Yes, you can build a Wi‐Fi-connected gas monitor that alerts your phone—but an ESP32 and MQ sensor should never be your home’s only gas alarm. The safest arrangement is a listed residential combustible-gas detector for the actual life-safety function, with Home Assistant adding remote push notifications or optional SMS. An ESP32/MQ-5 build is useful for learning and supplementary monitoring, but it is not a certified replacement.
Choose the safe architecture first
| Approach | Best for | Local alarm | Remote alert | Main limitation |
|---|---|---|---|---|
| Listed detector plus smart input | Home safety | Yes | Yes | Relay or integration compatibility varies by model |
| ESP32 plus MQ-5 | Learning and prototyping | Must be added | Yes | Not certified; readings require warm-up and calibration |
| Commercial smart detector | Convenience | Usually | Depends on the model | May depend on cloud services and app compatibility |
For a construction or home-improvement project, the recommended design is:
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- Install a listed combustible-gas alarm according to its instructions.
- Choose a model or alarm system with a documented relay, dry-contact output, or supported smart-home integration.
- Connect that alarm state to Home Assistant through an appropriate, isolated input.
- Use Home Assistant for urgent phone notifications and, if required, an external SMS service.
The certified alarm must continue to sound locally if Wi‐Fi, Home Assistant, the router, or the internet fails.
Know which gas you need to detect
Natural gas is primarily methane and is lighter than air. Propane and LPG are heavier than air. That difference affects both the sensor choice and its mounting position.
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- Natural gas: generally requires high placement, near the ceiling or high on a wall, subject to the detector manufacturer’s instructions and local requirements.
- Propane/LPG: generally requires low placement, often within approximately 18 inches of the floor, subject to the product instructions and adopted code.
- Both fuels: one sensor and one mounting height may not adequately cover different fuel areas. Use fuel-specific listed detectors or a product explicitly rated for both.
NFPA committee material discusses high placement for natural-gas detectors and low placement for propane detectors. Confirm the current locally adopted code and the exact detector manual before installation: NFPA placement material.
A carbon-monoxide alarm does not detect methane or propane, and a smoke alarm does not detect combustible gas. Homes with fuel-burning appliances may need separate combustible-gas and CO protection. UL explains the distinction between these devices and the relevant standards at UL’s gas-detector certification page.
What certification means
Look for the exact model’s documentation and applicable listing—not merely a marketplace description or a generic “UL” logo. Relevant standards include UL 1484 for residential gas detectors, UL 2075 for gas and vapor detectors and sensors used in household gas-detection systems, and UL 2034 for carbon-monoxide alarms.
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Parts for an experimental ESP32 monitor
- ESP32 development board
- MQ-5 module for experimental natural-gas/LPG monitoring, or a sensor specifically matched to the fuel
- Power supply sized for both the ESP32 and the sensor heater
- Local buzzer and/or status LED
- Secure connectors or terminal blocks
- Ventilated enclosure that does not block the sensing element
- Home Assistant server and reliable Wi‐Fi
- Optional backup power for the network and Home Assistant host
MQ-5 modules are marketed for LPG, natural gas, and coal gas. MQ-2 modules are marketed for LPG, propane, hydrogen, and other gases. These modules respond to multiple vapors and are not selective, calibrated residential alarms. See the manufacturer information for MQ-5 and MQ-2.
An integrated option such as M5Stack’s Unit MQ uses an MQ-5 sensor and provides an I2C interface with sensor voltage, ADC readings, temperature, and firmware information. That can simplify software integration, but it does not make the completed project a listed alarm.
Power and enclosure precautions
MQ sensors contain a heated sensing element. They normally require continuous heater power, produce heat, and are not automatically suitable for battery operation. Do not describe the project as battery-friendly unless the exact sensor, duty cycle, current draw, temperature, and runtime have been measured.
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Do not put an unsealed hobby circuit directly in a location where leaked gas could create an ignition hazard. The safest arrangement keeps the listed detector responsible for detection and treats the hobby electronics as a separate monitoring layer. Use a suitable enclosure with unobstructed airflow, secure wiring, and no exposed conductors likely to contact moisture.
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Build the prototype as a monitored state machine
The functional arrangement is:
MQ-5 sensor → ESP32 → Wi‐Fi → Home Assistant → phone push notification or optional SMS
└→ local buzzer/LEDThe ESP32 should:
- Power and read the sensor continuously.
- Ignore readings during the documented startup warm-up period.
- Smooth noisy readings.
- Require a sustained threshold crossing instead of reacting to one sample.
- Drive a local buzzer immediately when its alarm state is reached.
- Publish a clear binary
gas_detectedstate. - Publish sensor health, Wi‐Fi status, uptime, and last-seen information.
- Report an unavailable or implausible sensor separately from a clear reading.
Use the analog value for trends, calibration, and troubleshooting. Do not treat an uncalibrated ADC value as a verified gas concentration or percentage of the lower explosive limit.
A basic hysteresis strategy prevents the alarm from chattering:
If reading exceeds ALARM_ON for CONFIRM_TIME:
state = gas_detected
If state is gas_detected and reading falls below ALARM_OFF
for CLEAR_TIME:
state = gas_clearALARM_OFF should be lower than ALARM_ON. Choose values only after observing the sensor’s clean-air baseline and response to an approved test stimulus; do not copy an arbitrary threshold from another build.
Configure Home Assistant alerts
Expose the prototype—or, preferably, the certified alarm’s isolated contact—as a binary sensor in Home Assistant. The current Home Assistant gas-detection trigger documentation shows both the interface workflow and YAML pattern: gas-detected trigger documentation.
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- What You Get: 1x PT199 Gas Leak Detector, 1x Storage Bag, 2x AAA Batteries, 1x User Manual. Detection range: 50-1,000 ppm (based on methane). Detection distance: 1-5cm
alias: "Urgent gas alert"
triggers:
- trigger: air_quality.gas_detected
target:
entity_id: binary_sensor.kitchen_gas
options:
behavior: each
for: "00:00:00"
actions:
- action: notify.send_message
target:
entity_id: notify.my_device
data:
title: "GAS ALERT"
message: "Combustible gas detected in the kitchen. Leave the building and call for help from outside."The entity ID and notification target will differ in your installation. Confirm the generated entity and notification service in Home Assistant before enabling the automation.
For a hobby sensor, a brief confirmation period may reduce nuisance alerts, but a long delay can postpone warning. Do not delay a life-safety alert merely to make notifications quieter.
Include fault and recovery notifications
- Send an urgent alert when gas is detected.
- Send a separate message when the state returns to normal.
- Use a cooldown or notification mode that prevents message flooding.
- Alert when the sensor becomes unavailable.
- Monitor the Home Assistant host and network if remote warning is important.
- Schedule a weekly or monthly test reminder.
Push notification is not the same as SMS. Push normally depends on the phone app and data connection. SMS requires a configured external messaging provider, sender identity, account credentials, network access, and usually per-message charges. Voice escalation is another option, but it adds more configuration and cost. Treat SMS as an optional second path, never as the only alarm path.
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MQ sensors are heated semiconductor sensors and should not be expected to give reliable readings immediately after power-up. The required warm-up and first-use burn-in depend on the specific sensor and operating mode. Follow its datasheet rather than applying one universal time.
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Before choosing a threshold:
- Run the sensor in clean air and record its baseline over time.
- Observe drift as temperature and humidity change.
- Check whether cooking vapors, alcohol, solvents, aerosols, smoke, or cleaning products cause a response.
- Choose separate alarm and clear thresholds plus sustained-duration rules.
- Record how long the sensor takes to return toward baseline.
Never release natural gas into a room or deliberately create a combustible mixture for testing. Safer options include the sensor’s documented procedure, a commercial calibration or test gas designed for that sensor, or a controlled manufacturer-approved test performed outdoors or by a qualified professional.
Test three separate things:
- Electronics: simulate the alarm input and confirm the ESP32 produces a local alarm and binary state.
- Sensor: use an approved test stimulus and verify a response.
- Complete system: confirm the local alarm, Wi‐Fi reporting, Home Assistant automation, phone notification, recovery message, and offline fault alert.
Place and maintain the device correctly
Follow the listed detector’s manual first. In general, natural-gas detectors belong high in the room, while propane detectors belong low. Avoid dead-air corners, concealed cavities, strong drafts, appliance exhaust, drains, splashing water, and locations where ventilation can bypass the sensing element. Place the detector where it can hear locally and where occupants can access its test control.
Garages, crawl spaces, utility rooms, RVs, and outdoor propane installations can have different temperature, humidity, ventilation, and code requirements. An indoor hobby module is not automatically suitable for those environments.
Review the system regularly. Test the certified alarm according to its instructions, periodically test the complete notification chain, inspect wiring and enclosure ventilation, review backup power, and replace the listed detector on its specified schedule. MQ sensors can drift or become affected by dust, humidity, solvents, cooking fumes, and age.
Troubleshooting
| Symptom | Likely causes | Action |
|---|---|---|
| Constant alarm | Warm-up, vapors, contamination, low threshold, poor power | Ventilate safely, inspect the installation and power, and do not dismiss repeated alarms without checking the fuel system. |
| No alarm | Wrong sensor, blocked sensor, failed heater, broken connection, threshold too high | Check the fuel match, heater power, sensor health, and approved test procedure. |
| Reading stuck at zero | Disconnected sensor, failed ADC connection, or firmware fault | Report it as a fault—not “gas clear”—and inspect the wiring. |
| Repeated notifications | Noise around one threshold or missing cooldown | Add hysteresis and notification control without creating a dangerous alert delay. |
| Device offline | Wi‐Fi, power, ESP32, router, or Home Assistant failure | Keep the local listed alarm independent and create an explicit offline alert. |
| Notification not received | Muted phone, Do Not Disturb, no data, expired token, disabled automation, or provider outage | Test the phone and notification service separately and retain the local siren. |
| Reading changes with cleaning products | MQ cross-sensitivity to vapors | Improve placement, record the interference, and do not claim selective gas measurement. |
Do not casually add an automatic shutoff
Closing a gas valve can be part of an engineered system, but it is not a simple relay project. Home Assistant documents valve-control automation, yet any automatic shutoff requires professional evaluation of the valve type, voltage and current, fail-safe behavior, manual reset, local code, installation requirements, and possible effects on emergency response. Never connect a hobby relay directly to a gas valve without verifying all of those conditions.
What to do when an alarm sounds
Do not operate switches, phones, plugs, or flames inside the suspected area. Leave immediately. From outside, call the gas utility or emergency services. Do not re-enter until qualified responders say it is safe.
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