Air Conditioning – Sizing Right Results in Savings and Comfort
A major objective of Austin Energy Green Building is to ensure that houses in Austin are cool and comfortable during our long, hot summers, yet as energy efficient as possible. Austin's high humidity makes that an even greater challenge.
In the past, it was common practice to oversize cooling systems. This covered up a lot of problems, such as leaky ducts, and often resulted in clammy, uncomfortable homes. With advances in construction techniques, more efficient windows, better insulation and other improvements, oversizing is unnecessary. However, some air conditioning contractors continue to install systems that are too large.
Bigger is Not Always Better
In truth, an air conditioning system with excessive cooling capacity is less efficient than one that is sized correctly. It is less effective in reducing humidity within the home, a key factor in increasing comfort. Not only are larger units more expensive to purchase, the homeowner will be paying for that inefficiency for the life of the unit. Furthermore, oversized air conditioning increases peak electrical demand, forcing electric utilities to increase their generating capacity. The cost of that unnecessary expansion is passed along to you, the ratepayer.
Why it is Important to Correctly Size your System
Air conditioning systems have two main functions: to cool and to dehumidify. When the system first comes on, it immediately begins cooling the air; however, dehumidification happens more slowly because it takes time for the air to circulate over the coil and release moisture. Large systems may lower the temperature to your thermostat’s set point so quickly that it cycles off before dehumidification can occur. The humidity could still be high, which makes you feel uncomfortable. So you lower the thermostat another degree, which causes the system to operate on an even shorter cycle, removing less humidity. Just as stop-and-go driving uses more gas and wears out your car, short cycling of your cooling system uses more electricity and shortens its life.
No More "Rule-of-Thumb" Sizing
Whether you are building a new house or renovating an existing one, the first step in ensuring that your cooling system is sized correctly is to have your air conditioning contractor perform a Manual J calculation. This takes into account all the factors in a house that affect the cooling load; local climate, the orientation and square footage of the structure, the number, size and location of windows, the amount of insulation, the desired indoor temperature, and how much outside air infiltrates the house. The calculation tells the contractor (and you) how much capacity you will need.
Design Appropriately for Central Texas
The square footage of living space per ton of cooling is a measure of a home design's appropriateness for Austin's climate. Austin Energy Green Building will not rate a home that cannot meet a standard of 500 sq ft or more per ton of cooling based on an accurate Manual J calculation. Some exceptions may be made for very small houses, due to availability of equipment. In order to receive a rating of 3 stars or higher, a home must have at least 600 sq ft per ton of cooling.
Ducts Play a Big Role Too
An important key to ensuring that your cooling system keeps you comfortable is that the duct system be designed to deliver sufficient conditioned air to all rooms of the house. Your contractor should perform a Manual D calculation, which determines the cooling needs of all the rooms and indicates the correct duct size for each.
Location, Location, Location
Do you go up in your attic on a summer day to keep cool? Of course not! Unfortunately, that is where many cooling systems are installed. Attics in Central Texas can reach 145F in the summer, placing additional demands on systems installed up there. Any leaks in the ducts just add to the problem.
If you are designing or building a new house, try to keep the system and ducts within the conditioned space; i.e., the space that is heated and cooled. That way, any conditioned air leaking from the ducts will leak where we need it. If you must put the system in the attic, installing a radiant barrier, a lighter-colored roof, and proper attic ventilation (continuous soffit and ridge venting) will reduce demands on the system. Also, have your contractor perform a duct pressure test to reveal any leaks in the system. Leakage should be held to under 10%.
Reduce Your Home's Cooling Load
Solar screens or vegetative shading on east- and west-facing windows can result in a substantial reduction in cooling demand. Installing compact florescent bulbs can reduce the heat from interior lighting (and use less electricity). Ceiling fans can allow you to set your thermostat a couple of degrees higher and still be comfortable – just make sure you turn them off when you leave the room. Check to see if you need additional insulation in your attic, and if you don’t have a radiant barrier up there, install one. And ask your contractor about systems that are especially efficient at dehumidification.
Small is Beautiful
"Ever bigger machines, entailing ever bigger concentrations of economic power and exerting ever greater violence against the environment, do not represent progress: they are a denial of wisdom. Wisdom demands a new orientation of science and technology towards the organic, the gentle, the non-violent, the elegant and beautiful." E.F. Schumacher
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