Methane detectors are mounted 30–60 cm from the ceiling because methane's molecular weight of 16.0 is well below air's 28.9, so the gas stratifies at the highest point of an enclosed space [S2][S4]. Propane (C3H8, MW 44) is roughly 1.5x heavier than air, and OEM guidance places propane sensors at 10–30 cm above the floor, which is the most common specification mistake when engineers default to a single height [S2][S3][S4]. Hydrogen sulfide (H2S, MW 34.1) sits in the "near-air" band and is mounted in the breathing zone, 122–183 cm above the floor, because H2S is toxic at low ppm rather than stratified by density alone [S2][S5].
The selection rule is the gas molecular weight versus air: lighter-than-air gases rise, heavier-than-air gases pool, and near-air-density gases (MW 28–32) mix evenly and should be sampled where people breathe [S2][S3]. A reference ceiling-band list from CO2Meter places hydrogen (MW 2.0), helium (4.0), methane (16.0), ammonia (17.0), and nitrogen (14.0) in the same "near-ceiling" group, while propane, butane, CO2, H2S, and gasoline vapors fall into the "floor" group at 12–18 in (30–46 cm) [S2]. Macurco, RKI Instruments, Honeywell, and GasDog converge on the same density-driven logic for fixed gas detector siting [S1][S3][S4][S6].
Methane Sensor Height: Near-Ceiling, 30–60 cm Down
Methane (CH4, MW 16.0) is the textbook lighter-than-air combustible, and GasDog's installation guidance specifies 30–60 cm below the ceiling for fixed methane detectors, paired with placement directly above the leak source rather than at room centre [S4]. Honeywell's 301-EM associated-sensor note uses identical logic: combustible gases lighter than air mount high, heavier-than-air gases (including propane) mount within 30 cm of the floor [S1]. The typical spacing between methane sensors on a coverage grid is 30–40 ft (9–12 m) according to an industry rule of thumb cited by RKI Instruments, though actual spacing must be derated where ceiling obstructions, beams, or HVAC diffusers create stagnation zones [S6].
A common field error is mounting the methane head in the breathing zone because the spec sheet mentions "1.5–1.8 m." That height is correct for CO and H2S, not methane; a methane leak at floor level near a valve will stratify upward and may take several minutes to reach a mis-located head, depending on room volume and ventilation rate [S4]. For mechanical rooms and boiler rooms, ceiling-high mounting within 30–60 cm of the slab is the conservative default, and CO2Meter's chart places methane in the same ceiling band as hydrogen and helium because all three share molecular weights well below the air benchmark of 28.9 [S2].
Propane Sensor Height: 10–30 cm Above the Floor
Propane (C3H8, MW 44) pools at floor level, and the CO2Meter chart specifies fixed propane sensors at 4–6 in (10–15 cm) off the floor, with Macurco's OEM guidance broadening the band to 6–12 in (15–30 cm) [S2][S3]. GasDog's heavier-than-air rule places LPG and propane detectors 30–60 cm from the floor, and Honeywell's 301-EM guidance uses the "approximately 30 cm from the floor" anchor for all combustible gases heavier than air [S1][S4]. The spread of 10–30 cm across the four sources is the realistic engineering tolerance: stay in this band rather than chase a single number, and bias low (10–15 cm) when the floor is the dominant leak plane, such as cylinder manifolds and forklift-LPG change-out stations [S2][S3].
Propane behaves like butane (MW 58.1) and gasoline vapour (MW ~60) for siting purposes, so a single low-band sensor location covers the heavier-than-air combustible class [S2]. Avoid placing the propane head directly in the high-velocity plume of a regulator vent, because condensed LPG droplets and 100% LEL concentrations can poison catalytic-bead and IR sensors; offset by 1–2 m horizontally and rely on diffusion sampling, as flagged in GasDog's interference-avoidance guidance [S4]. For LPG cylinder storage rooms, CO2Meter and Macurco both treat the floor band as non-negotiable, and any "high mount because the room is small" compromise should be rejected at the design review [S2][S3].
H2S Sensor Height: Breathing Zone, 122–183 cm
Hydrogen sulfide (H2S, MW 34.1) is heavier than air by about 18%, yet it is treated as a breathing-zone gas because the acute-toxicity threshold (10 ppm OSHA PEL) and the IDLH value (100 ppm) are reached in the worker's inhalation path long before density-driven stratification dominates a normally ventilated room [S2][S5]. GasDetection.com's fixed toxic-gas siting table lists H2S at "Breathing Level" alongside CO, NH3, SO2, NO2, and Cl2, all gases whose hazard is inhaled rather than pooled or stratified [S5]. The breathing-zone band is 4–6 ft (122–183 cm) per Macurco and gasdetection.com, equivalent to 1.2–1.8 m, which is the same range GasDog quotes for carbon monoxide at 1.5–1.8 m in underground-parking layouts [S3][S4][S5].
For H2S service, the dominant siting variables are release geometry and ventilation, not density: monitor at the worker breathing plane near the manifold, skim tank, or wellhead, and add a low-band sensor if the room has poor floor-level ventilation where H2S could pool to flammable concentrations above 40,000 ppm [S5]. Critical Environment Technologies' 2025 guidance notes that mounting height can also be driven by the specific sensor technology (electrochemical cells have a slightly slower response when mounted in dead-air pockets), which is why a 1.5 m nominal height is preferred over the absolute floor for H2S even though density alone would suggest 30 cm [S7].
Coverage Spacing, Interference, and Calibration Anchors
Spacing between fixed gas detector heads is 30–40 ft (9–12 m) on a coverage grid in open industrial areas per the RKI Instruments rule of thumb, and 5 m (16.5 ft) is the maximum typical spacing cited by MSA for fixed gas detection in HVAC and mechanical-room service [S6][S8]. Both numbers are derated for beam-and-joist ceilings, partial enclosures, and areas where airflow modelling shows short-circuiting past the sensor [S4][S8]. The Macurco chart, Honeywell 301-EM guide, and RKI blog all agree that the sensor must be sited within line-of-sight of the most likely leak point: valves, flanges, compressor seals, cylinder regulators, and tank vents [S1][S3][S6].
Interference discipline is consistent across the four sources: keep heads away from strong drafts, steam pipes, heater exhausts, and electromagnetic sources from VFDs or large motors, and avoid placing a combustible-gas head directly in the high-velocity jet from a pressure relief valve, which can condense LPG droplets or expose LEL sensors to >100% LEL and lock the bridge output [S4]. For wiring, a separate decision governs whether to use a 4–20 mA loop or a digital bus to carry the signal back to the controller, and that choice interacts with the mounting-height rule only at the head-count level, since each detector still occupies one spatial position on the leak grid. Calibration gas is typically 50% LEL methane in air for combustible heads and 25 ppm H2S in N2 for toxic electrochemical cells; both are independent of mounting height but are the most common commissioning omission when engineers focus on placement and forget bump-testing at the installed location [S5][S7].
Decision Matrix: Methane vs Propane vs H2S Siting
The three target gases compared on density band, height above floor, primary hazard, and sensor technology: methane sits in the ceiling band at 30–60 cm below the slab with catalytic-bead or NDIR sensing, propane in the floor band at 10–30 cm with catalytic-bead or NDIR, and H2S in the breathing zone at 122–183 cm with an electrochemical cell [S1][S2][S3][S5]. For pure leak detection, the simple rule is MW versus 28.9, but the engineering decision is which axis dominates: density stratification for methane and propane, inhalation exposure for H2S [S2][S4]. When the same room contains all three (a gas-processing skids with a propane-fired reboiler and an H2S-containing acid-gas stream), three separate heads at three different heights are required; there is no single "compromise" height that satisfies all three gases [S4][S5].
For a utility room with a natural-gas boiler and no H2S source, the head count is one methane sensor near the ceiling and a CO sensor at 1.5 m, since CO is a combustion byproduct with MW 28.0 that follows the breathing-zone rule [S2][S4]. For an LPG cylinder store with forklifts, the spec is a low-band propane head at 15 cm, a CO head at 1.5 m, and an optional low-band oxygen sensor, because nitrogen purging in enclosed stores can produce an asphyxiation hazard even when the propane is well-vented [S2][S5]. The pattern is clear: each gas gets its own head at its own height, and a fixed gas detector specification should list the per-gas height in mm, not a single "room height" for the entire safety system. Combustible-only sites may also benefit from a redundant combustible gas detector on the opposite wall, since a single methane head cannot cover a long, narrow boiler room at 9–12 m spacing. Sites with worker exposure across multiple toxic species should evaluate a multi-gas detector at breathing-zone height to consolidate H2S, CO, and O2 sampling without sacrificing the per-gas siting rule for methane or propane stratification.
Limits of Density-Only Siting
Density-driven siting assumes still air, which is rarely true in practice. HVAC diffusers, doorways, and thermal plumes from hot equipment can override density stratification, and a methane leak at floor level in a high-ceiling boiler room may still rise to a ceiling-mounted head within seconds if a supply diffuser is pushing air upward at the leak point [S3][S4]. Conversely, propane pooling in a cold corner of a loading dock may not reach a 30 cm sensor if the floor is recessed and the sensor is mounted on a 20 cm plinth, which is why a portable gas detector bump test is recommended before relying on the fixed system in unusual geometries [S3].
For ambient-air reference, the CO2Meter chart anchors the molecular weight of fresh air at 28.9 and uses 12–18 in (30–46 cm) from the floor as the standard low band and 48–60 in (122–152 cm) as the standard breathing-zone band, with 30 cm below the ceiling as the standard high band [S2]. A height gauge or laser meter is the right tool at commissioning to verify these bands rather than eyeballing the head location against a 2x4 stud, and the calibration certificate should record the as-installed height for every detector. A gas detector installed in the wrong band will still respond to a sufficiently large release, but the alarm will fire later than the design intent, which is precisely the failure mode the density rules exist to prevent.
Track two signals for the next design review: (1) whether your local AHJ enforces IEC 60079-29-2 selection-installation guidance for combustible-gas heads alongside the OEM height chart, and (2) whether the project specification states the per-gas mounting height in millimetres or only a generic "per manufacturer," since the latter is the most common cause of methane heads landing at 1.5 m and propane heads at ceiling height on the same P&ID.