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Simple modifications to an electric water heater can save space and money in your SDHW system.

One of the first challenges, when installing a solar domestic hot water (SDHW) system, is where to put the solar storage tank. Many homes just don't have the space for an extra tank. For households installing a SDHW system with an external heat exchanger and 100 square feet (9.3 m2) of collector or less, using a single tank may be the solution. Here is how you can retrofit an oversized electric water heater to accommodate both solar hot water storage and electric backup in one tidy package.

The one-tank strategy may cost you some efficiency if solar storage volume is undersized for the collector area. Insufficient storage volume causes collectors to operate at higher temperatures, which increases heat loss and lowers efficiency. The system may also shut off earlier in the day as the tank reaches its temperature limit. Stay within the generally accepted guidelines in the storage volume table.

An 80 gallon (303 l) electric tank will give you about 55 gallons (208 l) of solar storage and 25 gallons (95 l) of electric backup. That corresponds to approximately two, 3 by 8 foot (2.3 m2) collectors in the Midwest or a single 3 by 8 in the

Southwest. The largest single tank readily available has a 120 gallon (454 l) capacity, which will give you approximately 80 gallons of solar storage and 40 gallons (151 l) of backup. However, a 120 gallon tank will typically cost more than two smaller tanks.

The Retrofit Strategy

When you turn on a hot water tap in your house, hot water comes from the top of the tank and cold water enters the bottom of the tank from the supply line. A standard electric water heater has two 4,500 watt heating elements,

Storage Volume Recommendations

Region

Gallons per ft.2 of Collector Area

Region

Gallons per ft.2 of Collector Area

South & Southwest States

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