May 14, 2012

Is Your Home and AC System Ready for Cooling Season? Part II

Ramona Madhosingh-Hector, Urban Sustainability Agent
James Stevenson, Energy Educator

Last week we examined ways to keep heat out of the home to keep the HVAC system from working harder than necessary. This week we will explore ways to make the system itself run as smoothly and efficiently as possible.

Terms to Help Get You Started
  • HVAC    Heating, Ventilation, and Air Conditioning equipment 
  • SEER    A measure of efficiency for air conditioning units; the higher the SEER number, the more energy efficient the unit is in cooling the air 
  • SHR    A measure of efficiency for air conditioning systems ability to remove moisture or humidity; the higher the SHR number, the less capable the system is in removing humidity 
  • Air handler    The indoor unit that moves the air through the heating/cooling system 
  • Cooling load & Load calculation    Measurements that calculate what size system is appropriate for a particular structure given its square footage, ductwork analysis, insulation, windows, etc. 
  • Condenser    The outdoor unit that keeps the refrigerant cool
  • Aluminum/foil tape    Specialty tape with an acrylic-based adhesive that performs consistently under extreme temperatures 
  • Mastic    A thick paste that provides a permanent seal at duct joints and connections; sometimes used in conjunction with a fiberglass mesh tape 
Let’s start with the thermostat. This is where your choices can make a huge difference. UF recommends setting the thermostat between 78°F-80°F and using ceiling or other fans to keep comfortable in the summer. Note however that higher temperatures and a lack of humidity control (i.e. turning the AC off for periods of time in the summer) can allow conditions that make the growth of harmful molds possible.

A programmable thermostat can make a big difference in HVAC operational costs. There are many makes and models. Choose one that has settings that make the most sense for your family’s weekly schedule. If you have a programmable thermostat in your home, make sure you are using it properly; not just as a high-tech on/off switch! When shopping for a programmable thermostat, look for the EnergyStar® logo. This will indicate that the particular model has proven to perform in an energy-saving way when used properly.

Percentage Increase
In Cost
80°F $100 -
78°F $116 +16
76°F $132 +32
74°F $148 +48
72°F $164 +64
70°F $180 +80

The Thermostat controls the “guts” of the HVAC system. The air handler, which may be located inside the home, is where air is blown across coils filled with very cold gas. This is your cooled air “supply.” These coils can become covered in dust and debris as they are constantly wet during the cooling season. Dirty coils in the air handler are the single most common cause of poor AC efficiency.

The pressurized refrigerant gas in the coils themselves is put in when the system is installed. It is worth having the refrigerant’s “charge” checked periodically by a licensed contractor. It is possible for a system to be over or under-charged, leading to inefficiencies.

The outdoor coil in the compressor unit also has to be checked from time to time. Make sure the outdoor unit is kept free from debris and has a 3’ clearance from plants or other barriers to air flow. Often the compressor is “hidden” behind plantings or a structure to keep the unit from view, but without adequate circulation, the heat that has been removed from the home has no easy escape, resulting in the system’s need to run for a longer period of time. Check the fins of the condenser as well. These very thin metal strips are where the heat is removed from the system. They are easily bent or damaged, resulting in inefficiency. A fin comb can be purchased to straighten these delicate structures out.

Have you had your ductwork inspected? Leaking ducts can allow cooled air to escape into the attic, wasting that valuable commodity. More importantly, leaky ductwork can allow heat to enter your home, making the AC have to work harder. There are many causes for leaky ductwork, so a periodic inspection is a very good investment. Our local utility companies offer incentives to have this work done, check with yours for more details. This can be done on an annual basis to prevent a loss of up to 40% of your conditioned air.

Leaks in ductwork are most likely to be found around connection joints, around the air handler and near vents. Ductwork should be held together with mastic or acrylic-foil tape, which has been approved for your type of ductwork. If you see duct-tape, that familiar grey fix-all fabric tape with a rubber adhesive, your ductwork is not properly sealed. This product, despite its name, becomes dry and brittle over time, leading to failure.

Finally, ceiling fans can be found in most Central Florida homes and can make a big difference in your comfort when used properly. A ceiling fan can allow you to increase your thermostat setting by 4°F without feeling any difference in your comfort. Keep the blades clean for most efficient operation. Many fans have a directional control. To operate most effectively, fans can be switched to rotate counter-clockwise in summer and clockwise in winter. The effect of the fan is only detectable by your skin. When air passes across your exposed skin, a bit of moisture is evaporated away making you feel cooler than the ambient temperature. This is known as the wind-chill factor; more recognizable when referring to cold winter temperatures “feeling” colder in the wind. The same effect can make us more comfortable in warm weather. Note, however that ceiling fans have no effect on the temperature of the home. Therefore they should not be left on in an unoccupied room or home.

As conditioning air both in summer and winter accounts for up to 50% of your power bill, it makes sense to keep that system working as smoothly as possible. If it is time to replace an old system, or if your home has changed in size (closing-in a porch for example) it is very important to know the facts. Determining the size of the system requires a complex calculation based on the size of your home, configuration of supply/return registers, even window type plays a role. For more information on proper sizing and HVAC selection, please see Energy Efficient Homes: Air Conditioning, FCS3262, 2008.

Energy Efficient Homes: Ceiling Fans 
Energy Efficient Homes: Air Conditioning
Energy Efficient Homes: The Duct System
Energy Efficient Homes: Easy Steps to Improving Your Home’s Energy Efficiency

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