Why the volume of make-up is the most important number in a closed loop, how quickly it dilutes the treatment, and how to track down where the water is going.
A sealed loop should take almost no water. Whatever make-up it does take is water that has left the loop — through a leak, a relief valve, a pump seal, a drain-down, or a coil that burst. Measure it, record it, and treat a rise as a fault.
Every gallon of make-up replaces a gallon of treated loop water with untreated water. As a fraction of the loop:
Share of the loop replaced = make-up volume ÷ loop volume
Example. A 5,000-gallon loop treated to 1,000 ppm inhibitor takes 50 gallons a day of make-up — 1 % of the loop every day.
The same loop taking 50 gallons a month would lose only about 1 % of its inhibitor in that month.
A closed loop should replace only a small share of its volume in a year. If the make-up in a year approaches the loop’s own volume, treat finding the leak as more urgent than adjusting the treatment.
The loop volume is needed for dosing and for judging make-up. If the design documents do not give it:
| Where water goes | What to check |
|---|---|
| Relief valves | A weeping relief valve on a boiler, expansion tank, or heat exchanger — check discharge piping for flow or scale trails. Often caused by an over-pressurized or waterlogged expansion tank |
| Pump seals and packing | Drips and white or green deposits at the seal |
| Drain-downs | Maintenance draining and refilling sections without telling anyone. Ask, and log drain-downs |
| Coils and heat exchangers | A failed coil leaks into air handlers or into another circuit — a loop that loses water while another system gains it |
| Automatic fill valves | A fill valve that is stuck open, or set above the relief pressure, can pass water continuously to a relief valve |
| Hidden piping | Underground or in-wall leaks. Isolate sections and watch the pressure fall |