Air Conditioning - Bigger is not better!

Tuesday, October 6th, 2009

Hello again from Olson Defendorf Custom Homes

Today we are talking about air conditioning. Bigger is not better! We will tell you why an oversized air conditioner isn’t better for you in comfortable indoor temperatures and monthly electric bills. 

Clammy home
In our hot and humid climate here in Austin, where your air conditioner can run from March-April until November-December, the most important goal is to remove humidity from inside your home. Removing humidity creates drier air, and drier air is easier to cool and feels more comfortable. The way an air conditioning unit removes humidity from your home is by staying on. The coils get cold and remove all of that nasty moisture. An oversized unit can’t dehumidify the air because it only comes on for a short period of time, blasts arctic air, and then shuts off. This is called short cycle. 
Monthly utility bills
When an oversized air conditioner short cycles it draws a huge amount of electricity every time the compressor starts up. Once it is running the electric use is minimal. It’s the start up that uses the most energy and increases your electric bill. When you unit is constantly coming on, blasting cold air, and then shutting off it is drawing much more energy that a properly sized unit would use. A properly sized unit will come on fewer times during the day and stay on longer, using less energy and lowering your electric bill.

So now you know what not to do. Here are some pointers to make sure you get the right sized air conditioner and get the most bang for your buck.

1. Make sure your builder uses the Air Conditioning Contractors of America’s Manual J to calculate how much cooling you need for your home. The old way of sizing A/C units was based on square footage. This method is outdated, and is a main reason for having oversized units. Manual J accounts for square footage, ceiling heights, window type, window locations, insulation, orientation of the house, number of occupants, etc. to determine the right size.
2. Be wary of the lowest bid. You might get a cheap upfront bid because the contractor isn’t spending the time to calculate the Manual J, but it will not pay off in the long run. Usually you get what you pay for in air conditioning. 
3. Check duct leakage. Air flow can be restricted for many reasons, but one of the biggest problems is leakage from joints in the ducts themselves. On older homes, Troy and I have seen over 50% leakage on some homes! The City of Austin requires a blower duct test per code to make sure your ducts are sealed.
4. Get a higher efficiency unit (SEER – Seasonal Energy Efficiency Ratio). The State of Texas requires a minimum 13 SEER when replacing or installing new units. The higher the SEER the more efficient your system. We always put a 14 SEER with R-6 ducts. You can upgrade up to a 16, 18, 20 SEER. We also recommend looking at a two stage system. You can have a multi-stage blower and even a multi-stage compressor. These can significantly lower your electric bills.

Lastly, let’s talk summers. Summers can be brutal in Texas, especially in Austin 2009 where we had 68 days over 100 degrees; many were record breaking 105-106 degree days. Your air conditioner should be sized to cool around 23-25 degrees below the exterior temperature. On a 106 degree day this would mean a properly sized home should keep the interior temperature at 81 degrees during the heat of the afternoon! A good system in Austin is sized to keep your home 75 degrees when it is 99 outside. 

We would like to thank Home Energy magazine and Marble Falls HVAC for their contribution to this blog.

Hopefully this blog enlightened you on your air conditioning system. Now you can impress your friends at the next party with all of your A/C knowledge! Let us know if you have any questions.