How to Determine the Correct Size for a New HVAC System

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Servicing an HVAC Unit
Servicing an HVAC Unit

Homeowners today have a good selection of different types of HVAC systems to consider. The best type is usually determined by your environment. Swamp coolers are made just for use in hot dry locations. Central air conditioner and heaters work well for most of the country. Heat pumps are very efficient but are expensive to install. The list goes on and on. The thing they all have in common is that they should be properly sized for the structure they will serve.

Furnaces that are too large tend to cycle off and on continuously. A central system that is too large may be less effective at dehumidification than a correctly sized AC unit. And it goes without saying that a larger unit costs more going in. A too-small system works too hard to keep up. But a properly sized system will do the right job for the environment at maximum efficiency.

Contractors Shouldn’t Rely on Rule of Thumb

Too many contractors rely on “experience” or an “educated guess. Bad idea. It’s much better to use worksheets designed by the Air Conditioning Contractors of America (ACCA). They are far more accurate because they take into account individual actual conditions. These include the amount and type of insulation, the size of the home, the size and glazing type of windows, air leakage, lighting, and home appliances.

To make these calculations, the International Residential Code, which is the dominant building code in the U.S. should be followed. It requires that heating and cooling equipment be sized with the help of Manual J, or a similar approved methodology. However, the rule is too often ignored. Studies have demonstrated that heating and cooling equipment is far too often over-sized, often by as much as 200%.

So before you hire a contractor, this is one thing you should quiz him about. Does he follow this protocol (even though your local code may not require it)? After all, a new HVAC system is a big investment and it’s your investment, not his.

Other Ways to Determine HVAC Size

If you really want to be sure that all the factors are being figured correctly, do it yourself and follow Manual J. You will need to determine the U-factors of building components such as windows, doors, insulated walls, determine the “outdoor design temperature” for your area, take an estimate about airtightness, and finally use a heat-loss formula to determine how much energy in Btu your home loses through the building exterior.

You can also hire a pro. This might be a certified HERS rater, a mechanical engineer, or an energy consultant. You may be spending a bit more time and money up front, but generally speaking, contracting a trained professional third party who has no vested interest in selling you a particular brand or size of unit is far preferable to trusting a seat-of-the-pants estimate that might be questionable.

In any event, always be sure that you and any contractor are on the same page before any work begins. Whenever this much money is involved it is always in your best interest.

I hope this information on determining the correct size for a new HVAC system has helped you. If so, pass the URL along to your friends. Thanks for visiting!


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About the author:

Photo of Kelly R. SmithKelly R. Smith is an Air Force veteran and was a commercial carpenter for 20 years before returning to night school at the University of Houston where he earned a Bachelor’s Degree in Computer Science. After working at NASA for a few years, he went on to develop software for the transportation and financial and energy trading industries. He has been writing, in one capacity or another, since he could hold a pencil. As a freelance writer now, he specializes in producing articles and blog content for a variety of clients. His personal blog is at I Can Fix Up My Home Blog where he muses on many different topics.


Lower Electric Bills with an A/C Misting System


Cool-N-Save A/C condenser misting paddle
Cool-N-Save A/C condenser misting paddle

Summertime is here and the living is easy. Except perhaps for that high electricity bill from the incessant cycling of the air conditioner. Sure, you’ve beefed up the insulation, sealed leaks and perhaps installed radiant barrier foil in the attic. But there’s more you can do; you may be able to lower your electric bills with an A/C misting system.

Can a Condenser Coil Mister Save the Day?

I have been intrigued by this concept for a while now and with the South Texas temperatures flirting with triple digits and rising utility rates I decided this was the year to implement it. I already understood the cooling power of evaporating water, having worked with cooling towers and chilled water systems while working as an Engineering Supervisor for Marriott.

 

But this is a different application although the basic science is roughly the same. The one I installed at my home yesterday is the Cool-N-Save basic kit. Their claim is that the mist will lower the temperature of the air around the coils by as much as 30°F. This means that your HVAC system will not have to work as hard, consequently becoming more efficient and saving you money. Have I saved money yet? I don’t know! I just installed it yesterday. But I do have historic data and I’ll update this post when I have some hard numbers.

Curious about the science? Heat flux, thermal flux, heat flux density, or heat flow rate intensity is a flow of energy per unit of area per unit of time. Call it what you will, heat flux is at its most efficient whenever the outdoor ambient air temperature is lower than the refrigerant flowing through inside the A/C coils. So the evaporative effect of the misting lowers the ambient temperature in the area around the condenser coils and bingo! Efficiency prevails.

How the Cool-N-Save Coil Mister Works

Cool-N-Save mister water filter
Cool-N-Save mister water filter

The basic kit comes with:

  • 1  Control valve and paddle
  • 3  2′ Misting arms (tubing from the control valve/paddle to the misting nozzles)
  • 3  Brass misting nozzles
  • 1  Cool Release water treatment filter (3 month life)
  • 1  20′ flexible water feed line
  • 1 Garden hose adapter

The only thing I added was a brass splitter with cut-off valves for my outdoor spigot so I can use the mister and garden hose concurrently.

A dedicated garden hose attaches to the supplied garden hose adapter whose tubing enters the filter. Another hose (cut from the 20′ line) connects the filter to the control valve on the paddle. The three misting arms connect to the control valve/paddle and dangle down three sides of the condenser respectively terminated by the misting nozzles. Use zip-ties in all the obvious places.

When the A/C cycles the fan in the condenser unit kicks off. This blows up the paddle and enables water flow to the misting nozzles. It’s that elementary, Watson. (Pardon the inevitable Sherlock Holmes reference.)

Regular readers of my energy efficiency  articles know that I stress incremental changes; it is generally many small to large changes, not just one huge one that will reap savings. An A/C misting system is my latest energy tweak. I’ll be back in touch with results.


Looking for more great content? Visit our partner sites:

The Green Frugal

Running Across Texas

 

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I offer article and blog-writing services. Interested? Hire Me!


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