Technical Feasibility 

Study, CO Sensing Safety 

Systems for Appliances

AHRI Study - September 8, 2010

Presented: June 3, 2014 to CPSC Forum

Larry Brand, Gas Technology Institute

larry.brand@gastechnology.org
AHRI CO Sensors for Appliances 2

Summary

>AHRI study conducted in 2009 - 2010 by GTI

> Objective:

─ Understand the types of CO sensor technologies available 

─ Establish a technical baseline for integrating CO sensors into 

appliances

─ Identify critical areas needing further investigation

>Approach:

─ Literature survey and manufacturer survey with analysis

> Results:

─ No sensor technology was fully adequate for this application

─ Temperature and humidity limits, sensor poisoning, lifespan, and 

cost are factors that need to be addressed 
AHRI CO Sensors for Appliances 3

Objective

>Provide a technical baseline for evaluating the feasibility of 

applying CO sensor safety systems to gas appliances

─ Establish the technical requirements for the application

> Identify critical areas needing further development or 

research

Temperature limits Contaminant resistance

Accuracy Reliability

Calibration needs Durability

Service life Initial cost

Operating cost
AHRI CO Sensors for Appliances 4

Manufacturer Survey

Nine surveys returned Sensor criteria based on survey responses

Type of GasFired 

Equipment 

Manufacturer

Number of

Responses

Boilers 4

Furnaces 4

Water Heaters 2

Infrared 

Heaters

1

Criteria Range

Temperature -40 to 500°F

Humidity Up to 100%

Normal CO Sensor 

Range

0 to 400 ppm

Maximum CO 

Sensor Value*

3000 ppm

Lifespan 20 years

Accuracy 5%

Electrical Voltage 24 VAC

Several respondents noted that the CO sensors would need to be impervious to different 

contaminants that could be caused by condensate in the flue or household cleaning 

products: chlorides, phosphates, out-gassing of binders, manufacturing oils, hydrochloric, 

carbonic, hydrofluoric, nitric, and sulfuric acids.
AHRI CO Sensors for Appliances 5

Literature Survey – 1996 and 2003

> 1996 – GRI report concludes that no commercially 

available CO sensor met all the technical requirements

> 2003 – GTI literature survey on available technologies

─ Solid state ceramic

─ Copper oxide with alkaline metal

─ Tin oxide in a sensor array

─ Tin oxide with platinum or palladium doping

─ Catalytic bead sensor

─ Gallium lead diode laser absorption sensor

─ Zirconia sensor
AHRI CO Sensors for Appliances 6

Literature Survey 2003

Type Supplier Sensor Element Status

Catalytic Bead Nemoto Platinum/Alumina/Catalyst Available

MOS* Figaro SnO2/RuO2/Charcoal Available

MOS Figaro SnO2/Pd/Ir doping Available

IR Comag IR “SmartScan” Infrared Available

Electrochemical Sixth Sense “SureCell” 3-electrode Available

Catalytic Bead Tokyo Gas Platinum/Alumina/Catalyst Field Trials

MOS Los Alamos 

National 

Laboratory

Ceramic/Noble Metal Patent Applied For

MOS Sigma Delta 

Processing

U. of Pavia Multi-Sensor Laboratory

MOS Micro-fabrication EVE Group SnO2/Pd doping Laboratory

MOS/Processing U. of Barcelona SnO2 Array Laboratory

Mid-IR BRD Stanford U. Quantum Cascade Laser Laboratory

Electrochemical Loughborough U. Nafion/Pt/Au Laboratory

IR Stanford U. GaSb Diode Near IR Laser Laboratory

MOS Osaka Gas CuO/Na2CO3 doping Laboratory

Conclusion: MOS, electrochemical, and catalytic technologies widely available but 

not suitable for gas appliance safety circuits.
AHRI CO Sensors for Appliances 7

Literature survey 2006-2010

>Galatsis (2008) identifies 3 types of CO sensors 

as predominant: metal oxide or semiconductors 

(SMO), electrochemical (EC) and infrared or 

optical (IRO)

SMO sensors have a small heated element that cause oxidizing gases such as CO to react with a 

metal oxide film. The film’s conductivity is measured and is proportional to the gas 

concentration. SMO sensors tend to be small, reliable, durable and inexpensive, but have poor 

gas selectivity and can be influenced by temperature and humidity, which could be an issue for 

flue gas sampling. 

EC sensors have an electrode in contact with a liquid electrolyte. Sample gas is diffused into the 

electrolyte which changes the electrical potential of the electrode proportionally to the gas 

concentration. EC sensors tend to be small, but can have poisoning and temperature issues.

IRO sensors have an optical sensor that changes in light transmission properties based on the 

concentration of sample gas present. IRO sensors tend to be small, low power consumption, 

good selectivity and have longer life spans compared to other sensors. However, IRO sensors 

are not as common as the other types, generally cost more and have slower response times.
AHRI CO Sensors for Appliances 8

Literature 2006 – 2010

Source: Galatsis et al (2008)
AHRI CO Sensors for Appliances 9

Sensors Available 2004

Manufacturer Sensor Type Products Applications

City Technology LTD* EC Sensors Gas Sensors

Comag IR IRO Sensors/Alarms HVAC control, ventilation

E2V** SMO, EC, IRO Sensors Gas Sensors

Figaro SMO, EC Sensors Gas Sensors

FiS Inc SMO Sensors Gas Sensors

Kidde EC Sensors/Alarms Nighthawk brand residential CO 

alarms

KWJ Engineering Inc EC Sensors Gas Sensors

Monox*** NA Sensors Gas Sensors

Nemoto & Co., LTD EC Sensors Sensors for residential use and 

boilers

Quantum Group Inc IRO Sensors Sensors for Costar brand CO 

Alarms

Note shift from semiconductor/oxide to electrochemical sensors
AHRI CO Sensors for Appliances 10

Sensors Available in 2010

>Market moved toward SMO and EC sensors with 

new materials or chemicals on small surfaces

─ Micro machined, micro platforms, nanoparticles or 

nanowire sensors

─ Smaller surfaces to reduce heating and cooling times 

and reduce power

>All in early development stage in 2010 – need 5 

more years of development. Suitable sensor life 

key missing requirement.
AHRI CO Sensors for Appliances 11

2010 Sensor Shortfalls

>Figaro (SMO) and City Technology (EC) sensors 

evaluated.

─ Figaro limited to applications without high heat or 

humidity due to lower sensor life

─ City Technology sensor life impacted by high 

humidity. Limited to temperatures below 105°F and 

RH between 15 to 90%
AHRI CO Sensors for Appliances 12

Limits on Appliance Application

>None of the sensors listed had a suitable life in 

an environment greater than 300°F

>The sensors would have to be placed 

downstream of the heat exchanger(s) in the 

appliance
AHRI CO Sensors for Appliances 13

Summary

Requirements

> Temperature Range: -40 to 500° F

> Humidity Range: 0% to 100% RH

> Normal CO Sensor Range: 0 to 

400 ppm

> Maximum CO Sensor Value: 3000 

ppm

> Lifespan: 20 years

> Accuracy: ±5%

> Electrical Voltage: 24 VAC

Findings

> Sensor range, maximum sensor 

value, accuracy and voltage are not 

limitations 

> Temperature and humidity levels in 

the appliances are barriers

> Sensor poisoning due to flue gas 

contaminants is a barrier

> Current life span of sensors is 6 

years; well short of the 20 year life 

expectancy of some gas appliances
AHRI CO Sensors for Appliances 14

Summary

SMO: 

Semiconductor

/Metal Oxide

EC: 

Electrochemical

IRO: 

Infrared

/Optical

Size No Issues No Issues No Issues

Reliability No Issues No Issues No Issues

Durability No Issues No Issues No Issues

Cost Potential Issues Potential Issues Issues

Temperature/Humidity 

Influenced Potential Issues Potential Issues Insufficient Data

Selectivity Issues No Issues No Issues

Poisoning Issues Potential Issues Insufficient Data

Life Issues Issues Potential Issues

Response Time No Issues No Issues Potential Issues

Power Usage No Issues No Issues No Issues

*Matrix based on usage in CO sensing on gas fired appliances and in 

comparison with each other type