by • September 4, 2014 • UncategorizedComments (10)16475

Rookie Builder Mistakes Part 2 HVAC

This is a topic that’s hard for me to cover in one blog post!  I’m coming up on my 20th year as a professional builder and I think that my understanding of HVAC design & execution is something that I’ll always be learning.  So, let’s start with some obvious mistakes that aren’t even visible to most builders or buyers.

Mistake #1 – Oversized Systems.

I would venture to guess that 75%+ of the homes built in America this year will have HVAC systems that were designed on the first day of HVAC rough-in.  The HVAC sub took the house’s square footage and divided by his sf/ton ballpark number and started to run ducts all in one day.  Here’s an example from my area here in the Hot/Humid South.  Most AC subs use 500sf/ton of house so a 2000 sf ranch style house would typically get a 4 Ton AC system installed.  The problem with this is that every house has different orientations, different windows/doors, and very different insulation/air sealing than a house built even 5 years ago.  Most houses in Texas have too large an AC unit for the house’s normal loads and so that 4 ton machine in a 2000 sf house might actually be 1 (or even 2) whole ton(s) too large.  Too large of an AC means that on most days the AC will cool the house rapidly (maybe only needing 7-10 minutes to cool) and all those short cycles are terrible for dehumidification, will shorten equipment life, and are using more energy than is really needed to cool the house.  Longer equipment run times are actually better with AC compressors.  See this great blog post by my blogger buddy Allison Bailes for a detailed analysis on this subject.

The Smart Builders know that their houses need a true HVAC design done by an ACCA accredited professional.  I have a third-party HVAC designer do an Energy Model and Manual J (load calculations) on every house I build.  Then I take those to my HVAC contractor to solicit a bid.

Mistake #2 – Bedrooms with no Returns

Every room in the house that has a duct needs a path for that air to get back to the AC return.  In the past, Builder have relied on the under-cut of the door to provide a return path but that’s a rookie mistake.  If you were to calculate the supply & return from a bedroom you’d be surprised at how high you’d need to undercut that door!

Smart builders insist on either Ducted Returns in bedrooms or “Jumper Ducts”.  See the picture of a Jump Duct in one of my houses.

12x12 Register in the ceiling of a Media Room connected by flexduct to a 12x12 Register in the hallway acts as a return air pathway

12×12 Register in the ceiling of a Media Room connected by flexduct to a 12×12 Register in the hallway acts as a return air pathway


Mistake #3 – Air sealing & Weatherization

Weatherization & Penetrations.  It’s a rookie builder mistake to let these type of things happen on your houses.


The Smart Builders know that trades must complete their work before claddings are installed.  He’s a great flashing tip for you too.  I order a Quickflash HVAC boot for every line set that penetrates my WRB.  I usually have them onsite for the HVAC subs to install.  They run about $30 each but make a perfect weather-seal at freon line penetrations AND they look alot better than the alternatives.


Quickflash Boot does a great weather seal around the line-set!


The stucco cladding looks great up against this boot. They are paintable, but I’m always hesitant to paint plastic. Black looks just fine most of the time.


Rookie Mistake leaving it up to the installer to figure this out!

Mistake #4 – Ducts outside of conditioned space

Here in the Southern US we don’t typically have basements so you’ll find most HVAC systems either in hot attics or in smelly crawlspaces.  Putting ducts in these locations is a major Rookie Mistake.  Think about this for a minute, your AC cools the air down to 60 degrees only to send that air through ducts in your attic that might run 120+ degrees (and that’s with a radiant barrier installed).  The ducts are insulated to R-8 but there might be a delta of 60 degrees between inside/outside those ducts.  Also, remember that all ducts have some leakage.  Well built new houses might leak 5-10% (older ducts often are tested above 20% leakage).  So, if your 1600 CFM HVAC system has 10% duct leakage you are losing 160 CFM to the hot attic every time your blower turns on.  This loss also depressurizes your house and that 160CFM deficit is often made up by air blowing in around ceiling registers, under doors, and backdrafting through bath exhaust ducts.

Ducts in Hot Attics is a major rookie mistake.  Don't put ducts outside of your envelope!

Ducts in Hot Attics is a major rookie mistake. Don’t put ducts outside of your envelope!

Smart builders either bring their ducts into the conditioned space, or they move the envelope outward so that the ducts are inside.   There are a myriad of ways to bring ducts inside the envelope but this needs to be thought through before construction begins!

Spray foam at the roofline is an easy way to encapsulate your attic and bring ducts inside.

Spray foam at the roofline is an easy way to encapsulate your attic and bring ducts inside.

Stay tuned for part 3 of my series on Plumbing mistakes…


Matt Risinger
– Risinger Homes in Austin, TX

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Risinger Homes is a custom builder and whole house remodeling contractor that specializes in Architect driven and fine craftsmanship work. We utilize an in-house carpentry staff and the latest building science research to build dramatically more efficient, healthy and durable homes.
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  • Cameron Taylor

    I just found your blog, Matt, and already like what I see. I’m also a Texan, although in DFW vs. Austin. But the mistakes you’ve brought out here are ubiquitous in residential construction. Keep up the good work calling for improvements to our buildings!
    Quick question: my brother lives in Austin and I first saw the metal flex duct used around the refrigerant line set at his house. Is that an Austin builder thing? I’ve never seen it used anywhere else, even in typical RNC here in DFW.

    • Cameron, That flex duct boot is pretty common here in Austin. I assumed it was the same elsewhere. Where in DFW are you building? I’m considering a project in Dallas, I’d love to get your take if you have a minute. Matt

      • Cameron Taylor

        Matt, I saw you at summer camp this year but did not have a chance to introduce myself before it was over. I am not a builder but I am intimately involved with buildings, particularly a museum who keeps me on board to maintain its critical interior environment and building related infrastructure. I have built a residential structure and have renovated several others in my time. I am also an adjunct HVAC instructor at our county’s community college, teaching residential system design, including Manual J. My interest in building science has multiple motivations…just sorry I can’t be of better help regarding the DFW builder market….although I think you’d do well here with your emphasis on building performance.

  • Ian Stringer

    Great article. In Canada the Job cannot begin without an approved heat loss calc and duct design. Spray foaming the attic is a good option, however this increases the condition-able area considerably. This will probably solve the over sizing issue but will also cause oversized utility bills. In my opinion attics are best left as attics and architects should design a home with an approved HVAC design in hand. This will also eliminate the ” I cant have a bulkhead there!” conversation.
    Just my two (Canadian) cents worth.

    • Ian, Always great to get a viewpoint from our Northern Neighbors. Remember however that most of my blog posts are oriented to our Hot/Humid climate zone so take what I say and filter it with your Building Science Critical Thinking sieve to see what’s right for you up there. Keep reading and commenting! Best, Matt

  • Andrew Taylor

    Matt – I’m curious about who you use for your HVAC designs? I’m in the Houston area and would like to talk to your designer if you don’t mind recommending him.

    • Andrew, I use Positive Energy here in Austin. They do work all over the country and are very reasonable for excellent design work.
      Can’t recommend them highly enough, but don’t throw them too much work as I need them for all my houses! Best, Matt

  • Sammy

    Hi Matt!
    If you could see my attic you’d just die. The HVAC system for the upstairs is up there (the system for the main level is in the crawl space), and it’s a bloody mess! We have continuos soffit venting, along with a ridge vent, that seems like it does no good moving the hot air up and out (I live in Alabama). I’ve been researching attic ventilation for a while and considered roof fans, but it seems like they’d just pull conditioned air from the house into the attic— and then right outside! I really like the idea of bringing that space into the envelope, but is it an easy retrofit? Also, I’m assuming this newly encapsulated space would have its own register, return, and that a tight seal would be required around areas like pull down attic stairs?
    Thanks you kindly!

    • Sammy, That’s a pretty typical attic for the South. Yes a retrofit is possible, but you’ll need a good contractor who knows the ins/outs. Including combustion safety of any gas appliances! I would definitely recommend NOT doing the power attic vent. Conditioned attics are the best, but you can have unintended consequences if not done properly. Another option might be to duct blast your existing duct work to see how leaky it’s performing. Then do duct sealing, sealing of any/all penetrations to the ceiling line, add additional insulation, and possibly adding a radiant barrier. Should be a less costly version of an energy upgrade than going to a conditioned attic. I wish you the best, Matt

  • I agree that in doing this king of project or installation the best way to call is the experience one not a first timer.It is best to rely on the experience one to avoid so many mistakes.