What a Solar Hot Water Storage Tank Does
Every solar water heating system that serves a home needs a place to hold the heated water until someone opens a tap. That job belongs to the solar water heater storage tank. According to the U.S. Department of Energy’s 2003 consumer guide, all solar water heaters and solar space heaters except pool heaters have some kind of storage. In a pool system the pool itself acts as the storage vessel, but for domestic hot water a dedicated, well-insulated tank is essential.
The Florida Solar Energy Center’s 2006 manual explains that the storage tank sits between the solar collector and the home’s hot water taps. Cold water enters the tank, is heated by energy captured in the collector, and is then drawn off for use. In an active system a pump moves water or a heat-transfer fluid between the collector and the tank. In a passive thermosiphon system the tank is placed above the collector so that warm water rises naturally into it while cooler water sinks to the collector inlet. In either case the tank is the central reservoir that determines how much solar-heated water is available at any moment.
Understanding the tank’s role also means understanding what it must withstand. The FSEC manual notes that a temperature-pressure relief valve is always plumbed to the solar storage tank because the tank can experience both high temperatures and high pressures. The DOE guide adds that solar storage tanks have an additional outlet and inlet connected to and from the collector, which distinguishes them from a standard water heater tank. For more background on how all the pieces fit together, see our DIY Solar Water Heater Components article.
One-Tank Versus Two-Tank Systems
When planning a solar water heating system you will encounter two common storage arrangements: a single tank that combines solar storage and backup heating, or two separate tanks where the solar tank preheats water before it reaches a conventional water heater. The DOE’s 2003 guide describes both layouts. In a two-tank system the solar water heater preheats water before it enters the conventional water heater. In a one-tank system the backup heater is combined with the solar storage in one tank.
The FSEC manual’s system-type diagrams show that in a two-tank arrangement the solar storage tank feeds preheated water to the inlet of a standard auxiliary water heater. When the sun has done its job, the backup heater may not need to fire at all. On cloudy days or during high-demand periods the backup heater raises the water temperature the rest of the way. A one-tank system places the backup heating element or gas burner inside the same vessel that stores the solar-heated water.
Which layout fits depends on your existing equipment, space and budget, so discuss it with your installer. Both designs require that the storage tank be well insulated and that all connections to the pressurized potable-water supply follow local plumbing and building codes. Work on a home’s drinking-water system may require a permit and a licensed plumber. For help thinking through the planning stage, visit our DIY Project Planning section.
Heat Exchangers: Inside or Outside the Tank
In an indirect system the fluid that circulates through the solar collector never mixes with the household water. Instead, it transfers its heat through a heat exchanger. The FSEC manual describes several heat exchanger designs and explains that they can be either internal or external to the storage tank.
Internal Heat Exchangers
A coil-in-tank heat exchanger is immersed in the lower section of the storage tank, according to the FSEC manual. The hot collector fluid flows through the coil and gives up its heat to the potable water surrounding it. A wraparound-tube heat exchanger uses tubes welded to the outside of the water vessel, also on the lower half of the tank. A wraparound-plate design, sometimes called a double jacket, uses a separate plate welded around the lower half of the tank. All three keep the heat-transfer fluid physically separated from the drinking water while allowing heat to pass through the metal wall.
External Heat Exchangers
External designs sit outside the tank. The FSEC manual describes a tube-in-tube heat exchanger where one fluid moves through an inner tube while a second fluid moves in a different direction through the space between the inner and outer tubes. A shell-in-tube heat exchanger sends potable water through multiple tubes while the collector-loop fluid passes over the tubes through the shell. A side-arm heat exchanger is a tube-in-shell design mounted on the side of the storage tank; it allows thermosiphoning of water from the tank through the heat exchanger, which initiates natural circulation without an extra pump.
Single-Wall and Double-Wall Requirements
The choice between single-wall and double-wall construction matters for safety. The FSEC manual states that nontoxic fluids such as propylene glycol can be used with a single-walled heat exchanger, but ethylene glycol, a toxic fluid, should be used only with a double-walled heat exchanger. Double-wall heat exchangers often incorporate small passageways between the two walls to provide leak detection; if a leak occurs, fluid flows out through those passageways and becomes visible. The FSEC manual also notes that heat exchanger materials must be compatible with the system’s fluid and piping, and that copper is by far the most common material because it prevents corrosion and conducts heat well. For more on material choices that protect your water supply, read Drinking Water Safety and Materials for Solar Water Heaters.
Safety Parts Every Storage Tank Needs
A solar storage tank operates under conditions that can push temperature and pressure beyond safe limits. Several components work together to protect the tank, the plumbing and the people in the house.
Temperature-Pressure Relief Valve
The FSEC manual explains that a temperature-pressure relief valve, also called a P&T valve or T&P valve, protects system components from excessive pressures and temperatures. It is always plumbed to the solar storage tank as well as any auxiliary tank. In thermosiphon and integral-collector-storage systems where the solar tanks are located on a roof, those tanks may also be equipped with a T&P valve because some jurisdictions consider them storage vessels. The manual notes that these valves are usually set by the manufacturer at 150 psi and 210 degrees Fahrenheit. A T&P valve located inside a building must drain to the outside; the FSEC manual advises following local code requirements if you are uncertain.
Expansion Tank
Most indirect systems that use a heat-transfer fluid other than water also incorporate an expansion tank, according to the FSEC manual. As the heat-transfer fluid temperature rises it expands. The expansion tank provides room for this change in volume. Inside the tank a flexible bladder maintains a constant pressure on the fluid while allowing it to expand and contract as it heats and cools. The expansion tank prevents damage from over-pressurization and eliminates the potential creation of a vacuum inside the collector loop. The FSEC manual adds that some solar systems also have an expansion tank on the cold water supply line when a check valve or back-flow prevention device is present on that line.
Scalding Prevention
Water inside a solar storage tank can reach temperatures high enough to cause serious burns. The FSEC manual describes a tempering valve that mixes cold water with the hottest water from the storage tank to deliver water at a preset temperature. However, the manual explicitly states that the intended use of this valve is to conserve hot water, not as a safety device. An anti-scald valve also mixes cooler water with hot water but functions as a safety device by closing off the flow into the house if the hot or cold mixing supply fails. For a deeper look at protecting your household, see Preventing Scalding From Solar Hot Water.
Thermosiphon Tanks and Roof Load
In a thermosiphon system the storage tank must be located above the collector. The DOE’s 2003 guide explains that water flows through the system when warm water rises as cooler water sinks, so the collector must be installed below the storage tank. The FSEC manual’s description is similar: cold water from the bottom of the tank flows to the bottom of the collector, and as the sun heats the water it becomes lighter and rises into the top half of the tank.
This arrangement puts a heavy, water-filled tank on the roof. The DOE guide cautions that contractors must pay careful attention to roof design because the water in the storage tank is heavy. The roof structure must be evaluated to confirm it can carry that load safely. Before placing any tank on a roof, consult a structural engineer or a qualified contractor and check local building code requirements. Our Roof and Structural Safety for Solar Water Heaters article covers what to consider.
Tank Sizing Relative to Collector Area
The DOE’s 2003 consumer guide offers a sizing guideline that many contractors have used. It suggests about 20 square feet (2 square meters) of collector area for each of the first two family members, then for every additional person add 8 square feet (0.7 square meters) if you live in the Sun Belt or 12 to 14 square feet (1.1 to 1.3 square meters) if you live in the northern United States. For active systems the size of the solar storage tank increases with the size of the collector, typically 1.5 gallons per square foot of collector. The guide then gives these rough tank-size categories: a small 66-gallon system for one to three people, a medium 80-gallon system for three or four people, and a large 120-gallon system for four to six people.
The FSEC manual approaches sizing from the heat-exchanger side, noting that for packaged and approved or certified systems the manufacturers have sized the heat exchanger and matched its performance to the collector array and flow rates. It warns that under-sizing the heat exchanger is a common mistake of home-built systems. Both sources present their numbers as guidelines from their respective publication dates: the DOE guide is from 2003 and the FSEC manual is from 2006 and focused on Florida conditions. Use them as a starting point for discussion with a professional, not as a substitute for a detailed load calculation based on your household’s actual hot-water use and local climate.
Insulation and Heat Loss
A storage tank that loses heat quickly forces the backup heater to work harder. The DOE guide states that most solar water heaters require a well-insulated storage tank. Check that the tank insulation is intact and that the pipe connections to and from the tank are insulated too. For guidance on insulation materials and techniques, see Insulation and Materials.
Parts to Inspect on a Storage Tank
The table below lists key parts associated with a solar water heater storage tank, what each part does, and what to look for during a visual inspection. The information is drawn from the FSEC manual’s component descriptions. Remember that anything connected to a pressurized domestic water supply must comply with local plumbing codes; a licensed plumber should handle repairs or replacements that involve the potable-water side of the system.
| Part | Purpose | What to Check |
|---|---|---|
| Temperature-pressure relief valve | Opens to release water if temperature or pressure exceeds safe limits | Look for corrosion, mineral buildup or signs of leakage at the discharge pipe; confirm the drain line runs to a safe outdoor location |
| Expansion tank | Absorbs volume changes in the heat-transfer fluid as it heats and cools | Check the tank’s pressure charge if the manufacturer’s instructions allow; look for external rust or dents |
| Heat exchanger (internal or external) | Transfers heat from the collector loop fluid to the potable water without mixing the two | On double-wall units, check the leak-detection passageways for any fluid; inspect accessible fittings for corrosion or scale |
| Tank insulation jacket or blanket | Slows heat loss from stored water to the surrounding air | Look for gaps, compression, water stains or rodent damage; confirm insulation is dry and fully in place |
| Anti-scald or tempering valve | Mixes cold water with hot to deliver water at a safer, preset temperature | Check for leaks around the valve body; confirm the outlet temperature is within a safe range using a thermometer at a nearby tap |
Frequently asked questions
Can I use a standard water heater tank for solar storage?
A standard tank lacks the extra inlet and outlet connections that a solar storage tank needs for the collector loop. The DOE guide notes that solar storage tanks have an additional outlet and inlet connected to and from the collector. Check the tank manufacturer’s instructions and local code before using any tank for solar storage. Any modification to a pressurized water vessel should be reviewed by a licensed plumber.
Does a solar storage tank need its own pressure relief valve if the collector loop already has one?
Yes. The FSEC manual states that a temperature-pressure relief valve is always plumbed to the solar storage tank as well as any auxiliary tank. A pressure relief valve on the collector loop protects the collector and piping from over-pressurization, but it does not protect the storage tank from the combination of high temperature and high pressure that can build up inside the tank itself. Both devices are needed in their respective locations.
How do I know if my roof can support a thermosiphon tank?
The DOE guide cautions that the water in a roof-mounted storage tank is heavy and that contractors must pay careful attention to roof design. Only a structural engineer or a qualified building professional can determine whether your roof can carry the load. The evaluation should account for the full weight of the tank when filled with water, plus any additional dead loads from mounting hardware and live loads such as snow or wind.
For more on the components that connect to your storage tank, browse our Storage category page.
