Fixed and Wall Mounted Gas Detection

When is a fixed gas detector actually required by code?

Less often than most of this industry will tell you. There are three cases where the model codes require detection outright, and a larger group where detection is an alternative to ventilation or simply good practice.

We think the most useful thing a distributor can do is tell you which is which, because a system sold on a code requirement that does not exist is a system you may have specified wrong.

Required outright. Carbon dioxide enrichment systems over 100 pounds, or any size with a remote fill, under 2018 IFC section 5307.4.3, which requires detection and ventilation together. Refrigeration machinery rooms under 2018 IMC section 1105.3, which requires a refrigerant detection system with audible and visual alarm actuated at or below the threshold limit value. Indoor storage and use of toxic and highly toxic compressed gases under 2018 IFC section 6004.2.2.10, which requires detection at or below the permissible exposure limit.

Detection or ventilation, your choice. Bulk carbon dioxide for beverage dispensing over 100 pounds, under 2018 IFC section 5307.3, where a detection system is permitted in lieu of mechanical ventilation. Enclosed parking garages and repair garages under 2018 IMC section 404.1, where the alternative is running the ventilation continuously at full volume.

Recognized hazard, no fixed detection mandate. Lead acid battery rooms, where 2018 IFC section 1206.2.12.1 requires ventilation rather than detection. Welding shops, where 29 CFR 1910.252(c) requires ventilation. Wastewater lift stations, where the OSHA obligation is to test and monitor for entry. Bakery proof rooms. Grain and agricultural spaces.

Check the code edition your jurisdiction has adopted before relying on any section number here. Local amendments are common and the code official decides.

How do I choose between a detector with a display and one without?

Ask whether anybody will ever be standing in front of it.

A local display is useful to a technician commissioning the system, calibrating a sensor, or trying to decide whether a panel alarm is a real leak or a drifting cell. If the detector is in an occupied space or a room people service, the display earns its cost.

If the detector is in a pit, a duct, a compressor room or anywhere nobody goes, the reading needs to travel to where the people are, and the display at the sensor is money spent on nothing. That is the case for a transmitter and a remote controller.

The SI-100C is the transmitter without a display. The SI-200E adds the display, RS-485 alongside the 4-20mA output, and ATEX and IECEx approval. The SI-200A is the intrinsically safe version for corrosive and highly toxic gases. The SI-301 draws its sample to the sensor for ducts, cabinets and enclosures.

Where should a fixed gas sensor be mounted?

At the height the gas goes, which is decided by density, not by convenience.

Heavier than air, mount low. Carbon dioxide is about 1.5 times the density of air, and both IFC 5307.3.2 and 5307.4.3 require CO2 sensors within 12 inches of the floor. Hydrogen sulfide, chlorine, propane and butane also settle.

Lighter than air, mount high. Hydrogen is the extreme case at a fourteenth the density of air and belongs at the highest point in the room. Methane and ammonia also rise, although a cold ammonia release behaves as a dense cloud first and then rises as it warms, which is why IMC 1105.3 asks for placement where refrigerant will concentrate rather than naming a height.

Near neutral, mount at breathing height and worry about spacing instead. Carbon monoxide has a molecular weight of 28 against about 29 for air and goes where the air goes.

In every case, put the sensor near the credible leak source as well as at the right height, and make sure a technician can reach it to calibrate it. A sensor that cannot be reached will not be maintained.

What does a fixed system consist of?

Four parts, and the detectors are only the first.

Detectors at the gas. A controller where people are, which provides the display, the alarm relays and the interlock logic. Field wiring, which for a 4-20mA loop is a different install from an RS-485 run. And the thing the system actually does when it alarms, which is usually starting ventilation and shutting a supply valve.

That last part is what most specifications underestimate. Under IFC 5307.4.3.1 the low level alarm in an enrichment room has to stop the flow of CO2 and start the exhaust. Under IMC 1105.6.4 an ammonia detector actuation has to take the machinery room to emergency ventilation. Under IFC 6004.2.2.10.2 a toxic gas alarm has to shut off the gas supply. The detector is the input to a control sequence, and the sequence is the compliance.

Controllers are covered by the SI-100IM for multi channel systems and the SI-200I for single channel. If you send us the detector list, we will put the panel bill of materials together.

Which industry pages should I read?

These are organized by what accumulates in your building rather than by what we would like to sell you.

Indoor grows and greenhouses, for CO2 enrichment, where detection and ventilation are both mandatory. Breweries, bars and beverage systems, for bulk CO2, where detection substitutes for ventilation. Refrigeration plants, ice rinks and cold storage, for ammonia machinery rooms.

Repair shops, upfitters and parking garages, for carbon monoxide and nitrogen dioxide. Battery rooms and data centers, for hydrogen. Laboratories and semiconductor facilities, for silane and the hydride gases.

Welding and fabrication shops, for shielding gas asphyxiation, and for the line between what a gas monitor detects and what needs fume extraction. Bakeries and food production, for proof room CO2 and oven CO, and for why no gas sensor detects flour dust.

Primary sources

M Squared Safety Solutions, Inc., 981 Calle Negocio Suite 200, San Clemente, CA 92673, 949-954-6581. Every code and regulatory statement on this page was read against the cited source on 18 September 2026. Model code section numbers are cited to the 2018 International Fire Code and International Mechanical Code. Jurisdictions adopt and amend these codes separately, so confirm the edition in force where the work is. This page describes what the regulations say. Whether a given provision is satisfied at a particular facility is a determination for that employer and the code official.