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Glycol/Closed-Loop Solar Hot Water

In closed loop systems, there are two separate fluids the flow in two loops that are not interconnected. In the first loop, the loop that flows through the solar collector, a fluid that is resistant to freezing, such as propylene glycol is often used. Though the loops never come into contact during normal operation, a non-toxic fluid is used so that if a leak occurs, there is no chance of harm.

In the second loop is the potable water. This loop usually begins in the solar storage tank where the heat exchanger is located. The heat exchanger is responsible for transferring heat from the first loop to the second. Water from the solar storage tank is potable and is usually routed to a second tank for secondary heating as necessary.

The advantage of a glycol system is that it has a much lower freezing point that water and can thus be used in cooler climates.  These systems also share many of the advantages of open-loop systems such as low profile solar collectors and flexible design.

There are two drawbacks to glycol systems. The first is that they are less efficient than direct systems. So heat is inevitably lost during the heat exchange process, somewhat diminishing the efficiency of the system as a whole.

The second drawback is the glycol itself which, after repeated heating and cooling cycles, can become viscous and difficult to circulate. Glycol must be replaced periodically, much like the antifreeze in a car. Unlike antifreeze in car, the glycol in a hot water system does not have preservatives and chemical stabilizers added to it because they are toxic. Glycol must be changed on a yearly or every other year basis in most cases to ensure peak operation.

As a final point it should be noted the glycol systems are pressurized. That is to say, there is no pump for the first loop (only the second to bring in potable water). As such, these systems, when drained and refilled, must be pressurized and all air must be evacuated. Again, this is just as in car. This is usually best accomplished by a professional.

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