FPS-500 and FPS-900 Operation & Alarms – South-Tek Systems

Applies to: South-Tek Systems N2-BLAST® FPS-500 and FPS-900 fire protection nitrogen generators.
Not the FPS-900-RS, which is a different unit with a different alarm set — check the serial-number label before using this article.

This article covers normal operating behavior, the controller display, alarm conditions, purity verification, and troubleshooting for the FPS-500 and FPS-900. Maintenance schedules and filter procedures are covered in the separate maintenance articles linked below. Both models use the same controller; the programs are proprietary and password-protected from the factory, so do not alter the controls or setpoints without South-Tek's written consent.

Controller display and buttons

The controller display shows the unit status on the top line and run hours below it. Five buttons sit under the display.

SET — opens the passcode screen / BlastOff simulation.
NXT (Next), labelled F1 on some units — scrolls through filter hours, program version, and ambient temperature. On units built with the optional oxygen sensor or pressure transducer it also reaches those setpoint screens; on a standard unit it does not.
− (Minus), labelled F2 on some units — also the Filter Reset button. It is the Down / minus key — identify it by the legend, not by counting key positions.
+ (Plus), labelled F3 on some units — increments values during data entry.
ENT (Enter) — confirms an entry.

ℹ Note — button labels vary Depending on the vintage of the unit, the five keys are legended SET · NXT · − · + · ENT or SET · F1 · F2 · F3 · ENT. They are the same keys — F1 is NXT, F2 is (−), and F3 is (+). The filter reset is always the Down / minus key. Take care not to press and hold SET while hunting for it: SET opens the passcode screen, which is the entry point to the alarm test that deliberately signals the fire panel.

What the screen shows

The full strings are FILTER CHANGE, FILTER RESET, Air n2 bypass and BLAsstoff 9H.Run. Shorter forms of the same words appear on some builds, so both are listed below — RUN/RUNNING, STDBY/STANDBY and the filter and BlastOff strings are the same states. Indications vary a little between the two models and between build vintages. If what you see does not match, photograph the screen and send it with your ticket.

Display	Meaning	What to do
RUN / RUNNING	Generating nitrogen and filling the storage tank.	Normal. Runs until the tank reaches the cut-out pressure.
STDBY / STANDBY	Tank is up to pressure; the compressor is off and the unit is idling.	Normal. Resumes automatically when pressure falls to the cut-in point.
FALM / FILTER CHANGE	Filter alarm — the controller's 1,000-hour filter timer has expired. On the FPS-900 it also trips the common alarm contact to the panel.	Maintenance reminder; the unit keeps running, so plan the change rather than doing it on the spot. Changing filters means shutting the generator down, isolating it and proving the filter section is at zero pressure on the gauge — so the downtime is a fire protection impairment, and the full procedure is in Annual Filter Replacement — FPS-500 & FPS-900 (FRP-007). Notify the building owner and the monitoring company before you shut the unit down, and tag the system. Afterwards, tell the monitoring company you are about to reset the timer — the reset pulses the common alarm contact — then hold the Down / minus key about 7 seconds until the beep and the reset confirmation. Before you leave, confirm the unit is back in Run or Standby, the alarm has cleared at the panel, and the riser is at supervisory pressure.
FILTER RESET	Filter timer successfully cleared.	Confirmation only — appears together with an audible beep once the timer clears.
BOFF / BLAsstoff 9H.Run	BlastOff® leak alarm — the generator has run about 9 hours continuously.	Treat as a suspected leak. Leak-check before resetting (see below).
Air n2 bypass	Manual air bypass is active — the handle is turned and the riser is on compressed air, not nitrogen.	Expected during the 30-minute fire protection piping fill test. At any other time, investigate — corrosion protection is suspended. Return the handle to Run and confirm the alarm clears.
(blank / dark display)	Power loss. There is no screen to read — the controller is unpowered. Like the bypass and BlastOff® alarms, and the filter alarm on the FPS-900, it reports through the single pooled common alarm contact, which de-energizes.	Opening the disconnect for service is a fire protection impairment — it stops make-up nitrogen and holds the alarm in at the panel. Notify the building owner and the monitoring company first and tag the system. Before you leave: restore power, confirm the unit returns to Run or Standby, confirm the alarm has cleared at the panel, and confirm the riser is back at supervisory pressure.

Normal operation

In Run mode the unit produces nitrogen and supplies it to the storage tank. It automatically enters Standby when the tank reaches the cut-out pressure, and resumes generating — with no operator input — when pressure falls to the cut-in point. Cut-out and cut-in are set by a mechanical pressure switch at the factory. The unit enters Standby at 70 PSIG nominal, with unit-to-unit variation of roughly 65–75 PSIG, and returns to Run once tank pressure falls 7–10 PSIG below its own cut-out point. The storage-tank pressure gauge typically reads between 60 and 70 PSIG, and a unit whose cut-out sits at the top or bottom of the band will swing a little wider than that and still be correct. This automatic cycling is normal. On a standard unit there is no setpoint screen to read — the values live on the switch itself. So record this unit's own cut-out and cut-in pressures off the storage-tank pressure gauge at commissioning, and treat a reading as a fault when it has moved from that baseline, or when the unit stops cycling properly — it will not reach Standby, or will not restart. In that case suspect the pressure switch — P/N 100317, which is in the critical spares kit. Units built with the optional pressure transducer do expose these setpoints on the controller. Either way, do not adjust them without consulting South-Tek.

What healthy runtime looks like

Runtime is the single most useful health indicator on these systems, because a nitrogen generator only runs when the sprinkler system is losing gas. How to use it:

Record the run hours at commissioning and at every inspection. What counts as normal depends on the volume the unit is supervising, so the reading that matters is this unit's own trend against its own baseline — not an hours-per-day figure carried over from another job.
A steadily rising daily average is the early warning. Compare each reading against the baseline taken when the system was last known to be tight.
About 9 hours of continuous running trips the BlastOff® alarm — the only runtime threshold that raises an alarm on these models. If a unit alarms at a noticeably different runtime, confirm the build with South-Tek rather than assuming a fault.

A rising trend usually means a developing leak long before it becomes an alarm. Extended running is expected and normal for a short period after the system is drained and refilled, after a trip test, or after piping work — the generator is simply refilling the system. If runtime stays high once the system is back at supervisory pressure, investigate for leaks.

Controller functions

Filter change reset: after replacing the filter elements, press and hold the Down / minus key until you hear the beep and the display confirms the reset. That is roughly 7 seconds. If it does not take, release and repeat. No passcode is required. The reset also pulses the common alarm contact for about one second. Tell the monitoring company before you press it, or it gets logged as a fault, and confirm the panel is back to normal afterwards.
BlastOff simulation (alarm test): this test deliberately signals the fire panel. Notify the monitoring company and the building owner first and have the point placed on test. Then hold SET for 7 seconds to open the passcode screen, press SET once to begin data entry, enter 6557 with the Plus/Minus buttons, and press ENT. On the resulting HOLD SET screen, hold SET for 5 seconds; where the dry contact is fitted it trips for 2 seconds. Afterwards confirm the contact has restored, confirm the panel reads normal, and have the point taken off test before you leave.
Next-button navigation: press NXT to view filter hours, program version, and ambient temperature. Setpoint screens appear only on units fitted with the optional oxygen sensor or pressure transducer.

Stopping the system

⚠ Warning — the unit starts on its own Standby does not mean stopped. Under automatic control the compressor and the solenoid valves start with no operator input the moment tank pressure falls to the cut-in point, and the display gives no advance notice. For any work at, in, or on the cabinet — soap-testing fittings, checking drain lines, checking shipping bolts, changing filters — open and lock the branch-circuit disconnect first — that is itself a fire protection impairment, so notify the building owner and the monitoring company and tag the system before you open it, and restore and verify before you leave — then isolate and bleed trapped pressure. Wear safety glasses whenever the cabinet is open with the machine running, and hearing protection whenever it is running.

Close the gas outlet valve on the nitrogen storage tank; the unit then fills to its cut-out pressure and drops into Standby. Closing that valve stops nitrogen reaching the riser, so it is a fire protection impairment — notify the building owner and the monitoring company, tag it, and keep the window short. Note that Standby is not off: the unit stays energized and will restart by itself. Before you leave, reopen the valve, confirm the riser is back at supervisory pressure, and confirm the unit is in Run or Standby.
In an emergency, move the On/Off toggle on the upper-left control panel to Off to shut the unit down immediately. That toggle is the unit's main power switch — a two-pole 15 A circuit breaker switch — so it does de-energize the unit. It is not the branch-circuit protection and not a lockable disconnecting means, and the supply conductors landing on the terminal strip stay live: for any service work, open and lock the branch-circuit disconnect. Shutting the unit down is a fire protection impairment — it stops make-up nitrogen and trips the common alarm contact at the panel. Notify the building owner and the monitoring company, tag it, and before you leave, restore power, confirm the unit returns to Run or Standby, confirm the alarm has cleared at the panel, and confirm the riser is back at supervisory pressure.

▲ Attention Do not count on the generator's air bypass to keep the riser supervised while you work. On these models the bypass air comes from the unit's own internal compressor, so it delivers nothing once the unit is de-energized. For longer work, arrange another supervisory source or extend the impairment declaration to cover the whole job. Turning the handle to bypass also trips the common alarm contact and puts the riser on compressed air, not nitrogen, with corrosion protection suspended — so notify first, keep it short, and before you leave return the handle to Run and confirm the bypass alarm clears.

Separately: shutting the generator down does not depressurize the sprinkler system, and the piping stays at supervisory pressure. Before any service work, isolate and bleed stored pressure and follow your site lockout/tagout procedure. Full detail: Safety Guidelines.

Alarm conditions

Power loss: if the generator loses power for any reason, the common alarm contact de-energizes and reports at the panel. Opening the disconnect for service does the same thing, and that is a fire protection impairment: notify the building owner and the monitoring company first and tag the system. Before you leave, restore power, confirm the unit returns to Run or Standby, confirm the alarm has cleared at the panel, and confirm the riser is back at supervisory pressure.
Bypass (Air n2 bypass): the handle on the left side of the cabinet has been turned to manual air bypass. This raises an audible and visual alarm and clears when the handle returns to Run. Expected during the 30-minute fire protection piping fill test — at any other time, investigate. The riser is on compressed air, not nitrogen, and corrosion protection is suspended until the handle is returned. Bypass is not a substitute for nitrogen supervision while the unit is down; see Stopping the system.
BlastOff® leak alarm: triggers on a suspected leak or nitrogen overdemand at about 9 hours of continuous run. It sounds the buzzer and shows on the controller. Power-cycle the unit to reset — after you have found the cause, not before. The panel is already in common alarm from the BlastOff itself, and the power cycle drops the generator and holds that alarm in, so notify the building owner and the monitoring company first and tag it. Afterwards confirm the unit returns to Run or Standby, confirm the alarm has cleared at the panel, and confirm the riser is back at supervisory pressure. Repeated resetting without a fix shortens the generator's life and means the piping is not holding the nitrogen it should.
Filter alarm: a maintenance reminder when the controller's 1,000-hour filter timer expires. The system keeps running. Reset after changing filters.

That is the complete set on a base FPS-500 or FPS-900. A compressor or low-pressure alarm is not standard on those two models — if a customer expects one, either it is a fitted option or the unit is an FPS-900-RS, which carries a standard low-pressure alarm. Confirm the model from the serial number before troubleshooting an alarm a customer says is missing:

Option

What it adds

Equipment alarm

A pressure switch on the air side that reports when air pressure falls away — a compressor that has stopped, tripped, or lost its supply. Its trip point and deadband are set per order and are field-adjustable, so read the setting off the switch and record it rather than assuming a value, and expect the alarm to hold in until pressure has recovered past the deadband. These are compressed-air settings with nothing to do with the nitrogen cut-in and cut-out.

Pressure transducer

A settable pressure setpoint and a low-pressure alarm. Without it, there is no nitrogen low-pressure alarm on the unit.

ℹ Note — model scope The FPS-500 and FPS-900 use a manual air bypass (turn the handle) and carry BlastOff® leak detection. Automatic air bypass, a touchscreen controller, and the extended BlastOff® II–IV features are found on the larger integrated models (FPS-1650 and up).

Alarm monitoring and BMS tie-in

Both models offer a single voltage-free common alarm contact — one Form C set, provided as a user connection where it has been landed, and there is no separate bypass or filter contact on this family. Confirm the contact is actually wired to the panel before relying on it — the notification steps throughout this article depend on it being landed. That one contact pools bypass, BlastOff® and power loss — and, on the FPS-900, the filter alarm as well, so the panel sees an alarm without knowing which one it is; the controller display is what tells you.

Output	What it reports
Common alarm Form C dry contact, both models	Bypass, BlastOff® and power loss, pooled — and the filter alarm as well on the FPS-900. Because power loss drives it, the contact reports by de-energizing — which is what makes it a supervisory contact rather than a status light. In the healthy state one leg is held closed and the other open; the leg wired for supervision opens on alarm and on loss of power. Confirm the terminal numbers and the contact's voltage and current rating against the unit's own schematic before landing anything on it — do not assume it will carry a notification-appliance circuit.
Analog output 4–20 mA, option	Where fitted, one configurable analog channel. On a unit without the optional sensors the only available signal is ambient temperature; oxygen percentage, trace O₂ in PPM and supply pressure need the oxygen analyzer or pressure transducer. Get the engineering-unit scaling — what value corresponds to 4 mA and to 20 mA — from South-Tek, and confirm the signal type against the unit's schematic before wiring a BMS input. Receiving equipment is by others.
Modbus option	Where fitted, the controller communicates Modbus RTU over RS-485 — slave ID 1, 8 data bits, no parity, 2 stop bits. Modbus RTU over RS-485 is the communication path on these models. If a project needs Modbus TCP/IP, BACnet, or an Ethernet connection, confirm availability with South-Tek before specifying it, and confirm the option is on the unit before designing an integration around it. Confirm the baud rate with South-Tek as well — do not assume a standard rate. Detail: Modbus Communication – FPS Nitrogen Generator.

Work from the terminal numbers on the unit's own schematic when landing this wiring rather than from normally-open / normally-closed habit, document the terminals used on the commissioning form, and function-test the contact end to end — trip the condition and confirm the panel or BMS actually receives it. The BlastOff® simulation described above exists for exactly this. Function-testing drives a real signal to the panel, so coordinate with the monitoring company and the building owner first, and confirm the panel is back to normal and off test before you leave. Wiring detail is in Fire Alarm & BMS Wiring for FPS Generator Dry Contacts.

⚠ Warning Generator alarms are supervisory signals only. They are not life-safety trip signals and must never be wired in a way that interferes with the operation of the fire alarm or sprinkler system.

ℹ Note — the relief valves are not adjustments These units carry a 115 PSI / 73 CFM safety relief on the pressure-switch assembly, a further relief valve on the filter assembly, and one supplied on the nitrogen storage tank. They are protective devices. If one is lifting, the cause is upstream — investigate it. Never block, plug, adjust, or substitute a relief valve. A relief on the nitrogen side that lifts is discharging nitrogen into the room, so treat any lifting relief as a possible oxygen-deficiency hazard: ventilate before you stay to investigate, and treat one lifting in a closet, pit, or small mechanical room as an oxygen-deficiency hazard — leave, ventilate, and do not re-enter without a calibrated meter reading 19.5% O₂ or higher.

Verifying nitrogen purity

⚠ Warning — nitrogen displaces oxygen Sampling discharges nitrogen. Nitrogen is colorless, odorless, and gives no warning — sample only in a well-ventilated space, never in a closet, pit, or sealed room, and never enter a confined space downstream of a vent without a calibrated meter reading 19.5% O₂ or higher.

Nitrogen purity is what the system is ultimately delivering, and unlike most corrosion measures it can be checked directly. South-Tek generators are specified to 98.5%+ nitrogen; a Quick-Check® handheld purity sensor reads oxygen content at a sample port or, where an AutoPurge (APS) is installed, at the APS port. Verify purity at each inspection and record the reading.

On a newly commissioned system, do not expect 98.5% immediately — the generator drives oxygen out of the piping over an initial period as nitrogen displaces the air, and how long that takes depends on the system volume and the purge arrangement. Judge an early reading against the commissioning record for that system rather than against 98.5%. If purity has not reached specification by the first scheduled inspection, stop treating it as commissioning and investigate it as a fault. A reading below spec on an established system points to a leak drawing in air, an APS device that has been valved shut or plugged, or a system left in air bypass.

Troubleshooting quick reference

Symptom	Likely cause	What to check
Runs continuously / will not reach Standby	Leak downstream, or demand exceeds the unit	Run the isolation check below; verify the system was sized for the largest single zone
Reaches Standby, then restarts frequently	Small ongoing leak or AMD passing gas	Soap-test fittings; verify the air maintenance device setting and that it seats. The unit restarts by itself from Standby — open and lock the branch-circuit disconnect before working at the cabinet. That is an impairment — notify first, restore and verify before you leave.
Repeated BlastOff (BOFF) alarms	Unresolved leak	Do not keep resetting — run the isolation check under Leak checking below and locate the leak
Unit will not start; display blank or showing no Run / Standby indication	Power or wiring issue	Confirm the branch circuit is live and the 15 A circuit breaker switch on the cabinet (the On/Off toggle) is On. Anything past that is electrical work: open and lock the branch-circuit disconnect before removing the cover or touching the terminal strip. This is an open-type control panel and the supply conductors stay live with the cabinet toggle Off. Qualified personnel only.
Purity reads below 98.5%	Air ingress, APS closed/plugged, or system in bypass	Confirm the bypass handle is in Run; where AutoPurge devices are fitted, check each one is open and not plugged; then leak-check
Noisy or vibrating cabinet	Shipping bolts still installed, or loose mounting	Confirm the four painted shipping bolts were removed and the cabinet is secured at both flanges. The unit restarts by itself from Standby — open and lock the branch-circuit disconnect before working at the cabinet. That is an impairment — notify first, restore and verify before you leave.
Water at the cabinet or drain	Normal condensate, or a blocked drain line	Verify both 1/4" OD drain lines at the bottom right of the cabinet run to a site drain and that neither is kinked, capped, or plugged. The unit restarts by itself from Standby — open and lock the branch-circuit disconnect before working at the cabinet. That is an impairment — notify first, restore and verify before you leave.

Leak checking

BlastOff provides early warning, but confirm leaks physically: listen, and spray soapy water around connection points. Remember the unit starts on its own from Standby — for any work at the cabinet itself, de-energize at the branch-circuit disconnect and lock it out first. The isolation check below is different: it is an observation test and is run with the unit powered, so keep out of the cabinet while it runs. If a leak is found, isolate the area and relieve pressure before repairing — bleed to atmosphere only in a well-ventilated space. Nitrogen is colorless, odorless and displaces oxygen with no warning: never bleed into a closet, pit, or sealed room, and never enter a confined space downstream of a vent without a calibrated meter reading 19.5% O₂ or higher. If nothing is obvious, isolate sections and watch for pressure loss.

⚠ Warning — this test impairs a fire protection system Closing the nitrogen tank ball valves on a system in service stops supervisory gas reaching the riser. Treat it as an impairment: notify the building owner and the monitoring company, tag it, and keep the window to the length of the test. Whatever the result, restore the valves before you leave the cabinet. A diagnostic that ends with the nitrogen supply valved off is worse than the leak you were chasing.

To determine whether a leak is between the generator and the tank:

With the unit powered on, close both ball valves on the nitrogen tank and note the tank pressure.
Let the system run; it should reach Standby within about 10 minutes. If it has not after roughly 15, stop the test, reopen both valves and restore the system — a unit that cannot make cut-out against closed valves has an internal leak or a compressor fault, and should not be left running against closed valves.
Wait 5 minutes. If it stays in Standby, the leak is downstream of the generator — if the tank pressure changed, it is in the storage-tank fittings; if it did not change, check the building piping.
If the system returns to Run, the leak is inside the cabinet or in the lines back to the tank.
Restore before you leave. Reopen both tank ball valves, confirm the tank recovers to the cut-out pressure, confirm the unit returns to Standby, and confirm the riser is being supplied again. Clear the impairment tag and notify whoever you notified at the start.

Maintenance

Maintenance procedures for these models are covered in their own articles:

FPS-500 and FPS-900 Preventive Maintenance — the quarterly and annual service schedule.
Annual Filter Replacement — FPS-500 & FPS-900 (FRP-007) — kit contents and the step-by-step filter change.
FPS-500 / FPS-900 Critical Spares Kit — parts worth keeping on hand.

Filters are replaced once per year or every 1,000 run hours, whichever comes first. The generator must be shut down, isolated, and the filter section proven depressurized on the gauge before any filter work, so plan the downtime as a fire protection impairment. Notify the building owner and the monitoring company before the unit goes down, and tag the system. After the change, clear the alarm by holding the Down / minus key until the beep and the reset confirmation — about 7 seconds — telling the monitoring company first, because the reset pulses the common alarm contact. Before you leave, confirm the unit returns to Run or Standby, the alarm has cleared at the panel, and the riser is back at supervisory pressure. Full procedure: Annual Filter Replacement — FPS-500 & FPS-900 (FRP-007).

When to get support

Contact South-Tek if the system will not enter Standby, repeatedly returns to Run without demand, keeps tripping BlastOff after a leak check, will not hold 98.5% purity, or shows signs of internal leakage or recurring alarms. Have the model, serial number, current run hours, and a photo of the controller screen ready — it speeds up the ticket considerably.

Manual downloads

Need help? South-Tek Systems Service — Phone: (910) 415-1880 · Open a support ticket at support@southteksystems911.zendesk.com or through the Help Center.