How a closed loop is kept full and at pressure, why its pressure falls, and how an automatic fill valve and a repressurizer differ.
Never over-pressurize a closed loop. Keep fill pressure well below the relief valve setting and below the rating of every component. See Pressurized Systems.
A closed loop must stay full and above atmospheric pressure at its highest point. If it does not:
A common rule of thumb for the cold fill pressure, measured at the fill point:
Fill pressure ≈ height of the highest point above the gauge × 0.433 psi/ft + about 4–5 psi
Example. The highest coil is 46 ft above the fill point: 46 × 0.433 ≈ 20 psi, plus 5 psi = about 25 psi cold fill pressure. The extra few psi keep the top of the system positive, so air vents vent rather than draw air in.
Use the design fill pressure where the system documents give one.
Water expands as it heats. The expansion tank absorbs that expansion so pressure rises only moderately between cold and hot. Its air or gas charge must match the fill pressure.
| Cause | Pattern |
|---|---|
| A leak | Pressure falls steadily; make-up rises |
| Air venting | After a fill or maintenance, pressure falls as trapped air is vented, then stabilizes |
| Cooling | Pressure falls as the loop cools and the water contracts — normal, and recovers on warming |
| Relief valve lifting | Pressure falls after each heat-up; often a waterlogged expansion tank |
| Automatic fill (pressure-reducing) valve | Repressurizer (glycol feeder) | |
|---|---|---|
| Feeds | Potable water, straight from the supply | Premixed fluid from its own tank |
| For | Water-only loops | Glycol loops, or any loop that must not be diluted |
| Backflow | Needs a backflow preventer to protect the potable supply | No connection to the potable supply |
| Measuring what it fed | Only with a meter on the fill line | Read the tank’s gallon markings; log them regularly |
| Stops when empty | Never — it keeps feeding a leak indefinitely | Stops and alarms at the tank’s LOW LOW level |
The Repressurizer senses loop pressure and runs its pump from its tank of premix whenever pressure falls below its low set point, stopping at the high set point. It will not feed from an empty tank, and raises an alarm at the tank’s LOW LOW level — so a leak shows up as a falling tank, not as a slowly diluted loop. See Repressurizer Operation.