Solar Water Heater Component Comparisons: Collectors, Tanks and Valves

Choosing the right components for a solar water heating system can feel like learning a new language. Unlike picking a standard electric or gas water heater, a solar thermal setup is a collection of specialized parts that must work together. This guide walks through the main component categories, explains what each piece does, and gives you clear questions to ask an installer so you can compare proposals with confidence.

Solar Collectors: The Engine of the System

Solar collectors capture the sun’s energy and convert it to heat. According to the Florida Solar Energy Center’s 2006 manual, most active systems use flat-plate collectors, which are designed to heat water to medium temperatures. A flat-plate collector consists of an insulated enclosure, a transparent glazing cover, an absorber plate, and flow tubes that carry the heat-transfer fluid. The glazing lets short-wave solar energy in but blocks long-wave heat from re-radiating out, while insulation behind the absorber reduces heat loss through conduction.

Evacuated-tube collectors are a different design that the FSEC manual describes as capable of delivering higher temperatures. Each tube contains an absorber surface surrounded by a vacuum that greatly reduces heat losses. In a heat-pipe design, a fluid inside the pipe evaporates, rises to transfer heat to a manifold, and then condenses and returns by gravity. Integral collector storage (ICS) and batch collectors combine the collector and storage in one unit, with one or more tanks inside an insulated, glazed box.

The ICC-SRCC provides third-party certification for solar thermal collectors to standards recognized by building codes across the United States. When you review a proposal, ask the installer whether the collectors are certified by the ICC-SRCC and which standard they meet. Also ask what temperature range the collector is designed for and how its efficiency was rated. For more background, see our solar collectors guide and the collector components page.

Storage Tanks and Heat Exchanger Setups

The storage tank holds the water heated by the collector. The FSEC manual explains that in warmer climates a single solar storage tank often includes an electric heating element placed in the upper third of the tank. This placement encourages temperature stratification: the hottest water stays at the top for use, while cooler water settles at the bottom to be sent back to the collector. Vertical tanks generally stratify better than horizontal tanks, though roof-mounted thermosiphon systems sometimes use horizontal tanks for aesthetic and mounting reasons.

In two-tank systems, a solar storage tank without any backup heating source is plumbed in series to a conventional auxiliary water heater. This arrangement is common in residential and light commercial applications. The FSEC manual notes that gas water heaters are not usually used as the solar storage vessel because the bottom-mounted burner would heat the whole tank and reduce the solar contribution. Instead, a solar preheat tank feeds the gas backup tank.

Indirect systems use a heat exchanger to transfer energy from the collector loop’s heat-transfer fluid to the potable water. Common designs include tube-in-tube, shell-in-tube, coil-in-tank, and wraparound types. The FSEC manual states that systems with heat exchangers can experience a 10 to 20 percent loss in total efficiency, but the trade-off may be worthwhile for freeze protection. A critical distinction is between single-wall and double-wall heat exchangers. Double-wall units have two barriers separating the fluid streams and often include a visible leak path between the walls. The FSEC manual advises that nontoxic fluids such as propylene glycol can be used with a single-walled heat exchanger, while ethylene glycol, a toxic fluid, should be used only with a double-walled heat exchanger.

Ask the installer whether the tank is vertical or horizontal and how stratification is managed. For indirect systems, ask whether the heat exchanger is single or double wall and whether it is sized to match the collector array. Our storage tank overview covers these choices in more detail.

Freeze Protection Approaches

Freezing is one of the biggest hazards for a solar water heating system. The FSEC manual describes several strategies. Indirect systems use a non-potable heat-transfer fluid, most commonly a glycol solution, that will freeze only at extremely low temperatures. Glycol fluids must be checked periodically to ensure they remain chemically stable, because overheated glycol can become acidic. The Solar Oregon maintenance guide also warns that degraded glycol can eventually cause pinholes in copper coils.

Direct systems in mild climates may use freeze-protection valves. These valves, described in the FSEC manual, are installed on the collector return line near the roof penetration and open at near-freezing temperatures to bleed warm water through the collector. A check valve must be used with this approach to prevent tank water from bypassing the collector. Drain-back systems, another option, rely on a vacuum breaker and gravity to drain the collector loop when the pump stops, removing water from areas exposed to freezing air.

When you interview an installer, ask which freeze protection method the proposed system uses and how it performs during a power outage. If the system uses glycol, ask how often the fluid should be checked and what pH range indicates it is still safe. See our cold weather protection guide for additional context.

Safety Valves: T&P, Pressure Relief, Tempering, and Expansion

A solar water heating system connected to a home’s pressurized drinking-water supply must include several safety devices, and installation must follow local plumbing and building codes, usually requiring a permit and a qualified, licensed installer. The FSEC manual details the key valves.

A temperature-pressure relief (T&P or P&T) valve protects the storage tank from excessive temperatures and pressures. The FSEC manual states these valves are usually set by the manufacturer at 150 psi and 210 degrees Fahrenheit. They are always plumbed to the solar storage tank and, if located inside a building, must drain to the outside. A separate pressure relief valve is installed on the collector loop in any system where the loop can be isolated from the tank. Its pressure rating, typically 125 psi, must be lower than the rating of all other components it protects.

A tempering or mixing valve blends cold water with the hottest water from the tank to deliver water at a preset temperature. The FSEC manual notes that a standard tempering valve is intended to conserve hot water, not to act as a safety device. An anti-scald valve, by contrast, also shuts off flow if the hot or cold supply fails. The Solar Oregon guide confirms that an anti-scald valve is required by code and mixes super-hot water with cold water before it reaches household faucets.

Indirect systems that use a heat-transfer fluid other than water typically include an expansion tank. As the fluid heats and expands, the expansion tank provides room for the volume change, preventing over-pressurization. The FSEC manual explains that the tank contains a flexible bladder that maintains constant pressure on the fluid.

Ask the installer to point out every safety valve in the proposed design, explain its set point, and confirm that the discharge lines are routed to a safe location. Also ask whether an expansion tank is included on the collector loop and, if local code requires one, on the cold-water supply line.

Controls and Sensors

In active systems, the controller acts as the brain. The FSEC manual describes three common types. A differential controller uses two thermistor sensors, one at the collector and one near the bottom of the storage tank. When the temperature difference reaches a preset value, often adjustable by the installer between roughly 5 and 20 degrees Fahrenheit, the controller starts the pump. When the difference falls to about 3 to 5 degrees Fahrenheit, the pump stops. Many differential controllers also incorporate a freeze-control function and a high-temperature limit setting that turns the pump off when the tank reaches a preset temperature.

A photovoltaic controller uses a small PV module mounted next to the thermal collector. The module generates DC electricity only when the sun shines, so the pump runs only when solar energy is available. The FSEC manual notes that pump speed is proportional to the amount of power generated, which can increase heat collection efficiency. A timer controller simply runs the pump during the brightest part of the day, approximately 9 a.m. to 4:30 p.m., and requires routine maintenance of the feed and return valve to prevent sediment buildup.

Ask the installer which controller type is proposed, what the on and off differential settings will be, and whether the controller includes freeze protection and high-limit functions. Also ask how the sensor wiring will be routed to avoid interference from household voltage lines. Our controls and monitoring guide explains these options further.

Component Comparison at a Glance

The table below summarizes the main component categories, the options you are likely to encounter, and the key questions to ask an installer for each. Use it as a checklist when you review bids.

For a broader look at how these components fit into a complete system, visit our components overview article and the solar water heater buying guide. The full component comparisons category has additional detailed breakdowns.

Frequently asked questions

Do I need a double-wall heat exchanger?

The answer depends on the heat-transfer fluid your system uses. According to the FSEC manual, nontoxic fluids such as propylene glycol can be used with a single-walled heat exchanger. If the system uses ethylene glycol, which is toxic, a double-walled heat exchanger should be used. Local plumbing codes may also dictate the requirement, so ask your installer to confirm what your jurisdiction requires.

How do I know if a collector is certified?

The ICC-SRCC provides third-party certification for solar thermal collectors in North America. Its certifications are designed to help manufacturers comply with building code, regulatory, and incentive program requirements. You can ask the installer for the collector’s ICC-SRCC certification number or look it up in the organization’s online directory of certified products.

Why does my proposal include an expansion tank?

The FSEC manual explains that most indirect systems using a heat-transfer fluid other than water incorporate an expansion tank. As the fluid temperature rises, it expands. The expansion tank provides room for this change in volume and prevents damage from over-pressurization. Some direct systems also include an expansion tank on the cold-water supply line when a check valve or backflow prevention device is present.

Ready to compare specific models and specifications? Browse the full set of guides in our component comparisons section.

Sources

• Florida Solar Energy Center: Solar Water and Pool Heating Manual (FSEC-IN-24, 2006)
• Solar Rating and Certification Corporation (ICC-SRCC)
• Solar Oregon: Maintenance and Safety Considerations