Gas detector for natural gas vs propane-butane: what's the difference and what it means for your choice
Why there is no one universal gas detector for everything
When selecting a gas leak detector, most customers focus on the price, brand or the design of the box hanging on the wall. In practice, however, the very first question that both the technician and the investor should ask is completely different: what type of gas is the device designed for and will it be deployed – for natural gas (methane, CH4) distributed through the gas network, or for propane-butane (LPG) from a pressurized cylinder or tank? Although these two gases are often colloquially referred to as simply "gas", they behave diametrically differently in terms of chemistry, physics and safety risk. A detector that works perfectly for methane may be practically useless for a propane-butane leak if it is incorrectly placed – and vice versa.
This article discusses the technical basis of this difference, explains what it means for the real selection, installation and setting of the detector, and provides practical recommendations from everyday installation practice. If you are considering the selection of a specific model, we recommend supplementing the information with the articles How to choose a gas leak detector for a household and business and AVANSA 100M vs AVANSA 150MC vs AVANSA 200M: comparison and recommended use.
Physical and chemical differences between natural gas and propane-butane
The basis of the entire problem lies in the molecular weight and density of the gas in relation to air. Natural gas is largely methane (CH4), which has a molar mass of approximately 16 g/mol. Air has an average molar mass of about 29 g/mol. Methane is therefore significantly lighter than air – its relative density is approximately 0.55 to 0.6. On the other hand, propane (C3H8) has a molar mass of about 44 g/mol and butane (C4H10) about 58 g/mol, which in practice means a relative density of 1.5 to 2.0 in relation to air. Propane-butane is therefore almost twice as heavy as air.
Density and direction of spreading in space
This seemingly "dry" physical data has a completely concrete practical consequence: in the case of a leak, natural gas floats and accumulates near the ceiling in an enclosed space, spreading under ceiling cornices, into the space above the kitchen line, into the corners of the ceiling near the chimney inlet. Propane-butane, on the other hand, practically immediately starts to slide down to the floor in the case of a leak, flowing into pits, channels, under floor coverings, into the space under cabinets, into shafts and technical channels, where it can accumulate for a long time without anyone visually or by smell registering it.
Explosive limits and detection sensitivity
The second important parameter is the concentration of gas in the air at which an explosive mixture occurs – the so-called lower explosive limit (LEL, Lower Explosive Limit) and upper explosive limit (UEL, Upper Explosive Limit). For natural gas (methane), the LEL is approximately 5 vol. % and the UEL is approximately 15 vol. %. For propane, the LEL is approximately 2.1 vol. % and the UEL is approximately 9.5 vol. %, for butane the LEL is approximately 1.4 vol. % and the UEL is approximately 8.4 vol. %. In the case of the common propane-butane mixture used in household and cabin cylinders, the LEL is calculated to be around 1.5 to 2 vol. % in practice.
This implies something very essential for the setting and calibration of the sensor: propane-butane reaches an explosive concentration at approximately half to a third of the proportion in the air compared to natural gas. The detector must be sensorically set so that it triggers an alarm in time – the alarm level is usually set at 20 % LEL to provide sufficient reserve before a truly dangerous concentration occurs. If the sensor is calibrated and set for methane, but propane-butane actually accumulates in the space, the evaluation of the concentration may be distorted and the alarm may come later than is safe.
Consequences for the placement of the detector in practice
It is precisely the different density that is the reason why natural gas detectors and propane-butane detectors are mounted at completely different heights. This topic is also discussed in detail in a separate article on the placement and installation height of gas detectors according to the type of gas, here is only a summary of the principles that have proven themselves in practice:
- Natural gas detectors (methane) are mounted high – ideally in the upper third of the wall, 15 to 30 cm from the ceiling, never near the floor. In the case of sloped ceilings or beams, they are placed at the highest point where the gas actually reaches, not in a "dead" corner behind a beam.
- Propane-butane detectors are mounted low – 10 to 30 cm above the floor level, near the expected source of the leak (near the PB cylinder, at the pipe crossing the floor, in a technical channel). They should never be mounted on the ceiling, as the heavy gas would practically never be detected there.
- In combined spaces (e.g., a boiler room with natural gas and an adjacent storage area with PB cylinders for backup use), it is necessary to use two separate detectors at two different height levels, not one compromise in the middle of the wall.
Sensor technologies and their suitability for individual gases
Gas detectors on the market use several different types of sensors that differ in the principle of detection, sensitivity to specific gases and lifespan. To understand the selection, it is good to know at least a basic overview:
- Catalytic sensor (pellistor) – works on the principle of catalytic combustion of gas on the surface of a heated element, changing its resistance. It is relatively universal for most flammable hydrocarbons (methane, propane, butane), but sensitivity to different gases varies, and therefore the manufacturer always specifies which gas the specific product is calibrated for. A disadvantage is that it needs enough oxygen in the surrounding area and can be "poisoned" by silicones or hydrogen sulfide, losing sensitivity without visible warning.
- Semiconductor (MOS/MOX) sensor – a cheaper technology, very sensitive, reacts to a wide range of gases and vapors, which is both an advantage and a disadvantage – it is more prone to false alarms due to vapors from cleaning products, alcohol, sprays.
- Infrared (NDIR) sensor – measures the absorption of infrared radiation at a specific wavelength typical for a given molecule, so it is very accurate and selective, especially for methane. It is not prone to "poisoning" by chemical vapors, has a longer lifespan, but is more expensive, so it is mainly used in industrial and professional applications.
For a typical household or small business, we most commonly encounter catalytic or semiconductor sensors calibrated by the manufacturer for a specific type of gas in practice. Therefore, it is always necessary to know when placing an order whether it is a connection to natural gas from the public network, or a space with a propane-butane cylinder or tank – this is the basic question the technician should always ask first when selling, even before considering a specific model or accessories, such as a solenoid valve.
How the difference manifests in the real selection of a specific detector
In the atria.sk catalog, you can find, for example, the Gas Detector AVANSA 100M, which is intended for installation in apartments, family homes, boiler rooms, and technical areas, and the Detector AVANSA 150MC with extended functions suitable for more demanding operations. The difference between individual models of the AVANSA series is discussed in detail in the article AVANSA 100M vs AVANSA 150MC vs AVANSA 200M: comparison and recommended use – it is important to emphasize that when ordering, it is necessary to choose a variant calibrated for the gas that is actually used or stored in the given space.
If the detector is to automatically close the gas supply after detecting a leak, it is appropriate to choose a complete solution with an integrated closing element, for example, Gas Detector AVANSA 100M + solenoid valve G1/2" DN15 for smaller DN15 pipelines, Gas Detector AVANSA 100M + solenoid valve G3/4", DN 20 for medium boiler room pipelines, or Gas Detector AVANSA 100M + solenoid valve G1" DN25 for larger industrial pipelines. The choice of the correct valve dimension is described in the article How to choose the correct solenoid valve for the detector (G1/2", G3/4", G1"), and the difference between a standalone detector and a solution with a valve is then discussed in the article Gas detector with solenoid valve vs standalone detector: what to choose.
Wiring diagram of a detector with a solenoid valve
The principle works as follows: the detector continuously measures the gas concentration in the air, and when the set threshold is exceeded (typically around 20% LEL), it sends a signal to close the solenoid valve mounted directly on the pipe, thus cutting off the gas supply to the appliances before the concentration rises to a dangerous level.
Comparison of natural gas and propane-butane in a table
| Property | Natural gas (methane, CH4) | Propane-butane (LPG) |
|---|---|---|
| Relative density compared to air | ≈ 0,55–0,6 (lighter) | ≈ 1,5–2,0 (heavier) |
| Behavior in case of a leak | risks upwards, accumulates at the ceiling | falls down, accumulates at the floor, in pits and channels |
| Lower flammability limit (LEL) | ≈ 5 vol. % | ≈ 1,5–2,1 vol. % |
| Upper flammability limit (UEL) | ≈ 15 vol. % | ≈ 8,4–9,5 vol. % |
| Typical odorant | THT (tetrahydrothiophene) | ethyl mercaptan |
| Common source in practice | gas connection, boiler, stove, public network | PB cylinder, LPG tank, caravan, cottage, mobile cooking |
| Recommended detector installation | upper third of the wall / ceiling | lower third of the wall / floor |
Practical scenarios from everyday installation practice
Over the years of solving customer orders with gas detectors, several typical situations repeat, which well illustrate why distinguishing between natural gas and propane-butane is crucial:
- An apartment in a housing estate with a boiler and stove connected to the public network. This is clearly natural gas – the detector is installed at the ceiling of the kitchen or in the boiler room, never at the floor. Common mistakes in the field: the technician has become accustomed from other jobs to install detectors low and automatically uses the same approach here, practically disabling the detection.
- A cottage or recreational building without a gas connection, with a PB cylinder under the sink or near the stove. It is essential to install the detector low, ideally directly at floor level at the storage location, because that is exactly where the gas will accumulate in case of a leak from the cylinder valve or hose.
- A boiler room in an industrial or commercial building with a larger natural gas pipeline. In addition to the correct installation height, a combination with an automatic closing valve is also considered here, so that during night or weekend absence of staff, it is not only relying on the sound of the alarm.
- A garage or workshop with a caravan equipped with an LPG system. Even short-term parking of a caravan with a connected cylinder can cause the accumulation of heavy gas at the floor of the garage – in a closed concrete space without ventilation, this is a real risk that we encounter more often than it might seem.
- A restaurant kitchen with a combination of natural gas on the stove and a backup PB cylinder on a portable stove. Here, it is necessary to solve two separate detectors at two levels, because one universally placed device cannot reliably monitor both leak scenarios at the same time.
Decision-making process when choosing
A simplified procedure that has proven effective in practice when advising customers is as follows: first determine the type of gas in the given space, then choose the sensor calibration and installation height accordingly, then assess whether it is appropriate to add an automatic closing valve, and only then consider the specific model and its additional functions (e.g., connection to the fire alarm system, GSM module, output for external signaling).
Common mistakes in selection and installation from the service perspective
When handling complaints and service interventions, the same mistakes tend to repeat themselves and can be summarized into several points:
- Installation of gas detectors at a low height "as it is common" – in reality, this significantly reduces the likelihood of detecting a leak in time, because lighter gas accumulates at the top and reaches the floor-mounted sensor only after the entire room is filled.
- Mixing up calibration – ordering a model calibrated for one gas type for a space where another gas is actually used. Although the sensor may react to both gases, the accuracy and speed of concentration evaluation differ significantly.
- Installation too close to a heat source, window, or air vent, where air flow distorts the actual concentration at the sensor location – the detector may report falsely low values even during a real leak.
- Underestimating the need for an automatic shut-off valve in spaces without constant supervision – a detector without a valve can only alert via a siren but cannot stop the gas supply if no one is present.
- Lack of regular inspection and calibration – sensors have a limited lifespan and sensitivity gradually decreases; this topic is discussed in detail in the article Maintenance, Calibration, and Lifespan of Gas Detector Sensors.
If a customer experiences repeated false alarms or, conversely, has reason to suspect that the detector is not detecting real leaks, we recommend reading the article Common Faults and False Alarms of Gas Detectors, where typical causes are analyzed (vapors from cleaning agents, humidity, dust, air flow during ventilation, sensor aging).
Frequently asked questions about the difference between natural gas and propane-butane when selecting a detector
Is there a detector that reliably detects both gases at once?
Some catalytic sensors respond to a wider range of flammable gases, but with different sensitivity for methane and for propane/butane. If there is a real risk of leaks of both types of gas in the space (e.g., natural gas on a stove and a backup PB cylinder), it is recommended to install two separate detectors at two different height levels, rather than relying on one compromise device.
Can I install a propane-butane detector near the ceiling to keep it "out of children's reach" or away from a wet floor?
No, such placement would practically exclude the detector's functionality. Propane-butane accumulates near the floor during a leak, so a detector near the ceiling would register the leak only after the entire room is filled with gas, which is not acceptable from a safety perspective. If the sensor needs protection from humidity or mechanical damage, the solution is an appropriate cover or a protected mounting bracket, not changing the installation height.
How can I find out which gas is actually used in my building?
Natural gas is connected to the public gas network, has its own connection and a contract with the gas supplier, and is measured by a gas meter. Propane-butane is delivered in the form of pressurized cylinders or from a private tank (e.g., in detached houses outside gasified areas) and is not connected to the public network. In case of uncertainty, it is easiest to check the contract with the energy supplier or ask directly on site during an inspection where the connection comes from.
Is a separate detector needed even for a small PB cylinder used only occasionally on a portable stove?
Even a small leak in the hose or regulator can gradually create a dangerous concentration in an enclosed space (e.g., a garage, storage room, cabin), since propane-butane is not diluted by ventilation as effectively as lighter gas and accumulates near the floor. When used regularly or for longer periods in an enclosed space, a separate detector is a reasonable investment, especially in places without windows or with minimal ventilation.
Does the type of gas also affect the selection of a solenoid valve?
The type of gas does not directly affect the valve dimensions – they depend on the pipe diameter and flow rate (G1/2", G3/4", G1"), as discussed in the article How to Choose the Right Solenoid Valve for a Detector. However, the type of gas indirectly influences where the detector with the valve should be placed and how quickly it should respond, which is reflected in the overall design of the safety system.
What if the gas source in the building changes over time, for example from a PB cylinder to a gas connection?
In such a case, it is necessary to reassess the placement and calibration of the detector – a detector originally installed low for propane-butane will no longer function properly after switching to natural gas. We recommend performing a safety system inspection with every change of gas source, not just the piping itself.
Conclusion
The difference between natural gas and propane-butane is not just a theoretical note for technicians – it directly determines where the detector should be installed, what type of sensor it should contain, and how quickly and reliably it can alert to a real danger. Lighter natural gas rises and requires installation near the ceiling, heavier propane-butane sinks and requires low installation. Both gases have different explosive limits, which affects the setting of the alarm threshold of the sensor. When selecting a specific device from the range, for example Gas Detector AVANSA 100M or Detector AVANSA 150MC, it is therefore always the first step to accurately determine which gas is actually used or stored in the given space – only then does it make sense to consider a specific model, an additional solenoid valve, or the method of installation. In case of doubts, it is advisable to consult the selection directly with a technician or review the complete category of detectors and related technical articles in the Knowledge Center.
Do you have a question about this topic?
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