Solar water heaters draw on free energy from the sun, but even in sunny climates there will be stretches of cloudy weather, early mornings, and periods of high demand when the solar system alone cannot deliver enough hot water. The U.S. Department of Energy’s 2003 consumer guide explains that a conventional water heater is typically used as a backup to provide a steady supply of hot water whenever the solar contribution falls short. How that backup heat is integrated into the system, and when it is allowed to run, greatly affects how much solar energy you actually use and how much you spend on backup fuel.
Backup Arrangements: One Tank vs Two Tanks
According to the Department of Energy’s 2003 guide, there are two basic ways to combine solar storage with a conventional heater. 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 Florida Solar Energy Center (FSEC) manual from 2006, which focuses on Florida installations, provides additional detail on how these configurations behave.
| Arrangement | How it works (DOE guide) | Question to ask |
|---|---|---|
| One-tank (integrated backup) | The backup heater is combined with the solar storage in one tank. | Where is the backup element or burner, and how is its thermostat set? |
| Two-tank (separate preheat) | The solar water heater preheats water before it enters the conventional water heater. | Is there room for two tanks, and how will the existing water heater be used? |
The DOE guide describes both systems; in either case the backup is a conventional electric or gas water heater or element. Your installer can explain which arrangement suits your space and existing equipment.
How Backup Settings and Timing Affect Solar Heat Use
The FSEC manual’s problem-assessment module includes a lifestyle analysis that directly links hot-water usage patterns to backup energy consumption. According to the 2006 manual, homeowners can “minimize their back-up heating expense” by shifting most hot-water use to late afternoon and early evening, when the solar storage tank is at its highest temperature. Taking showers and running the dishwasher in the evening, and using only small amounts of hot water in the morning, helps the backup heater stay off longer.
The same FSEC section suggests a simple way to confirm that the solar portion of the system is doing its job: turn off or disconnect the electric backup element, usually by switching off the appropriate circuit breaker. When the backup is off, any hot water you get is coming entirely from the sun. While this is primarily a troubleshooting step, it illustrates that the backup thermostat setting and the backup on/off state directly control how much solar heat is actually used. Some systems have a switch near the tank for this purpose; others require you to flip the breaker at the electrical load center. Leaving the backup off permanently could leave you without hot water during cloudy periods or at times of high demand.
The FSEC material also highlights that thermosiphoning can inadvertently activate the backup heating element. Without a properly functioning check valve, cooler water in rooftop collectors at night can sink and displace warm water from the storage tank, causing the tank temperature to drop and the backup heater to turn on. The FSEC manual advises that a well-maintained check valve prevents this unwanted circulation.
The Role of Tempering and Anti-Scald Valves
Because solar preheat can raise water to temperatures well above the backup heater’s normal setpoint, delivering that very hot water directly to the faucets can be a scalding hazard. ENERGY STAR notes that batch collectors, in particular, can heat water to high temperatures when household demand is low, and that “a tempering valve is your protection from scalding at the tap.” Solar Oregon, a nonprofit advocate for solar energy, states that during the summer solar water may be hotter than is safe for the home and that an anti-scald valve, required by code, will be installed to mix super-hot water with cold water before it reaches the home’s faucets.
The FSEC manual from 2006 makes an important distinction: a mixing or tempering valve (Figure 36 in that manual) mixes cold water with the hottest water from the storage tank to deliver water at a preset temperature, and its intended use is to conserve hot water, not to serve as a safety device. An anti-scald valve (Figure 37) also mixes cooler water with hot water but, unlike a standard mixing valve, it will close off the flow into the house if either the hot or cold supply fails. Solar Oregon says an anti-scald valve is required by code where it works; check your local code. Regardless of the exact valve type, the principle is the same: solar-heated water can easily exceed the backup heater’s thermostat setting, and a blending valve helps keep delivery temperatures safe.
What to Keep in Mind for Your Own System
Every solar water heating setup connected to a home’s pressurized drinking-water supply must follow local plumbing and building codes, and installation usually requires a permit and a qualified, licensed installer. The Department of Energy’s consumer guide stresses that its booklet is not a technical design manual and recommends working with an experienced solar heating contractor.
The backup heating arrangement you choose affects not only upfront costs and space but also long-term solar utilization. A dedicated backup heating overview can help you weigh one-tank and two-tank designs. You may also want to review system selection factors such as household demand and climate, and our guide on how to choose a solar water heating system walks you through certifications and contractor selection. Paying attention to the backup controls, and understanding when the backup heater is allowed to run, is a straightforward way to get more hot water from the sun and less from the utility. And safety components like a properly sized anti-scald valve and pressure-temperature relief valve protect both the system and the people using it. Regular inspections, as covered in routine maintenance, help catch issues such as a stuck check valve that could otherwise cause the backup heater to operate more than necessary.
Frequently asked questions
Can I turn off my backup heater to save energy?
As the FSEC manual suggests, temporarily switching off the backup element is a good way to check that the solar system is working, but it is not a strategy for permanent energy saving on its own. If you leave the backup off, you may run out of hot water on cloudy days. Use this method periodically to see how much solar energy your system is capturing, then restore normal backup operation.
Is a tempering valve really necessary if my backup thermostat is already set low?
The backup thermostat does not control how hot the sun makes the water. ENERGY STAR says water in a batch collector can become very hot when demand is low and calls the tempering valve your protection from scalding at the tap. Solar Oregon points out that anti-scald valves are required by code for this reason.
Is a two-tank backup system always better than a one-tank system?
Not in every situation. The Department of Energy’s guide presents both as valid options. A one-tank system saves space and simplifies the plumbing, while a two-tank arrangement separates preheat and backup. The right choice depends on factors such as available space, existing equipment and hot-water use.
For more detailed information on integrating backup heat into a solar water heating setup, visit our Backup Heating section.
