The Gould AquaBoost II holds the pump outlet near 50 psi, but gauges P2, P4, and P5 reportedly rise toward their 100-psi stops during periods with no hot-water draw, then fall back near 50 psi when hot water is used. Treat this first as a trapped-volume or thermal-expansion problem to investigate—not as proof that the circulator pumps are operating in parallel.
Record a safe pressure baseline
Before changing pump speed or valve positions, determine what the gauges actually measure and whether they can safely indicate the pressure in question. A gauge that reaches its mechanical stop only shows that pressure has reached or exceeded its readable range; it does not establish an exact 100-psi system pressure.
- Identify each gauge location and range. Match P2, P3, P4, and P5 to the installed piping drawing. Record the gauge range, condition, and whether a check valve or closed valve separates it from the booster outlet or another gauge.
- Compare with a known-good test gauge. Use a suitable pressure-rated gauge at an approved test point. Record simultaneous cold conditions, recirculation-pump operation, and booster outlet pressure. Do not rely on a pegged gauge for a pressure limit.
- Stop if pressure exceeds a component rating or a gauge pegs. Have a qualified plumbing or controls professional assess the system before further testing. Do not loosen a fitting or open a drain to release suspected pressure.
Pass check: You can identify the measuring range and location of each gauge, and a serviceable gauge provides a readable pressure at a known test point. If not, correct the measurement problem before interpreting the pressure pattern.
Map the pressure zones and flow paths
Draw the actual domestic-water path through the booster, hot-water coil, mixing valve, recirculation pump, check valve, return, and cold-water feed. Mark each gauge and every valve. The installed drawing and field piping must agree; a mixing-board sketch alone may not show later changes.
| Observed condition | What it points toward | What to check |
|---|---|---|
| P2, P4, and P5 rise while P3 does not | A check valve or other boundary may separate pressure zones; the gauges may not see the same trapped volume. | Trace the check-valve direction and confirm which side of it each gauge occupies. |
| Pressure falls when hot water is drawn | Drawing water may relieve pressure from a heated or otherwise trapped section by creating flow and changing its pressure boundary. | Record whether the fall occurs at all gauges or only some, and whether the boiler coil is heating at the time. |
| Gauge readings change with circulator speed | The circulator may alter local differential pressure or flow through a restrictive path. | Compare suction- and discharge-side readings and note the operating speed. |
Mark valve positions as found. Do not copy a partly open position as a target setting: a throttled valve can create an unintended flow path or restrict the intended return, but its function must be confirmed from the piping and equipment instructions.
Pass check: The field map identifies the direction of each check valve, the return route, the cold feed, and the gauge zones. Resolve any mismatch before changing a valve.
Separate circulator differential from system pressure
The Bell & Gossett NBF-25 circulator is a three-speed pump; it was initially left at speed two and later reduced to speed one. That change reportedly reduced the observed high readings from near 100 psi to about 70 psi. This correlation does not by itself establish the root cause.
A circulator adds pressure differential across its inlet and outlet while it moves water. It can shift local gauge readings, particularly where check valves, mixing valves, or partly closed valves divide the circuit. It does not explain away a pressure increase in a heated, isolated volume: that condition requires checking expansion accommodation and the system's pressure boundaries. The booster setting of about 50 psi is its reported outlet target, not a guarantee that every isolated point remains at that pressure.
- With a qualified person monitoring readable gauges, record booster outlet pressure and pressures immediately before and after the circulator at the current speed.
- Repeat under comparable water temperature and draw conditions at the other permitted speed settings. Change only one setting at a time.
- Compare the across-pump differential and downstream pressure. Use the circulator curve and actual operating conditions to assess whether the measured differential is plausible; do not infer it from the motor size alone.
Pass check: The pressure readings show whether the speed change affects only the local pump differential or also changes pressure in the suspected trapped zone. Restore the manufacturer-approved operating setting unless the system designer specifies otherwise.
Confirm whether heating traps a closed volume
When water heats, it expands. If check valves, closed valves, or a booster arrangement prevent that expanded water from pushing back into a pressure source or an expansion vessel, pressure can rise sharply in the confined section. A hot-water draw can then lower the reading by allowing water to move out of that section. The reported rise during no-draw periods and relief on a draw fit this mechanism and warrant a direct check.
Determine whether an expansion vessel is installed, whether it is connected to the affected potable-water volume, and whether its isolation valve is open. Verify that it is rated for the service and temperature. A vessel on a different side of a check valve may not protect the trapped section.
The suspected route described for this board includes flow from the recirculation pump through a check valve and a bypass toward the mixing valve and riser when there is no draw. If the return valve is partly open, the hot-water coil may heat a small loop. Trace that route in the field; the position and purpose of the valve cannot be determined safely from its cracked-open appearance alone.
Pass check: The system map identifies the heated volume, every boundary that can isolate it, and an expansion vessel hydraulically connected to that volume—or confirms that no vessel is present. If the vessel's connection or condition is uncertain, have it tested before relying on it.
Provide expansion control for the isolated potable volume
If testing confirms a closed heated volume without effective expansion accommodation, have a qualified installer select and install a potable-water expansion vessel suitable for the system. Size it using the actual volume it protects, cold fill pressure, operating temperature range, and allowable pressure. Read the vessel manufacturer's precharge and installation instructions; do not guess a precharge value from the 50-psi booster setting.
- Confirm the vessel's pressure and temperature ratings, potable-water suitability, and required connection arrangement.
- Locate the connection so check valves or closed valves do not isolate the vessel from the volume it must protect. Follow the booster, mixing-valve, and vessel manufacturers' piping requirements.
- Set and verify precharge using the specified method, with water pressure removed from the vessel water side when required by its instructions. Record the measured precharge and system cold pressure.
- Check that the vessel is not isolated by a closed service valve and that the connection is not blocked.
A pressure-relief device can provide a protective limit only when its setting, discharge arrangement, and component ratings are selected for the installation and applicable requirements. It is not a substitute for correctly sized expansion control; routine discharge signals a problem to diagnose.
Pass check: The installer documents vessel suitability, precharge, connection to the protected volume, and the pressure ratings of the connected components before heating tests resume.
Set the valves and verify normal recirculation
With the flow map and equipment instructions in hand, have the installer set each valve to its intended operating position. In particular, investigate the partly open valve on the suspected return/bypass route and the reported connection between recirculation and cold supply. A valve that is meant to be closed must not be left throttled as a substitute for correcting the piping; a valve that serves a required function must not be closed on assumption.
Confirm the direction and operation of the check valve, the mixing valve's required connections, and the hot-water return route. Use temperature and pressure readings at the relevant points to confirm circulation and mixing behavior. If the actual piping does not match the manufacturer's arrangement, resolve the design discrepancy before commissioning.
Pass check: Each valve position has a documented function, circulation follows the intended return path, and the mixing valve and check valve operate in their specified directions.
Complete the no-draw and draw verification
After pressure instrumentation, expansion control, and valve positions pass their checks, perform a controlled operating test. Record booster outlet pressure, affected-zone pressure, temperatures, circulator speed, and whether the boiler coil is heating. Use readable gauges rated for the expected range and watch for abnormal pressure rise; stop the test if readings approach equipment limits or become unreadable.
- Record the cold, no-draw baseline with the booster and recirculation operating in their approved configuration.
- Allow the hot-water system to reach its normal operating condition without a draw, while monitoring the same pressure points. Verify that pressure remains within the equipment's ratings and does not climb toward gauge overrange.
- Open a normal hot-water outlet and record the pressure response. Confirm that the readings behave consistently with the mapped pressure zones and return to a stable operating condition after the draw ends.
- Repeat the no-draw observation after the draw. Check for a renewed pressure rise, relief discharge, vessel leakage, or a change in readings that tracks pump speed instead of temperature.
End-to-end pass check: During heating with no draw and during a normal draw, all gauges remain readable, pressures stay below the lowest applicable component limit, no relief device discharges, and the pressure does not repeat the reported rise toward 100 psi. If pressure rises again, isolate the affected zone and recheck its boundaries, vessel connection, valve positions, and gauge accuracy before returning the system to service.
Frequently asked questions
What happens if the hot-water loop has no expansion vessel?
Heating water in a volume isolated by check or closed valves can raise pressure because the expanding water has nowhere to go. Confirm the boundaries and install a correctly selected vessel connected to the protected volume.
What happens if I lower the circulator from speed two to speed one?
It can change flow and local pressure differential, as the reported readings changed from near 100 psi to about 70 psi. That change does not establish that thermal expansion is controlled; compare pump inlet/outlet readings and test pressure during a no-draw heating cycle.
What happens if I leave the return valve partly open?
An unintended bypass or restricted return can change circulation and may allow a small loop to heat, but the correct position depends on the actual piping design. Trace the line and confirm the valve's function and required position from the system documentation before adjusting it.