Low-Cost Electrochemical Toxic Gas Sensors for Residential and Light Commercial Applications

NAP-78A High Temperature Sensor for Monitoring

CO / H2 / Flammable Gas levels in Boiler Flues / Ducts / Chimneys


Detectable gas:

CO and H2 in boiler flues and Stacks,

Other flammable gases at high temperature

Detection range:

0 – 2%

Recommended Voltage

2.0V +/- 0.1V

Current drawn

140mA +/- 10 mA

Minimum Sensitivity

4mV / 1500ppm


+/- 100ppm

Response time (90%):

< 8 seconds

Maximum Long Term Drift:


<+/- 2mV Signal/year


<+/- 2 mV / month

Temperature Range:

 -20 +200 degrees C

Temperature Drift (zero)


Humidity range:

0-100% RH, non condensing


0.9 – 1.1 atm


Effectively linear

General Description:

The NAP-78A Gas Sensor from Nemoto is designed to continuously monitor the flue gases generated by gas or oil fired domestic hot water and central heating boilers in order to ensure the safety of the device and also to limit the environmental impact of the boiler under fault conditions.

The sensor will detect the presence of high levels of carbon monoxide and/or hydrogen in the flue gases generated by the boiler. High levels of these gases in the flue during normal operation occur as a result of incomplete combustion, and hence indicate a faulty boiler which could pose a safety hazard within the dwelling in which it is fitted. The signal from the sensor can be used either to trigger an alarm, or to automatically turn the boiler off to ensure the safety and well-being of the people within the building.

The NAP-78A is particularly useful as an indicator of a blocked or partially blocked flue, whether in the flues air intake or in its exhaust stage. Recent changes in standards relating to domestic water heating boilers, particularly the European Standard EN 483, have highlighted the necessity of accurately detecting this fault condition, and the NAP-78A sensor accomplishes this with ease, ensuring compliance with this section of the standard.

The sensor uses the well proven Catalysed Pelletized resistor (Pellistor) method, which is well established for the detection of Flammable gases in general ambient safety applications. In the case of the NAP-78A the device is specially modified, including a special resistor matrix mounted in a separate box, to give stable and reliable performance when mounted inside a boiler flue working at high temperatures.

The NAP-78A sensor is well proven as a reliable real time warning of unsafe combustion conditions within boilers, and all the specifications and performance data in this manual are designed with this primary application in mind. However, note that the NAP-78A sensor can also be used in a wide variety of other applications where the monitoring of flammable / explosive gases is required within ductwork or flue stacks at high temperature.