Scale is the hard, chalky deposit that forms when water with high mineral content, often called hard water, is heated inside a solar water heating system. According to the Florida Solar Energy Center’s 2006 manual, the combination of dissolved minerals and the elevated temperatures produced by a solar collector can accelerate scale buildup in system piping, fittings, valves, collectors, storage tanks and heat exchangers. Left unchecked, scale reduces heat transfer, restricts water flow and can lead to expensive repairs.
What is scale and where does it form?
In a solar water heater, the hottest surfaces are inside the collector and any heat exchanger. The Florida Solar Energy Center’s 2006 manual, written for Florida conditions, notes that scale can accumulate in collector flow tubes, storage tanks, and the narrow passages of a heat exchanger.
In a direct system, the same potable water that comes out of your taps circulates through the collector. That means every bit of hardness in your water supply passes through the hottest part of the system. Indirect systems keep the potable water separate from the collector loop, so scale on the collector side is less likely, but the heat exchanger and storage tank can still develop deposits on the water side.
How water quality affects different system types
The FSEC manual states that indirect systems are typically used in areas with freezing temperatures or “areas that have water that is very high in mineral content.” The reason is straightforward: a closed loop filled with a treated heat transfer fluid avoids exposing the collector to raw hard water. Direct systems, by contrast, send household water straight through the collector, so they are much more exposed to scaling in hard-water regions.
ENERGY STAR’s guidance for solar water heaters reinforces this point. It advises that if you live in an area with hard water, you may need to add de-scaling agents to the water every few years. The program suggests running a vinegar solution through the collector or hot water loop every three to five years as one way to manage mineral buildup. This recommendation applies to both direct systems and the potable-water side of indirect systems.
Signs that scaling may be hurting your system
Scaling rarely announces itself with a single obvious symptom. Instead, you may notice a gradual decline in performance. The FSEC troubleshooting checklist links several common complaints to mineral buildup:
- Not enough hot water, even on sunny days.
- Flow blockages that cause the pump to cycle on and off unexpectedly.
- Reduced flow visible on a flow meter, or a smaller temperature difference between the collector supply and return pipes.
When a heat exchanger becomes scaled or clogged, the manual’s corrective action is to back flush and clean it. If collector flow tubes are clogged, the recommendation is to flush the collector tubing. In storage tanks, sediment and corrosion can accumulate, requiring the tank to be flushed or components to be replaced. Any of these signs should prompt a closer look at water quality.
Maintenance and prevention approaches
Routine attention to water quality can prevent most scaling problems. ENERGY STAR’s consumer guidance suggests adding a de-scaling agent such as a vinegar solution to the collector or hot water loop every three to five years. The FSEC manual describes back flushing heat exchangers and flushing the collector loop to remove dirt and scale. Have this work done by someone familiar with the system; any work that involves opening the pressurized potable-water side must follow local plumbing codes and may require a licensed plumber and a permit.
For more on routine upkeep, see our DIY Maintenance and Troubleshooting section and the detailed article on DIY Solar Water Heater Maintenance and Troubleshooting. The Water Quality and Scaling category page collects all related guidance.
Heat transfer fluid condition in closed-loop systems
Indirect systems rely on a heat transfer fluid, often a food-grade glycol, to move heat from the collector to the storage tank. Solar Oregon’s 2015 maintenance article warns that glycol that has been overheated can become acidic. That acidity can eventually cause pin holes in copper coils, leading to leaks and pressure loss. Solar Oregon recommends checking the pH of the glycol every year or two and having your installer show you how to do it.
The FSEC manual adds that glycol fluids must be checked periodically to ensure they remain chemically stable. It describes using pH indicator strips or color charts to monitor the fluid’s condition. The manual also advises inspecting a sample of the fluid for rust, scale, water deposits, sludge or gummy materials, any of which can signal degradation or a leak. If the fluid is breaking down, the system may need to be drained and recharged according to the manufacturer’s instructions.
Drinking-water safety and material requirements
Any part of a solar water heater that touches potable water must meet the lead-free requirements of modern plumbing codes. The FSEC manual notes that double-wall heat exchangers are required when a toxic heat transfer fluid is used, providing an extra barrier between the fluid and the drinking water. Even with non-toxic fluids, all wetted materials should be approved for potable use. Our Water Quality and Potable Materials article explains these requirements in more detail.
Solar-heated water can reach temperatures high enough to scald. Solar Oregon points out that an anti-scald valve, also called a tempering valve, is required by code to mix super-hot water with cold water before it reaches the taps. This valve is a critical safety component, and its proper operation is part of any water-quality and system-safety review. Learn more at Scalding Prevention.
Common scaling issues, signs and responses
| Issue | Sign | Response | Source |
|---|---|---|---|
| Scale in heat exchanger | Reduced hot water output, poor heat transfer | Back flush and clean the heat exchanger | FSEC manual troubleshooting checklist |
| Mineral deposits in collector flow tubes | Low or no flow, pump cycling, lower performance | Flush collector tubing | FSEC manual troubleshooting checklist |
| Hard water scaling in direct system piping | Flow blockage, pressure loss | Flush system, check effluent for dirt and scale | FSEC manual troubleshooting checklist |
| Glycol becoming acidic | Pin holes in copper coils, leaks, pressure drop | Check pH every year or two; replace fluid if acidic | Solar Oregon maintenance article |
| General hard water buildup | Gradual loss of efficiency | Add de-scaling agent (e.g., vinegar solution) every three to five years | ENERGY STAR solar water heater guidance |
Frequently asked questions
How often should I de-scale my solar water heater?
ENERGY STAR suggests running a vinegar solution through the collector or hot water loop every three to five years if you live in a hard-water area. The exact interval depends on your water hardness and system design. A qualified solar contractor can help you set a schedule based on local conditions.
What are the signs that my heat transfer fluid needs to be replaced?
Solar Oregon advises checking the pH of the glycol every year or two. If the fluid has become acidic, it can corrode copper and cause pinhole leaks. The FSEC manual adds that you should inspect a fluid sample for rust, scale, sludge or a gummy consistency. Any of these signs means the fluid is degrading and may need to be drained and replaced according to the manufacturer’s directions.
Does a drainback system eliminate scaling problems?
Drainback systems, described in the FSEC manual, often use distilled water as the heat collection fluid, which keeps household water out of the collector loop. However, the potable water side of the system, including the storage tank and any heat exchanger, can still develop scale if the household water is hard. Regular inspection of the water side remains important.
For more detailed guidance on maintaining water quality in your solar water heater, visit our Water Quality and Scaling category page. You can also explore the broader DIY Maintenance and Troubleshooting section and the pillar article on DIY Solar Water Heater Maintenance and Troubleshooting.
