Hydronic Heating Maintenance: What to Know About Chemical Treatment and Water Quality
Key Takeaways
- Perform seasonal maintenance checks, including flushing, cleaning, and inspecting safety components, to ensure system reliability before winter.
- Regularly inspect and maintain pumps, sensors, and control systems to detect early signs of wear or leaks that could lead to system failure.
- Offer water chemistry and system health assessments as part of your service to increase customer satisfaction and generate additional revenue.
Proper hydronic heating maintenance goes beyond just adjusting the thermostat; it involves managing the water chemistry flowing through the pipes to protect boilers, pumps, and heat exchangers.
Summer and early fall offer heating contractors an opportunity to enhance their business and reputation. While homeowners enjoy warm-weather activities, this season should also be a prime time for maintenance and customer engagement. Successful businesses focus on delivering consistent product and service quality, emphasizing customer satisfaction through proactive problem prevention. This makes these seasons the perfect time for annual maintenance checks on hydronic heating systems before winter returns.
Even if customers prefer to ignore their heating systems during the warmer months, a unique maintenance program that highlights water quality could capture their interest.
Why Water Chemistry Matters
When new water is introduced into a system, either during installation, leaks, or repairs, it brings minerals and oxygen. Over time, that can lead to internal corrosion, mineral scaling, and sludge buildup. Hydronic heating systems corrode without proper maintenance and become less efficient, leading to additional service calls and a shorter lifespan. Replacing a heating system can be very expensive, costing home and building owners thousands of dollars.
Maintaining proper pH levels is key. Most metals in these systems — copper, cast iron, and steel — perform best with a higher pH (around 9-10). If the system includes aluminum components, you must maintain a lower pH, usually between 7.0 and 8.5.
A corrosion inhibitor will help increase pH levels and reduce wear on internal components. Some inhibitors are suitable for all metals, including aluminum, cast iron, and stainless steel.
An annual pH test is a bare minimum. In commercial applications or older systems, checking pH twice a year is a smart move that can help you catch problems before they start.
Start with a visual reading by opening the drain valve and bleeding off some liquid. If it appears clear, proceed with a pH test. To run a pH test, you can use test strips that provide a color-coded readout. If you need a more precise reading, say, for an aluminum system, use a digital pH meter.
Levels of freeze protection and corrosion protection should also be checked annually. Use test strips to check overall protection, including the levels of propylene glycol and corrosion protection.
Service Tips for Steam and Closed-Loop Systems
Steam systems come with their own set of challenges. Even in a well-functioning setup, you’ll lose some water through steam. That means you’re constantly introducing fresh water, and with it, more minerals and oxygen. If those elements aren’t treated, corrosion and sludge can build up fast.
Larger steam boilers are typically used in multifamily or commercial applications. Years ago, these large steam boilers held hundreds of gallons of water, but as time progressed, steam boilers became more efficient, and more compact units are now available.
As a steam boiler heats up, water becomes steam flowing through the system. When that steam condenses, it will return to the boiler. However, newer, more compact boilers don’t have the same capacity as their older counterparts, leading to quicker water depletion.
To address this issue, oftentimes a reservoir tank is installed along with components that sense when the boiler water levels are low or completely depleted. These components automatically shut the boiler down, allowing water to return to the boiler or reservoir tank giving it time to refill. Essentially, they prevent the unit from firing if there's insufficient water. Some of these components are electronic; others use a mechanical ball that floats inside the unit, rising and falling with the water level.
Timers on these systems indicate a shutoff for a set period, usually one to three minutes, to allow all condensate and water to return to the boiler. If the return water is insufficient, electronic and solenoid valves will open to let more water in, introducing oxygenated water that can lead to corrosion or a pump will deliver water to the boiler from the reservoir tank. This is why it's critical to flush these safety components periodically to mitigate mineral buildup.
Oatey recommends using a cleaner, especially if the system isn’t dirty enough to warrant a complete flush. These cleaners are designed for cleaning and conditioning steam and hot-water boilers in hydronic heating systems while containing silicone that protects pumps by lubricating seals, maximizing efficiency, and preventing valve clogging and corrosion.
Always monitor your system's pH levels. If the first liquid sample in your visual pH reading shows significant rust or sludge, it may require a more aggressive approach, such as flushing with a system restorer.
Sludge often points to corrosion in the heat exchanger, fittings, or piping. It’s also advisable to add a corrosion inhibitor once a year to help condition new water entering the system.
Regular maintenance, including the use of cleaners or corrosion inhibitors at least once a year — typically at the beginning or end of the heating season — is crucial for the longevity and efficiency of steam boilers.
Newer residential hydronic systems are typically designed as closed-loop systems. It's important to avoid opening and flushing a closed-loop system unless absolutely necessary. The reason for this caution is that the primary enemy of such systems is oxygenated water, which can lead to corrosion and other issues.
However, like a car, closed-loop hydronic heating systems require regular servicing to prevent problems and maintain efficiency. Subtle clues that a system might be experiencing issues include a rumbling noise indicating that minerals are starting to collect within the system. Even in a closed loop, a component remains that feeds water into the system as needed.
Additionally, your customer might unknowingly have a system leak that automatically introduces oxygenated water over time from a pressure regulating valve. This can create numerous complications and alter the pH level within the system, contributing to corrosion.
For a closed-loop residential boiler to function effectively, the circulating liquid must maintain the appropriate pH levels. A yearly pH test is recommended to identify potential leaks or sludge buildup and determine if the system requires flushing with a cleaner.
Beware of Sludge
Discolored water isn’t always a sign of trouble. In fact, slightly brown water is normal in systems with cast iron or copper components. But sludge is another story.
Sludge forms when metals corrode or minerals settle at low points in the system, typically the boiler or heat exchanger. Left unchecked, sludge buildup can impair heat transfer, block drain valves, and even push cast iron boiler sections apart. That’s why corrosion inhibitors matter. They don’t just prevent damage; they help keep the system clean and efficient in the long term.
Modern systems often include air separators and debris filters that help trap particulates before they cause damage. If your system has one, make sure it’s being cleaned at least once a year.
Annual Service Checklist
Whether you’re maintaining a single-family boiler or a system in a multi-unit building, an annual service checklist should go well beyond a quick visual once-over.
Here’s a rundown of what a complete inspection should include:
- Visual inspection: Look for signs of leaks, staining, or old corrosion. Check joints, pipes and the boiler body;
- Burner service: Remove and clean burners. Check and replace thermocouples or thermopiles, if needed;
- Sensor checks: Inspect ignition, flame, and water level sensors;
- Exhaust piping: Look for rust on galvanized venting or cracks in PVC piping, depending on system type;
- Control system test: Inspect and calibrate aquastats. In commercial setups, check outdoor temperature sensors and high-limit safety controls;
- Pump check: Inspect for leaks or noise. Different pump types wear differently, so know what you're looking for; and
- Water quality test: Use test strips or a digital tool to check pH levels and, if antifreeze is present, glycol concentration.
Don’t forget to check any drain valves or low-point filters, particularly in older systems. These can clog with sediment and make flushing a major headache later on.
Freeze Protection
When antifreeze is needed, especially in snow-melt systems or properties prone to freezing, choosing the right type and concentration is critical.
Antifreeze does more than just protect against freezing temperatures. A hydronic heating system must also maintain chemical balance, resist corrosion, transfer heat efficiently, and remain pumpable under extreme conditions. Selecting the right product depends on system metallurgy, desired freeze protection, glycol concentration limits, and local climate.
These factors determine what to use:
System material compatibility and pH requirements: Different metals require specific antifreeze formulations to avoid damage. Most systems with copper, steel, or cast iron are compatible with -100, with a pH of 8.5–10.0 and 9.0–10.0, respectively. For aluminum components, choose a solution formulated with a lower pH (7.0–8.5) to prevent degradation.
Freeze and burst protection: Another factor is the freeze-protection level required for the heating system or the desired temperature coverage. Different anti-freeze types offer different freeze-protection levels, so it is essential to choose the right type to ensure adequate protection.
Viscosity and pumping efficiency: Lower viscosity means easier circulation and better energy efficiency. Choose lower viscosity blends for systems with smaller pumps or longer loops.
Heat transfer performance: High heat transfer coefficients are excellent for performance-focused applications.
Glycol quality and content: Some boiler manufacturer warranties state that antifreeze made with virgin propylene glycol (not recycled) is required, or the warranty is voided. Recycled propylene glycol is captured and cleaned after use (mainly for de-icing aircraft). It is therefore considered a lower grade of propylene glycol and may still contain other ingredients even after cleaning.
Be aware of the PG ratio (ratio of propylene glycol to water in the antifreeze solution). This impacts both viscosity and compatibility with manufacturer limits. Many manufacturers limit the total glycol content to minimize strain on the system. It’s important to differentiate the “100” on the label, which does not mean "100% glycol."
Testing with a refractometer or digital meter ensures proper concentration, especially in older systems or after a partial drain-and-fill. Regular testing is necessary, as antifreeze can degrade over time and lose effectiveness in preventing freeze and corrosion damage.
Missed Opportunity
If you don’t check pH, offer corrosion treatment, or test antifreeze levels as part of standard service, you’re missing an opportunity to earn additional revenue and show customers you’re thinking ahead.
The products are affordable, easy to apply, and offer real value. Even if the system is running fine today, protecting it from long-term wear and chemical imbalance can make all the difference in long-term performance, efficiency, and lifespan.
Bottom line: Preventive maintenance isn’t just about checking boxes; it’s about understanding what’s happening inside the system and taking the right steps to keep it running clean. Start with water quality, don’t skip the chemistry, and give every system the attention it deserves.
About the Author
Jim Eichenser
Jim Eichenser is an inside sales and tech support specialist at Oatey Co. and has decades of plumbing and boiler installation and service experience.