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Solar Hot Water

Solar water heating, solar hot water and solar thermal are terms used to describe systems that use the sun’s energy to heat liquids. Unlike solar PV systems which use the light of the sun to generate electricity, solar hot water systems use the warmth of the sun to heat liquids without using electricity. Solar thermal systems can heat water for a wide variety of uses, including swimming pools, radiant heating, and domestic, commercial or industrial hot water. This proven and reliable technology offers long-term performance with little to no maintenance.

Investing in a solar hot water system is a smart choice for most homeowners. To get the most for your money, you´ll want a properly sized system that offers the best performance in your climate. The right solar water heating system can provide all, or at least a significant amount, of your household hot water needs for much the year, saving you 60 to 70 percent off your water heating costs (when compared to standard electric hot water). Solar hot water collectors are more than three times as efficient at producing energy from the sun, and the system cost is typically far less than for a solar electric (PV) system. When combined with federal, state, and utility incentives that are available in many areas, these systems offer a quick payback—in some cases, only four to eight years.

First Reduce, Then Produce

Your best hot water savings will come from no or low cost options, so before you invest in solar hot water, take these steps:

  • Turn your hot water setting down. Hot tubs are usually set at around 106°F, but most water heaters are set between 140 and 180°F. Try reducing your hot water tank setting to 125°F and see how you do.

  • Bundle up! Insulated water heater “blankets” can reduce energy loss by 25% to 45%, which means this inexpensive measure will pay for itself quickly. But be careful with natural gas or propane fired water tanks that use an open flame to heat the water. You will need to provide a space for air at the bottom of the tank, and at the top where the flue exits the tank.

  • Fix those drips. They may not look like much, but they are a constant and persistent drain on your water heating load, and they waste water too.

  • Use flow restrictors and faucet aerators to reduce your hot water consumption.

  • Find other ways to use less hot water. Wash in cold water whenever possible, and only wash full loads of clothes and dishes.

  • Insulate your hot water pipes.

Solar Hot Water Basics

Solar hot water systems typically include a set of solar thermal collectors, a water storage tank, interconnecting pipes and a fluid system to move the heat from the collector to the tank. A solar hot water system may use electricity for pumping the fluid, and have a reservoir or tank for heat storage and subsequent use.

In temperate climates solar hot water systems may provide insufficient warmth for structural heating systems or domestic hot water, since in winter, when most heating is needed, little heat is available from the sun. This limitation can sometimes be resolved by using an evacuated tube system, or by storing solar heat in the ground or in groundwater (seasonal thermal store).

Solar water heating systems are either active or passive. An active system uses an electric pump to circulate the fluid through the collector; a passive system has no pump and relies on thermosiphoning to circulate water. The amount of hot water a solar water heater produces depends on the type and size of the system, the amount of sun available at the site, and the installation angle and orientation.

Solar hot water systems are also characterized as open loop (also called “direct”) or closed loop (also called “indirect”). An open-loop system circulates household (potable) water through the collector. A closed-loop system uses a heat-transfer fluid (water or diluted antifreeze) to collect heat and a heat exchanger to transfer the heat to the household water. A disadvantage of closed looped systems is that efficiency is lost during the heat exchange process.

Active Systems

Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors. They are usually more expensive than passive systems but are more efficient. Active systems are usually easier to retrofit than passive systems because their storage tanks do not need to be installed above or close to the collectors. If installed using a solar PV panel to run the pump, an active system can operate even during a power outage.

Open-Loop Active Systems

The simplest of all active systems, open-loop systems use pumps to circulate household potable water through the collectors. This design is efficient and lowers operating costs, but is not appropriate if water is hard or acidic because scale and corrosion will gradually disable the system.

Open-loop active systems are popular in regions where freezing never occurs. Flat plate open-loop systems should never be installed in climates that experience sustained periods of subzero temperatures. The Apricus AP solar water heater can be installed in an open loop in areas that experience sub-zero temperatures as long as the solar controller has a low temperature function.

Closed-Loop Active Systems
These systems pump heat-transfer fluids (usually a glycol-water antifreeze mixture) through the solar water heater. Heat exchangers transfer the heat from the fluid to the water that is stored in tanks. Double-walled heat exchangers or twin coil solar tanks prevent contamination of household water. Some standards require double walls when the heat-transfer fluid is anything other than household water.

Closed-loop glycol systems are popular in areas subject to extended subzero temperatures because they offer good freeze protection. However, glycol antifreeze systems are more expensive to purchase and install and the glycol must be checked each year and changed every few years, depending on glycol quality and system temperatures.

Drainback Systems

Drainback systems use water as the heat-transfer fluid in the collector loop, and a pump to circulate the water through the solar water heater. When the pump turns on, the water is sent from the reservoir back through the collector and heat exchanger, passing heat to the water in the solar tank. When the pump turns off, the solar water heater drains the water into a reservoir, which ensures freeze protection and also allows the system to turn off if the water in the storage tank becomes too hot. Drainback systems are effective and reliable, even on the hottest and coldest days of the year. They require the least amount of maintenance of any active system, operating twenty years or more without needing service.

A downfall of drainback systems is that the solar water heater and plumbing must be carefully positioned to allow complete drainage to eliminate freezing. Another potential downside is that larger pumps are usually required to provide sufficient head pressure to pump the water up to the collector, especially if you’re pumping water two stories or more. The result is electricity usage is slightly higher than in a sealed closed or open loop system.

One way around the height problem is to place the reservoir in the attic, reducing the height the pump has to lift. However, if it’s located in a place where the pipes going to and from the reservoir could freeze, glycol must be added. This is also done when long, horizontal pipe runs that do not allow drainback to occur quickly.

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Passive Systems

Passive systems move household water or a heat-transfer fluid through the system without pumps. Passive systems have the advantage that electricity outages and electric pump breakdowns are not an issue. This makes passive systems generally more reliable, easier to maintain, and possibly longer lasting than active systems. Passive systems are typically less expensive than active systems, but are also less efficient due to slower water flow rates through the system.

Thermosiphon Systems

A thermosiphon system relies on natural convection to circulate water through the collectors and to the tank, and is widely used with both flat plate and evacuated tube absorbers. As water in the collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, cooler water in the tank flows down pipes to the bottom of the collector, causing circulation throughout the system. In this type of installation, the storage tank must be mounted above the collector so that thermosiphoning can occur.

During periods of no sun, the cool fluid in the base of the collectors is not heated, does not rise, and thereby stops circulation of the antifreeze. In this manner, thermosiphon units achieve the operational characteristics of active systems but without the pumps and controls.

The advantage of this system over the batch heater is that solar heat is stored in a well-insulated tank, so hot water can be used any time, without the penalty of overnight losses. The disadvantages of this design are poor aesthetics because the tank is located and visible on the roof, and because the weight of the tank may create structural integrity issues.

Batch Heaters

Batch heaters are a simple passive system consisting of one or more storage tanks placed in an insulated box that has a glazed side facing the sun. Batch heaters are inexpensive and have few components, but only perform well in summer when the weather is warm, usually providing less than 40% of a family’s yearly demand.

Batch water heaters combine the collector and storage tank in one box. On a sunny day, sunlight travels through the glazing and strikes the tanks, which are painted black or covered with a heat-absorbing material. As the tanks absorb the sun’s energy, the water inside heats up. This “preheated” water goes directly into a conventional water heater.

Because of their relatively low cost and simplicity, the batch heater is a popular choice for homes in moderate climates where freezing is not much of an issue, because it provides the best value for heating domestic water. Commercially manufactured batch heaters are relatively low cost. Crude batch heaters can even be homemade. If batch heaters are installed on the roof, weight has to be taken into account. Commercial batch heaters can weigh 200 pounds (90 kg) dry, and when filled with 40 gallons (150 l) of water, more than 320 pounds (145 kg) is added.

The appearance of batch systems has been a major drawback, but units are now available that fit into the roof structure and resemble a skylight. Others can be enclosed within interior spaces, so they are less obtrusive and are protected from freezing.

Solar Thermal Collectors

There are basically three types of thermal solar collectors: flat-plate, evacuated-tube and concentrating.

Flat-Plate collectors, which have been around since the 1950s, are the most common type of solar collector in use throughout the world. These collectors are flat, much like a traditional solar panel, and consist of an insulated, weatherproof box under one or more glass or plastic covers (called glazing), a set of tubes, and a dark-colored absorber plate on the bottom.

How a Flat-Plate Collector Works

As sunlight passes through the glazing, the absorber plate heats up, changing solar energy into heat energy, heating a heat conducting fluid (air, anti-freeze or water) as it flows through the pipes in the collector. The absorber plates are commonly painted with “selective coatings,” which absorb and retain heat better than ordinary black paint. Absorber plates are usually made of metal—typically copper or aluminum—because the metal is a good heat conductor. Copper, which is a better conductor and less prone to corrosion than aluminum, is also used, but it is more expensive. In locations with average available solar energy, flat plate collectors are sized approximately one-half- to one-square foot per gallon of one-day’s hot water use.

Because the flat plate collectors have water moving directly through them, they are less freeze protected than an evacuated tube system, and are therefore usually recommended only for climates where freezing is not an issue. Additionally, because the flat plate collector is flat, it has a much narrower time frame for which to heat water with the power of the sun. The biggest advantage to flat plate collectors is that they are less costly than an evacuated tube hot water heater.


The main use of flat-plate technology is in residential buildings where the demand for hot water has a large impact on energy bills, such as in homes with a large family, in multi-family housing units, or where the hot water demand is excessive for other reasons.

Typical commercial applications for flat-plate collectors include laundromats, car washes, military laundry facilities and food establishments. This technology can also be used for space heating if the building is located off-grid or if utility power is subject to frequent outages. Solar water heating systems are most likely to be cost effective for facilities with water heating systems that are expensive to operate, or in operations such as laundries or kitchens that require large quantities of hot water.

Unglazed flat-plate collectors are commonly used to heat water for swimming pools. Because they do not need to withstand high temperatures, less expensive materials such as plastic or rubber can be used. They also do not require freeze-proofing because swimming pools are generally used only in warm weather or can be drained easily during cold weather.

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Evacuated tube solar collectors are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. They have rows of parallel transparent glass tubes, each of which contains a collector tube. As sunlight passes through the evacuated glass tubes it heats up the collector and, ultimately, a solar working fluid (water or an antifreeze mix—typically propylene glycol) in order to heat domestic hot water, or for hydronic space heating.

Evacuated tube collectors have several advantages over flat plate collectors.

  • They are more efficient because their rounded shape allows for 360 degree solar collection, and therefore the energy absorbed is approximately constant over the course of a day.

  • High quality units can efficiently absorb diffuse solar radiation present making them efficient even under cloudy conditions.

  • They have the same performance in similar light conditions summer and winter.

  • The vacuum within the evacuated tubes reduces convection and conduction heat losses, allowing them to reach considerably higher temperatures than most flat-plate collectors.

  • The tubes themselves do not contain any water or glycol, just a vacuum sealed area that transfers heat to a manifold through a copper heat pipe.

  • Gaps between tubes allows snow to fall through the collector, minimizing the loss of production in some snowy conditions, even though the lack of radiated heat from the tubes prevents effective shedding of accumulated snow.

There are several types of evacuated tubes:

Glass-Glass tubes consist of two glass tubes which are fused together at one end. The inner tube is coated with a selective surface that absorbs solar energy well but inhibits radiative heat loss. The air is withdrawn (“evacuated”) from the space between the two glass tubes to form a vacuum, which eliminates conductive and convective heat loss. These tubes perform very well in overcast conditions as well as low temperatures. Because the tube is 100% glass, the problem with loss of vacuum due to a broken seal is greatly minimized. Glass-glass solar tubes may be used in a number of different ways, including direct flow, heat pipe, or U pipe configuration.

Glass-Metal tubes consist of a single glass tube. Inside the tube is a flat or curved aluminum plate which is attached to a copper heat pipe or water flow pipe. The aluminum plate is generally coated with Tinox, or similar selective coating. These type of tubes are very efficient but can have problems relating to loss of vacuum. This is primarily due to the fact that their seal is glass to metal, and the heat expansion rates of these two materials is dissimilar. Glass-glass tubes although not quite as efficient glass-metal tubes are generally more reliable and much cheaper.

Glass-glass – water flow path tubes incorporate a water flow path into the tube itself. The problem with these tubes is that if a tube is ever damaged water will pour from the collector onto the roof and the collector must be “shut-down” until the tube is replaced.

Evacuated tube solar collectors are now an affordable and much more efficient alternative to either batch or flat plate collectors.

Please note:

  • In lower quality systems moisture can enter the manifold around the sheet metal casing, and may eventually be absorbed by the glass fiber insulation and find its way down into the tubes. This can lead to corrosion at the absorber/heat pipe interface area or freezing ruptures of the tube itself if the tube absorbs water.

  • The high stagnation temperatures in evacuated tube collectors can cause antifreeze to break down, so care must be used if selecting this type of system in temperate climates.

Concentrating Collectors

These types of solar hot water collectors are typically parabolic troughs that use mirrored surfaces to concentrate the sun’s energy on an absorber tube (called a receiver) containing a heat-transfer fluid, or the water itself. This type of solar collector is generally only used for commercial power production applications, because very high temperatures can be achieved. It is however reliant on direct sunlight and therefore does not perform well in overcast conditions.

Other Solar Hot Water System Components

Storage Tanks

A solar water tank can just be a modified water heater, but it is usually larger and very well-insulated. The cold water that would normally go directly to your conventional water heater enters the solar tank and solar-heated water exits. In closed-loop systems, the water is heated by contact with a coil of pipe containing the water or antifreeze that circulates through the collectors. In open-loop systems, the potable water is directly circulated through the collectors. The preheated solar water is then plumbed back to the cold side of your existing hot water tank, which now functions as a backup. Whenever hot water is turned on in the house, the preheated solar hot water is moved from the solar tank to the backup heater.

Backup Water Heater

A backup water heater, which is often just a conventional domestic hot water heater, ensures that hot water is available whether the sun shines or not. But not all backup water heaters use a tank. A tankless water heater eliminates the cost of keeping water warm between uses. Solar hot water systems and tankless water heaters are a winning combination, by reducing your hot water heating cost 75% or more. Not all tankless heaters can be used as a backup heater for solar.

Circulating Pump

Circulating pumps move water or antifreeze between the solar collector(s) and the storage tank. They are used in active systems, but are not required in batch or thermosiphon systems. The size of your system and the distance and height between the collector(s) and the storage tank will determine which pump is your best choice. AC pumps use standard household power; DC pumps are powered from a DC source, often a solar panel. Good pumps can last as long as 20 years, even with heavy use.

Heat Exchanger

Heat exchangers, which are used in closed-loop solar hot water systems, enable the transfer of heat from one fluid to another without the two mixing. Internal heat exchangers are located inside the tank and are not visible. External heat exchangers are usually a pipe within a pipe.

Expansion Tank

Closed-loop systems require an expansion tank to allow heated fluid to safely expand. Without an expansion tank, pressure builds and eventually something will blow! The size of the expansion tank needed depends on the total volume of fluid, which is determined by the number and size of collectors, and the length and diameter of the pipes in the solar loop. In most cases, a 15 gallon expansion tank is adequate, but it never hurts to go larger, especially for systems with a large collector footprint.


A control is needed in active systems using a pump to turn the pump on to circulate water. A differential thermostat control system or a DC-powered pump can be used for this function. The differential thermostat controller compares heat sensor readings from the storage tank and collectors and switches the pump accordingly. A DC pump is usually powered by a solar-electric panel connected directly to the pump. The DC pump setup is very simple: when the sun comes out, water is heated in the solar hot water system and the pump is on. When the sun goes down, water is no longer being heated and the pump shuts down.

Controls are not needed in batch heater systems, where energy is moved by simple water pressure, or in thermosiphon systems, where energy is moved naturally by heat rising.

Isolation Valve

Every solar water heating system should have valves so you can manually isolate your solar tank in the event of a problem, while still allowing your backup water heater to remain in service. Isolation valves should be placed in both the incoming and outgoing potable water lines to the solar tank. The isolation valves can also be plumbed to bypass the backup gas or electric water heater, allowing them to be turned off during the seasons when your solar hot water system can supply 100 percent of your hot water, thus eliminating standby loss.

Mixing (Tempering) Valve

A mixing valve is installed right after your backup water heater and before your faucet, to be used on sunny days when water in collectors can reach scalding temperatures. If the water coming out of the backup heater is too hot, the tempering valve opens to mix cold water back in and prevent scalding. The temperature of the hot water can be set by the user on most valves.

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Mounting Your Solar Hot Water System

The most common mounting systems for solar collectors are roof mount, ground mount, and awning mount.

Roof Mount

  • The most common mount used in residential applications.
  • Horizontal brackets are used for positioning.
  • The surface area and weight of the solar water panel must be considered.

Ground Mount

  • Installation is usually as simple as placing support poles in the ground.
  • In most instances brackets can be used for positioning.
  • The hot water solar panels’ size and weight is much less a concern.

Awning Mount

  • Attaches the collector to the top of an outside wall.
  • Horizontal brackets are used for positioning.
  • Overall surface area and weight of the panel is somewhat of a concern.

When choosing a mounting system, roof mounts are usually the cheapest option, If weight is an issue, ground mounts can be a good choice. Wall mounts are another solution that can work well in some situations.

Orientation and Tilt

The position the hot water solar panels are installed in is commonly referred to as the collector orientation; the tilt of the collector is simply the angle at which it is installed.

The collector orientation and tilt should maximize the sun’s radiant heat. Find the sunniest spot for your collectors, one that has little to no shading between 9 AM and 3 PM. Facing collectors up to 30 degrees east or west of true south and at your site’s latitude plus 15 degrees tilt, generally will still yield results within 15 percent of optimum. Any nominal losses from tilt, orientation, or even shading can usually be overcome by adding more collectors.

Sizing Your Solar Hot Water System

You should try to size your solar hot water system to provide 100 percent of your hot water in the summer and about 40 percent of your hot water year-round. Average hot water use in U.S. homes is 15 to 30 gallons per person per day. This primarily includes bathing, clothes washing, and dishwashing. Your commitment to efficiency will have a lot to do with your actual usage.

Below are some generally accepted rules of thumb for solar hot water sizing based on climatic region:

  • In the Sunbelt, use 1 square foot of collector per 2 gallons of tank capacity (daily household usage).
  • In the Southeast and mountain states, use 1 square foot of collector per 1.5 gallons of tank capacity.
  • In the Midwest and Atlantic states, use 1 square foot of collector per 1.0 gallon of tank capacity.
  • In New England and the Northwest, use 1 square foot of collector per 0.75 gallon of tank capacity.

Based on these rules of thumb, a household of four with an 80 gallon storage tank will need approximately 40 square feet of collector in the Southwest, 55 square feet in Southeast, 80 square feet in the Midwest, and 106 square feet in the Northeast and Northwest.

These are big ballpark calculations however, and will be affected by incoming water temperature, hot water temperature setting, actual usage, and the intensity of the solar resource at your site.

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