Could certainly be that they are not sensitive as most other devices. They are kind of useless if they finally trip and then your house blows up when you turn on a light switch. 
Natural gas is a safe gas compared to most. You need a 5% to 15% ration with air for it to be flammable, anything out of this range and it won't ignite. Compared to propane that will ignite at almost any concentration with air.
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Ya. I was over the top. You'd just never wake up from you nap.
I have a natural gas grill that I tested mine with years ago before there was an inbuilt driver as a few of us were developing one. I turned in the gas and out the sensor in it with the lid closed and it did detect the gas after a bit.
This said all of my Heiman gas detectors started beeping recently and have now failed. Lasted only a few years. I have since bought the Shelly NG sensors to try. I have these integrated via Home Assistant and HADB back to HE.
I have some NG ones that I purchased a few years ago and I did test them to be sure they worked. Mine is right next to the stove on the wall and it detected NG when I opened the valve on the burner.
I would say it detected within 30 seconds after turning on the gas to the burner.
Update: the Heiman NG sensors will also detect this. 
I test with this:
It worked with these devices:
- Manufacturer _TZE204_iuk8kupi, Model TS0601 [VERY, VERY spammy - 6-8 log entries per second, non-stop; I keep it only because it returns numeric values, but I only enable it for 30 secs every 5 minutes]
- Manufacturer _TYZB01_0w3d5uw3, Model TS0204 [Heiman clone]
I'm using the Homi/Heiman Gas Detector driver for this device:
Manufacturer: _TYZB01_0w3d5uw3
Model TS0204
Everything works OK, but I can't get debug log messages to stop, even though the "Debugging?" preference is OFF. Can this be fixed?
Example:
dev:112025-07-22 07:44:22.406 PMdebug
Model: TS0204
dev:112025-07-22 07:44:20.340 PMdebug
Manuf: _TYZB01_0w3d5uw3
dev:112025-07-22 07:44:18.336 PMdebug
Gas Sensor - Garage Explosive Gasses TU is clear
dev:112025-07-22 07:44:16.343 PMdebug
Zone State is enrolled
dev:112025-07-22 07:44:14.393 PMdebug
Zone Type is Gas Sensor
dev:112025-07-22 07:44:12.279 PMinfo
Debugging is off.
dev:112025-07-22 07:44:12.279 PMinfo
Check in time: 1800 secs.
dev:112025-07-22 07:42:25.600 PMdebug
Model: TS0204
dev:112025-07-22 07:42:23.589 PMdebug
Manuf: _TYZB01_0w3d5uw3
dev:112025-07-22 07:42:21.587 PMdebug
Gas Sensor - Garage Explosive Gasses TU is clear
dev:112025-07-22 07:42:19.788 PMdebug
Model: TS0204
dev:112025-07-22 07:42:19.585 PMdebug
Zone State is enrolled
dev:112025-07-22 07:42:18.373 PMdebug
Model: TS0204
dev:112025-07-22 07:42:17.729 PMdebug
Manuf: _TYZB01_0w3d5uw3
dev:112025-07-22 07:42:17.619 PMdebug
Zone Type is Gas Sensor
dev:112025-07-22 07:42:16.507 PMdebug
Model: TS0204
dev:112025-07-22 07:42:16.371 PMdebug
Manuf: _TYZB01_0w3d5uw3
dev:112025-07-22 07:42:15.729 PMdebug
Gas Sensor - Garage Explosive Gasses TU is clear
dev:112025-07-22 07:42:15.527 PMinfo
Debugging is off.
dev:112025-07-22 07:42:15.527 PMinfo
Check in time: 1800 secs.
dev:112025-07-22 07:42:14.496 PMdebug
Manuf: _TYZB01_0w3d5uw3
dev:112025-07-22 07:42:14.369 PMdebug
Gas Sensor - Garage Explosive Gasses TU is clear
dev:112025-07-22 07:42:13.800 PMdebug
Zone State is enrolled
dev:112025-07-22 07:42:12.514 PMdebug
Gas Sensor - Garage Explosive Gasses TU is clear
dev:112025-07-22 07:42:12.391 PMdebug
Zone State is enrolled
dev:112025-07-22 07:42:11.732 PMdebug
Zone Type is Gas Sensor
dev:112025-07-22 07:42:10.469 PMdebug
Zone State is enrolled
dev:112025-07-22 07:42:10.368 PMdebug
Zone Type is Gas Sensor
dev:112025-07-22 07:42:09.672 PMinfo
Debugging is off.
Platform Version: 2.4.2.134
Hardware Version: C-8 Pro
Your numbers are not accurate. The lower explosive limit and upper explosive limits are as follows:
methane 4.4/16.4
propane 2.1/10.1
Perhaps you are thinking of acetylene, which has a LEL of 2.5 and an UEL of 100.
