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What does the ACD-EN multi-sensor detect, and how does its CO, smoke and heat functions work?

The ACD-EN combines optical smoke, heat and CO sensing in one addressable device; the factory default mode 9A gives no separate CO or heat alarm, so select a CO or COHb mode if you need one.

Direct answer

The ACD-EN is an addressable multi-sensor that combines optical (smoke) sensing, heat sensing and a carbon monoxide (CO) sensing element in a single device. It can detect carbon monoxide (CO) and can provide a COHb (carboxyhaemoglobin) toxic-threat function, but only in the operating modes that include those functions — not in the factory default mode 9A. Its smoke, heat and CO elements can be applied in different combinations by selecting one of the device's EN 54-certified operating modes. Confirm the exact mode for each application against the current ACD-EN documentation.

Applies to

  • Products / Models: ACD-EN, ACD-EN(WHT)
  • Firmware / Version: All versions. The behaviour depends on the operating mode set at the control panel, not on device firmware.
  • Region: UK / Europe
  • Standards: EN 54-5 (heat), EN 54-7 (smoke), EN 54-26 (CO), EN 54-29 (smoke and heat multi-sensor), EN 54-30 (CO and heat multi-sensor), EN 54-31 (smoke, CO and optional heat multi-sensor), CEA 4021. Which parts apply depends on the selected operating mode; EN 54-26 covers CO-only operation and does not apply to combined-sensor modes. BS 5839-1 (recording the basis for the selected response characteristics).

What the ACD-EN detects

  • Carbon monoxide (CO) via a monitored CO cell (manufacturer-stated design life: up to 10 years, dependent on environmental conditions and on installation in accordance with Hochiki's installation guidelines; the panel is given an end-of-life warning for the cell).
  • COHb toxic threat (carboxyhaemoglobin accumulation in the bloodstream) as a separate function from the CO alarm, monitoring both rapid high-concentration and prolonged low-concentration CO exposure to assess cumulative physiological risk to occupants.
  • Smoke via an optical sensing element.
  • Heat via a heat element supporting fixed-temperature and rate-of-rise responses.

How the sensor functions relate

  • The optical (smoke), heat and CO elements are distinct sensing functions within one addressable device.
  • The CO/COHb function is provided in addition to the combined smoke and heat multi-sensor function, not as a replacement for it.
  • How these elements drive a fire decision depends on the selected operating mode (see below).
  • For how a multi-sensor compares with optical, heat and CO detectors when choosing a detector type, see How to choose the right type of fire detector.

Selecting an operating mode

  • The ACD-EN supports 24 operating modes, each identified by a mode code, that determine which elements lead the fire decision and which act as supporting sensors. All 24 modes are LPCB approved to EN 54. Mode availability depends on the control panel: confirm that the panel supports ACD mode selection (and day/night settings) before specifying a mode.
  • The factory default is mode 9A (+S/FT/CO +RFA): the optical smoke element makes the fire decision, and the fixed-temperature heat and CO elements are used only to adjust/enhance the smoke threshold. In mode 9A the panel does not raise a separate heat or CO alarm, and the COHb toxic-threat function is not active. If the application requires a CO alarm or COHb indication, select a mode that includes them (for example 9D +CO, 9C +COHb, or a combined mode such as 8D or 99) and confirm control panel support.
  • The + symbol denotes the main operating mode of the sensor, the / denotes the element enhancing that mode (enhancing means this mode will not activate the sensor alone, the + will make the decision.
  • Other modes allow, for example, smoke-only, heat-only (fixed temperature and/or rate of rise), CO-led, or full multi-element operation.
  • Day and night modes can be configured independently, so the device can apply different behaviour by time of day (for example a care home may use mode 99 by day and 8D at night). Day/night selection is control panel dependent.
  • Choose the mode from the current Application Note 152 — ACD Multi-Sensor with CO: Mode Selection Guide for the application; do not assume the default mode suits every environment.
  • Record the mode chosen for each device, and the reason for choosing it, in the system's operating and maintenance manual. Where day and night modes differ, record both. BS 5839-1 recommends that the basis for selecting a detector's response characteristics is recorded, and that the choice is agreed with the other interested parties following the fire risk assessment.

When to contact technical support

  • If you are unsure which operating mode is appropriate for the application or environment (for example, dusty, steamy, or sleeping-risk areas).
  • If CO or COHb alarms occur without an identifiable cause.
  • If the CO cell is approaching or has exceeded its stated design life and you need guidance on replacement or re-certification.

Safety note

This article is general guidance for Hochiki Europe products. Design, installation, commissioning, maintenance and modification of fire detection and alarm systems should be carried out by, or under the responsibility of, a competent person in accordance with the applicable standards (for example BS 5839-1 in the UK). Statutory requirements imposed by the relevant enforcing authority also apply. Always refer to the current product manual and data sheet for the exact model before carrying out any work.

References

Last reviewed: 2026-10-20 — Reviewed by: Hochiki Europe Technical Support