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.
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.
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.
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!
Stay tuned for part 3 of my series on Plumbing mistakes…
– Risinger Homes in Austin, TX
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