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Ask The Expert- Need To Change My A/C System

July 28th, 2008 · 5 Comments · Ask the expert, Green Houston Topics

We were sent excellent question this past week and one that I personally believe that about half of the entire City of Houston probably has this time of year.  Clayton Farmer with Alternative Power Solutions and Kathleen Reardon with RDArchitecture both hopped in to answer right away…

Here was the Question:

Hi, I need to change the Air conditioning / Heating system in my home but I would rather consult with someone besides a salesperson that is simply trying to sell me a product instead of a real solution. I have a thirty year old two story home that has the typical problems of a system that was installed in an era of cheap energy (i.e. very inefficient) but I think its overall operation could be improved with the right system. I am hoping that you could refer me to someone that could help me find a good, affordable solution. So, if you know someone, could you either forward this email with your comments or reply back and I would appreciate it very much

This is a question that has several different answers to it and one that is not answered easily.  Kathleen is quick to mention that it involves the Envelope of the Home (air that leaks into and out of the home) along with the A/C Ducts, etc. Here is Kathleen’s explanation:

The best solution to this one is a combination of things.  Make sure the duct system is not leaking and is well insulated.  If not – repair it, or consider replacing it.  Ideally duct work insulation should have an R value of 8.  Install a new a/c system with 16 SEER or greater.  And then make sure the home is well insulated, walls and attic.  Check for air leaks between the air conditioned space and attic or outside.   I’m not sure about the affordable part – I think just put the specifications together and shop around. A good a/c company will be able to do a manual J form to size the system well, which is important.  A system that is too big will not provide dehumidification.

Clayton replied with a lengthy and extremely informative response that brings up a bunch of good points so I will post the whole comment here:

I certainly agree that it is best to consult with a professional who understands the principles of building science and HVAC load calculation & performance rather than a sales person who is most likely not familiar with these concepts.

Houses built prior to mid 1990’s exhibit a large amount of air infiltration primarily due to an insufficient air barrier. Evidence of this can be easily found throughout the home. It is also true that these homes have large amounts of HVAC system leakage from the inefficient design and inadequate installation of the ductwork, air handlers, and plenums. An energy audit can help identify these locations and provide a plan of action for addressing all significant deficiencies. Typically, these homes suck in a great deal of outside air thru the air infiltration points when the HVAC system is running because the home becomes negatively pressurized and the neutral pressure plane rises to the top of the home. This is most commonly due to duct leakage, air flow constriction and static pressure. A quick explanation: your air return draws air from the home into the HVAC system thru the ductwork into the coils and out thru the supply ductwork back into the home. Along the way some of this air is lost and expelled into unconditioned space as well as being slowed down by turns in the duct work and long runs to the extremities of the home. The outside air must be sucked in to equalize the pressure imbalance from the outdoor environment. This greatly effects indoor air quality, HVAC efficiency, and utility usage. A well installed HVAC system will help lower the neutral pressure plane  within the home when the system is running. When your new system is installed, make sure you request a “duct leakage test “ or a duct blaster test” to ensure your HVAC system has less than 5% leakage.

The next thing to consider is system size, system efficiency and runtime. The primary purpose of an AC system in this climate is to remove moisture within the home, a byproduct of this is cool air. Having an oversized system (which is very common on homes older than 10 years) causes the unit to “short cycle” (running for 15 minutes and shutting off for 5-10 minutes then kicking on again and repeating the cycle). This “short cycling” causes the home to meet your temperature preferences before it sufficiently removes the moisture. It also causes hi utility bills since the amount of power required to start the stationary compressor in the AC condensing unit is massive compared to the amount of power it actually takes to run the system after start up. It is similar to automobiles in that you get better gas mileage on the highway than in the city because of all the starting and stopping.  I would recommend having your contractor resize your system using current residential load calculation software. This will minimize “short cycling” and maximize moisture removal within your home. Since you are planning to change the system then it would be a great time to upgrade to a more efficient unit(s). However, it does not make a lot of sense to install a good high efficiency unit(s) if the home itself is inefficient. It is important to ensure that the thermal barrier is aligned properly and the air barrier is performing at peak efficiency for your home which will help maximize the efficiency and value of your new HVAC system. This involves other contractors and very reasonable cost but will greatly benefit the home and the HVAC system by reducing thermal loss and air infiltration. Correcting these type of deficiencies will increase the overall value of your home, lower energy usage, and reduce payback time.

The above is just a brief description of things to consider while undertaking your project. My goal was to be informative and brief, I can certainly provide much more detailed and technical information if you would like.

I have had the pleasure of working with many terrific HVAC companies during my time in the home building industry. I can definitely recommend a few that are well trained, educated on current technologies and building science concepts, and most importantly honest with great customer service. I am not sure if it is appropriate to recommend specific individuals through this unbiased informational site. I will leave that up to the moderator. Please feel free to contact me at the below number/ email if there is anything I can help you with. Thanks you for contacting us and I hope we have been helpful.


Air Team- Darrin Curtis, owner

AirTron (Sterling Air)- Glen Pollock, General Manager

I hope that this clears up some confusion and helps out a little bit.  If you have any questions…leave a comment, start a discussion.  This is a tricky subject, for sure.

Thank You to Clayton, Kathleen and Sam for emailing in the question.  Did we leave anything out?  If anyone has a reputable A/C Contractor that you would like to recommend, feel free to comment in the sections below.

Clayton Farmer
Alternative Power Solutions
Phone: 281-802-1224

Kathleen Reardon

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5 Comments so far ↓

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