Heiman C1-M Carbon Monoxide Alarm

A worthwhile smart upgrade in a Thread home

Heiman C1-M is a small carbon monoxide alarm with a useful extra: it can report an alarm into an existing Matter-over-Thread home. That makes it interesting for someone who wants a CO warning in their usual smart-home app without adding a Heiman bridge. It is less compelling if the main requirement is simply an audible warning in the room.

Heiman sells it in the US through its own Amazon store. Consider it for a home that already has the necessary Thread equipment. Buying that equipment solely for this alarm makes the proposition harder to justify.

Small enough to be unobtrusive

The 70 mm round body is only 37 mm deep. It uses a replaceable CR123A battery, so there is no power cable or outlet to work around. A physical test/hush button and the local sounder remain useful without a smart-home connection. Heiman specifies at least 85 dB at ten feet.

This is a CO-only alarm. It neither replaces a smoke alarm nor serves as a CO₂ room-air monitor. There is also no numerical display on the device: if you want to read a concentration without opening an app, choose an alarm with its own screen. Do not assume that a manufacturer's app graphic means every Matter platform shows the same readings.

The exact C1-M appears in an Intertek authorization covering UL 2034 and CSA 6.19. That establishes a much more useful product identity than a generic Matter badge. This review covers that North American model; the European HS-720ES-M is a different purchase, not simply another case for the same alarm.

The battery story needs more clarity

The retail listing's ten-year battery claim is difficult to accept at face value. Heiman's current C1-M table instead gives seven years, while its older C1 manual gives three. These are conflicting manufacturer and seller estimates, not battery life we have measured.

Treat the CR123A as a replaceable consumable and follow the instructions supplied with the actual unit. The sensor's ten-year end-of-life interval is a separate limit: fitting a new cell does not restart the alarm's service life. If you want a sealed battery with one unambiguous replacement schedule, this is an unnecessarily confusing product to choose.

Keep the benefit in proportion

C1-M has Works with Home Assistant certification. That makes it a credible candidate for a local dashboard and configured notifications. You still need a Matter controller and Thread border router, and an alert reaching your phone depends on the chosen platform and notification settings.

I would not buy several on the assumption that one automatically makes every other alarm sound. Heiman identifies the separate C1-R as the radio-interconnected version; a shared Matter network is not evidence that C1-M does the same thing.

For comparison, First Alert's basic CO400 costs about $22 from the First Alert Store. It supplies a local CO alarm without Matter for much less. Pay the difference when connected notification is a feature you will use. Keep a working, suitable CO alarm within its replacement date if it already meets your needs; Matter alone is not a reason to discard it.