9.3 Water treatment and scale prevention
Water quality is measured by a few simple numbers. Hardness is the amount of dissolved calcium and magnesium, given as mg/L (ppm) of CaCO₃. TDS (total dissolved solids) is all dissolved salts in mg/L. pH shows whether water is acidic (below 7) or alkaline (above 7).
Borewell water in many parts of India is hard. Hardness of several hundred mg/L is common in places like Rajasthan, Gujarat and parts of Bengaluru. A kettle used with such water gets a white crust; a heat pump gets the same crust inside its PHE, and fast, because the water is hot.
Scale forms faster at higher temperatures. That is why hot-water heat pumps above about 55 °C are most at risk. Open systems, where fresh make-up water is added all the time, bring in new hardness every day.
Typical water quality guide for heat pump circuits
| Parameter | Typical target (closed loop) | Why it matters |
|---|---|---|
| pH | 7.5–9.0 | Low pH corrodes copper; very high pH can attack aluminium and brazing |
| Total hardness | Below about 100–150 mg/L as CaCO₃ for hot water service (typical) | Scale on PHE |
| TDS | Below about 500 mg/L (typical) | Scale and corrosion |
| Chlorides | As low as practical; typically below about 200 mg/L for stainless steel at heat pump temperatures | Pitting of stainless plates |
| Suspended solids | Filtered by strainer, mesh typically 0.5–1 mm | Blocking of PHE channels |
These are typical values. Always use the limits in the unit's manual where available.
Ways to prevent scale
| Method | How it works | Good for | Limits |
|---|---|---|---|
| Closed loop with indirect PHE | Hard water never touches the heat pump PHE | Most sites with hard water | Needs a second heat exchanger in the tank |
| Water softener (ion exchange) | Swaps calcium and magnesium for sodium | Domestic and hotel hot water | Needs regular salt and regeneration |
| Reverse osmosis (RO) | Removes most dissolved salts | Small flows, very bad water | Wastes water; RO water can be corrosive, so adjust pH |
| Polyphosphate dosing | Keeps hardness in suspension | Moderate hardness | Must be food-safe for drinking water |
| Lower hot water set point | Less scaling at lower temperature | Where use allows | Check hygiene needs (legionella) |
| Y-strainer and dirt separator | Catches sand and rust | All systems | Must be cleaned monthly |
Worked example — How much scale comes in?
A hotel in Jaipur uses 5,000 L of hot water a day. Hardness is 300 mg/L as CaCO₃. If even a tenth of this hardness drops out as scale:
V = 5,000 L, H = 300 mg/L, f = 0.1 m = 5,000 × 300 × 0.1 = 150,000 mg = 150 g per day.
Over a year: 150 g × 365 ≈ 55 kg of potential scale. Even if most leaves with the water, some will settle in the PHE. This is why open systems on hard water need a softener or an indirect loop.
Remember
- Test hardness, TDS and pH at commissioning and every quarter.
- Hot water above about 55 °C scales fastest.
- An indirect closed loop is the best protection on hard water.
- Clean strainers monthly.
Most water-side problems in Indian sites come from hard, dirty or corrosive water, and they are cheaper to prevent than to clean.
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