Tuesday, May 7, 2013

Air conditioning 101


It's that time of the year....summer.   Air conditioners are starting to hum.  Some aren't.  They're broken.

Let me share with you some of what us homebuilders....er...."building scientists" *snicker* ....have learned regarding the proper a/c size and efficiency ratings you'll need to know if yours goes kaput.

Logic says buy the highest SEER (Seasonal Energy Efficiency Ratio) rated unit you can afford, the higher the number the better.  These days they begin at 13 SEER....that's rock bottom....up to 23 or 24 SEER. That is not necessarily a smart plan.  There are other variables that need to be considered.

If your home is relatively new and has a tight "envelope" (very good wall and ceiling insulation, good windows and doors, proper caulking, well sealed a/c ducts, etc), you can probably get by nicely with a 16/18 SEER unit.  

That's because the a/c won't come on that often or run for very long at a time due to the home's draft-free tightness.  If the a/c isn't running a lot, a 20 SEER won't save enough to justify the premium price over a 16 SEER unit.  Save the money and just buy the 16 SEER model.

On the other hand, if you have an older, under-insulated, drafty house, your a/c unit will probably come on often and stay on for a long time at a stretch. Then you should strongly consider a 20 SEER system.  

It will run enough, and therefore save enough, to justify the premium over a 16 SEER unit.  Of course, there are also other upgrades you should consider doing if your house is that deficient.

Logic also says, "If a 4-ton unit is good, a 5-ton unit is better", right?

Not really.  Just as a car needs to warm up and be driven a few miles and get up to speed before you begin to realize optimum mileage efficiency, an a/c unit needs to run at least 10 minutes at a time before it reaches peak performance.  

An over-sized a/c unit that comes on and then cycles off after only 5 minutes or so isn't even getting warmed up.  Your fancy 20 SEER system is just spinning it's wheels.  Don't "over-size".

So how do you know how "tight" your home is?  If you've been in your home a while you probably know if it feels drafty or not, and of course your utility bills can tell you a lot, too.  But the best way is to hire an energy auditor to come to your home and do a "blower-door test".  Spend a little, save a lot.



They'll temporarily seal up a door opening and, using a fan, depressurize your home.  They'll then compare the pressure inside and outside, and this will tell them how tight your house is.  Using this information a competent HVAC contractor can calculate the proper SEER rating and capacity your home needs.

Yeah, I know.  Boring post.  I just hate to see my friends get ripped off by slick talking salespeople yapping non-stop on TV/radio.  Be careful out there.

S

31 comments:

  1. I'll keep that in mind if I ever have a house. When the air conditioner for my apartment broke a year or two ago I'm not sure what model they put it in but it's a lot less rusty than the other ones nearby. Shame I can't take it with me when I move.

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  2. OK Scott, really, when does the funny stuff start?....I'm waiting...still waiting. Damn a six o'clock news post...from SCOTT??

    If we ever meet at a party, remind me not to ask you about air conditioners.

    Interesting post though, stuff I never thought of. who would have thought bigger isn't always better? Especially in Texas!
    Question about new air tight homes. Is it true that they present a danger of deadly, expensive to remove mold? Maybe another post when you run out of funny stuff...or banker bashing...which they often deserve.

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    1. OK, I promise....funny stuff tomorrow.

      Mold is caused by too much moisture trapped in the house. Control the moisture, no mold. Here in Texas our mechanical systems can usually deal with adequate dehumidification of the inside air. If your home is still too moist, consider a separate dehumidifier.

      I build with OPEN cell foam wall insulation (that lets moisture flow through), then use Tyvek Homewrap on the outside of the sheathing. Tyvek is a moisture membrane that lets water vapor from the inside out, yet keeps water droplets on the outside from passing through to the inside. In other words the house "breathes".

      The big no-no is CLOSED cell insulation in the walls, then a CLOSED cell foam sheathing like Dow Styrofoam on the outside. Either is OK alone, but NOT BOTH together. This traps any water that might get behind the sheathing (such as due to poor window caulking), and that is almost a guarantee of mold growth.

      TMI? ;)

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    2. TMI? Not at all, I asked for it. Interesting answer. Apparently from some articles I've read some builders don't know this and some inspectors may take short cuts. Some guy in Va. had mold so bad they had to knock his 500K house down.

      Thanks

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  3. When our builder grade models died we got 15 seers. Big, big difference.

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  4. Thanks for the warning. I'm off to check out what kind of air conditioner we have, even though most homes here in Portland don't have AC.

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  5. This is why I'm glad I rent :) Although of course I care about our energy usage - we use even billing & last year we ended up with two free months at the end of our billing cycle! So I guess we're doing ok.

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  6. I always wonder how people managed to live in the South before the arrival of air conditioning. Thanks for your informative post. I used to work for Trane when we lived in South Carolina, so I know a little about AC.

    Speaking of AC - we lived in Buffalo and it was time for me to buy a new car. I debated a lot of things (like, new vs. used, etc.) and finally decided on a new, very basic car without AC, stick shift, no radio, nothing automatic, small, two doors. As I said, very basic - but it got 40 mpg. Did I say no AC? - One week after I bought the car, my husband had a job interview. He got the job. The job transferred us to South Carolina. I drove the car with no AC for seven freaking years in South Carolina. God does have a sense of humor.

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  7. My place is, quite literally, a fibro shack. Carpets right on open floor boards, no insulation, tin roof. I wanted to put AC in the lounge room but it would have been practically impossible to do it, so I ended up putting a portable unit in the bedroom. It means there's only one bearable room in the whole house, but on the plus side my electricity bill didn't skyrocket!

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  8. This is great information. We just found the best air conditioning repair company. I'll have to see if my husband wants to try looking into an energy auditor. I think that might be a good idea since we just moved into our house. I'd hate to be trying to cool it and have all the cool air go right out of the house. Thanks so much for sharing this very useful information.

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  9. Nope, this is not a boring post! I can relate so much! I can't live during summer without having an air conditioner around! Regardless if my bill suffers, I just can't stand the heat. Good thing there are air conditioners that are designed to be energy efficient.

    Darryl Iorio

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  10. As a fact, the Goodman brand of Goodman air conditioners has been in the market for almost 30 years and this is not an easy number to achieve. Basically, when you hear the word Goodman air conditioner, you know you are dealing with a pro. Carrying its decades of expertise, we know that we are assured for their service and quality-based products. They wouldnít be in this industry for this long if they didnít provide a good service for their customers.

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  11. That's an awesome tool to use to see the air pressure in your home. I use my air conditioner in Baltimore MD and I had to take many factors into account when buying my unit but I never even thought to see how "tight" my house was.

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  12. Trying to help your friends and other people who would bump into your blog about practical things to consider when buying an AC is never a boring post. From the many choices of brands, types, and sizes of AC's out in the market, many would think that latest models and bigger sizes are always better. Clearly, that isn't always the case. There are other things to consider, and the tips you've shared are really helpful. Thank you!
    Harris Aire Serv

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  13. Air conditioning company Tampa Bay is exactly what happened and today he employs great people who take ownership of their part in the company and know how important the service industry is to the community they live in.

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  14. It’s good that people are actually aware of these things. Some homeowners who don’t bother with consulting before getting a heater tend to just go “bigger is better” on these things and end up with an underutilized heater and possibly costing them more, not just with the initial purchase but with resource consumption as well. Although I would think if the house’s insulation is faulty, I’d rather go for the repair that the higher SEER rated heater. I would probably save more in the long run.

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  15. Well, if you got air conditioner problems, it is better to have an estimate first on how much air conditioner repair will cost you this days.


    Air Conditioning Repairs

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  16. Such a useful info for HVAC and for everyone seeking knowledge about HVAC. Thanks.

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  24. When it's about home or small commercial cooling we think about electricity savings.I appreciate this blog to share this information.but in office what is the environment and culture? Are people dressed in business casual, jeans and t-shirts, or full-on suits? Get all concept of commercial air condition system in office from accutrolhvac.com.If you are finding human comfort and safety don't forget to get touch with our daily blogs..

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  28. Found your blog. It is full of really good information. Thank you for sharing. If you ever need service on air conditioning repair please visit us at rheemteamcomfort.com. We would love it if you would have a look at some of our blogs and let us know your thoughts.

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  29. When it's about home or small commercial cooling we think about electricity savings.I appreciate this blog to share this information.but in office what is the environment and culture? Are people dressed in business casual, jeans and t-shirts, or full-on suits? Get all concept of commercial air condition system in office from accutrolhvac.com.If you are finding human comfort and safety don't forget to get touch with our daily blogs..

    ReplyDelete