Duct Branching

[pjsullivan]

Dear Mr. Wiggins:

I am a professional real-estate inspector. I own my own company (visit my web site if you like). I am not a building inspector. I do many different kinds of inspections ... all the way from slab to finished product, and I am experienced and educated in many different aspects of residential and commericial construction. I am a "general practitioner" ... you are a "specialist". I know a few things about many subjects, whereas you know many things about a few subjects. I accept your apology, and I apologize for that "last laugh" jab.

I have a report to write ... What should I say about this duct design? Here's a first cut:

"The HVAC ductwork in the photo, although branched correctly, may not provide adequate airflow to the smaller diameter sections of the branches. Recommend a qualified HVAC professional evaluate and determine a course of corrective action."

I would like to be more specific about a remedy that uses the same materials. However, I do not wish to "pi** off" the builder's pro.

Any takers?

[Irishmist]

important clarification: tin fab's reference to "static regain method" was referring to my reference to a "graduated ductwork system" utilizing all rectangular fittings. Please don't confuse the system we are referring to with what is shown in this picture. We are talking about two entirely different systems. Correct me if I am wrong tin fab.

[Steve Wiggins]

(w) Specific limitations for ducts and vents. The inspector is not required to do the following:

(1) determine the efficiency, adequacy or capacity of the systems;

(2) determine the uniformity of the supply of conditioned air to the various parts of the structure;

(3) determine the types of materials contained in insulation, wrapping of pipes, ducts, jackets, boilers and wiring;

(4) operate venting systems unless ambient temperatures or other circumstances, in the reasonable opinion of the inspector, are conducive to safe operation without damage to the equipment; or

(5) operate a unit outside its normal operating range as reasonably determined by the inspector.

[pstu]

PJSullivan wrote:

>>I have a report to write ... What should I say about this duct design? Here's a first cut:
>>
>>"The HVAC ductwork in the photo, although branched correctly, may not provide adequate
>>airflow to the smaller diameter
>>sections of the branches. Recommend a qualified HVAC professional evaluate and
>>determine a course of corrective action."
>>
>>I would like to be more specific about a remedy that uses the same materials.
>>However, I do not wish to "pi** off" the builder's pro.

Like you I am in S.Texas, we are almost close enough to be neighbors. One of my good friends lives in Round Rock. I am a homeowner who tries to educate himself, perhaps you will forgive me for asking an ignorant question.

PJSullivan, I do appreciate an inspector who learns more than the minimum, and want buyers in general to be well protected against defects. But there is such a thing as raising false warning, and that is a holy PITA to the seller. During a recent home sale I had the unpleasant experience of an inspector raising false warnings. I want to ask you, what exactly is the basis for raising this as a homeowner problem?

I submit that the actual method of duct construction is not sufficient to say it "may" not provide adequate airflow. Of course if one of the duct experts such as Dash will give a contrary opinion, I will gladly defer to his judgement.

Requesting a Manual D analysis of the duct system would give professional evidence of its adequacy or inadequacy, but is that the norm anywhere in home inspections? Directly measuring supply airflow with a flow hood would also be professional evidence. But such a thing is so out of the ordinary that I have to wonder if it is really within the professional norms of the inspector.

It seems to me you have an equal basis for alleging any other deficiency of the HVAC system. How do you know the system is sized to the house's cooling or heat load? How do you know the airspeed is under the 700 fpm limit stressed by Manual D for flex duct? In each case the homeowner would benefit from having the system tested (except for cost of testing), but what is the basis for raising the original accusation? I submit that there is not enough basis.

My skepticism is colored by the fact my own attic ductwork resembles the picture you showed. I am not under any illusion that my house has great ductwork, but after several HVAC guys have looked it over, they have found various things to criticize but I don't remember any raising the objection you have. I have actually had measurements of supply duct flow made, and they are reasonable. After addition of some much needed return ducting, ESP for the system is in the range of .45-.60 inches of water column depending on filter loading, about .35 if the furnace is on low speed. Not ideal but with a variable speed fan not a source of big problems either. It seems if the construction method were going to cause pressure drop and airflow problems, the measured ESP would be higher and/or airflow would be lower.

Just a devils advocate thought. I am sincerely interested in learning what is right.

Best wishes -- P.Student

[Edited by perpetual_student on 01-08-2005 at 10:27 PM]

[pjsullivan]

First of all, I appreciate Mr. Wiggins for reminding me of TREC's standards of practice about what I am not required to do. Just because I am not required to do a thing does not mean that I am not free to do a thing; provided of course that I am well grounded in what I write. I know inspectors who often learn about a subject thoroughly, and offer opinions based on their well researched knowledge; however, they (and I) never imply that we are a professional. In fact, I always defer to the professional in my reports.

I'm simply trying to determine if something is installed correctly. The customer raised the issue, not I. He wanted me to research the subject, and see what I could find out.

As I'm sure you are all aware, homes in this part of the country are literally thrown together by immigrant workers who cannot even speak English. In fact, in my county, if you want a job as a building superintendent, you have to know how to speak Spanish (a sad situation). So, as a result, many homebuyers harbor a deep mistrust for builders because the customer rarely gets what they want because of miscommunication, etc. So, when the customer saw this (he's an engineer), he asked what I thought. I said "I don't know". He asked if I could find out if it's ok. I said "I would try". Hence, my question. I'm not trying to be a HVAC pro. That's why I came here. Isn't this forum for people who want answers to HVAC questions?

How about this ... forget the fact that I'm a home inspector. Pretend that this is my home. I have this ductwork. I'm concerned that the airflow may be inadequate. I have come here requesting a professional opinion about whether there is a concern or not. So far, I have received many different replies ... some say it's ok ... some say it's not. I've already spent way more time on this than I wanted to. Can anyone help me, or should I go somewhere else?

[pstu]

If you really want to learn what makes good duct design, you can buy the ACCA Manual D book for about $40. I did, it's not an easy read but the info is in there. What I expect you will find is that sizing is more important than type of construction. And I will say again that size needs to be measured against the job the HVAC system needs to do, how can you criticize the duct system without equally examining the AC system, and how can you criticize that with no knowledge of the heat load on the house? Lotta questions that are easier to ask than to answer.

Hope this helps -- P.Student

[-80guru]

PJ, how did you get in my attic? Seriously, That method used in the picture in my opinion is an abhorent way to install flex duct work. The Y branching method is fine if installed correctly. Is that amalgamation of ducts that I see installed correctly? NO! there is your answer. Is it going to be a problem? That is not a question anyone can answer except the technician who does an airflow analysis on that duct system. So tell your client to hire an HVAC professional to perform an airflow analysis. If that is not acceptable then you or anyone else cannot honestly answer if "It will be a problem".

[Steve Wiggins]

Originally posted by pjsullivan


"The HVAC ductwork in the photo, although branched correctly, may not provide adequate airflow to the smaller diameter sections of the branches. Recommend a qualified HVAC professional evaluate and determine a course of corrective action."

PJ, that statement is horribly inaccurate. You are going to have an a/c contractor dead in the middle of your xxxx.

Part of the problem here is there is no design requirement on the blueprints. You will find all sorts of other trades design but no hvac. The builders refuse to pay a design professional to draw out good design. Therefore all the decisions are left up to the installing contractor or his subs.

This notion you have of the smaller branches not getting enough air is weak. How do you know how much air should be coming out of the supply register? Even if you measured it with a balometer you still won't know because there is no requirement for that to be listed on the blueprints.

[mark beiser]

What it comes down to is whether the duct system was designed and sized properly for the fittings and materials used. I'll bet any of you a weeks pay that it is not, and the installer just tied all that together as the most convenient place to do it, using the least materials.

Ductwork run like that is very typical here in north Texas, and in my experience, is the cause of a lot of airflow issues.
Any time I see multiple wyes that close together in flex duct like that, I find badly uneven airflow.

At least they used metal wyes, those ductboard triangle things are total crap, and should be outlawed, lol.

[pjsullivan]

Thanks so much -80guru. I followed your advice, and informed my client that if he still has a concern regarding the ductwork, then he should contact an HVAC pro to perform an airflow analysis.

And thanks sincerely to all who replied. I will definitely be coming back here whenever I have HVAC questions.

[johnl45]

Originally posted by pjsullivan
Thanks so much -80guru. I followed your advice, and informed my client that if he still has a concern regarding the ductwork, then he should contact an HVAC pro to perform an airflow analysis.

And thanks sincerely to all who replied. I will definitely be coming back here whenever I have HVAC questions.

Now your acting like a true professional, good job.

[pstu]

A question I hope one of the more expert people will answer: Is the problem with this picture, partly that the Wyes are so close together? Can anyone explain why triangle boxes degrade airflow?

What can you see that will degrade airflow? Can you describe in a few words how it would be built better?

If there were a clear and simple answer, and the price were not thru the roof, I would probably pay to have it corrected in my own house.

Thank you -- P.Student

[dash]

There is nothing wrong with the wyes,or even tri-boxes,IF they are accounted for in the Man. D ,duct design.

In this case we have no information,to make a decision,as to what the airflow will be.

If you want to redo your duct system,it's Man. J,room by room,then a redesign using Man D.This will allow you to bring the ESP down to whatever you select,and provide even temperatures throughout the home.

[tinner73]

flex duct degrades the airflow. it is not smooth like the metal pipe. the smoother and squarer the duct, the better. the part here that gets me is even if you went with CHEAP round "stove" pipe it would be way better than this. it would be fairly close in price and you wouldn't need a
tin-shop. this way saves on labor mainly and what's more important than their bottom line anyway. i'm happy they don't do this around Chicago.

[pstu]

Dash, I have always consistently heard that message from you, "Manual D is the only way to be sure". Thanks for reassuring me that the universe is governed by natural laws and not by whims.

Although after the system has already been built, would not a flow hood test of each supply register (and maybe the returns too) give a valid after-the-fact answer?

Best wishes -- P.Student

[dash]

Originally posted by perpetual_student
Dash, I have always consistently heard that message from you, "Manual D is the only way to be sure". Thanks for reassuring me that the universe is governed by natural laws and not by whims.

Although after the system has already been built, would not a flow hood test of each supply register (and maybe the returns too) give a valid after-the-fact answer?

Best wishes -- P.Student

Testing would give you the total airflow delivered,thru the supply and return ducts,less any duct leakage.

This is valuable information,but you need to know what each rooms cfm should be(Man J),and what is the airflow thru the equipment(ie. without leakage).

[dash]

Originally posted by tinner73
flex duct degrades the airflow. it is not smooth like the metal pipe. the smoother and squarer the duct, the better. the part here that gets me is even if you went with CHEAP round "stove" pipe it would be way better than this. it would be fairly close in price and you wouldn't need a
tin-shop. this way saves on labor mainly and what's more important than their bottom line anyway. i'm happy they don't do this around Chicago.

Flex is not a problem,IF the Man.D design is for flex.Flex does not degrade airflow,poor design and install does.

[conrad1]

You are on the right path /do have an HVAC company check the design airflow airflow airflow 2nd only to unit size in importance the pic you showed actually looks better than most but that does not mean it is sized correctly to the unit or that the unit is sized correctly to the bldg. location have a manul j and manual d calc done

sorry dash I had not read the 3rd page before I posted you already settled it apologies for the redundancy

[Edited by conrad1 on 01-09-2005 at 11:59 AM]

[dash]

Looking at the photo,I'd check to make sure the wyes are insulated properly.

[tinner73]

dash...my arch nemesis...please tell me why an all flex system would be better than an all metal one.