Solar Hot Air Collectors

Designing a new home to optimize heating with the sun is fairly straightforward---orient the home correctly, insulate well beyond code requirements, and provide appropriate amounts of glazing and thermal mass.

Each day, the energy equivalent of more than nine gallons of gasoline falls as sunshine on the south-facing portions of my roofs---just one third of the total roof surface area.

According to the U.S. Department of Energy, solar hot air systems are most cost effective in cold climates with good solar resources, and are most economical when they are displacing more expensive heating fuels like oil, electricity, or natural gas. Solar hot air collectors will reduce home heating bills, while avoiding the pollution and greenhouse gases associated with burning fossil fuels for heating.

Solar space heating works best on days that are relatively sunny and cold---and when interior spaces demand heat.

To determine if your state offers any assistance for offsetting the cost of a solar space heating system, see the Database of State Incentives for Renewables & Efficiency at www.dsireusa.org. There is no Federal Tax Credit at this time for solar space heating.

Reasearch published by the real estate industry's Appraisal Institute indicates that for each dollar of operating costs saved annually, the value of the house goes up twenty times that. If first-year savings in natural gas costs is estimated to be $279--in theroy, the house would increase in value by $5,572.

Besides the positive financial gains, I also project intangible benefits---the pleasures of minimizing the use of the gas furnance, and feeling the solar-heated air coming through the vent. Las, but not least, I would lighten my environmental footprint, a satisfaction not measurable in dollars and cents.

If your home has sunny, south-facing wall or roof space, and you're looking to offset a portion of your heating bills without doing extensive remodeling, a solar hot air system may make sense (and save dollars) for you too.

Air collectors are being used successfully today for such diverse applications as composting toilets, heating water and benzene removal from the earth. Their main functions are for solar space heating of homes and buildings.

Blowers and Controls

When installed properly a 4 foot x 8 foot air collector can provide 25 to 50% of the energy needed to heat a 400 to 800 square foot space depending on climate and building construction. This makes air collector systems ideal for zone type heating systems.

Air Movement

Hot air rises and cold air falls. This concept of air movement, called natural convection, is important in any space heating project no matter what fuel is used.  If a solar heating system works with natural convection, hot air rising and cold air falling, then the result in comfort level can exceed expectations.

The Installation

Any good space heating system is designed around the fact that we live near the floor and the heat is at the ceiling. 

The best features of air collector systems are simplicity and reliability. The collectors are relatively simple devices. A well-made blower can be expected to have a 10 to 20 year life span if properly maintained, and the controls are extremely reliable. Since air will not freeze, no heat exchanger is required. By using packaged insulated flex duct, the installation can be accomplished by anyone with a little mechanical ability. All this adds up to a quick payback on the cost of the equipment and gives you years of warmth with safe clean solar heated air.

Water Heating With Air Collectors

The heat from the solar heated air is transferred to the water. Although air collectors are not quite as efficient as liquid collectors in heating water, the systems are simpler and require less maintenance than glycol water heaters,. An air collector solar water heater uses the same type of differential control as a liquid system, but the differential is usually higher to account for the lower heat capacity of air.

Collector Tilt and Orientation

The sun is most intense at noon every day when it is highest in the sky. The four to five hours in late morning and early afternoon are called the "Solar Window". This is when more than 80% of the total collectable energy falls on a solar collector. Collectors should face as true south as possible towards the solar window. A variation of up to 30* east or west of true south is acceptable. If the collector(s) are faced more than 30* off true south more collectors will be required. If a choice of east or west is available it is usually best to orient the collectors to the west. The outside temperatures are almost always warmer in the afternoon. Prevailing local weather patterns should also be considered in mounting the collector. For instance, if you are in an area where mornings are normally clear and afternoons are cloudy, it would be best to face the collector in a more easternly direvtion.

A properly orientated solar collector can increase system payback up to fifty percent quicker than a haphazardly installed system.