Sizing Air Conditioning
By Tim Carter
©1993-2007 Tim Carter
My friends - the Gibson's - purchased a tract home about 15 years ago. It was actually the sub-division model home. The front of the house has no less than 10 windows if I am not mistaken. One month ago their AC unit gave up the ghost. My HVAC man installed a new AC unit for them. The Gibsons had always complained about being uncomfortable. They thought the AC unit was low on Freon. It turns out the AC unit was never sized right from the get go. It was 1.5 tons too small!
To add insult to injury, the return air ducts from the second floor rooms were non-existent. There was just one central return duct in the second floor hallway. My HVAC man did a heat gain calculation to make sure that the proper sized unit was installed. He has a neat computer program that allows him to do this. You can do the same thing by hand - I have done it numerous times - and arrive at the same results.
One Size Doesn't Fit All
 Air ConditionersYORK designs and manufactures a complete line of heating, ventilation and air conditioning systems. Find a real pro to install your AC unit. www.york.com |
Tract houses often have similar sized AC units. The builder or HVAC person simply does one calculation and often applies it to all houses that are similar in size. Guess what? This doesn't work! In the case of my friends, the glass on the front of their house happens to face west south-west, the WORST possible direction! The blazing sun late in the afternoon cooks the inside of their house. I'll bet that a number of you reading this bulletin have similar situations.
Some HVAC contractors try to size AC units by doing a simple square foot analysis. They use a gross multiplier that ranges from 400 (older homes) to 1,000 (newer homes) sq. ft. of floor space per ton to arrive at total BTU heat gain or total tonnage. This method is unacceptable. If your HVAC person proposes this, find a qualified technician who understands heat gain and knows how to use Manual J.
The Bible - Manual J
Calculating heat gain is really complex. You must use a booklet called Manual J. This booklet takes you step by step through the process making you account for each and every source of heat. Many scientists have worked over the years to formulate the tables, formulas, and values that combine to form this bible of the air conditioning industry.
You can purchase a copy - and I URGE you to do so - if you want to really understand how air conditioning works. The ordering instructions can be found if you keep on reading.
The manual is actually a rather technical booklet, but if you follow instructions well, you can easily work out a heat gain computation of your own. You will at least get close. Don't get bogged down in all of the technical mumbo-jumbo within the manual. Simply take your measurements and crunch some numbers.
Finding a Professional
The same outfit that publishes Manual J also has a new program that will allow you to locate a professional who is dedicated to a high professional standard. They have a new certification program called the Air Conditioning Excellence (ACE) Technician Certification Program. Air conditioning contractors who are ACE certified meet a high standard because they have completed a comprehensive, industry education and examination process and have demonstrated technical expertise with numerous types of air conditioning (and heating) units in the field. They also have a thorough understanding of non-technical and safety issues that impact the equipment in your home. If you want a professional in your home, I suggest you look for someone who has this ACE certification.
Oversizing Equipment
Some people want to oversize their equipment so they are assured of being cool on those hot days. This can be a big mistake. You actually want to undersize your equipment ever so slightly. Researchers at Texas A & M found that air conditioners that were undersized by 10 to 20 percent are more efficient and more effective at removing water vapor (humidity) from your indoor air.
Now I will agree with them to a point, but I don't know if I would undersize my own equipment to the 20 percent benchmark! I would go 5 percent at the most....a totally unscientific approach since I have always sized the equipment on my jobs to match or slight exceed the calculated heat gain.
Oversizing a unit causes problems because the unit doesn't run long enough. You want your air conditioner to run for 15 - 20 minutes at a time. This allows the recirculating air to come into contact with the cold coil inside the air handler. This is how humidity is removed from the air. You will feel most comfortable when as much moisture is squeezed from the interior air as is possible. Short cycling air conditioners don't remove humidity well at all.
Calculating Heat Gain
The first thing you need to do to calculate the heat gain of your house is to purchase a copy of Manual J. This booklet is very likely at your local library and/or you may be able to borrow a copy from a local air conditioning distributor. The point I am trying to make is that it contains 116 pages of tables, examples, and other valuable information that is physically impossible for me to include in this tiny Bulletin.
My copy of Manual J has an awesome example computation. You can see the floor plan of the house and the computation sheet. This allows you to see how they arrived at all of the individual heat gain BTU's. The example computation will allow you to easily compute your own house's heat gain.
The following instructions are meant to HELP you make sense of the rather technical Manual J. Without the following notes, I'm afraid that you might give up in frustration.
You can purchase your own copy of Manual J by contacting the following Association. The cost ($35) is steep, but there is no other booklet like it:
Air Conditioning Contractors of America (ACCA)
2800 Shirlington Road
Suite 300
Arlington, VA 22206
www.acca.org
Call Toll Free - 888-290-2220
0 komentar:
Posting Komentar