Good heat starts
with good water.
The water inside a heating system affects how well it heats and how long its parts last. Mineral deposits can insulate hot metal, corrosion can shed debris, and trapped air can interrupt circulation. Here is how those problems develop and how treatment addresses them.
THE FIRST THING TO UNDERSTAND
The same water
goes around again.
A closed heating loop sends water from the boiler through the home and back. Because it reuses that water, recurring top-ups are a reason to look for a leak or a discharging safety valve.
Why constant topping-up hurts
Fresh water can bring in more oxygen and minerals. If a leak or safety relief valve keeps releasing water, the fill valve—the part that adds water—may quietly replace it. Fixing the loss matters as much as treating the water.
When to replace the fluid
A fluid sample shows whether mineral content, acidity or depleted protective chemicals need attention. Debris collected from a separator also helps the technician decide whether cleaning is needed.
The heating loop and your drinking water are different systems. Some boilers also heat tap water, either inside the boiler or through a sealed coil in a separate storage tank. A heat exchanger is the metal wall or coil that passes heat across while keeping the two water supplies apart.
Caleffi: the closed hydronic circuit ↗BEFORE ANYONE OPENS THE FILL VALVE
Choose fill water that suits the boiler.
The boiler’s manual gives water-quality limits for its heat exchanger. Compare a test of the home’s water supply with those limits before choosing softened, demineralized or blended fill water.
Hardness, salts and corrosion
Hardness measures dissolved calcium and magnesium, minerals that can form hard deposits. A salt-based softener exchanges those minerals for sodium. It can reduce deposits, but dissolved salts remain. That water still conducts electricity, which can support corrosion between different metals in a heating system.
What we check
Demineralization removes dissolved mineral salts. The technician chooses treatment by comparing the incoming water with the boiler’s limits, then checks the finished mixture, including any approved additives.
Caleffi idronics 18, pages 11–15: softening, conductivity and corrosion ↗
A real example: Lochinvar WHB fill water
The WHB installation manual, revision T, specifies fill-water hardness of 5–12 grains per gallon. Grains per gallon, shortened to gpg, is a measure of water hardness. The manual calls for correction below 5 gpg. That makes zero-hardness water from a household softener unsuitable as-is for this specification.
The cited WHB manual also gives other water-quality limits. Keeping the test results with the exact model’s manual makes later top-ups and service easier to plan.
Lochinvar WHB manual: Start-up / Fill water ↗What bad water
can look like.
Record these observations for the service visit. Don’t open or sample a hot, pressurized system yourself.
White, hard deposits
Hard mineral deposits can coat the hot metal, making it harder for heat to reach the water. They can also block narrow passages. Check the mineral content of added water and whether the system has needed repeated top-ups.
Black grit or sludge
Corrosion of iron parts can make black magnetic particles called magnetite. They can collect in water pumps, valves and heat-transfer passages. A magnetic dirt collector catches those particles; a technician still needs to find what is damaging the metal.
Cloudy water / recurring debris
Installation residue, corrosion and old contamination can all be involved. Sample condition, solids and system history help decide whether targeted cleaning or a fuller flush is justified.
Clear water / recurring failures
Test clear-looking water too. Fluid tests measure dissolved salts and pH—how acidic or alkaline the water is—and check the condition of corrosion-protection additives.
DIFFERENT PROBLEMS. DIFFERENT TOOLS.
Match the treatment
to the water condition.
Explore the treatment options and the checks used to choose and maintain them.
Collecting dirt and magnetic particles
What it does
Captures particles already circulating in the water. A magnet is particularly useful for particles from iron corrosion.
Service considerations
Remove the collected dirt using the device’s service procedure. Assess dissolved minerals and recurring oxygen entry through the appropriate water and system checks.
Cleaning and flushing
What it does
Removes residue or contamination when the condition of the system calls for it. The cleaner, method and rinse must suit the equipment and existing materials.
Service considerations
After cleaning, the system needs the specified rinse so leftover cleaner does not remain in the heating fluid. Repairing a leak or other contamination source helps keep the cleaned loop in good condition.
Water suitable for filling this boiler
What it does
Starts the loop with water prepared for the exact boiler and the other equipment connected to it. Test the actual fill supply before choosing treatment.
Service considerations
A household softener reduces hardness, while demineralization removes dissolved mineral salts. Select the treatment by testing the incoming water against the boiler’s limits, including any minimum hardness.
Chemicals that help protect metal
What it does
A corrosion inhibitor is a chemical added to help protect metal from damage. Use it only when permitted for the system, at the maker’s specified strength and with compatible products.
Service considerations
Correct leaks, remove debris and address oxygen entry alongside fluid treatment. Identify existing additives and use the product maker’s compatibility and testing instructions.
Air and oxygen control
What it does
Removing trapped air, collecting small bubbles and maintaining correct pressure all help. Oxygen-barrier tubing has a layer that limits oxygen passing through the pipe wall into the water.
Service considerations
Assess oxygen entry through the pipe materials. Where needed, specify suitable tubing or a heat exchanger to separate that circuit from vulnerable equipment.
Further reading: Caleffi water-quality guide · Fernox: checking inhibitor and cleaner residue · Find the boiler’s specific water-quality limits
FREEZE PROTECTION IS A MAINTAINED FLUID
Antifreeze needs
the right mixture.
When it earns its place
Glycol is the antifreeze ingredient mixed with water. Exposed pipes or spaces without reliable heat may need it. Use only a heating-system product and mixture strength allowed by the boiler and connected equipment. The required protection depends on the actual exposure.
What you take on
Glycol mixtures move and transfer heat differently from plain water. The pump and expansion tank must suit the mixture. Concentration means the percentage of glycol in the liquid. That percentage and the condition of the protective chemicals need periodic tests.
What should the technician test and record?
Record the product name, glycol percentage and freezing protection. Test acidity and corrosion-protection chemicals using that product’s method, then record any mixture correction and the result after circulation.
Repeated water additions dilute glycol. Adding more concentrate by guess can overshoot the needed protection. Never add automotive antifreeze, mix unidentified fluids or assume plain propylene glycol provides a complete set of chemicals needed to protect system metals.
KEEP THIS WITH THE EQUIPMENT
A useful service record
has actual findings.
Keep the test results with the boiler’s model number, fluid product and service date. The next technician can compare new readings with those earlier results.
Use this checklist to prepare for a service visit, then print a copy for your records.
What can you do
between visits?
- Look for new drips, stains, noises or recurring error codes without removing covers.
- Keep the burner’s fresh-air intake and exhaust openings clear.
- Tell your technician if you keep losing pressure or adding water.
- Keep service records and the correct equipment manual together.
- Follow the manufacturer’s schedule for qualified service and carbon-monoxide (CO) alarm care.
Gas odor, a CO alarm or suspected fumes requires the emergency response described in the main boiler guide. Don’t use cleaning chemicals, leak sealers or pressure adjustments as an experiment.
See what happens when one number changes. Explore how water carries heat, how antifreeze gets diluted and why valves need pressure. Start with guided examples or enter your own measurements, with explanations in both views. Open the calculators ↗
CHECK YOUR UNDERSTANDING
Can you explain it?
A glycol mixture leaked and was topped up with plain water. What changed?
Try answering in your own words, then compare your reasoning.
Show the answer & why
The added water diluted the glycol. After the leak is repaired, measure the mixture and compare its protection with the heating-fluid product’s data. Adjust the mixture and test it again after circulation.