Designing a perfect HVAC system for your home
The best AC system for cost and efficiency
Calculations for new homes or upgraded systems

In June I wrote an article about setting your home up for the summer while away for extended periods of time. In it I discussed security and setting up your air conditioning to control humidity and reduce the potential for mold. At the time I made the assumption you had an Air Conditioning unit that was running at peak efficiency, I also recommended having a maintenance service before leaving. So, how do you get a good system and know that it is good for your home. You may be building a new home or you may be replacing a system, I’ve learned something about AC systems over the years here in Sarasota.
It might be easy to say that you want the biggest and best, that way you can’t go wrong. It is my hope that after reading this that finding the right system for your home is a better way to think about your project. It can be confusing and my advice here may only apply to this area of the country, and even this area of Florida.
The size of the air conditioner depends on many factors and not just square footage of the home. It is based more on room volume and other environmental factors that a professional will measure in a heat load calculation. They will factor in shade for the home, open windows and the direction these windows face. From the heat load calculation they will determine the amount of British thermal units, or Btu’s required to cool the home. Since 12,000 Btu’s are required to melt a ton of ice in one hour, they will express your needs in terms of tons. A one-ton unit will be efficient for a home that requires 12,000 Btu’s, a two-ton unit for 24,000 Btu’s, and so on up to five-ton units.
If more than five tons of cooling are required in a residential space they will likely separate the home out into zones. The main section may have a four-ton unit, while the master bedroom area will have sufficient cooling with a two-ton unit. Programmable thermostats are important here. The system will run more efficiently if you determine the times of day you will be sleeping and program the master bedroom to be cooled during those times while the rest of the home will be cooled to the desired temperature only during the times you will be entertaining, eating and relaxing in the family room.
You likely will not see two tons stamped on the installed AC unit, nor will you likely see Btu’s recorded there. To determine what size the unit is you may have to refer to the running amperage that is stamped on the installation plaque. Since most residential AC units are installed as single-phase, 220 volt systems, you should be able to calculate the size based on the running amps. There, five running amps will be the equivalent of one ton or 12,000 Btu’s. A ten-amp system is two tons. If there is a 12.5 amp stamp, expect your system to be 2.5 tons.
There is an economical hazard in installing the wrong size AC for your home. Too little and the unit may run continuously. If the unit is too big, the home will be cooled down too quickly and humidity may not be removed from the home. Too much humidity and mold may form.

If you moved down from the north you might remember humidifiers that put moisture in the air during cold winter spells. Here in Sarasota we use dehumidifiers to remove the humidity, or moisture in the air that can cause mold and make us feel uncomfortable in the home. In many homes you will find a humidistat that is a stand-alone setting near the thermostat, or an integral part of the thermostat. The humidistat only reacts to relative humidity. There are settings on the humidistat to set the percentage of RH, but in most situations you will turn the humidistat to the maximum on position and let the system do the work for you. In situations where you may be leaving on vacation you can adjust the thermostat and humidistat for more efficient and safe environmental control. See my article on June 8 for more information and settings while away for extended periods of time. Humidistats are also used in humidifiers and even microwave ovens.
At times homes in our area may need an element of heat, pun intended. For that you will rely on heat strips in the heating portion of your Heating, Ventilating and Air Conditioning system, or HVAC system. Most homes in our area have an air conditioner with a heat strip. Comparatively, this system is less economical if you need more heat, but that may be one of the reasons why you moved to Sarasota or Longboat Key in the first place. The more north you move a heat pump is more economical because it uses any heat in the air to move into the home. I see heat pumps here when the homeowner requires higher temperatures in their homes. They cost a bit more to install and most residents here do not get a good return on their investment. Heat pumps are typically used locally to control the temperature of swimming pools. They provide heat in the winter and cooler water in the summer.
You can calculate the size of the heat strip in the AC. The US Department of Energy has classified our area as a hot-humid zone, we love it here for the warmth in the winter. But that classification also means that we can determine our heat strip needs by multiplying our square footage by 25. The multiplier increases the more north you go in the country and heat pumps become more economical. So, for a 2,000 square-foot home (25 X 2,000) you will need 50,000 Btu’s to heat your residence. Heat strips are sold by kilowatts, and a five kilowatt heat strip is sufficient to heat a home that requires 17,000 Btu’s. You can start there with the 2,000 square-foot home mentioned here. The calculation would be 50,000 divided by 17,000, which equals 2.94. Multiply 2.94 by the 5 KW per 17,000 Btu’s and your needs will be a 14.7 KW heat strip. Your contractor will also be able to calculate the size of the heat strip needed for use in your home or condominium. If the contractor recommends a 15 KW heat strip you can see that he may be selling the correct size for this particular home.
Now that you know what size AC and what size heat strip you need for your HVAC system, your contractor will likely ask you how efficient you want your system to be. The more efficient, the more it will cost to install but the energy savings may be worth the additional up-front cost. Your utility company, standards organizations and the federal government can help you there. Air conditioning, heating and refrigeration appliances are rated for efficiency by their Seasonal Energy Efficiency Rating (SEER) and determined by the ratio of cooling in Btu’s to watt-hours of energy consumed. The higher the number, the more efficient the unit is expected to perform. Most contractors will calculate your energy savings for different options, it is up to you to determine justification for the extra cost. I usually think in terms of the number of years that you expect to live in the home. If you are going to live their long term then a more efficient system may be the best ROI for you.

The ENERGY STAR program by the federal government certifies many appliances for energy efficiency. In order to qualify for the ENERGY STAR certification, the product must be 15 percent more efficient than conventional models based on the SEER number. Other home improvement products such as windows, roofing and insulation can also achieve ENERGY STAR status. Your home can have a higher resale value if it is certified as a green home with energy efficient products and systems.
Utilities also have incentives for upgrading or installing a more efficient AC unit such as the one through Florida, Power and Light. The local power provider for this area, FPL will issue a rebate if you install a complete system, the system has a minimum SEER of 15, and the system was installed by a Participating Independent Contractor.
Another way to save money on cooling costs is through proper use of your ceiling fans. I see ceiling fans running continuously, which is just consuming energy and not providing additional benefits. The AC system running properly will perform adequate circulation within the home during the day when you are not there. Running your ceiling fan while in the room is the most efficient use of the fan.
This area has many highly qualified air conditioning contractors. Please contact me at (941) 387-1840 if you would like information on a particular contractor or if you have a property to sell or buy.