Why Solar Hot Water Gets Dangerously Hot
Solar water heaters capture the sun’s thermal energy to heat water, and on bright summer days they can raise storage temperatures far above what a conventional water heater would deliver. According to ENERGY STAR’s explanation of how solar water heaters work, batch collectors, also called Integrated Collector-Storage systems, heat water in dark tanks or tubes inside an insulated box. When household demand is low, water can sit in the collector for long periods and become very hot. The result is that water leaving the solar storage tank may be much hotter than the safe limit for bathing, washing and other household uses.
What the CPSC Says About Tap Water Scalding
The U.S. Consumer Product Safety Commission (CPSC) has documented how quickly hot tap water can cause injury. In its guidance on avoiding tap water scalds, the CPSC states that most adults will suffer third-degree burns if exposed to 150-degree water for two seconds. Burns will also occur with a six-second exposure to 140-degree water or with a thirty-second exposure to 130-degree water. Even at 120 degrees, a five-minute exposure could result in third-degree burns. The CPSC urges all users to lower their water heaters to 120 degrees Fahrenheit and notes that a thermostat setting of 120 degrees Fahrenheit may be necessary for residential water heaters to reduce or eliminate the risk of most tap water scald injuries.
Who Is Most at Risk
The CPSC warns that the majority of injuries and deaths involving tap water scalds are to the elderly and children under the age of five. The agency stresses that a child should never be left unsupervised in the bathroom, even for a second, and that a caregiver’s presence at all times is the strongest protection against accidents and scalding for infants and young children. The CPSC also advises consumers to hand-check water temperature before use, especially when bathing infants and young children, and never to take hot water temperature for granted.
Tempering Valves and Anti-Scald Valves
A tempering valve, sometimes called a mixing valve, blends cold water with the hottest water from the storage tank so that water delivered to the home’s faucets is at a preset temperature. The Florida Solar Energy Center’s 2006 solar water and pool heating manual describes a mixing or tempering valve as a device that mixes cold water with the hottest water from the storage tank, delivering water at a preset temperature. The manual notes that the intended use of this valve is to conserve hot water, not as a safety device. An anti-scald valve, by contrast, also mixes cooler water with hot water to deliver water at a preset temperature but functions as a safety device, closing off the flow into the house if the hot or cold mixing supply fails. ENERGY STAR’s guide to solar water heaters describes the tempering valve as protection from scalding at the tap, explaining that it mixes in cold water to decrease the water’s temperature before it is delivered to the tap.
Where the Valves Go
The Florida Solar Energy Center’s 2006 manual provides installation guidance for these valves. It states that if a standard mixing valve is to be installed, it should be located below the top of the tank. Specific models of anti-scald valves can be installed in the piping above the tank. The manual also covers a pressure-temperature relief valve, which is always plumbed to the solar storage tank as well as the auxiliary tank. In thermosiphon and integrated collector-storage systems where solar tanks are located on a roof, those tanks may also be equipped with a temperature-pressure relief valve since they are in some jurisdictions considered storage vessels. These valves are usually set by the manufacturer at 150 psi and 210 degrees Fahrenheit. Temperature-pressure relief valves located inside a building must drain to the outside, and the manual advises following local code requirements if uncertain.
Code Requirements and Other Safeguards
Solar Oregon’s maintenance and safety considerations page states that during the summer, solar water may be hotter than is safe for the home, and an anti-scald valve, required by code, will be installed to mix super-hot water with cold water before being sent to the home’s faucets. Solar Oregon also lists other safety equipment, including pressure and temperature release valves and thermal expansion tanks, to protect the home and safety of the occupants. The Florida Solar Energy Center manual adds that a pressure relief valve, typically set at 125 psi, must be installed on the collector loop in any system where the collector loop can be isolated from the storage tank. Because it opens only with high pressure, it operates less frequently than a temperature-pressure relief valve and offers a higher degree of reliability for protecting the solar collector.
Checking Your Tap Temperature
The CPSC describes a straightforward method for checking tap water temperature at home. It recommends holding a candy or meat thermometer under the faucet for the most accurate reading first thing in the morning or at least two hours after water use. If the reading is too high, the thermostat on the water heater should be adjusted according to the manufacturer’s instructions, and the temperature should be checked again with the thermometer. The CPSC notes that consumers should consider lowering the thermostat to the lowest settings that will satisfy hot water needs for all clothing and dish washing machines. For electric water heaters, the CPSC explains that most have two thermostats, both of which must be set to a common temperature for proper operation, and that the current must be shut off before making adjustments.
Why a Licensed Plumber Matters
Any component connected to a home’s pressurized drinking-water supply must follow local plumbing and building codes and may require a permit and a licensed plumber. The Florida Solar Energy Center’s manual specifies that lead-free solder should be used for soldering system joints and components, and that local code requirements must be followed. The manual also states that pressure-temperature relief valve piping should be piped to the outside or into a suitable drain six inches above ground level, again following local code. Because solar storage tanks are heavy, a 100-gallon tank full of water weighs about 1,000 pounds according to the manual, and must be located on a well-braced area if it is above grade. A professional can assess the structural support, ensure the tank is placed where a leak will not cause damage, and confirm that all safety valves are correctly installed and routed.
Other Safety Components to Understand
Beyond scalding protection, a solar water heating system includes several components that prevent damage from pressure, freezing and overheating. The Florida Solar Energy Center manual describes a vacuum breaker, which admits atmospheric pressure into system piping to allow the system to drain, and freeze-protection valves, which are set to open at near-freezing temperatures and are installed on the collector return line close to where the line penetrates the roof. Warm water bleeds through the collector and out this valve to protect the collector and pipes from freezing. The manual also notes that a check valve must be used in all systems with freeze-protection valves to prevent tank water from bypassing flow through the collector. Solar Oregon adds that if a system uses a heat transfer fluid, usually food-grade glycol, the pH balance should be checked periodically because glycol that has been overheated can become acidic and eventually cause pin holes in copper coils.
The table below summarizes the key safeguards discussed in this guide, what each one does, and the source that describes its function.
| Safeguard | What it does | Source |
|---|---|---|
| Tempering or mixing valve | Mixes cold water with hot water from the storage tank to deliver water at a preset temperature; primarily conserves hot water. | FSEC 2006 manual |
| Anti-scald valve | Mixes cooler water with hot water to deliver water at a preset temperature and shuts off flow if the hot or cold supply fails, acting as a safety device. | FSEC 2006 manual; ENERGY STAR |
| Pressure-temperature relief valve | Protects system components from excessive pressures and temperatures; typically set at 150 psi and 210 degrees Fahrenheit. | FSEC 2006 manual |
| Pressure relief valve | Protects components from excessive pressures in the collector loop; typically set at 125 psi. | FSEC 2006 manual |
| Freeze-protection valve | Opens at near-freezing temperatures to bleed warm water through the collector and prevent freeze damage. | FSEC 2006 manual |
| Thermal expansion tank | Absorbs pressure increases caused by thermal expansion, protecting the home and occupants. | Solar Oregon |
Frequently asked questions
Does a solar water heater always need a tempering valve?
Solar Oregon states that an anti-scald valve is required by code and will be installed to mix super-hot water with cold water before it is sent to the home’s faucets. ENERGY STAR describes a tempering valve as protection from scalding at the tap for batch collector systems, where water can sit in the collector for long periods and become very hot. Local plumbing codes determine the exact requirement, but the safety function is widely recognized.
Can I adjust the temperature myself instead of installing a valve?
The CPSC provides instructions for checking tap temperature with a thermometer and adjusting the thermostat on a conventional water heater, but solar storage tanks can reach temperatures far above the thermostat setting of a backup heater during sunny periods. A tempering or anti-scald valve addresses the overheated water coming from the solar storage tank regardless of the backup heater’s setting. The CPSC also notes that consumers should consider lowering the thermostat to the lowest settings that will satisfy hot water needs, but this alone may not prevent scalding from a solar-heated tank in summer.
What else should I check to keep the system safe?
Solar Oregon recommends understanding how your system addresses freezing and overheating, and checking the pH balance of heat transfer fluid periodically if your system uses glycol. The Florida Solar Energy Center manual describes several valves that protect against excessive pressure and temperature, including pressure-temperature relief valves and pressure relief valves, and notes that a regular service schedule should be maintained for check valves because corrosion and scale may build up on internal components.
For more on staying safe around a solar thermal system, visit the Scalding Prevention category page, the broader DIY Solar Water Heater Safety section, and the pillar article DIY Solar Water Heater Safety: Scalding, Pressure and Codes.
