter. We like it cool at night. A little extra heat in the bathroom helps. 
 
However, in a cooling dominated climate, many people demand a cool bedroom in the summer. Even if peak loads are relatively small, you can't just dump conditioned air in the main living area and expect it to make its way to the bedrooms across closed doors.
Posted @ Saturday, April 23, 2011 12:03 AM by David Butler
David, a 9 or 10 ft plate(pressure boundary) with dropped ceilings in minor rooms and hallways sounds good to me. 
 
The passivhaus air delivery system that you designed/described makes sense to me also. 
 
I think a similar concept could be adapted to many floor plans...I think you are using the imported air handler in the way it was intended...A very positive aspect is that the air handler and the ducts are all in the same horizontal plane...It eliminates the First hard duct turn that comes with upflow air handlers.....Also it(the unit) is not taking up any floor area. 
 
 
 
This type of unit is very compact horizontally AND it appears to be serviced from the bottom. 
 
I imagine the unit could also be located in the top portion af a walk-in storage closet or even "IN" a hallway.... the service hatch would look very similar to an attic portal. 
 
 
 
question... would you spec the same type of equipment and duct layout for a HalfassivHaus? A high performance home that was affordable (HERS 50) but not up to PH snuff? 
 
Posted @ Saturday, April 23, 2011 7:44 AM by John Brooks
I am working on a course for duct design. What I have found is that the installers need to know the results of every twist, droop, turn and split off. Can they also calculate FPM per duct diameter? 
 
I guess years and years of sending the beginners out to install duct systems is proving unworkable. LOL!
Posted @ Sunday, April 24, 2011 11:51 AM by Christopher Cadwell
Hey Christopher, be sure your duct course addresses low velocity. Most contractors, when confronted with the lower airflows that result from a super insulated home, seem to think that the air will somehow get 'lost' in the ducts if the velocity is too low. 
 
This issue came up when we were working on last revision of Manual D. Hank added the following comments to address this concern: 
 
Table A1-1, last bullet "Low velocity is not an issue" 
 
Section A15-5 "There are scores of things to worry about when designing and installing a comfort system. Low velocity though a duct airway is not one of them." 
 
Unfortunately, I few HVAC contractors actually read the book. The key to fixing this problem is through training. It's not the installers I'm worried about. It's the designers, the owners, and unfortunately, many of the 'old school' trainers. We need more folks out there like you who are not stuck in the past.
Posted @ Sunday, April 24, 2011 12:24 PM by David Butler
Yeah, I know what you are saying. You got it. 
 
Also, so where does the low velocity lost air go, if it gets lost with a super tight duct system? LOL! 
 
What they really should be worried about is low velocity out of the register. I routinely see veteran mechanical engineers using registers that only have 250 Fpm velocity out of the register. That is just guaranteeing stratification. I just show them the engineering data from the grill manufacturer that states residential should be 500-750 fpm out of the face.  
 
With lower airflows then you end up with 6x6 or even 4x8 registers in bedrooms. Looks funny but saves money. The air comes out fast enough to mix the air in the room. 
 
Also what they, worried contractors, have not done is tested installed systems and found that most systems are only delivering 275-325 cfm per ton or .75 cfm per sq ft. Just by following the manufacturers sizing recommendations for air filter and return duct sizing gets you this poor result (too small of an area really). This already low air flow is on houses that have no special shells, and energy hog duct systems. The registers are way over-sized and the air does not mix well.  
 
Therefore if we reduce the heat load btuh per sq ft ratio by 60%, with a great shell and ducts, then the airflow should follow, as not as much air needed at all. Like less than half of the previous design or .5 cfm per sq ft or less. Just make sure the registers are sized right to mix the air. The quantity of air is so low that you can barely hear it anyhow. 
 
I have not specifically read Man J, guilty. I have read Man N, commercial load calculations, which is probably more in depth. 
 
Good stuff.
Posted @ Sunday, April 24, 2011 1:50 PM by Christopher Cadwell
I do recommend you get the J book. It's big, but mostly tables and examples. Not that much to read, really.  
 
I haven't read N, but commercial loads are usually dominated by internal people and equipment loads and ventilation. Not so much emphasis on shell. Not sure when N was last updated, but J8 completely revamped algorithms for radiant glazing loads (AED), infiltration and duct loads, as well as support for ICFs, SIPs and encapsulated attics.
Posted @ Sunday, April 24, 2011 2:06 PM by David Butler
Yeah Man J is on the list. Man N latest version does focus on the shell heavily, very similar to Man J, with AED, infiltration and duct loads. It sounds very similar to Man J, with mostly charts and references and an intense couple of first chapters. 
 
I use Rhvac for load calcs for residential. It does all that thinking. I do make sure the settings are correct, and manually set a lot of factors. It has worked out very well.
Posted @ Sunday, April 24, 2011 2:23 PM by Chris Cadwell
Christopher, regarding register throws, when you have bedrooms that only need 40 CFM or less, it's hard to get decent face velocity. The first time I encountered this problem I consulted with several trusted sources, including Hank R, and Dave Fedders (now retired technical guru at H&C). Everyone said the same thing: When the envelope is that good, face velocity and register throws become less important. Also, right-sized equipment system will have longer run cycles, thus keeping the air moving (especially with heat pump primary).  
 
Still, you want to select diffuser with smallest possible Ak factor in cases like this. I like the RoyalAir 531 floor register. It comes in 2x10 with Ak of 0.06 (!) so you can get a decent face velocity down to about 25 CFM.
Posted @ Sunday, April 24, 2011 2:32 PM by David Butler
Good points David, on the run times, etc. 
 
Thanks for the info on that grille. Normally I just dont even supply unless there is 75 cfm or more needed. Yeah the problem is the bedrooms, where I dont want to leave out a supply. I will have a use for this grille. 
 
What exactly is "Ak"? I assume that is design static difference of the grille in Iwc.
Posted @ Sunday, April 24, 2011 2:39 PM by Christopher Cadwell
you wrote: 
"I just dont even supply unless there is 75 cfm or more needed" 
 
Hmm.. with many "just" Energy Star homes I've worked on, I'm seeing 900 to 1200 sf per ton. In that case, only the largest rooms would need that much air. My normal cut-off for not providing a diffuser is about 15 CFM, like a walk-in close with no exterior wall. If, for example, a bath needs 25 CFM, I use smallest register and just don't worry about face velocity.  
 
Ak factor is the effective area in square feet. Divide CFM by the Ak factor to get face velocity. For example, the 2x10 has a gross opening of 0.139 SF, but the effective area (Ak) is 0.06 SF. See this paper by Dave Fetters: 
http://bit.ly/hHASdY 
 
The engineering section of the Hart & Cooley catalog has Ak factors for all of its products (except wood grilles, which should be banned).
Posted @ Sunday, April 24, 2011 3:00 PM by David Butler
Thanks for that on the Ak factor. I had to interpolate my own with TruAir to get some of ours. 
 
Hmm. I have about the same buth sizing per sq ft.  
 
Yeah, ...  
 
I use Gavin's philosophy on supply ducts. 2000 sq ft home only gets 6-8 supplies. This means almost never supply to bathrooms or laundry rooms, and for sure never to closets. 
 
I think even if you calculated those rooms as partitions, loads are so small and the transfer from partition walls and convection is sufficient to maintain a low temperature variance. 
 
In the end it is a matter of preference really. 
 
I do it mostly for minimizing duct surface area, and I have never had a comfort complaint yet.  
 
Keep in mind I am installing them for energy conscientious consumers. I do make a note to them and explain that no-one really "lives" in those spaces. Also I can install bath fan motion sensors, that can pull in btuh from the main house as well while the room is occupied. 
 
I know my customers, as in retrofits. I dont do new homes, where I cannot enlighten the customer on the duct design. I do talk about how they use those rooms and if they have any special needs (loads). 
 
There is just not enough scientifically tested results yet on this type of design, other than I know installers such as Gavin myself, and a few others who have been doing this for years and have not had any issues.  
 
Actually it would only take a few hobos. I am going to document this someday. 
 
Great conversation.
Posted @ Sunday, April 24, 2011 3:19 PM by Christopher Cadwell
> I dont do new homes 
 
Yeah, it makes a difference when you can have conversation with client.  
 
I only do new construction, where omitting supply from a bath is not an option. That being said, with current market conditions, I'm often able to interact with the actual owners. Spec homes are a thing of the past.  
 
In the passive home project referenced in my earlier comment, I omitted supply in half-bath. Instead, there's a stale air exhaust for the HRV, which will pull conditioned air under the door. In general, I prefer to install ducted returns in large closets rather than 'waste' supply air. For small closets, I encourage owners to use louvered doors to promote circulation, a tip from an old-timer (not that I'm that young).
Posted @ Sunday, April 24, 2011 4:23 PM by David Butler
I am new to this blog but not to this topic. Want to say this is a great discussion, people who analyze Manual D are all too few.
Posted @ Monday, April 25, 2011 10:31 AM by M. Johnson
Hi: We're thinking about having a high velocity AC system installed in our 107 year old soon-to-be-landmarked house in Chicago. We'll probably use a SpacePak system because we've got radiator heat and no ducts. The AC system will cool the first and second floors. (We're probably going to deal with the currently-unfinished attic / third floor separately.) We're talking to a very experienced SpacePak contractor. However, I'm a little worried because the contractor's tentative plan calls for only a single return -- at the top of the large front stairway. Is a single return like that enough? Or, should there be multiple returns, as with a conventional system. I'm particularly worried about non-return from the kitchen. The kitchen has big windows that get lots of afternoon sun. It also is at the back of the house, and pretty far from the main stairway (and accessed by two 30" doors). So, there won't be much air moving from the kitchen to the restu of the house for return purposes. Should we, somehow, try to put a supplemental return in the kitchen? Or is that not necessary? (I'm not sure HOW we'd do this regardless. But, I'd like to know WHETHER we should try.)  
 
 
 
Let me also say that the tenative plan is for some of the main ducting to be in an uninsulated part of the third floor. I know that the ducts-in-uninsulated-spaces idea is no-no to you. Thanks
Posted @ Wednesday, August 08, 2012 2:18 PM by Paul Wangerin
As many will tell you, avoid having a return in the kitchen. I believe it violates code most all places. 
 
Although I am not an HVAC pro, I studied the subject intensely for several years. Was active on HVAC-Talk board until they virtually banned non-professionals. At present you are invited to present any question on that board as a homeowner, and one of their pros will answer. But a non-pro cannot say anything. 
 
You really want to make sure there is adequate path for air to get to that return. Especially if doors are shut. Having about a 3-inch undercut is about enough area, but that is ugly and nobody does that. You will be needing some kind of transfer duct or vent, and that will enable a good design to be possible. 
 
Without that return air path, you might get chronically high or low pressures in particular rooms, and depending on the outside climate might create moisture problems. 
 
Hope this helps -- M. Johnson
Posted @ Wednesday, August 08, 2012 2:25 PM by M.Johnson
@Paul, a single return is actually preferable as long as there's a low-static return path to each supply. In multi-floor homes, a single return encourages air mixing between floors. The distance between supply and return is irrelevant. However, there must be an adequate return path from each room that can be isolated from the return. This can be achieved with jump ducts and/or transfer grilles. See BSC's report for sizing guidance: http://bit.ly/oQtlYh 
 
For bedrooms, you need to consider noise propogation. Tamarack makes a baffled transfer grille that greatly reduces noise (must be sized ~50% larger than indicated in above report). Insulated flex jump ducts also do a good job dampening noise. 
 
Of course, a fully ducted return system can be made to work, but it would increase the already high static of your high velocity blower. This would either reduce system airflow or increase blower energy (or both), depending on whether blower is constant CFM, constant torque, or PSC. 
 
In any case, as M.Johnson said, ducted returns are a bad idea in kitchens. If the kitchen can be isolated from the main return, then install a transfer grille. Direct returns should also be avoided in baths and laundries, but the design CFM for these rooms is typically small enough that a nominal 3/4" door undercut is sufficient. Door undercuts are good for up to 2 CFM per square inch of gap.
Posted @ Wednesday, August 08, 2012 2:52 PM by David Butler
Sorry that I inadvertently stepped into the "professionals" versus "homeowners" issue. I should have known better because in my own profession -- I'm a law school prof -- we have something very, very similar. Again, sorry. 
 
 
 
Also: In the olden days -- late 19th, early 20th centuries -- the doors between the kitchens and the rest of the house were always kept closed (to keep kitchen germs from spreading). We now keep both doors between the kitchen and the rest of the house open most of the time. So, it sounds like I'll have enough circulation from the kitchen. It's just that I was concerned because I've seen countless examples of numerous return vents in convention systems.
Posted @ Wednesday, August 08, 2012 3:12 PM by Paul Wangerin
Paul W.: Pros vs. homeowners is not an issue here. M. Johnson was talking about the HVAC Talk forum. Both he and David Butler have given you good advice here, and I agree that you don't need to worry about having a central return. Also, if you do have ducts in an unconditioned attic, which is often unavoidable in existing homes, make sure to keep them low in the attic, right above the insulation. If you're in a humid climate, don't bury them in insulation because you could have condensation problems.
Posted @ Thursday, August 09, 2012 6:48 AM by Allison Bailes
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