An addressable fire alarm control unit does not care what brand of device is sitting on the other end of a pair of contacts. It cares about one thing: the resistance it measures across a supervised input, and which address reported the change. Get that framing right and the integration question answers itself.
What does the panel actually see when the CO alarm sounds?
Nothing about carbon monoxide. The panel polls an addressable input monitor module on the signaling line circuit (SLC), the module reports its input as normal, active, or open, and the panel looks up what you programmed against that address. The CO alarm's relay module is just a dry contact in that loop.
That is why the compatibility worry usually dissolves. Input monitor modules exist precisely to bring foreign contacts into the fire alarm system, and they are used every day for:
- Sprinkler waterflow switches
- Sprinkler valve tamper switches
- Commercial kitchen hood suppression system release
- Generator run/fail status
- Fire pump controller status
- Distributed antenna system (DAS) monitoring
A Kidde or BRK CO alarm with its relay accessory is electrically no different from a tamper switch as far as the module is concerned.
Check before moving on: confirm on the CO alarm's relay module datasheet that you have a form A (N.O.) or form C dry contact, and that the contact is rated for the module's supervisory current — typically a few milliamps at low voltage. A relay intended to drive a 120 VAC fan damper will still work, but verify the contacts are gold-flashed or rated for dry-circuit switching, otherwise the module may read an intermittent open after a year of oxidation.
Electrically compatible, but is it code compatible?
This is the part that bites people. The question is not whether the contact will close; it is whether the authority having jurisdiction (AHJ) will accept a single-/multiple-station residential CO alarm as the sensing device on a commercial fire alarm system.
| Device class | Typical listing to verify | How it lands on the panel | Where it gets rejected |
|---|---|---|---|
| Single-/multiple-station CO alarm with relay accessory | UL 2034 | Dry contact into an addressable monitor module | AHJ requires a system-connected, panel-supervised CO device |
| System CO detector on the SLC | UL 2075 | Native addressable point, sensor supervised end to end | Must be from the panel manufacturer's compatible device list |
| Third-party CO panel / gas controller | Per its own listing | Alarm and trouble relays into two monitor modules | Trouble contact left unmonitored |
Ask the AHJ this before you order anything. If the answer is "system detector," you are buying a CO detector from the panel manufacturer's compatible device list and the monitor module becomes unnecessary. If the answer is "monitor the existing alarm," the module approach stands and brand does not matter.
Check: get the AHJ's answer in writing on the permit correspondence, and pull the panel manufacturer's compatible device list to confirm which SLC devices and modules are approved for that control unit revision.
Alarm or supervisory — which signal type do you program?
Do not default to alarm. The three panel states mean different things to the people who receive them:
| Signal type | What it means | Panel behavior you get |
|---|---|---|
| Alarm | Life safety event requiring evacuation or response | Notification appliances, latching, alarm transmission to central station |
| Supervisory | Off-normal condition in a protection system needing attention | Supervisory tone and LED, separate transmission path, typically no general evacuation |
| Trouble | A fault inside the fire alarm system — open circuit, missing device, ground fault, low battery | Trouble buzzer and relay |
A CO event is not a trouble. Trouble is reserved for faults within the fire alarm system itself. Whether the CO point annunciates as alarm or supervisory depends on occupancy, whether evacuation is the desired response, and what the local code amendments say — the AHJ decides, and both answers are defensible in different buildings. In many occupancies, sounding a fire evacuation for CO is the wrong response, because CO response is shelter/ventilate/evacuate-by-instruction rather than pull-station behavior.
Where the panel firmware supports a dedicated CO or carbon monoxide point type, use it. It gives the correct temporal-4 sounder pattern on compatible appliances and the correct off-site event class without you hand-mapping anything.
Check: the AHJ's decision is recorded, and the panel's point-type table contains the type you intend to program.
SLC monitor module or an onboard panel input?
Most addressable control units do have some hardwired inputs on the main board or on a zone expander — initiating device circuits that behave like a conventional zone, with the address assigned internally by the panel. They work, and on a retrofit they can save a long SLC pull.
| Option | Where it lives | Effect |
|---|---|---|
| Addressable input monitor module | Anywhere on the SLC, mounted in a 4" box at the CO alarm | Its own address, its own custom label, individually annunciated and disabled |
| Onboard IDC / zone input | Panel enclosure or expander card | Consumes a general zone; label granularity limited; requires a home run from the utility room |
Choose the SLC module when the CO alarm is remote from the panel or when you want the point identified by name on the LCD and at the central station. Choose the onboard input only when the wiring already exists and the zone count allows it. The deciding factor is almost always the cable run, not the electronics.
Check: count free SLC addresses on the loop and confirm the loop's device capacity and remaining current budget before adding the module.
How do you wire the contact into the module?
The monitor module input is a supervised circuit. It measures the end-of-line resistor through the field wiring, so an open wire reads as a trouble and a short reads as an active input.
- Set the module address with its rotary or DIP switches before mounting. Record it on the as-built.
- Land the SLC in and out on the module's line terminals, observing polarity.
- Wire the CO alarm's normally open relay contact across the module's IDC terminals.
- Install the end-of-line resistor specified by the module datasheet across the far side of the contact — at the CO relay, not at the module. Placing it at the module defeats supervision of the field wiring.
- Use the module's normally open, non-latching class B configuration for a supervisory or alarm input unless the panel programming calls for latching in the module itself.
- Keep the module and its field wiring in the same enclosure family as the fire alarm system, and do not share raceway with unrelated power conductors.
Check: with the loop powered, short the IDC terminals with a jumper. The panel must annunciate the point within the loop poll interval. Remove one field conductor; the panel must report an open/trouble on that address. If shorting works but opening does not, the EOL is in the wrong place.
What about the CO alarm's own power and end-of-life signal?
The panel is now watching one contact. It is not watching whether the CO alarm has power, whether its sensor has reached end of life, or whether someone unplugged it. A UL 2034 alarm on a 120 V receptacle in a utility room is exactly the kind of device that gets unplugged for a floor scrubber.
Two mitigations, in order of preference:
- Use a CO alarm/relay accessory that provides a separate fault or trouble contact, and bring it into a second monitor module programmed as supervisory or trouble. Now the panel reports both conditions on distinct addresses.
- If only one contact exists, power the alarm from a dedicated, labeled circuit and add it to the inspection checklist with the sensor's replacement date. CO sensing elements have a finite life stamped on the device; the panel will not remind you.
Check: pull power from the CO alarm. Either the second module reports, or you have accepted an unmonitored failure mode and documented it.
How do you program the point and route it off site?
The tag is right; the binding is wrong is the usual failure here. The module reports fine on the panel LCD, and the central station still prints "Zone 1 Fire." The point type and the communicator mapping are two separate settings.
In the communicator or dialer section, map that point to the panel's carbon monoxide or supervisory event class rather than letting it fall into the generic fire alarm group. What the receiver prints is what the operator acts on, and "FIRE ALARM ZONE 1" produces a fire department response to a CO event. Confirm the account number, the receiver line, and the reporting path — dialer, cellular, or IP — are all in service before testing.
Check: read back the programmed label on the panel LCD and confirm the point type displayed matches the AHJ decision.
When to call the manufacturer's application engineer
For the input side, rarely — a monitor module is a monitor module. It pays off on the output side, where the same project usually grows a requirement: shut down a fan, close a damper, or drive a remote annunciator hundreds of feet from the panel. Manufacturers keep relay devices that are not obvious from the catalog, including relays powered from a notification appliance circuit rather than the SLC, which lets you take a control output from the nearest NAC device instead of pulling a new home run to the panel. On long runs that difference is measured in conduit, not in relay cost.
Have the panel model, firmware revision, loop loading, and the exact sequence of operations in front of you when you call.
End-to-end verification
Test the whole chain, not the contact. Notify the central station and place the account on test first.
- Apply the CO alarm manufacturer's approved test method — the test button or listed test gas per the device instructions. Functional gas testing proves the sensor; the test button proves only the horn and relay logic.
- Confirm the CO alarm's local sounder operates and its relay transfers.
- At the fire alarm panel, confirm the correct address and custom label appear, in the correct state: supervisory queue or alarm queue as programmed, not trouble.
- Confirm the programmed output correlations behave — notification appliances silent for a supervisory point, active for an alarm point.
- Call the central station and have the operator read back the event as received: account, point number, and event description. It must say carbon monoxide or supervisory, not fire.
- Clear the CO condition, reset the panel, and confirm the point restores and the restoral transmits.
- Open one field conductor at the CO relay and confirm the panel reports a trouble on that specific address, then restore and confirm the trouble clears.
- Take the account off test and record the module address, point type, EOL value, and central station event description on the as-built and in the system record of completion.
Frequently Asked Questions
How do I connect a Kidde or BRK CO alarm to an addressable fire alarm panel?
Wire the CO alarm's relay accessory contact into an addressable input monitor module on the SLC, with the end-of-line resistor at the relay so the field wiring stays supervised. Brand compatibility between the CO alarm and the panel is not an electrical issue — the module only reads a contact.
How do I decide whether the CO point should be an alarm or a supervisory signal?
The AHJ decides, based on occupancy and the intended response. Program it as supervisory when CO should not trigger a general evacuation, as alarm when it should, and use the panel's dedicated CO point type if the firmware offers one.
How do I know if I need a system CO detector instead of a residential CO alarm?
Ask the AHJ before ordering. Single- and multiple-station alarms are listed to UL 2034; system CO detectors that sit natively on the SLC are listed to UL 2075 and must appear on the panel manufacturer's compatible device list. If the AHJ requires panel-supervised CO sensing, the monitor module approach is off the table.
How do I get the CO signal to display correctly at the central station?
Map the point to the panel's carbon monoxide or supervisory event class in the communicator programming, not the generic fire group, then have the operator read back the received event description during test. A correct panel label with a wrong communicator mapping still prints as a fire alarm.
How do I monitor the CO alarm's own failure or end of life?
Use a relay accessory with a separate fault contact and bring it into a second monitor module programmed as supervisory or trouble. With a single alarm contact, power loss and sensor expiry are invisible to the panel — put the sensor replacement date on the inspection checklist and feed the device from a dedicated labeled circuit.