Applies to: N2-BLAST FPS-1650, FPS-3250, FPS-5000, FPS-10000, FPS-16500 and FPS-22500 — the models with automatic bypass, driven from the display. FPS-500 and FPS-900 do the same job with a manual handle — see the last section.
Bypass mode holds the fire protection system at supervisory pressure by feeding it compressed air instead of nitrogen. On the FPS-1650, FPS-3250 and FPS-5000 that air comes from an external air compressor. On the FPS-10000, FPS-16500 and FPS-22500 it is taken from the generator's own feed air.
This article covers the case where you need to force it on and leave it on — the generator is down for troubleshooting or waiting on a part, and the system has to stay supervised in the meantime. That is a legitimate use of bypass and the equipment is built for it. It is also a job with a beginning and an end, and the end is the part that gets forgotten.
Two different things are both called bypass
| Automatic bypass | Forced (manual) bypass | |
|---|---|---|
| What starts it | The generator, on its own. | You, from the display. |
| When | During the 30-minute fire protection piping fill test, or when the generator cannot hold supervisory pressure on its own. | When the generator will be down long enough that it cannot be relied on — a repair, a part on order, extended troubleshooting. |
| How long | Minutes to hours. It ends itself. | Days or weeks, deliberately. |
| How it ends | Automatically, once bypass pressure reaches the cut-off setting. | When someone turns it off — or, unintentionally, at the next power interruption. See the warning below. |
If what you are dealing with is the automatic kind — a fill test, or a short spell where the generator fell behind — there is nothing to do by hand. It returns to nitrogen on its own. The setpoints that govern it are these:
| Model family | Bypass cut-in | Bypass cut-off |
|---|---|---|
| FPS-1650, FPS-3250, FPS-5000 | 50 PSIG | 70 PSIG |
| FPS-10000, FPS-16500, FPS-22500 | 50 PSIG | 60 PSIG |
ℹ Note — the cut-off is not the same on both families These are the factory defaults and they differ between the two families. Do not carry one family's numbers to the other. Both are set on the Bypass Alarm Settings screen, and both are read from the generator's own pressure sensor — they will not match the reading at the riser gauge.
They are site-adjustable by design: if the bypass air compressor shuts off below 60 PSIG, the cut-off has to come down to sit clear of it and the cut-in comes down with it.
They are site-adjustable by design: if the bypass air compressor shuts off below 60 PSIG, the cut-off has to come down to sit clear of it and the cut-in comes down with it.
A bypass event raises a Bypass alarm at the generator and is logged there. Whether anyone off site hears about it depends on whether the bypass dry contact was wired to the fire alarm panel or the building system at installation — on many jobs it was not. Check before you assume the event is monitored. See BlastOff and SMART-Trak Overview for N2-BLAST Systems.
The one thing that will catch you out
⚠ Warning — a power interruption silently ends bypass A valve forced on from the valve control screen stays on until someone turns it off there — it will not time out and it will not drop out on its own. But a power cycle resets the whole screen. After any power interruption every valve reverts to off — with the exception of the crossover valve, which is left on.
So on a bypass meant to run for weeks: the first outage, brownout, breaker trip or unplugged cord ends it. The bypass valve closes, air stops reaching the sprinkler piping, and because the generator is switched off there is nothing at the machine raising its hand about it.
What actually catches this is the riser's own low-air supervisory switch — the one wired to the fire alarm panel. Before you leave a system on forced bypass, confirm that switch is installed, wired, monitored and tested. On an extended forced bypass it is not a backstop, it is the primary alarm.
So on a bypass meant to run for weeks: the first outage, brownout, breaker trip or unplugged cord ends it. The bypass valve closes, air stops reaching the sprinkler piping, and because the generator is switched off there is nothing at the machine raising its hand about it.
What actually catches this is the riser's own low-air supervisory switch — the one wired to the fire alarm panel. Before you leave a system on forced bypass, confirm that switch is installed, wired, monitored and tested. On an extended forced bypass it is not a backstop, it is the primary alarm.
And know what the failure means. On a dry pipe system, air pressure falling far enough does not simply raise a trouble signal — the dry pipe valve trips and the piping fills with water. On a pre-action system it drops the supervisory circuit, and on a single interlock arrangement it can admit water too. This is why the checks below are not optional housekeeping.
Before you commit to an extended bypass
| Settle this | Why |
|---|---|
| Tell the owner and the monitoring company first | Stopping the generator opens a window with no make-up source before bypass takes over, and that alone can drop supervisory pressure far enough to signal. Notify the alarm monitoring company before you start and have the system placed on test. Whether this condition counts as an impairment is the building owner's impairment coordinator's call, and the AHJ's — not yours and not ours. The system stays pressurized and stays supervised, but the corrosion protection the owner is paying for will be off for the duration, and their insurer may have a view. Tell them, tell them roughly how long, and follow their direction. |
| Check the pressure the regulator is set to | A regulator in the bypass path sets the pressure delivered to the system — the compressor's own pressure does not reach the riser. Verify what that regulator is actually set to rather than assuming it is right for this system. It must sit above the low-air supervisory setpoint and the dry pipe valve trip point with margin, and it must not exceed the maximum air pressure the valve is rated for. Too high slows a trip; too low leaves the system signalling. |
| Confirm the air supply can run continuously | On an FPS-1650, FPS-3250 or FPS-5000 the external air compressor stops being a backup and becomes the only thing holding the system up. It needs to be sized and maintained for that — receiver drained, filters clean, able to cycle for the whole bypass. A compressor that fails while the generator is also down leaves nothing. |
| Sort out the moisture — and this depends on your model | Corrosion needs oxygen and water, and over weeks the water arrives with the air. Worse than corrosion: water that freezes in an unheated attic, parking deck or loading dock is an ice plug, and that is a water-delivery failure, not a housekeeping problem. On the FPS-3250 and above, bypass air passes through the dryer. On the FPS-1650 there is no dryer — see the warning below. Aim for a pressure dew point comfortably below the lowest temperature any part of the piping sees, not merely below room temperature. If the air compressor is oil lubricated, confirm coalescing filtration as well; oil carried into sprinkler piping attacks gaskets. |
| Agree who owns the purge-back and when it happens | Coming off bypass is a job in itself and the longer the run the bigger it gets. Agree who is returning to do it before you start, or it does not happen. |
⚠ Warning — on an FPS-1650, bypass air is not dried The FPS-1650 has no dryer. On that model, whatever moisture the bypass air supply carries goes straight into the fire protection piping for as long as bypass runs. On the FPS-3250 and above the bypass air passes through the dryer, so the exposure is far smaller.
An extended bypass on an FPS-1650 needs drying arranged some other way — a site air dryer, or an air maintenance device with drying, ahead of the connection. Without it, accept that water is going into the system and plan around it: drain the low points, drum drips and auxiliary drains far more often than usual, and on any system with piping in an unheated space, treat a long undried bypass as an ice plug risk and get the drying sorted before you commit to it.
On the FPS-3250 and above, switching the generator off does not switch the dryer off. The dryer keeps running on its own regardless of what the main screen says, so a forced bypass does not leave you feeding undried air. The only thing that stops it is the dryer's own power switch — and that switch is behind a warranty seal and is not to be touched.
If you find that seal broken or the dryer switched off, stop and treat it as a finding. The system may have been running without drying for some time, there is likely water in the piping already, and the warranty is affected. Call us before going further rather than simply switching it back on.
An extended bypass on an FPS-1650 needs drying arranged some other way — a site air dryer, or an air maintenance device with drying, ahead of the connection. Without it, accept that water is going into the system and plan around it: drain the low points, drum drips and auxiliary drains far more often than usual, and on any system with piping in an unheated space, treat a long undried bypass as an ice plug risk and get the drying sorted before you commit to it.
On the FPS-3250 and above, switching the generator off does not switch the dryer off. The dryer keeps running on its own regardless of what the main screen says, so a forced bypass does not leave you feeding undried air. The only thing that stops it is the dryer's own power switch — and that switch is behind a warranty seal and is not to be touched.
If you find that seal broken or the dryer switched off, stop and treat it as a finding. The system may have been running without drying for some time, there is likely water in the piping already, and the warranty is affected. Call us before going further rather than simply switching it back on.
▲ Attention — the corrosion clock runs the whole time Air is roughly 21% oxygen. The generator exists to keep that oxygen out of the piping, so a system on bypass is not paused — it takes on fresh oxygen for as long as bypass is on, and pushes it further into the system the longer it runs.
That is an acceptable trade to keep a system supervised while a generator is repaired. Set an end date rather than an open one: review it weekly, and if it is heading past a month, escalate rather than letting it become the way the building runs. And if bypass is being used because the generator cannot keep up rather than because it is broken, the problem is somewhere else — start with N2-BLAST System Leak Check Procedure.
That is an acceptable trade to keep a system supervised while a generator is repaired. Set an end date rather than an open one: review it weekly, and if it is heading past a month, escalate rather than letting it become the way the building runs. And if bypass is being used because the generator cannot keep up rather than because it is broken, the problem is somewhere else — start with N2-BLAST System Leak Check Procedure.
Forcing bypass on
⚠ Warning — leave the nitrogen outlet valve open Bypass air joins the line inside the generator, upstream of the nitrogen outlet ball valve, and leaves through that same valve. If you close it, no bypass air reaches the fire protection system — you will have shut the generator down and cut the riser's make-up supply at the same time, on a system that already needed help.
The operation manual carries a caution about valving off the nitrogen outlet before toggling valves on this screen. That caution is about the PSA cycle valves; it does not apply to the bypass control — confirmed with South-Tek service, August 2026. For bypass, the outlet valve stays open. For every other valve on that screen the caution stands, which is the next warning down.
The operation manual carries a caution about valving off the nitrogen outlet before toggling valves on this screen. That caution is about the PSA cycle valves; it does not apply to the bypass control — confirmed with South-Tek service, August 2026. For bypass, the outlet valve stays open. For every other valve on that screen the caution stands, which is the next warning down.
| Step | What to do | What you should see |
|---|---|---|
| 1 | Notify the owner and the alarm monitoring company, and have the system placed on test. | Nobody is dispatched on the pressure dip while you work, and the owner has made their own call on impairment status. |
| 2 | Write down: system supervisory pressure at the dry pipe or pre-action valve gauge, the low-air supervisory setpoint, and the valve's trip point. | The numbers you will judge the bypass against, now and on every check-in. |
| 3 | Confirm the nitrogen outlet valve is open and leave it that way. Confirm bypass air is available and the regulator is set correctly for this system. | A supply to switch to, at the right pressure, with a path to the riser. Forcing bypass with no air behind it holds nothing up and you will not find out until supervisory pressure has already fallen. |
| 4 | From the main screen, switch the generator off — press and hold the power button until it is no longer green. It must be off at the main screen for the forced valve to hold. | The generator stops. This stops nitrogen production, which is why the air supply is confirmed first. The cabinet is still electrically live and the storage tank is still pressurized. |
| 5 | Go to the Maintenance menu, then Valve Controls. | The valve control screen, listing the flow valves, crossover valve, equalizers, supply and exhaust valves, and Automatic BYPASS. |
| 6 | Toggle Automatic BYPASS on. Touch nothing else on this screen. | The bypass valve opens and bypass air begins feeding the system. It will stay open until someone turns it off here — or until the power is interrupted. |
| 7 | Stay with it until you have seen supervisory pressure hold or recover at the riser gauge, and until you have seen the air compressor cut in and cut out at least once. | Pressure held, and a supply that is actually cycling to maintain it. If pressure keeps falling, bypass air is not getting through — recheck the outlet valve, the regulator and the air supply rather than walking away. |
| 8 | Confirm at the fire alarm panel that the riser's low-air supervisory signal is being received, then take the system off test. | The alarm that will catch a bypass failure is proven to work before you rely on it for weeks. |
| 9 | Tag the generator: stopped deliberately, system on bypass air, date bypass started, and who to call. Note that the cabinet remains live and pressurized. | Nobody restarts it blind, and the length of the run is not a guess later. |
⚠ Warning — do not actuate any other valve on that screen The same screen drives the flow valves, the crossover valve, the equalization valves, and the supply and exhaust valves — the PSA cycle valves — and on some models a pressure dump valve. They are there so a technician can verify a solenoid during service, with the machine stopped and depressurized.
Driving them by hand on a pressurized generator can cross-connect the sieve beds, strand a bed at pressure, or vent the nitrogen storage tank. Venting the tank on a fire protection system takes away the buffer the riser depends on. If you are here to force bypass, the bypass control is the only thing you touch.
Driving them by hand on a pressurized generator can cross-connect the sieve beds, strand a bed at pressure, or vent the nitrogen storage tank. Venting the tank on a fire protection system takes away the buffer the riser depends on. If you are here to force bypass, the bypass control is the only thing you touch.
While it is running on bypass
Check it within the first 24 hours, then weekly at minimum, and any time you know the site has lost power. Name the person on site who is doing it.
| What can fail | What it does to the sprinkler system |
|---|---|
| Site power interruption | The forced bypass valve reverts to off and stays off. Air stops. The generator is switched off, so nothing at the machine reports it. This is the most likely failure of the lot on a multi-week run. Go back and force bypass on again. |
| Air compressor failure | Motor, pressure switch, belt or a tripped breaker — the only make-up source stops. Pressure decays to the low-air supervisory setpoint, then toward the trip point. |
| Dryer failure (FPS-3250 and above) | Air keeps flowing, so nothing alarms and no gauge moves, but water goes into the piping and collects at the low points. On an unheated system it can freeze. This is the one failure on this list with no pressure symptom at all — the only way to catch it is to look at the dryer, so look at it every visit. On an FPS-1650 there is no dryer to fail, which is its own problem: see the warning above. |
| Regulator drifting high | Over-pressurizes the system, which lengthens trip time on a dry pipe valve and can push past what the valve is rated for. |
| Bypass solenoid or valve failure | Air stops with no local indication, the same as a power loss. |
- Compare supervisory pressure against the number you wrote down at every check, not just "it looks about right".
- Drain the air receiver and its traps, and — more importantly — drain the system's own low points, drum drips and auxiliary drains. Condensate carried in with bypass air ends up in the sprinkler piping, not in the receiver.
- Expect purity to fall. A Quick-Check manifold or portable reading will show the system drifting toward air. That is bypass working as designed, not a second fault — but keep the readings, because they tell you how much purging is ahead.
- Keep the repair moving. The trade only makes sense while it is temporary.
Coming back off bypass
On an extended bypass this is most of the work. Putting the generator back on does not put the nitrogen back in. The piping is full of air and it has to come out.
| Step | What to do | What you should see |
|---|---|---|
| 1 | Confirm the generator is genuinely ready — repair complete, part fitted, fault cleared. Notify the monitoring company again and place the system on test. | Whatever put you into bypass is actually resolved, and the purge will not dispatch anyone. |
| 2 | Drain the system low points, drum drips and auxiliary drains before you purge, not after. | Water out. Purge first and you seal the condensate in under nitrogen. |
| 3 | On the valve control screen, toggle Automatic BYPASS back off. | The bypass valve closes. |
| 4 | Check every other valve on that screen reads off. If the site has lost power at any point during the bypass, expect to find the crossover valve on — that is the one the reset leaves energized. | All off. Restarting with a valve left driven brings the machine up in a state it was never meant to be in. |
| 5 | Restart the generator from the main screen: press and hold the power button until it turns green. | It starts and begins making nitrogen. |
| 6 | Let it build and reach standby, and confirm no bypass alarm is standing. | Normal operation. If it drops straight back into bypass, the underlying problem is still there — find it rather than forcing bypass again. |
| 7 | Purge the air out at the hydraulically most remote point, opening the vent only far enough to get flow without pulling supervisory pressure down toward the trip point. Let the generator keep up. After weeks on air, expect to repeat it — one sweep will not clear branch lines and dead legs. | A controlled, gradual exchange. Not a fast blowdown. |
| 8 | Verify purity at the far end of the system, not at the generator, against the value this system was commissioned at. Where no commissioning record exists, work to 98% nitrogen or better at the most remote sampling point and confirm the target with us. | Purity back where it belongs. Until it reads right at the far end, the system is still full of the air bypass put in. |
| 9 | Take the system off test, remove the tag, and record it: why bypass was used, the dates it ran between, any power interruptions, and the purity you finished on. | A record showing the system was returned to nitrogen, not just to pressure. |
⚠ Warning — nitrogen displaces the air you breathe Purging vents nitrogen, and nitrogen has no smell, no color and no warning. It is not poisonous — it kills by displacing the oxygen you breathe. Vent to outdoors or into a large open area, never into a small closed room, and ventilate riser closets, valve rooms and ceiling spaces while you work in them. If someone collapses in a space that may be oxygen-deficient, do not go in after them — call emergency services and leave entry to trained rescuers with supplied air.
▲ Attention — pressure restored is not protection restored The generator reads good purity at its own outlet long before the far end of the piping has come back, and the longer the bypass ran, the further through the system that air traveled. Verify at the far end and give it the time it needs — days rather than hours after a long bypass. See AutoPurge and Quick-Check Overview for N2-BLAST Systems for reading purity out in the system.
FPS-500 and FPS-900 — the handle
These models have no automatic bypass and nothing to set on a display. They do the same job with a manual bypass valve on the panel: move the handle to the bypass position and leave it there. On current FPS-500 and FPS-900 units the handle points up for nitrogen and out for bypass — but the placard on the unit governs, and handle legends have not been printed the same way across every drawing set and model generation. Read the placard and follow it if it disagrees with this article.
▲ Attention — no placard, or you cannot read it? Do not work it out by trial There is no safe way to identify the handle positions by operating the valve and watching what happens. Any such test puts air into the fire protection piping in order to find out whether you are putting air into the fire protection piping — and on some installations it returns the wrong answer outright, because where a nitrogen bypass has been fitted for pre-filling to meet code, the bypass leg carries nitrogen and a purity check reads backwards.
Get the handle positions from the installation layout drawing supplied with the system, or call us with the model and serial number. Once you have it confirmed, mark the handle so the next person does not have to ask.
Get the handle positions from the installation layout drawing supplied with the system, or call us with the model and serial number. Once you have it confirmed, mark the handle so the next person does not have to ask.
⚠ Warning — a handle raises no alarm There is no bypass alarm, no log entry and no dry contact behind a manual handle. Nothing anywhere records that the system is on air, and nothing will tell you if it stops. Tagging the valve, notifying the owner and monitoring company, and confirming the riser's low-air supervisory switch is monitored are not optional here — they are the only visibility the arrangement has.
What carries over from the rest of this article: the regulator setting, the continuous-duty and dry-air requirements for the air supply, the owner notification, the failure list, the check interval, and the whole purge-back and purity verification. What does not: the cut-in and cut-off table, the valve control screen warnings, and the power cycle behavior — a handle stays where it is put. See FPS-500 and FPS-900 Operation & Alarms.
Related articles
- FPS-1650, FPS-3250, and FPS-5000 Operation & Alarms
- FPS-10000, FPS-16500, and FPS-22500 Operation & Alarms
- FPS-500 and FPS-900 Operation & Alarms
- BlastOff and SMART-Trak Overview for N2-BLAST Systems
- N2-BLAST System Leak Check Procedure
- AutoPurge and Quick-Check Overview for N2-BLAST Systems
- N2-BLAST Fire Protection Systems Overview
Need help? Have the model and serial number, why bypass is needed and for roughly how long, and the current system supervisory pressure ready. South-Tek Systems Service — Phone: (910) 415-1880 · Open a support ticket at support@southteksystems911.zendesk.com or through the Help Center.