For Those Who Know: Water Is No Sideshow

Treating heating water isn’t rocket science – but it’s no autopilot task either. In practice, the biggest (and most expensive) mistakes aren’t caused by the technology itself, but by how it’s used.

Whether in new builds, retrofits, or renovations – if the water treatment is done sloppily or based on a “should be fine” attitude, you’ll usually pay twice: through unnecessary maintenance, damaged components, or even loss of warranty.

Mistake #1: Wrong Connection Point – Bypass on the Supply Line

A classic mistake that can’t be mentioned often enough:
The treatment unit is connected to the supply line instead of the return. Sounds trivial – but the consequences are serious.
In the supply line, the water may be too hot (over 80 °C), and such high temperatures can damage the resin – drastically reducing its exchange capacity and potentially causing failure of the treatment unit.

The correct setup: integration in the return line, ideally at a hydraulically favorable position to ensure even mixing in the heating circuit.

It also makes functional sense: the heating water is treated and filtered before it flows back through the heat generator. This helps prevent deposits in heat exchangers, scaling, or corrosion. Particularly in condensing boilers with aluminum or stainless steel exchangers, this protects both the technology and the warranty.

In short: The cleaner the water before the boiler, the lower the risk of efficiency losses or damage.

Mistake #2: Expired Resin – As If Nothing Happened

Mixed-bed resins are consumables. They have a finite capacity and lifespan.
If you’re using resin that’s been sitting idle in a basement for three years, poor readings shouldn’t come as a surprise.

Rule of thumb:

Replace the resin at least once per year – even if it hasn’t been fully used.
For systems in regular use: Monitor conductivity and pH regularly to identify the right time for a replacement.

WaterBoy measuring case

The UWS WaterBoy measuring case contains everything the HVAC specialist needs to measure data in accordance with the VDI 2035, ÖNORM H 5195-1 and SWKI BT 102-01 standards in two L-BOXXES.
To the product

Mistake #3: No Flushing Before Initial Filling

If it cannot be ensured that the system is free of chips, sand, or other coarse contaminants, it should be flushed before commissioning in accordance with DIN EN 14336, e.g., with normal drinking water.

Heaty Ferriline No. 2

Complete unit for professional bypass treatment, sludge and magnetite filtration in the hot water area incl. MAGella twister
To the product

Mistake #4: Additives in the Water – Invalid Measurements

Adding glycol, cleaners, or other chemical agents to the heating water skews the measurements.
Conductivity becomes unreliable, and pH levels can shift unpredictably. The problem: you no longer know whether the water complies with standards – and reacting to deviations becomes guesswork.

Recommendation:

Only use additives that are explicitly VDI 2035-compliant
Always interpret measurements according to the manufacturer’s guidelines
When in doubt: send a sample to a lab or consult the chemical supplier

Mistake #5: Uncontrolled Replenishment – Deviation Guaranteed

The system was filled correctly, water values were fine – but after a few weeks or months, conductivity spikes and pH drops out of range. What happened?

Simple: Water was added – without control, desalination, or backflow prevention.
Often this happens manually, using drinking water straight from the tap.

What many underestimate: even small volumes of untreated water can disrupt the chemical balance in the heating system – especially in low-volume or mixed-material systems.

Recommendation:

Always top up via mixed-bed resin, e.g., with our Heaty Complete models
Incl. system separators according to EN 1717
Log refill volumes and monitor them regularly

Heaty Complete Professional

The Heaty Complete PROfessional is the world’s first smart IoT top-up system incl. Cloud connection – fully automatic, leakage protection and digital water meter.
To the product

Mistake #6: No Documentation – No Protection

The system is running, water values are fine – but nothing was documented. And that becomes a problem the moment a fault occurs or the manufacturer requests evidence.

Without proper documentation, any retrospective water analysis is worthless. And the response comes quickly:
“Water quality was not verified – warranty denied.”

Therefore, essential:

Log all water values (pH, µS/cm, °dH) in the system logbook
Record the resin replacement date
In existing systems: record current conditions regularly (e.g., during maintenance)

It takes less than five minutes – but could save thousands of euros in the long run.

One Thing You Should Always Do: Re-Test

Even if everything is properly installed and the resin is fresh: never blindly trust the system. Heating water must be monitored – not just during initial filling, but regularly during operation.

At minimum, measure:

Conductivity (µS/cm)
pH level
Total hardness (°dH)

At the first sign of deviation: re-treat before any damage occurs.

Conclusion: Cut Corners Here and You’ll Pay Twice

Clean, properly treated heating water is like insurance for the entire system.
It protects not just components and materials, but also you as the installer – from callbacks, warranty issues, and expensive service work.

Our recommendation:

Don’t just rely on good equipment – ensure clean implementation.
Because in the end, it’s not about what you’ve installed – but what actually flows through the system.

Header image: Alexander Fox | PlaNet Fox via Pixabay

Expensive and Stressful: Common Mistakes in Heating Water Treatment