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Solar Domestic Hot Water systems are a great way to save money, cut down on the use of fossil fuels and do a big favor for the environment. We have install many of these systems over the last two years and they work very well, even in the middle of winter.
I decided to install drain back systems because I like their simplicity and their easy maintenance. The average home owner can very easily keep track of the water in the sight glass and add water if needed. They perform well and when properly installed are pretty much bullet proof. I like that.
AET AE-40 collectors on roof
The system components consist of Flat Plate collectors:
AET AE-40 collectors
These are Alternative Energy Technology AE-40 collectors. They are elevated slightly from the roof pitch to facilitate snow removal and better drain back performance. They are also tilted to the left so that the water drains out of the bottom of the collectors when the pump is off. This is a very important detail to avoid freeze damage.
pipe to and from the collectors on the roof
The piping is 3/4 L copper tubing insulated with closed cell (AKA Rubatex) R-5 foam insulation. Where ever possible, the insulation is slid over the ends of the pipe instead of slit lengthwise. The ends and any slit pieces are glued together with special glue called R-420. The exterior runs are covered with PVC jacket to protect the insulation from UV damage and improve the system appearance.
AET 10 gallon stainless steel drain back tank and pumps
The drain back tank is mounted on a shelf attached to the basement wall. This is a 10 gallon stainless steel drain back tank with an internal heat exchanger. It has a sight glass which is marked with the proper fluid levels for when the system is running and when it is off.
The solar loop pump is a TACO 009BF5. I use bronze pumps in the solar loop of a drain back because the water gets sloshed around quite a bit and becomes oxygenated. A cast iron pump will rust and foul the site glass. It also keeps the solar loop a “potable water system” and thus avoids and questions about the single wall heat exchanger in the drain back tank. The pump is mounted below the lowest fluid level in the drain back tank. At the very bottom of the solar loop is the drain valve.
The storage tank loop is a TACO 006B4. This is a slightly larger pump that normal because the storage tank is located about 15 feet away in another room. This configuration is slightly unusual, however, it was the only way to fit the solar system in a crowded basement.
In the storage tank loop there is an air vent at the highest point in the loop to bleed out any air that may become trapped in that loop. Trapped air can cause pump cavitation and or reduce the flow in the loop. For maximum efficiency, the loop needs to move about 4-6 gallons per minute from the bottom of the storage tank through the heat exchanger and back to the top of the storage tank.
80 gallon solar storage tank
The storage tank is an 80 gallon off the shelf unit with a 12 year tank warranty. It has electric back up elements which are not connected because the home owner has an indirect oil fired tank connected to their home heating system.
The system controller is a DTC-2 by IMC. I really like these controllers because they have temperature reading for the storage tank and the collectors. They also have variable set points for the high limit and temperature on differential.
Watts 1170 tempering valve on output SDHW system
Finally, the output to the backup heating tank has a Watts 1170 tempering valve. This is very important because the solar storage tank temperatures can get very high durring the summer months. With out a tempering valve scalding water can be sent to the showers and sinks in the house.
Every time I commision one of these systems, I think to myself “There is less oil.”
