The Pros and Cons of Drain Down and Drain Back Solar Water Heaters

A drain-back solar water heating system drains water in panels into a tank when there's no heat available from the sun. The panels are empty of water, then, and cannot freeze. A non-pressurized tank is used to capture this water, and a pump refills the panels when the sun's warmth is detected. This type of system requires little maintenance and is durable.

What are some advantages to the drain-back solar water heating system?

• Since panels are empty they do not overheat.
• Thermosiphon will get the heat to your waterheater.
• Durable
• Requires little maintenance; and
• Cannot freeze.

What are the disadvantages to the drain-back solar water heating system?

• The disadvantages are most evident in off-grid systems, where the energy used in pumping is relatively high. This is because the pump must be sized to fill the collectors daily rather than just circulate water through them. As well, the tank must be located below the panels so the water that is drained back will have a place to go. This is my favorite choice of a system for freezing climates.

What are the advantages to the drain-down system?

• High efficiency,
• Freeze-proof, and
• It uses a small pump with small energy use.

What are the disadvantages to the drain-down system?

• Lots of expensive parts, including a complex controller,
• Requires periodic inspection and maintenance, and
• The daily dumping of water from the collectors onto the ground can be problematic.

How much does a new drain-back system cost?

The drain-down solar water heating system is a variation on the drain-back system. In the drain-down system, the water is dumped onto the ground. It uses a Sunspool™ valve to fill the panels for operation. The same valve, when it reaches a lower temperature, opens to dump the water that's in the panels onto the ground. This is a fairly common design you may have seen in older systems.

A drain-back system may cost approximately $3,500 in parts and labor. This amount is the same a homeowner may spend in 8.5 years of electricity use and 15 years for propane use for a "regular" hot water system. However, these figures do not include the costs of maintenance, rising costs of utility, or lost interest on investment.

Sources:
Infinite Power Renewable Energy of Texas
Build it Solar
Interview
Arizona Solar Center