Arizona Solar Center Blog

Commentary from Arizona Solar Center Board Members and invited contributors.

While blog entries are initiated by the Solar Center, we welcome dialogue around the posted topics. Your expertise and perspective are highly valued -- so if you haven't logged in and contributed, please do so!

Solar Hot Water


Hot water.  It is a regular part of our daily lives.  It is used to clean our clothes, wash our dishes...



and to bathe and relax us.  It is used to heat our buildings and even extends use of our swimming pools into the winter months.

But then, hot water doesn't come that way naturally, unless you have a natural hot spring.  Water must be heated in order for us to meet these purposes.


HOT WATER - IMAGE 05 In the past, fires heated water for the variety of cooking, cleaning and bathing uses.

Today we use electricity and/or natural gas to excite water molecules to such a point that it becomes hot. Electricity is generated at some usually distant point source location such as a river whose force spins turbines, or near a coal resource to fire up generators, or even at isolated and highly security conscious nuclear plants.


Natural gas is captured and processed and readied for delivery to the consumer, where it is then turned, creating heat which can be used directly for maintaining comfort, or to transfer that heat energy to another medium - like food, or water HOT WATER - IMAGE 07

The remote site generation of electricity and/or capturing of gas both require transfer to get the product to the consumer. This transfer requires a sophisticated and complex network to assure both quality and quantity needs are met.

Transport of energy always has some losses of product and efficiencies along the way but most arrives ready for use. HOT WATER - IMAGE 10

In the recent past, serious issues and questions have arisen regarding environmental impacts, resource access, energy distribution, and energy cost - issues regarding safety, stability, and security.

Today, some utility companies are incorporating renewable energy systems of wind and sun in their generation of electricity. These renewable energy farms continue the approach of centralized collection, generation, and complex distribution system, but the locations are much closer to the end use consumer, often within the boundaries of communities they serve. In Az. communities like Tucson, Springerville, Prescott, Phoenix and Yuma, utility solar plants are springing up, due in part to Arizona's mandated Energy Portfolio Standard which designates that Arizona utility companies must derive a prescribed about of their energy from solar and renewable energy resources.



The use of the sun to meet people's needs isn't restricted to the actions of large utility companies. More energy, in the form of sunlight, falls upon the roof of a typical house than the entire house uses! Solar energy is the most democratic of energy sources - available to everyone and doesn't require a sophisticated and complex system of extraction, conversion, and transport for people to use. Best of all it is free and directly under your control.

HOT WATER - IMAGE 15 Many Arizonans use the sun's energy to meet daily hot water requirements for bathing, washing, space heating, pool heating, and heating of buildings, and many more are interested.


The benefits of using the sun to heat water include: 
  • Solar water heating reduces the amount of energy required from the utility company thereby reducing monthly bills
  • Less energy demand means less using up of finite oil and gas resources, reduction in the infrastructure required to create and deliver energy to users
  • In replacing other energy resources, it will add to the reduction of pollution, improve air quality, and lessen negative impacts on the environment
less energy... less pollution...
  • Solar water heating is direct, simple, safe and within the individual's direct control
  • Solar water heating will meet all needs when incorporated appropriately
  • Water will be hot, and some even claim it is healthier



  • To quote an Arizona utility - "Just a portion of your house's roof receives more solar energy than you need to heat house hot water all year long. To take advantage of that pollution-free energy you need a solar water heating system."


There’s nothing magic or mysterious about heating water with the sun - a lot of hot water with simple operation and little maintenance, and monthly energy bills will be reduced - a sort of reimbursement for your investment, something a traditional water heating system doesn't provide, and when savings surpass initial investment - it is free!! What more could you ask for?


Using sunlight to heat water is simple and has been done by Arizonans for quite some time. A "batch" water heater was discovered on an outbuilding of the historic Tempe Bakery, and Phoenix's historic Ellis -Shackleford house had a Day/Night solar water heating system, a replica of which remains.  In fact, solar water heaters first appeared in the west around the turn of the century and were heavily used, not only in Arizona but also in Los Angeles, as people heated their water naturally.



HOT WATER - IMAGE 22Today, with energy supply, stability, environmental, safety, security and cost issues and concerns, and the desire for energy independence in Arizona, there is increasing incorporation of solar because of product improvements, stable costs, a well defined industry, consumer protections, state oversight, financial incentives in the form of tax credits and even utility rebate programs.

Solar water system - A solar water heater system has a number of component parts:


  • Basically, there is the collector - used to capture the heat in sunlight, and...
  • the water storage tank which is part of both the heat collection loop, storage, and distribution when hot water is needed. A tank is not, nor may no, be necessary for hot water applications as in pool heating and some radiant floor heating installations.
  • Additionally there are elements of a solar water heating system that are applicable. These include an auxiliary heating system used in periods of additional hot water demand; and...
  • a control system for monitoring and coordinating the operation of all a solar system’s components in more sophisticated systems. This controller optimizes heat collection, minimizes heat loss, and provides freeze and over-heating protection to the system.


Solar collector - What is it?  Simply - a container with a glass cover, which allows sunlight to impact the interior surface.


HOT WATER - IMAGE 26Arizonans are very familiar with the direct heating action of sun through windows - an uncovered car in the summer, and even a sunny winter day, can be unbearably hot.  Sunlight moves through the windshield and impacts interior surfaces (seats, dashboard, steering wheel, etc.) and the resulting heat is resisted from escaping by the glass, and the car interior heats up to sometimes intolerable levels.

HOT WATER - IMAGE 27This is the same action that occurs with a solar collector in a solar water heating system.

Solar collectors capture the sun’s light energy conversion into heat, which then heats water or another heat transfer fluid. The collection of the sun’s energy happens at the collector’s dark color interior absorbing surface, under the glazing. As the absorber heats from exposure to sunlight, water moves through the absorber, picking up heat and carrying it to storage or for direct use. There are variations of this basic water heating system utilizing highly efficient heat transfer fluids through the collector then through an exchanger where the heat is transferred to the water to be used. Since the glazing reduces heat loss to the outside air, colder climate conditions may warrant multiple glazings to increase heat retention capabilities.

What About Hot Water Storage?  Like typical water heating systems, the storage tank holds heated water, but in a solar system, water is heated by continued circulation through the collector and is always hot.  Solar hot water system tanks may be integral to the collector element, as in a direct heat batch water system;  mounted in direct connection with the collector...

batch system


HOT WATER - IMAGE 30or separate all-together.  Solar hot water tanks can be a primary means of hot water storage, or used as a preheater, feeding into a regular tank. In all cases, solar tanks are highly efficient, and better  insulated than standard tanks and are usually of larger capacity than regular tanks, in order to provide large hot water storage capacity for nighttime use and days of limited sunlight. Some solar water heating systems can use an existing water heater tank for additional storage. In other, cases existing water heaters should be replaced with a solar tank or a combination of a solar tank and auxiliary storage tank.  Residential solar tanks are commonly available in 3 sizes.  Custom tanks can be provided for special conditions or for larger applications.

Storage tank size is directly related to consumption of hot water and amount of storage desired for days of low or no solar access . Arizona families typically use about 20 gallons per day per person. Once a daily consumption rate is estimated, one can multiply this number by the number of storage days desired - and identify the total amount of storage capacity desired.  This calculation will aid in determining tank size (s) and configuration of the hot water storage system.

Super hot water - Solar water heating systems can generate water much hotter than conventional water heaters so a mixing valve is usually incorporated at the tank area. This is a protective measure of temperature adjustment, by adding cool water to the hot water from the storage tank when necessary during hot water use.

Cold climate impact - Because solar equipment is exposed to outdoor conditions, freezing is an issue which is well mitigated in modern solar water heating systems. When water freezes, it expands - this is why water-filed pipes break during cold weather. Just as in all plumbing exposed to the elements, freeze protection is important, even in areas that experience mild winters with above freezing temperatures. The solar water heater absorber panel is an efficient solar collector and can also be a re-radiator at night. State requirements mandate that safeguards must be built into all solar systems sold Arizona.

HOT WATER - IMAGE 31 A variety of solar hot water approaches are used in Arizona. All have the means of capturing the sun and heating water for use - they vary in the details of solar capture, transport of captured heat, and approach to storage and storage placement. Basically there are 2 fundamental approaches - Direct and Indirect  Direct heat exchange is when the water to be used is heated directly by the solar collector. Indirect heat exchange involves heating an efficient heat transfer medium other than the water, then transferring the gathered heat from the collection medium to the water to be used. Direct heat transfer is highly effective, and even more so when attention is given to water quality. Calcium carbonate residue (scale) can form on the inside of heated containers like tea kettles and in a solar collector. Scale reduces collector performance and may shorten a collector’s life.

Water treatment, a common installation due to Arizona's hard water, often mitigates the condition, and regular care and maintenance is always a good practice. Indirect heat transfer solar systems, beside providing higher heating and lower heat loss, are another approach to dealing with scaling and freezing. Fluid heated in the collector is typically, propylene glycol, a non-toxic antifreeze compound. This heated liquid is in a separate line and loop from the water to be used or stored. The glycol flows through the collector and heats up from the sun's impact, then flows to a heat exchanger where it goes up its captured heat, then goes back to the collector for another round of heat gathering. The heat exchanger transfers the heat collected in the glycol  to the water to be used, which circulates, in a separate piping system loop to the storage tank where it gives up its heat and returns to the heat exchanger for another transfer. As a result, there are two separate fluid loops, one that gathers the sun's heat, and the other which contains water to be used.


Passive Systems

- Batch or Integrated Collector/Storage (ICS) Systems
HOT WATER - IMAGE 33The simplest of systems - water in a dark container exposed to the sun . Contents will get hot, and in an Arizona summer, get very hot. This is the basis of an ICS and batch/ bread box system which combines collector and storage in a single unit. Water flow only occurs when hot water is drawn off.  Direct heating of the tank or tanks, makes this system compact, simple, and effective, and can be used typically as a preheater to a regular water heater or as some Arizonans have done, to meets all needs. These units do not rely on equipment and external energy to work.  Water pressure brings water to the tank. Water sits in the dark colored tank exposed to the sun and gets hot.  When hot water is removed, it is replaced by an equal amount of "new" water.

The "batch" approach has been used in Arizona for quite some time. Evolved improvements in the design have enhanced the effectiveness in water heating and storage. Newer ICS systems incorporate a number of small-diameter storage tanks connected in parallel or series to expose more surface area to the sunlight, heating the water at a faster rate. Improvements in glazings and containers have made the systems more efficient in heat retention and the issue of freezing is mitigated by the pure volume of the water. Some systems use evacuated glass tubes (like a thermos bottle) around the collector to keep heat loss to a minimum.  As a result, ICS systems do not usually operate at temperatures high enough for scale build-up to clog the system.

- Thermosyphon Systems
HOT WATER - IMAGE 34 Hot water  rises and cold water settles. This is because hot water is less dense than cold water due to its molecular "excitement" in being heated. In a standard water heater, colder water is at the bottom of a tank. When heated it becomes less dense and rises to the top of the tank, while being replaced by cooler, upper water, which is, in turn heated, rises, etc.. This cycle is called a convective action.  A thermosyphon solar hot water heating system incorporates natural convection to move fluid heated by the collector to a storage tank. To do this naturally, the storage tank is located at a point higher than the collector. Cool water from the bottom of the tank flows to the bottom of the collector where it is heated making it rise to the top of the storage tank. This process is continuous and occurs whenever there is enough sunlight to warm the liquid in the collector. As a result, thermosyphon systems do not need pumps and for that reason they are considered a passive system .Thermosyphon systems can be used to directly heat and store water, as well as indirectly heat water with the use of an an antifreeze/heat exchanger loop.

- Active Systems/Forced Circulation Systems

HOT WATER - IMAGE 35 These applications, called active systems because a pump is used to move fluid through the solar collector, allow hot water storage to be placed at any convenient location within the building. Forced circulation systems transfer heat either directly by water circulating from the collector to the tank, or indirectly by use of a heat transfer fluid at the collector and transferring that collected heat via a heat exchanger to water in the storage tank. Variations of a forced circulation include Open Loop and Closed Loop systems.

Open loop forced circulation systems transfer heat directly to water to be used. A sensor monitors the storage tank temperature and in the collector loop. When water in the collector loop is hotter than the water in the storage tank, the pump is activated and water from the tank is circulated through the collector. State requirements stipulate provision of equipment to prevent freeze damage, and open loop systems come with recirculation and/or drain down configurations, as well as with freeze plugs or a “dribble” valve.

  • A recirculation system controller activates the pump when collector temperature nears freezing, and storage tank hot water circulates through the collector loop to raise its temperature.
  • A drain down system has a valve located at the bottom of the collector loop which opens when the temperature drops near freezing, and all water in the collector is automatically drained from the collector and piping, into the tank.
  • A freeze plug is simply a valve that opens when the pressure in the collector rises above a certain point. As water changes from liquid to ice, it expands which forces the freeze plug to open and relieve that pressure, thereby avoiding freeze damage to the solar collector and piping.
  • A “ dribble” valve is much like a freeze plug. Composed of a material that shrinks when it gets cold, it opens, allowing water to drain from the collector. Open loop, and closed loop, systems also are installed with a check valve, which allows fluid in the collector loop pipe to move in only one direction in the collector to prevent undesired reverse siphoning and loss of heat when the sun is not available.

- Closed Loop

HOT WATER - IMAGE 36 Closed loop forced circulation systems transfer heat to water to be used in a 2 loop method. In one loop fluid not susceptible to freezing, is heated at the collector and circulated to a heat exchanger which removes the gathered heat and transfers it to a loop containing the water to be used and/or stored, and the collection fluid is circulated back to the collector. There are two separate fluid loops, one for the heat collecting liquid, and the other for the water to be used.  Separately, each moves through the heat exchanger which implements the heat transfer process. A system controller turns the circulating pump on when the collector fluid is hotter than the storage tank water. There are two primary types of closed loop systems: the drain back and the non-freeze.

Drainback forced-circulation systems have an additional tank (drain back tank) for ensuring protection against freezing. When the pump is off, collector fluid drains into the drain back tank. Non-freeze forced circulation systems use an antifreeze-water mixture in the collector loop. The antifreeze mixture provides protection against very high and low collector operating temperatures. An expansion tank is usually included on these systems to allow the collector loop fluid to expand and contract without damaging the pipes.

HOT WATER - IMAGE 37 Arizona is a great location for solar water heaters because of year around bright sunny, cloud-free days. Some installations, still in operation today, have provided up to 100% of the daily hot water requirement and been in constant use for over 20 years. Others have realized energy savings ranging from 75-90% with a modicum of back-up. As a general rule, savings depends on the system, and amount of hot water demand, and timing of use. If large amounts are needed, or lots of early morning hot water is necessary, inclusion of auxiliary heating may be desirable and the amount of electricity or gas used for this is dependent on the capacity and type of storage, and especially amount of and timing of demand.

Seasonal conditions also impact upon the effectiveness of a system.  Summer, spring & fall have more exposure to the sun than winter with its short daylight hours. Summer conditions easily provide 100% of the requirement, while winter, with its less exposure to the sun may necessitate larger storage capacity or ancillary heating and back-up system in colder climates.


- Collector Placement



Collectors are best located in an unshaded area where there is unobstructed access to the sun throughout the year. The ideal location, of course, is the roof. A solar hot water system should be located to minimize piping runs between collector and storage, and, as in all good hot water design, between storage and end use. This reduces materials, and cost, as well as heat loss in the pipes. Collector placement considerations include:

  • A collector facing true south gains equal amounts of sunlight in the morning and the afternoon. If more hot water is desired in the morning, the collector should face somewhat east of true south, and if hot water is more desirable later in the day or early evening, the collector should face west of true south.
  • Collector performance is maximized when placed perpendicular to the sun. Typically, a collector is placed to operate at its optimum during the winter, with its short days of sunlight exposure, lower sun angles, and colder temperatures. For this reason the upright angle of the collector is important in maximizing solar heating of water during wintertime conditions.
  • Optimum collector angle and angle of a roof may not be compatible. This condition may require a support system like a rack or integration into the building form. It is important to note some subdivisions have restrictions regarding equipment on rooftops, and considerations regarding aesthetic integration and maintenance of style. In these cases, rack mounted collectors meet resistance, and resolution may be in the form of collectors placed flush with the roof. While this may reduce optimum performance of a hot water system, it will still provide an extensive amount of solar heated water. It is said that the difference between an ideal angle and a flush roof angle is about $30 per year in savings.


  • Energy bills will be lower due to less demand of electricity and/or gas. Savings are directly proportional to efficiency of the system, cost of local gas and electricity, and amount of hot water used.
  • Solar energy replacement for heating water means reduction of gas and/or electricity to be provided by the supplier and avoided new and costly generation and transmission systems.
  • Solar water heating replacement of electricity and gas systems results in avoiding additional pollution created by generating electricity and burning gas - a solar water heater avoids the equivalent pollution of .3 cars per year.
  • Utility providers have various incentive and buy-down programs for solar incorporation and utilization.
  • Local, county, and state government provide incentives for incorporation of solar energy equipment. The State has a tax credit of up to $1000 for the purchase and installation of approved solar water heating systems, and there is no sales tax. The community of Marana waives building permit fees for solar photovoltaic and hot water installations.
  • A solar water heating system is a good investment. Return on this investment will be reduced energy bills and a cleaner environment over the lifetime of the system.



The major elements of a certified solar water heating system should last at least 20 years with proper use, care, and maintenance. Other components, such as vents and mixing valves, may need occasional replacement but are relatively inexpensive and easily replaced. To ensure the best performance, a diligent maintenance practice, like one people use for their cars, is recommended. This includes:
  • Flush all tanks once a year.
  • Annual or bi-annual maintenance check-up by a certified service technician.
  • Keeping the glazing clean and unobstructed.
  • Check for leakage at pipes leading to and from the collector.
  • Check the insulation on the pipes and at all joints.
  • For active systems, allow plenty of space around the pump and other equipment in order to extend its lifetime.
  • Check antifreeze in closed loop systems.
  • If performance drops off, check for consumption pattern changes. If the consumption pattern has not changed, check the system's maintenance manual and/or contact a service representative.

A solar water heater can deliver your hot water all year round - In much of Arizona we are fortunate that we have fewer cloudy and cool days than almost anywhere else in the country, so solar energy can carry much of the load.

  • All system component and system must meet State requirements. Contact the Az. Dept. of Commerce Energy Office for information
  • Az. Registrar of Contractors, the Better Business Bureau and the Az. Solar Energy Industries Association are information sources about solar companies and certified installers.
  • Az. Dept of Revenue and Az. Dept. of Commerce Energy Office are information sources re: approved solar systems and tax benefits
  • A properly installed, approved system must have warranties for the equipment and installation
  • Parts and labor for the entire system for a minimum period of two years from the date of installation.
  • Warranty against freezing for a minimum of five years.
  • Solar Contractors License (State of Az. Registrar of Contractors)
  • Certified Solar Technicians to do the work (Az. Solar Energy Industries Association)
  • Product Meets Az. Dept. of Commerce Energy Office Certifications and Requirements Installation Requirements
  • Meets or exceeds all applicable Codes
  • Conforms to Arizona adopted guidelines and standards that require residential solar water heating systems installed in Arizona to be certified under the Solar Rating and Certification OG-300 rating system.  This certification ensures compatibility of components and provides comparative information for the consumer.  Information can be attained at the State Department of Commerce Energy Office, from solar companies, and at

Arizona Revised statues 33-439 prohibits Homeowner Associations from imposing restrictions that effectively prohibit the installation and use of solar energy.  Recent Arizona court rulings have upheld consumer rights to install solar energy devices, while noting that Homeowner Associations can identify reasonable restrictions as long as they do not significantly increase a solar system's cost or diminish its' efficiency. For further information go to, or contact the Az. Dept. of Commerce Energy Office, or contact the Arizona Solar Energy Industries Association at 888-253-8180.


HOT WATER - IMAGE 44 Conventional water heating uses electric energy or gas. Gas is burned directly in the water heater, but the electric energy released into the water in the form of heat is usually generated by burning a fuel at a central power plant. Burning hydrocarbon-based fuels (such as coal, oil, or natural gas) emits oxides of carbon (Cox), nitrogen (Nox) and sulfur (Sox). Solar water heaters significantly reduce pollutants and contribute to a more clean and healthy environment.

This presentation was constructed by the Arizona Solar Energy Association for the Arizona Solar Center, Inc. under contract with the Az. Dept. of Commerce Energy Office, funded by the Dept. of Energy Million Solar Roofs program. Materials and information were provided by a number of sources, including in big part the Arizona Public Service Consumer’s Guide to Solar Water Heating.

NOTE:  Financial support for this presentation has been provided by the Arizona Department of Commerce (Energy Office) and the U.S. Department of Energy through (DOE) Grant No. DE-FG51-01R021250. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the Energy Office or U.S. DOE. The State of Arizona and U.S. DOE assume no liability for damages arising from errors, omissions or representations contained in this presentation.
All About Solar Cookers 7
Solar Hot Water: A Primer