Solar Hot Water Basics

Solar hot water systems use basic principles and components to capture incoming solar radiation and heat water for domestic and other uses. Over the years, a variety of system designs have been developed and tested to meet specific consumer needs and environmental conditions. Sometimes, particularly for a new solar customer, the vocabulary and options can seem a bit overwhelming. This section provides background information on the common uses for solar hot water systems, system types, sizing and system components. Solar hot water professionals can help you further determine the system that is best for you.

Frequently Asked Questions
How are Solar Hot Water Systems Used?	What are the Most Common Solar Hot Water System Designs?	What are the Most Common Components in a Solar Hot Water System?
Domestic Hot Water	Closed Loop Glycol	Collectors
Space Heating	Open Loop Drainback	Tanks
Pool Heating	Open Loop Seasonal Batch	Controls / Circulators
Commercial Applications		Heat Exchangers
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Solar hot water systems are most commonly used to heat water for basic household needs such as laundry, bathing, dishwashing and cooking. These systems are commonly referred to as "domestic hot water systems".

Domestic hot water systems typically use solar energy to pre-heat the water that is incoming to a conventionally fueled heating tank. The warmer the water from the solar heater, the less conventional fuel will be needed to provide the household's hot water needs. During the summer months in Vermont, a properly sized solar hot water system will provide almost 100% of a household's needs. In the winter, or during extended cloudy periods, the amount of hot water provided by the sun may be 30% or less. Vermont's solar hot water professionals tend to install systems that meet between 60% and 70% of the annual load.

The size of the solar collectors suitable for your site will depend upon the manufacturer and your hot water usage, but typically they will require between 50 and 100 square feet of mounting area. The most common mounting technique used today is install the collectors flush with the roof on a south facing exposure. Alternatives, such as ground mounting, or rack mounting on gable ends are also sometimes possible.

Solar water heaters can also be used to provide space heating. The same set of solar collectors can be used to provide hot water for both space heating and space heating needs, although space heating will generally require a much greater collector area and storage capacity. Additional controls and heat exchangers are also needed. Due to these extra costs, and because sunshine is relatively scarce when heating loads are highest (for example at night and during the winter) solar energy is more often used to heat domestic water than it is for space heating.

A solar water heating professional with past experience in installing systems sized to provide space heat can help you consider the pros and cons of various options. You should also make sure to carefully consider passive solar and other building efficiency measures that will reduce your heating loads to help you take maximum advantage of the available solar resource.

Solar heaters are often the most economical way to heat a swimming pool. Compared to conventional pool heaters using propane or oil, solar pool heating systems can pay for themselves in four years or less. If you currently don't heat your pool, a solar heating system can provide an economical way to extend your pool season, starting earlier in the spring and extending later into the fall.

Solar pool heaters work by circulating pool water directly through collectors and then rerouting the warmed water to the pool. System controllers sense when collectors are warmer than the pool water, and open valves diverting water from the pool circulator through the collectors and then back into the pool. The controller can be set to automatically keep the pool temperature anywhere between 65 and 100 degree Fahrenheit. The collectors used for pool heating systems are often less expensive than those used for domestic hot water systems, providing significant economic advantages.

Roof mounted solar pool heating system.
Credit: DOE/NREL and Aquatherm Industries

Commercial facilities with high hot water demands and access to a good southern exposure can be great candidates for solar hot water. Restaurants, bakeries, beauty salons, health clubs, and hotels are all potentially good sites. A commercial installation generally makes use of the same system design and components as residential systems, including a conventional back-up for hot water heating during high load and low sun periods. Solar hot water professionals can help you determine the applicability of solar for your site.

Closed Loop - Glycol System
Closed loop systems use a heat-transfer fluid to collect heat and a heat exchanger to transfer the heat to household water. Active closed loop systems use electric pumps, valves, and controllers to circulate the heat-transfer fluid, usually a glycol-water antifreeze mixture, through the collectors. This glycol-water antifreeze mixture makes closed-loop glycol systems effective in areas subject to freezing weather. For this reason, closed loop systems are preferred for year round use in Vermont.

Closed Loop - Drainback System
Drainback systems use water as the heat-transfer fluid within the collector loop. The water is forced through the collectors by a pump and then is drained by gravity to the storage tank and heat exchanger. These systems have no valves to fail and when the pumps are off, the collectors are empty, thereby assuring freeze-protection and auto shut-off if the water in the storage tank becomes too hot.

Open loop, seasonal, batch
Open loop systems heat and circulate household (potable) water directly in collectors prior to distribution in the household. One type of open loop system is a batch heater that is simply a black tank filled with water and placed inside a south-facing, insulated, glazed box, where it absorbs solar energy. The tank may incorporate a selective surface that that absorbs sun well but inhibits radiant loss. In climates where freezing occurs, batch heaters must either be protected from freezing or drained for the winter. Batch heaters are inexpensive and have few components, therefore they require less maintenance and experience fewer failures. These systems are good economical choices for seasonal applications such as summer camps.

Solar hot water systems are made up of collectors, storage tanks, piping, controls, and in some cases pumps. Active systems use pumps to circulate water or other heat transfer fluid through the system. Passive systems have no pumps and rely on gravity or natural convection to circulate water depending on the system.

Collectors
The most commonly used collector is the flat-plate collector. It is an insulated, weatherproofed box, made of metal or plastic, containing a dark absorber plate beneath a translucent cover (typically tempered, low-iron glass). Copper piping carrying heat exchange fluid travels in an S-shaped pattern between the absorber plate and translucent cover. Typically, metal collectors are more sturdy, fire resistant, and expensive to manufacture and ship than plastic collectors. Contact your local solar contractor to learn which collector is best suited for your application.

Tanks
Most solar hot water systems require a well-insulated storage tank. Many systems use converted electric water heater tanks for storage or plumb the solar storage tank in series with a conventional water heater.

Controls, Circulator
A differential temperature controller monitors the temperatures at the solar collector outlet and at the storage tank. When the collectors are warmer than the tank, the control turns on a circulator which circulates a heat-transfer fluid, usually water or a water-glycol mixture, through the collectors and back to the heat exchanger located in or adjacent to the storage tank.

Heat Exchangers
A heat exchanger transfers heat from the heat-transfer fluid (usually either water or a water-glycol antifreeze mixture) to the household water supply. The heat exchanger itself is a series of copper coils submerged in the water storage tank or a compartment outside of the storage tank.