The majority of people in North America and the world live in climates where freezing temperatures are at least a possibility. Solar collectors are normally installed outside and copper tubes are subject to bursting when water freezes and expands. This alone makes untreated water unsuitable as a heat transfer fluid in solar collectors. Ethylene Glycol, Propylene Glycol, Silicone Fluid and a variety of oils have been used successfully in the past as heat transfer fluids. The lower specific heat (ability to hold heat) of silicone and bray oil and their limited availability and high price discourage wide usage today. Ethylene Glycol, better known as car antifreeze, is a good choice except for its toxicity. We prefer a fifty percent solution of non toxic propylene glycol and water as a heat transfer fluid in freezing climates.

A glycol system contains two entirely separate piping systems which are called loops. The two piping systems or loops are connected through a device called a heat exchanger. A simple, economical heat exchanger can be constructed of a 1/2" copper pipe inside a 3/4" copper pipe. If the hot glycol solution is made to flow through the larger tube and cold water through the smaller, the heat will be exchanged and the water heated. This is called a single wall heat exchanger. Single wall heat exchangers are prohibited by the national plumbing code since breaching one pipe could possibly contaminate the domestic water loop if the glycol loop contained a toxic fluid. All glycol systems should have a design where two pipe walls must fail in order for the glycol loop fluid to mix with the potable (drinkable) water system. Although double wall heat exchangers are more complex and expensive, they should always be used in these systems.

A low head pump (3' to 12') is used to circulate the glycol solution. The water in the DHW (Domestic Hot Water) loop can be circulated through the heat exchanger with another low head pump or configured to thermosyphon. Thermosyphoning uses the physics that hotter liquids are lighter and will naturally rise. If the heat exchanger inlet connection can be placed and connected under or near the bottom of the storage tank and the outlet piped to the top of the tank the water will thermosyphon through the heat exchanger. The water will only flow if it is being heated by the glycol loop.

When any liquid is heated it expands physically. In open loop systems this expansion is not a problem since the extra pressure is absorbed through the entire piping system in the house, well system or city water system. The glycol loop is closed and an expansion tank of the proper size is required to prevent excessive pressure in the loop.

A differential control is needed if two pumps are used in the glycol system. If the system can be installed with a thermosyphon DHW loop a simple snap disc switch or light sensing switch will normally be workable. If P.V., D.C. power is used with a D.C. pump(s) and the components are sized correctly a control is usually not necessary.

Periodic maintenance of glycol systems must address two things, the condition of the glycol and the hardness of the domestic water. Glycol solutions can turn acidic after a few years of use. The solution should be checked with Litmus paper or a P.H. meter every three to five years. Acidic glycol solutions should be drained, purged with water and refilled with a new solution. Acidic solutions are usually caused by overheating the glycol loop above 225°F. If the solar system is not used for an extended period of time, the collector loop should be drained and the fluid stored or the collector glazing should be covered. If hard water with high mineral content is circulated through the DHW loop, the tubes in the heat exchanger will eventually clog. This should be checked every three to five years depending on water hardness and corrected with a light acidic solution (vinegar or intech 52) circulated through the tubes until the deposits are gone. A heat exchanger that can be taken apart can be cleaned with a wire brush. A correctly designed solar water heater of this type can produce hot water in virtually any climate for decades.

Home Heating Systems using liquid collector glycol systems

About us - AAA Solar has been distributing solar and wind energy products for over twenty two years. We have sold or installed over ten thousand solar and wind energy systems. We distribute a full line of alternative energy products including solar electric systems, wind turbines, solar hot water heaters, solar home heating systems, swimming pool heating systems and solar ovens. Many systems are available in kit form for the do-it-yourselfer. Our installation and service department is a fully licensed and bonded New Mexico contractor with plumbing, electrical and heating and air conditioning licenses. We install and service solar systems and high efficiency heating and cooling appliances throughout the state of New Mexico. We occasionally install and or service systems in neighboring states. Please call us or e-mail for more details.

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