10 Considerations for Smart Solar-Heating System Design: Page 3 of 4

7. Percent of Load

Calculate your heating loads. Each person in the home will typically use about 20 gallons of hot water per day. This usually covers bathing, cleaning, and laundry. But “typically” does not fit all: If you have a big bathtub, add accordingly. If you are more conservative than most, then subtract accordingly. If you’re not sure how much hot water you use on a daily basis, install a water meter on the cold water line that feeds your existing water heater. Master Meter makes quality water meters (about $60) that can be bought online.

Determining your space-heating load is more complex. First, do not use your current heater’s size—your boiler may have a 100,000 Btu per hour input, but this does not mean it consumes 100,000 Btu per hour since it cycles on and off. This could be one cycle lasting two minutes every hour in the spring and fall, to one cycle lasting 50 minutes every hour during the height of the heating season. This fluctuating load makes determining the heating load difficult.

The best way to calculate heating load is to use your monthly heating bills or have a heat loss calculation done for your building for every month of the heating season. This information will help you determine how much energy is needed to keep your building comfortable. Next, determine how much of this heating load you want to cover with solar during the winter months. Some people choose to cover all of the winter heating load, but for a more moderate and efficient system, most people aim for covering the spring and fall seasons. This is a common practice where the winter months bring cloud cover. In areas like the Southwest, however, a high solar fraction is possible even in December.

For pool heating, knowing the pool’s surface area is most important, because this is where the majority of heat loss occurs. Is it an outdoor or indoor pool? What months of the year will it be heated? Outdoor pools can gather heat from the sun but can also lose heat to cool ambient temperatures, wind, and evaporation. Indoor pools lose less heat because they are sheltered, but depending on the ventilation system of the building, a lot of heat could be pumped outside to keep the humidity levels low inside the pool room. Another big factor is using a pool cover, which can save up to 50% of the daily heating energy. It works like a comforter, keeping the pool cozy and warm, and, at the same time, slowing evaporation. Armed with this information, you can use a sizing program provided by most solar pool heating manufacturers to determine how many collectors you will need. If you have a separate electric or gas meter for your existing pool heater, you can look at your utility bills to determine the amount of energy it currently consumes.

8. Collector Type

Choose your collectors. The common types are unglazed, glazed flat plate, and evacuated tube. Each works best for a certain job in a certain climate. Unglazed collectors are generally used for year-round pool heating in a warm climate or for seasonal pool heating in cooler climates. Flat plates can be used for many jobs, such as year-round indoor pool heating and domestic water and space heating in most any climate. Evacuated tubes can be used for the same jobs as flat plates, but work best for higher-temperature applications in cloudy or very cold climates.

Once you have decided on a collector type, you will need to choose the model. Many factors should go into your decision, such as sizes available, energy output, and the quality of the components and finished product. Consider the collector’s (and manufacturer’s) reputation, its country of origin (if you like to keep it local), warranty, and, least of all, price. If you simply shop by price, more than likely you will end up disappointed with your purchase. What you are really looking for is the most Btu you can get per dollar. If your collector is cheap, but only lasts a few years, it has not paid you back. Select a collector that balances performance and cost.