Although drainback systems work well in cold climates, most active systems in use today are pump-driven systems that employ polypropylene antifreeze as the heat transfer fluid. These systems are, therefore, indirect or closed-loop systems (Figure 3-11). In order to prevent the mixing of the heat transfer fluid with domestic hot water, these systems require heat exchangers. They transfer heat from the antifreeze to the water in the solar hot water tank.
Closed-loop antifreeze systems require other components as well, some of which are not required in other SDHW systems. For example, as shown in the diagram below, closed-looped antifreeze systems require an expansion tank. Located in the antifreeze loop, it accommodates the expansion of the antifreeze as it heats up during normal operation, preventing pressure from building to dangerous levels inside the closed-loop portion of the system. When the antifreeze heats up it expands, creating internal pressure. Rather than splitting the pipes open, excess is shunted into the expansion tank, reducing pressure. When pressure drops, the antifreeze empties from the expansion tank.
This system uses a small electric pump to propel antifreeze (food grade propylene glycol) through a closed loop. Heat from the antifreeze solution is released at the heat exchanger, warming water in the solar hot water tank.
Closed-loop antifreeze systems require other components as well, some of which are not required in other SDHW systems. For example, as shown in the diagram below, closed-looped antifreeze systems require an expansion tank. Located in the antifreeze loop, it accommodates the expansion of the antifreeze as it heats up during normal operation, preventing pressure from building to dangerous levels inside the closed-loop portion of the system. When the antifreeze heats up it expands, creating internal pressure. Rather than splitting the pipes open, excess is shunted into the expansion tank, reducing pressure. When pressure drops, the antifreeze empties from the expansion tank.
Closed-loop antifreeze systems also require a fill valve, a valve that allows service personnel to drain the antifreeze if it goes bad and then refill it.
Pros and Cons of Pump Systems with Antifreeze
Closed-loop antifreeze systems are popular, well understood by those who have been in the industry for a while, and reliable. They work well in all climates, hot or cold, and in cold climates provide excellent protection against freezing.
On the downside, closed-loop systems are the most complex of all hot water systems on the market today. They have more parts and, almost without exception, the more parts there are, the more chances there are for things to go wrong.
Another small downside of this system is that its use of a heat exchanger means that it functions slightly less efficiently than an open-loop pump-driven system in which water serves as the heat exchange fluid. In addition, propylene glycol needs to be replaced by a professional from time to time. This isn’t a job for most homeowners.
What is a Solar Hot Water System?
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