Freeze Protection For Closed Loop Systems
Solar hot water systems are a great way for homeowners to heat their water using renewable energy. Homeowners in northern climates, however, must be sure that their system contains a solar antifreeze solution.
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The most common solar antifreeze solutions used are ethylene glycol and propylene glycol. Propylene glycol is the preferred choice because it is not considered toxic. Glycol-water mixtures have a 50/50 or 60/40 glycol-to-water ratio. The solar antifreeze circulates through an independent loop between the collector and the hot water storage tank and prevents the fluid from freezing. The solar water heating system is then safe from damage and continues to function in low temperatures.
Because the solar antifreeze circulates on an independent loop, it never comes into contact with the water that is being heated. This means that there is no chance of the household’s drinking water being contaminated with the solar antifreeze solution. Solar hot water systems that use antifreeze are just as safe as any other.
Using solar antifreeze does have its disadvantages, however. First, systems that use solar antifreeze (i.e. active and indirect solar water heating systems) are more complex than those that do not (i.e. passive and direct or active and direct systems). This means that a system which uses solar antifreeze is more expensive, larger, and harder to maintain than a direct solar water heating system. Unfortunately, the use of antifreeze is unavoidable in areas with severe winters. The only alternative is to shut the system down and drain it automatically when temperatures are low (known as drainback-systems).
Systems that use solar antifreeze also have more maintenance needs. Antifreeze does not last forever; the solar antifreeze solution should be drained and replaced every three to five years.