What a solar water heater component list really means
Solar water heating is solar thermal, not photovoltaic electricity generation. A system captures the sun’s heat and moves it into water you use for showers, dishes and laundry. Every part along that path has a job, and each job comes with physical demands: high temperature, sustained pressure, possible freezing, corrosion from water chemistry or ultraviolet exposure outdoors. The Florida Solar Energy Center’s 2006 training manual and the U.S. Department of Energy’s 2003 consumer guide both walk through those parts in detail. This article organizes that information so you can evaluate what each component must handle before you buy or build anything. It is not a step-by-step construction guide. Anything connected to a pressurized drinking-water supply must follow local plumbing and building codes and may require a permit and a licensed plumber.
Before diving into individual parts, it helps to see the whole picture. Our DIY System Components category page groups every major subsystem, and the articles linked there go deeper on each one.
Collectors: the heat-gathering front line
According to the FSEC manual, solar collectors capture the sun’s electromagnetic energy and convert it to heat. Three collector types appear across the DOE guide, the ENERGY STAR program’s explanation and the FSEC materials: flat-plate, evacuated-tube and integral collector-storage (ICS, also called batch) collectors. Each type faces different temperature, pressure and durability demands.
Flat-plate collectors
The FSEC manual describes flat-plate collectors as devices designed to heat water to medium temperatures, giving roughly 140 degrees Fahrenheit as a representative figure in its 2006 text. A flat-plate collector contains an enclosure, glazing, insulation, an absorber plate and flow tubes. The glazing, usually tempered glass, must let short-wave solar energy in while blocking long-wave heat from radiating back out. The insulation behind the absorber must withstand high stagnation temperatures; the FSEC manual specifically warns that insulations like Styrofoam are not suitable. The absorber surface may use a semi-selective black paint or a spectrally selective coating that reduces re-radiation. Flow tubes, typically copper, carry the heat-transfer fluid away from the absorber. The whole assembly sits outdoors year-round, so the enclosure material must handle UV exposure, rain and wind loads. For more on collector construction choices, see Collectors.
Evacuated-tube collectors
FSEC states that evacuated-tube collectors are designed to deliver higher temperatures, giving roughly 300 degrees Fahrenheit as a reference point in its 2006 manual. Each tube consists of a highly tempered glass vacuum tube that serves as both glazing and insulation, with an absorber surface inside the vacuum. ENERGY STAR notes that these collectors can operate in temperatures as low as minus 40 degrees Fahrenheit and can even work in overcast conditions. The vacuum greatly reduces conductive and convective heat loss, and ENERGY STAR notes that individual tubes are replaced as needed. In a flooded evacuated-tube design, the heat-collection fluid flows directly through the absorber tube. In a heat-pipe design, a fluid evaporates inside the tube, rises to a manifold where it transfers heat, then condenses and returns by gravity.
ICS and batch collectors
In an ICS or batch system, the collector and storage are one unit. The FSEC manual describes ICS collectors as typically incorporating four-inch or larger horizontal metal tanks connected in series, coated with a selective or moderately selective absorber finish and enclosed in a highly insulated box with multiple glazing layers. A simpler batch collector may be a single black-painted metal tank inside an insulated, glazed enclosure. ENERGY STAR warns that water can remain in the collector for long periods when household demand is low, making it very hot, and states plainly that a tempering valve is your protection from scalding at the tap. The DOE’s 2003 guide says ICS systems should be installed only in mild-freeze climates because the outdoor pipes could freeze in severely cold weather. The tanks themselves need to be suitable for potable water and able to withstand both high storage temperatures and the pressure of the household water supply. Our Batch Heaters article covers these designs in more detail.
Storage tanks: holding heat without losing it
The DOE guide says most solar water heaters require a well-insulated storage tank, and that solar storage tanks have an additional outlet and inlet connected to and from the collector. The FSEC manual adds that the water can be stored in any vessel suitable for high temperatures, and that tanks can be pressure-rated depending on the application and whether a backup heating system is used.
FSEC describes the most popular tanks as steel with an inner glass, stone or epoxy lining to protect against galvanic corrosion from dissolved oxygen in the water. Steel tanks also use an internal sacrificial anode rod, typically aluminum or magnesium, to attract oxygen away from the steel. Stone-lined, stainless steel or copper tanks normally do not require anode rods. Where hard or highly conductive water is stored, the anode rod should be replaced periodically, and the tank should be drained periodically to remove sediment. Solar storage tanks should be well insulated, especially if installed in an air-conditioned space. If installed outdoors, FSEC says the tank should be protected from environmental conditions and UV degradation by a UV-resistant plastic, stainless steel or aluminum jacket. Painted or powder-coated steel jackets require continuous maintenance in coastal or high-moisture areas.
In a single-tank system, the backup electric element is deliberately located in the upper third of the tank to encourage temperature stratification, with the collector return tube opening below that element. In a two-tank system, the solar storage tank preheats water before it enters a separate auxiliary tank that contains the backup heat source. FSEC notes that stratification is generally better in a vertical tank than a horizontal one, though roof-mounted thermosiphon systems may favor horizontal tanks for mounting reasons. For a closer look at tank selection and placement, see Storage.
Piping and insulation: the circulatory system
Piping carries heat-transfer fluid between the collector and the storage tank. The FSEC manual says piping between the tank and collectors should be as short as possible and insulated, with at least 1/2-inch rubber-type insulation in its installation guidance, and that exterior insulation must be protected from ultraviolet rays. It warns that UV exposure and extreme system temperatures speed up the breakdown of exterior pipe insulation, so insulation should be inspected and replaced as necessary. The Piping article discusses material choices and routing considerations in depth, and Insulation and Materials covers what different insulation types can tolerate.
For active direct systems that circulate potable water, FSEC states that pumps must have bronze or stainless steel housings and impellers to prevent corrosion; cast iron pumps in contact with potable water will rust. In indirect systems that use glycol or other heat-transfer fluids, cast iron pumps may be acceptable, but the fluid’s compatibility with every gasket, seal and metal in the loop must be verified. Glycol-compatible materials include Teflon, Viton and EPDM, according to the FSEC manual.
Controls and sensors: deciding when to run the pump
Active systems need a way to turn the circulator on only when the collector can add heat. The FSEC manual describes three control approaches: differential controller, photovoltaic controller and timer controller.
A differential controller compares temperatures from two sensors, one at the collector’s hottest point and one at the tank’s coldest point. FSEC says the pump typically switches on at a 5-to-20-degree-Fahrenheit difference and off when the difference drops to about 3 to 5 degrees Fahrenheit. Most controllers use 10K-ohm thermistors; at 77 degrees Fahrenheit a 10K thermistor should read roughly 10,000 ohms. Sensor wiring must be kept away from sharp edges and from 110V or 220V wiring to avoid electrical interference, and exterior sensor wiring should be protected from environmental degradation.
A photovoltaic controller uses a PV module mounted adjacent to the thermal collector. The module generates DC electricity only when the sun shines, so the pump runs only when solar energy is available. FSEC notes that pump speed is proportional to the power generated, which can increase heat-collection efficiency. A timer controller simply runs the pump during the brightest part of the day, roughly 9 a.m. to 4:30 p.m. in the example FSEC gives, and requires that the collector feed and return lines be positioned at the bottom of the solar tank to avoid losing stored heat on cloudy days. The Controls and Sensors article explains these options further.
Valves and safety devices: protecting people and equipment
Pressurized solar water heating systems use valves that handle temperature, pressure, flow direction and freeze events. The FSEC manual lists the key types and their roles.
Temperature-pressure relief
A temperature-pressure relief valve, also called a T&P or P&T valve, protects the storage tank from excessive temperature and pressure. FSEC says these valves are typically set by the manufacturer at 150 psi and 210 degrees Fahrenheit. They are always plumbed to the solar storage tank, and in some jurisdictions also to roof-mounted tanks in thermosiphon or ICS systems. T&P valves located inside a building must drain to the outside; local code requirements should be followed. ENERGY STAR calls a tempering valve your protection from scalding at the tap: it mixes cold water into the hot water to lower its temperature before it reaches the tap. The Scalding Prevention page covers this in detail.
Pressure relief, air vents and vacuum breakers
A pressure relief valve protects components from excessive pressures that can build up when the collector loop is isolated from the storage tank. FSEC states that any system where the collector loop can be isolated must have a pressure relief valve on that loop. An air vent allows trapped air to escape and prevent air locks that would restrict flow; it is usually installed at the uppermost part of the system. In indirect glycol systems, FSEC recommends a manual air vent that the installer closes after charging, because an automatic air vent can eventually vent dissolved air bubbles and cause the loop to lose pressure. A vacuum breaker prevents a vacuum lock during drainage, allowing the collector loop to drain fully by gravity when needed.
Check valves, isolation valves and freeze valves
Check valves prevent thermosiphon heat losses at night by stopping reverse flow. Isolation valves let you shut off parts of the system for service without draining the whole loop. Freeze-prevention valves, used in some direct systems, open to bleed warm water through the collector when temperatures approach freezing. The FSEC manual also describes drain valves for emptying the collector loop or tank. All these valves must be rated for the temperatures and pressures they will encounter and must be compatible with the system’s fluid chemistry. The Pressure and Temperature article and the Winterizing DIY Systems guide cover freeze-protection strategies and pressure safety.
Expansion tanks
Most indirect systems that use a heat-transfer fluid other than water incorporate an expansion tank. As the fluid heats up, it expands; the expansion tank’s flexible bladder maintains constant pressure while accommodating the volume change. FSEC warns that the expansion tank must be sized for the total potential expansion of the fluid, or elevated pressures will cause the pressure relief valve to open and result in complete pressure loss in the solar loop. Some direct systems also use an expansion tank on the cold-water supply line when a check valve or backflow-prevention device is present.
Heat exchangers and heat-transfer fluids
Indirect systems keep the collector-loop fluid separate from the potable water. A heat exchanger transfers the collected heat between the two loops. FSEC describes several designs: tube-in-tube, shell-in-tube, coil-in-tank, wraparound-tube, wraparound-plate and side-arm. The material must be compatible with both fluids; copper is the most common because it resists corrosion and conducts heat well.
The critical safety distinction is between single-wall and double-wall heat exchangers. FSEC states that nontoxic fluids such as propylene glycol can be used with a single-walled heat exchanger, but toxic fluids such as ethylene glycol must be used only with a double-walled heat exchanger. Double-wall designs often include small passageways between the two walls to provide visible leak detection. FSEC also notes that undersizing the heat exchanger is a common mistake in home-built systems.
Heat-transfer fluids themselves must be checked periodically. Glycols contain inhibitors that prevent the fluid from becoming corrosive; FSEC says glycol fluids must be checked periodically following the manufacturer’s instructions, for example by checking pH with strips or color charts. Some fluids should not be diluted because dilution changes freeze protection levels. Synthetic and silicone oils have long lifetimes and low toxicity but lower thermal conductivity and higher viscosity, which increase pump and heat-exchanger demands. Their low surface tension can also make leaks a recurring problem. If aluminum is used anywhere in the system, FSEC warns that no copper, brass or similar alloys should be used unless the heat-transfer fluid is non-ionic or an ion trap is placed upstream of each aluminum component, and ion traps require periodic servicing.
What each component must withstand: a summary table
| Component | Primary job | What it must handle |
|---|---|---|
| Flat-plate collector | Absorb solar energy, transfer heat to fluid | Extremely high temperatures (FSEC says insulation such as Styrofoam is not suitable); outdoor weather; glazing must pass short-wave sunlight and hold back re-radiated heat |
| Evacuated-tube collector | Collect heat with minimal loss | Higher delivery temperatures (about 300°F per FSEC); vacuum integrity; ENERGY STAR says they operate as low as -40°F |
| ICS/batch collector | Collect and store hot water in one unit | High storage temperatures; household water pressure; freeze risk limits use to mild climates per DOE |
| Storage tank | Hold solar-heated water, maintain stratification | Potable-water compatibility; high temperatures; internal corrosion; outdoor UV and moisture if exterior-mounted |
| Piping and insulation | Move fluid between collector and tank with minimal heat loss | Loop temperatures; outdoor UV exposure, which FSEC says speeds up insulation breakdown |
| Pump/circulator | Move heat-transfer fluid through the collector loop | Fluid chemistry (bronze or stainless steel for potable water); head and flow requirements; operating temperatures |
| Differential controller and sensors | Run pump only when collector can add heat | Sensor wiring must avoid electrical interference and outdoor degradation; sensors must read accurately at loop temperatures |
| T&P relief valve | Protect tank from over-temperature and over-pressure | Typically set at 150 psi and 210°F per FSEC; must drain to outside if indoors |
| Pressure relief valve | Protect isolated collector loop from over-pressure | Loop stagnation pressures; must be present wherever the collector loop can be isolated |
| Air vent and vacuum breaker | Release trapped air, allow gravity drainage | Manual vent recommended for glycol loops to avoid pressure loss; must be positioned vertically at high points |
| Check valve | Prevent reverse thermosiphon flow at night | Loop temperatures and flow direction; must not stick open or closed |
| Freeze-prevention valve | Protect collector and piping from freeze damage | Activates near freezing; must not leak or fail closed in cold weather |
| Expansion tank | Accommodate fluid expansion and contraction | Must be sized for total potential fluid expansion; bladder must tolerate fluid chemistry and temperatures |
| Heat exchanger | Transfer heat from collector loop to potable water | Double-wall required for toxic fluids per FSEC; material must resist corrosion from both fluids; must be sized adequately |
| Tempering valve | Prevent scalding at the tap | Mixes cold water into hot outlet; ENERGY STAR calls it your protection from scalding, especially with batch collectors |
Putting it together safely
Every component choice connects to safety and code requirements. The DOE’s 2003 guide and the FSEC manual both describe systems that interact with pressurized household plumbing and high-temperature fluids. A T&P relief valve that drains indoors, a missing tempering valve on an ICS system, or an undersized expansion tank on a glycol loop can create serious hazards. The Codes and Professional Review article explains why local building codes and a licensed plumber’s review matter before you connect anything to your home’s water supply. The Water Quality and Potable Materials page covers material choices that keep drinking water safe.
Freeze protection deserves special attention. The FSEC manual lists five basic methods: indirect antifreeze fluid circulation, draining (automatic drain-down, manual drain-down or drainback), water flow, thermal mass and auxiliary heating. A system built for a warm climate and later exposed to a hard freeze without proper safeguards can rupture collectors and piping. The Winterizing DIY Systems article walks through what each freeze-protection approach requires.
Frequently asked questions
Do I need a double-wall heat exchanger for my DIY system?
According to the FSEC manual, it depends entirely on the heat-transfer fluid you choose. Nontoxic fluids such as propylene glycol can be used with a single-walled heat exchanger. Toxic fluids, notably ethylene glycol, must be used only with a double-walled heat exchanger. The double-wall design provides a visible leak path between the two walls so a leak can be spotted before the toxic fluid reaches the potable water.
Can I use a regular water heater tank for solar storage?
The DOE’s 2003 guide says solar storage tanks have an additional outlet and inlet connected to and from the collector, which a standard water heater tank may not have. The FSEC manual adds that solar tanks are designed to encourage temperature stratification, with the cold inlet and collector feed at the bottom and the collector return below the backup heating element. A conventional tank may not have the right port locations or insulation levels for solar service.
What is the biggest material mistake in home-built solar water heaters?
The FSEC manual points to several common errors: using Styrofoam-type insulation that cannot withstand collector stagnation temperatures, using cast-iron pumps in contact with potable water, undersizing the heat exchanger, and mixing copper or brass with aluminum components without a non-ionic heat-transfer fluid or properly serviced ion traps. Each of these mistakes can lead to corrosion, pump failure or poor heat transfer.
For more on planning a system that fits your site and hot-water needs, start with DIY Project Planning and the articles on Hot Water Needs and Site Assessment.
