How closed recirculating loops — hot water heating, chilled water, process cooling, and glycol systems — lose water, why every gallon of make-up matters, and what the controller does to protect them.
A closed loop circulates the same water round and round, sealed from the air. Unlike a cooling tower, it does not evaporate, so it does not concentrate and needs no blowdown. Ideally, it would need no make-up at all.
| Open cooling tower | Closed loop | |
|---|---|---|
| Water lost | Evaporation, drift, and blowdown — by design | Only through leaks, drain-downs, and relief valves — by accident |
| Make-up | Large and continuous | Should be close to zero |
| Treatment | Fed continuously, in proportion to make-up | Charged once to a high level, then topped up |
| Main risk | Scale, deposits, and biological growth | Corrosion and biological growth, made worse by fresh make-up |
Make-up is the problem in a closed loop. Every gallon added dilutes the inhibitor, and brings in dissolved oxygen, hardness, and bacteria. A loop that takes a lot of make-up corrodes, however good the treatment program.
| Job | Why | How |
|---|---|---|
| Measure make-up | Make-up is the leak detector: a rise means water is being lost | A water meter on the make-up line, read by the InteliHive — see Make-Up and Leaks |
| Hold pressure, and feed premixed fluid | Keeps the loop full and the pumps from cavitating, and adds glycol mix rather than plain water | A Repressurizer — see Pressure and Fill |
| Alarm on loss | Catch a leak before the loop is diluted or drained | Alarms on make-up volume, on low pressure, or on a low feed-tank level |
| Feed inhibitor | Replace what is lost with the make-up | By hand through a pot feeder, or Volume Dosing on the make-up meter |
| Monitor corrosion | A closed loop’s failures are corrosion failures | A corrosion rate probe, alongside coupons |
| Page | Covers |
|---|---|
| Make-Up and Leaks | Why make-up is the key number, how fast it dilutes inhibitor, and finding the leak |
| Glycol Loops | Why glycol, which kind, keeping the concentration and inhibitor right, and topping up |
| Pressure and Fill | Fill pressure, expansion tanks, why pressure drops, and automatic fill vs a repressurizer |