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. Continue Reading
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