Why DIY solar water heater risks matter before you build

A solar water heater is a solar thermal system, not a photovoltaic panel system. It uses the sun’s energy to heat water or a heat transfer fluid, and that heated water can reach temperatures that are dangerous in a home. Understanding the risks and limitations first helps you decide whether a DIY project is appropriate for your house, your climate, and your skills. This guide explains the main hazards and what typically manages them, based on published guidance from solar energy organizations and government agencies.

Short version: The main risks are scalding, pressure buildup, freeze damage, roof leaks, and performance limits. Tempering or anti-scald valves, pressure and temperature relief valves, freeze protection, careful roof work, and realistic expectations about backup heating all matter.

Solar Oregon’s maintenance and safety guidance says that freezing and overheating are the biggest hazards in a solar water heating system. That statement is a useful frame for the whole topic. A DIY system can work well, but only if you plan for what happens when the water gets very hot, when the pressure rises, when temperatures drop below freezing, and when the roof has to support the collector.

This article is part of the site’s Limitations and Risks section. If you are still learning how these systems work, start with the DIY Solar Water Heater Basics page and the pillar article on DIY Solar Water Heater Basics: How They Work and What to Weigh.

Scalding from very hot stored water

Solar-heated water can become much hotter than water from a typical conventional water heater. ENERGY STAR’s solar water heater guide explains that in a batch collector, also called an integrated collector-storage or ICS system, water can remain in the collector for long periods when household demand is low. That water can become very hot.

According to the U.S. Consumer Product Safety Commission, most adults will suffer third-degree burns if exposed to 150 degree Fahrenheit water for two seconds. Burns can also occur with a six-second exposure to 140 degree water or 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 users to lower 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.

Solar Oregon says that during the summer, solar water may be hotter than is safe for the home. An anti-scald valve, which Solar Oregon says is required by code, is installed to mix super-hot water with cold water before the water reaches the home’s faucets. ENERGY STAR describes a tempering valve as protection from scalding at the tap. The tempering valve mixes in cold water to decrease the water’s temperature before delivery.

The Florida Solar Energy Center manual from 2006 makes a useful distinction between two similar-sounding valves. A mixing or tempering valve mixes cold water with the hottest water from the storage tank and delivers water at a preset temperature. The FSEC manual says the intended use of that valve is to conserve hot water, not to act as a safety device. An anti-scald valve also mixes cooler water with hot water, but the FSEC manual says it functions as a safety device because it closes off the flow into the house if the hot or cold mixing supply fails.

For a closer look at this topic, see Scalding Prevention and the broader DIY Solar Water Heater Safety page.

Pressure and temperature relief

A solar water heater is connected to a home’s pressurized water supply, and heat increases pressure. The FSEC manual says a temperature-pressure relief valve, also called a pressure-temperature relief valve or P&T valve or T&P valve, protects system components from excessive pressures and temperatures. The manual says this valve is always plumbed to the solar storage tank as well as the auxiliary tank.

The FSEC manual, which is a 2006 Florida-focused document, says these valves are usually set by the manufacturer at 150 psi and 210 degrees Fahrenheit. Those are that source’s stated guideline figures, not a universal current specification. The manual also says temperature-pressure relief valves located inside a building must drain to the outside, and that if you are uncertain, you should follow local code requirements.

Solar Oregon lists pressure and temperature release valves and thermal expansion tanks as safety equipment that protects the home and the occupants. The FSEC manual describes a separate pressure relief valve that protects components from excessive pressures in system plumbing. In any system where the collector loop can be isolated from the storage tank, the manual says a pressure relief valve must be installed on the collector loop.

Anything connected to a home’s pressurized drinking-water supply must follow local plumbing and building codes and may need a permit and a licensed plumber. The Pressure and Temperature page covers this area in more detail.

Freeze damage and which system types are vulnerable

Freeze damage is one of the most serious risks for a DIY solar water heater. The FSEC manual says that except for solar systems in the tropics, all systems require some type of freeze protection method. The manual lists five basic methods: indirect antifreeze fluid circulation, draining, water flow, thermal mass, and an auxiliary heater.

ENERGY STAR explains that direct systems circulate water through solar collectors and are preferable in climates where it rarely freezes. Closed-loop, or indirect, systems use a non-freezing liquid to transfer heat from the sun to water in a storage tank. The non-freezing fluid passes through a heat exchanger, and ENERGY STAR says these systems make sense in freezing climates.

Batch or ICS systems are particularly vulnerable. ENERGY STAR says batch collectors are incompatible with closed-loop circulation systems and are generally not recommended for cold climates. The U.S. Department of Energy’s 2003 consumer guide also says ICS systems should be installed only in mild-freeze climates because the outdoor pipes could freeze in severely cold weather. That guide is from 2003, so treat its statements as the guidance published at that time.

The FSEC manual describes freeze-protection valves that are set to open at near freezing temperatures. These valves are installed on the collector return line close to where the line penetrates the roof. Warm water bleeds through the collector and out the valve to protect the collector and pipes from freezing. The manual notes these valves need water pressure to work, so on well water or cistern systems low-pressure relief valves should also be installed.

Drainback systems offer another approach. The FSEC manual says drainback systems provide freeze protection and high-limit protection because the collectors empty by gravity when the system pump is not operating. When installed correctly, the manual says these systems provide a fail-safe method for protecting the collectors and piping from freeze damage.

If you live in a cold climate, review Winterizing DIY Systems and the system-type explanations in DIY Designs before choosing a design.

Leaks, roof penetrations, and roof load

Roof-mounted collectors create penetrations in the roof, and every penetration is a potential leak point. The FSEC manual says the most demanding aspects of installing roof-mounted collectors are the actual mounting and roof penetrations. It says to always follow accepted roofing practices, be familiar with local building codes, and communicate with the local building inspector.

The FSEC manual recommends sealing and flashing pipe and sensor penetrations in accordance with good roofing practices. It says to use permanent sealants such as silicone, urethane, or butyl rubber. The manual also says the collector or its supports should be anchored to the roof rafters or trusses, not the sheathing. Water should not gather or pool around roof penetrations, and roof curbs may be required.

Wind load is another structural concern. The FSEC manual says a mounted collector is exposed to wind forces, and in parts of the world vulnerable to hurricanes or extreme wind storms, the collector and its mounting structure need to withstand intermittent wind loads up to 146 miles per hour. That corresponds to a pressure of about 75 pounds per square foot, according to the manual. It also says winds and thermal contraction and expansion may cause improperly installed bolts and roof seals to loosen over time.

The FSEC manual advises against mounting collectors near the ridge of a roof or other places where wind load may be unusually high. Mounting collectors parallel to the roof plane helps reduce wind loads and heat loss, according to the manual.

Roof work itself is hazardous. The Roof and Structural Safety page explains what to consider before climbing onto a roof. The Codes and Professional Review page covers permits and professional input.

Overheating and stagnation

Solar collectors can get extremely hot when the system is not moving heat away. The FSEC manual says collectors must be able to tolerate stagnation temperatures, and that this can be as high as 400 degrees Fahrenheit for some solar collectors. That is a figure from the 2006 FSEC manual and describes what some collectors may experience, not a target for a DIY build.

Solar Oregon says overheating is one of the biggest hazards. It also says glycol that has been overheated can become acidic and eventually can cause pin holes in copper coils. Solar Oregon recommends having an installer show you how to check the system every year or two, particularly the pH balancer if the system uses a heat transfer fluid such as food-grade glycol.

Stagnation is closely tied to system design. A system that keeps collecting heat when no hot water is being used can reach high temperatures quickly. That is one reason pressure relief, tempering, and proper materials all matter at the same time. The Controls and Sensors page explains how controllers and sensors help manage heat collection.

Performance limits: climate, season, and backup heating

A solar water heater does not produce the same amount of hot water every day. The U.S. Department of Energy’s 2003 consumer guide says solar heating technologies can be used in any climate, but the type of system you choose depends on your site, the climate you live in, installation considerations, cost, and how you would like the system to be used. The guide says you usually need an unshaded area that faces south, southeast, or southwest, such as a roof.

The FSEC manual explains that cold supply water changes how much energy a system needs. It gives an example from its 2006 Florida-focused manual: the cold water supply temperature in Miami is approximately 75 degrees Fahrenheit, while the cold supply in Toronto is approximately 45 degrees Fahrenheit. More total solar energy is required to raise the Toronto system to the same end-of-day tank temperature, according to the manual. To achieve the same end-of-day tank temperature in Toronto, the manual says you must increase the collector area.

Because solar output varies, a backup heater is usually part of the system. The DOE guide describes systems in which the solar water heater preheats water before it enters a conventional water heater. Solar Oregon says the hot water from a solar water heater system will be sent through your water heater, which may not even turn on if the solar water is hot enough. That means the conventional heater is still there for cloudy days and high-demand periods.

For help estimating how much hot water you actually use, see Hot Water Needs. For site and climate considerations, see Site Assessment.

Risk summary table

Risk What manages it Who should handle it
Scalding from very hot water Anti-scald or tempering valve, lower storage temperature Plumber or experienced installer familiar with local code
Pressure or temperature buildup Pressure and temperature relief valve, expansion tank Plumber or experienced installer
Freeze damage Indirect antifreeze loop, drainback, freeze-protection valves Designer or installer who understands the local climate
Roof leaks or structural failure Flashing, sealants, anchoring to rafters or trusses Roofer, structural professional, or experienced installer
Overheating or stagnation Proper controls, pressure relief, heat-tolerant materials Designer or installer
Insufficient hot water in some seasons Backup water heater, realistic sizing Homeowner with professional sizing help

What this means for a DIY project

A DIY solar water heater is not just a plumbing project. It is a pressurized system on or near your house, and several failure modes can injure people or damage the building. The FSEC manual says a 4-by-10-foot solar collector can act like an airplane wing in strong winds, so it must be structurally well connected and securely attached to its mounting surface. That warning applies whether you buy a collector or build one.

The FSEC manual also says solar water heating systems must not contaminate water that may be used in food preparation or in human contact activities. For this reason, the manual says solar water heaters with heat exchangers frequently employ two walls to isolate the potentially dangerous heat collecting fluid from the water to be heated. If you are choosing materials, the Water Quality and Potable Materials page explains what potable-water-safe means.

Anything connected to a home’s pressurized drinking-water supply must follow local plumbing and building codes and may need a permit and a licensed plumber. Before you commit to a build, read Is DIY Right for You and Planning a DIY Solar Water Heater: Needs, Site and Materials.

Frequently asked questions

Can I prevent scalding without a special valve?

The U.S. Consumer Product Safety Commission says consumers should consider lowering the thermostat to the lowest settings that will satisfy hot water needs. However, solar-heated water can be much hotter than a conventional heater’s set point, and Solar Oregon says an anti-scald valve is required by code. Relying only on a thermostat setting is not the same as having a valve that mixes cold water before the tap.

Are batch heaters safe in cold climates?

ENERGY STAR says batch collectors are incompatible with closed-loop circulation systems and are generally not recommended for cold climates. The U.S. Department of Energy’s 2003 consumer guide says ICS systems should be installed only in mild-freeze climates because the outdoor pipes could freeze in severely cold weather. If you live where temperatures drop below freezing, a different system type is usually more appropriate.

Do I still need a backup water heater?

Yes, in most cases. Solar output changes with weather and season, and the U.S. Department of Energy’s 2003 guide describes systems in which the solar water heater preheats water before it enters a conventional water heater. Solar Oregon says the hot water from a solar water heater system will be sent through your water heater, which may not even turn on if the solar water is hot enough. The conventional heater remains available when solar heat is not sufficient.

For more on the limits of DIY solar water heating, see the Limitations and Risks category page.

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