First of all, a DHW or SDHW system is a solar heating system that heats your water that is used for drinking, bathing, laundry, etc. Usually, but not always, this water gets heated in a water tank, often called the solar pre-heater tank, that is separate from the standard gas or electric water heater. If the solar system cannot heat it to the desired temperature, the standard water heater is then called upon to bring the water up to the final desired temperature.

Next, a solar space heating system simply refers to a solar system that is used to heat the space in which you are living. This can be accomplished in several ways. Probably the simplest way is when a solar collector is heated by the sun. Air is then drawn over the collector, heated, and then distributed into a room that needs to be heated. This type of system, called an air system, has no heat storage system and, therefore, supplies heat only when the sun is shining.

A different type of air system will collect heat in a similar fashion to that described above, but in addition, will deliver heat to a heat storage system. Such a storage system usually is made up of a large pile of rocks contained in a strong vented box. As the air, heated by the solar collectors, is passed over the rocks, the rocks are in turn heated. Later in the day when heat is needed to warm a room but not available directly from the solar collectors, the flow of air is directed over the rock storage. The rocks give up their heat to the air, which is then delivered to the room.

Although not very popular, there are also air systems that are used to heat DHW. In such a system, air, heated by the solar collectors, is directed over a device called a heat exchanger. The heat exchanger is very similar to an automobile radiator. As the heated air flows over the heat exchanger fins which are in contact with tubes containing water, heat is transferred through the fins to the water that is being pumped through the tubes.

More common are the liquid space heating systems. This simply means that some sort of liquid is used to collect heat from the solar heat collectors. These different liquids, called the heat collection fluids, will be discussed in more detail below. The heat collection fluid is used to heat water which is usually stored in some sort of storage tank. When heated to a high enough temperature, the water in the storage tank can be used to heat a living space.

There are two common ways to accomplish this. One way is to pump the heated water to a unit called a heat convector. This is a box that is usually mounted along a wall and contains a small fan and yes, you guessed it, a heat exchanger. The fan blows cool room air over the heat exchanger while a pump pushes heated water from the storage tank through the pipes within the heat exchanger. During this process, the air passing over the heat exchanger is heated and directed back into the room.

A second common way to deliver heat from a liquid system to a living space is with a standard forced air furnace blower fan. This works just like the method described in the paragraph above except in this case, the fan is the furnace fan, and the heat exchanger is mounted in either the cold or warm air plenum leading to or from the furnace. Warn or hot water is pumped to the heat exchanger from the solar storage tank which has been heated by the heat collection fluid. This type of system is usually controlled so that when solar heat is available (solar storage water temperature is above some pre-set level) the living space will be heated with solar-heated water, and when the solar heated water is below the pre-set level, the standard furnace will take over to heat the house.

The remaining terms mentioned above, closed-loop-antifreeze systems, drain-down systems, drain-back systems, and recirculation systems refer to the method in which heat is removed by a liquid from the solar collectors.

A closed-loop-antifreeze system will use some type of liquid that won't freeze at the minimum expected temperatures that will be experienced by the solar collectors. The most common of these liquids is a mixture of propylene glycol and water. Less commonly used are a synthetic oil called Bray Oil and silicone oil. Mineral oil has also been used in some installations. A closed-loop-antifreeze system is totally closed, i.e., once the liquid is pumped into the collector heating loop and brought up to the proper operating pressure, the appropriate valves are closed. If all joints are tightly sealed and barring a piping system failure, no air will enter the system and no liquid will leak out. The liquid will be circulated through the collectors and heated when the sun is shining on them. As the liquid is heated, it will then carry this heat to the solar storage tank where the heat will be given up to the cooler water within via a heat exchanger. This type of heat collection loop is found in both SDHW and space heating systems.

Another method of collecting solar heat is called a drain-down system. Water is the liquid that is pumped through the solar collectors for this type of system. The water must not be allowed to stay in the collectors when there is any chance of freezing temperatures. A valve, called a drain-down valve, is used with these systems and allows the collector loop to drain the water out to prevent a collector freeze up. This drained water is usually routed to a floor drain and hence, wasted. The drain valve, being an electro-mechanical device, is subject to failure and if a failure occurs during a period of freezing weather, the collectors are likely to freeze. Collectors can be repaired, but it is an expensive repair job. Consequently, drain-down systems are not widely used in colder parts of the country.

The drain-back system also uses water as the heat collection fluid. However, this type of system is designed so that as soon as power to the fluid circulation pump is removed, the water will drain from the collectors back into a storage tank. At the time of draining, air contained within the drain-back tank, goes up to the collectors as the water from the collectors drains into the tank. For both the drain-down and drain-back systems it is critical that the collectors and all pipes leading to and from the collectors be sloped to allow the water to drain completely . Any water trapped within the collectors or improperly sloped pipes will be subject to freezing and an expensive repair bill.

The final liquid heat collection system is called a recirculation system (or recirc system). Water, the heat collection fluid used with this type of system, is never automatically drained from the heat collection loop. As a result, such a system should never be used where freezing temperatures are to be expected on a regular basis. These systems should be limited to the very southern parts of the U.S. Provisions for manual draining of the system are a must since even the very southern parts of the U.S. can experience a freeze now and then. Some recirculation systems have controls that allow the heat circulation pump to turn on to circulate the water through the collectors during freezing conditions. This simply circulates warm water from the solar storage tank to the solar collectors, and this keeps the collectors from freezing. This works well as long as there is power to keep the pump running and enough warm water in the solar storage tank to last out the freeze spell.

Further in depth descriptions of DHW, Space Heating, Pool, and Spa/Hot Tub heating are contained on their respective information pages. Just click on the type of system that is of interest to you for more information.

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