Many homeowners know they need a smoke alarm.
Fewer realize that a smoke alarm and a carbon monoxide alarm protect against different hazards.
That distinction is important because carbon monoxide does not behave like smoke, and a smoke detector should not be expected to detect CO.
Likewise, a CO alarm is not a substitute for smoke detection.
This is why combination smoke and carbon monoxide alarms have become an attractive option for residential safety.
The HM-636THS is designed specifically as a combination Smoke/CO Alarm, incorporating separate smoke and carbon monoxide alarm functions in one residential device.
Smoke and CO Are Different Hazards
The simplest way to understand the difference is:
Smoke = fire detection
CO = carbon monoxide detection
The smoke portion of the alarm responds to smoke that reaches the smoke sensor.
The CO portion responds to carbon monoxide at the CO sensor.
The product documentation explicitly states that the smoke alarm is not designed to detect gas, heat, or flames, while the CO alarm is not designed to detect fire or other gases.
That means homeowners should never assume that one type of alarm can replace the other.
Why Smoke Alarms Are Necessary
Residential fires can develop quickly.
Smoke is often one of the earliest detectable products of combustion.
The product information explains that smoke particles vary in size and quantity during fires and that smoke alarms are designed to detect combustion particles.
That is why smoke alarms are fundamental to residential fire safety.
Why CO Alarms Are Necessary
Carbon monoxide presents a different challenge.
CO is invisible and odorless.
People cannot reliably detect it without an alarm.
Potential sources include fuel-burning appliances, fireplaces, heaters, gas cooking equipment, dryers, and vehicles.
A home can therefore have a dangerous CO problem without the visual or olfactory clues associated with a fire.
Does a Smoke Alarm Detect CO?
No.
A smoke alarm is designed to detect smoke.
The product documentation explicitly states that the smoke portion is not designed to detect gas.
This is one of the most common misunderstandings in home safety.
Does a CO Alarm Detect Fire?
No.
The CO portion is designed for carbon monoxide.
It is not intended to detect fire or other gases.
That is why a home needs appropriate protection for both hazards.
What Is a Combination Smoke and CO Alarm?
A combination alarm places both functions into one device.
Instead of installing separate smoke and CO alarms in the same location, a combination device can provide both types of detection.
The HM-636THS does this in a residential alarm with:
· Smoke detection
· CO detection
· Separate alarm patterns
· Test/Hush button
· 10-year battery
· Low-battery warning
· End-of-life warning
· Alarm-memory indication
What Are the Benefits of a Combination Alarm?
1. One Device, Two Detection Functions
A combination alarm reduces the number of separate alarm housings needed in locations where both functions are appropriate.
2. Easier Household Recognition
The alarm uses different signal patterns for smoke and CO conditions.
For example, the CO alarm uses four beeps every five seconds during its active alarm condition.
3. Long Battery Life
The HM-636THS uses a 10-year non-replaceable lithium battery.
4. One Maintenance Routine
Homeowners can incorporate the combination alarm into the same weekly testing routine.
Does a Combination Alarm Mean One Device Is Enough for the Whole House?
No.
This is an important point.
The product guidance states that the alarm only senses smoke or CO that reaches its sensors.
An alarm installed upstairs may not provide adequate early warning for a problem occurring in the basement.
That is why the recommended minimum protection includes every sleeping area, every bedroom, and every level of the home.
Where Should Combination Alarms Be Installed?
Recommended locations include:
· Every level
· Every bedroom
· Hallways near sleeping areas
· Near stairs
· Additional rooms where appropriate
The product guidance recommends combination alarms inside bedrooms, near sleeping areas, at the top of stairs, and at the bottom of basement stairs.
What About Closed Bedroom Doors?
A closed bedroom door can slow smoke and CO movement.
For this reason, the product guidance specifically emphasizes alarm installation inside bedrooms when doors may be closed during sleep.
This is a simple but important concept:
An alarm outside a bedroom is not necessarily equivalent to an alarm inside the bedroom.
Smoke Alarm vs. CO Alarm: Different Emergency Responses
The emergency response also differs.
If the smoke alarm sounds:
· Get everyone out.
· Follow the family escape plan.
· Do not go back inside.
· Call the fire department from outside.
If the CO alarm sounds:
· Move immediately to fresh air.
· Account for everyone.
· Do not re-enter.
· Call the local emergency service or 911.
· If the alarm reactivates within 24 hours, have qualified personnel investigate the source.
What Does UL 217 Cover?
The smoke portion of the product is identified as conforming to UL 217.
UL 217 is the relevant standard identified in the product documentation for the smoke alarm function.
For consumers, the important lesson is to distinguish between certification for the smoke function and certification for the CO function.
What Does UL 2034 Cover?
The CO portion is identified as conforming to UL 2034.
UL 2034 addresses single- and multiple-station carbon monoxide alarms.
Therefore, when evaluating a combination alarm, homeowners should look for appropriate standards covering both detection functions.
Why 85 dB Matters
The HM-636THS specifies an alarm sound level of at least 85 dB at 10 feet.
However, loudness alone does not guarantee that every person will wake up.
The product documentation notes that alarms may not awaken every sleeping individual, especially when doors are closed or other sounds interfere.
Families should therefore practice their escape plans and consider how children, older adults, or people who may need assistance will respond to an alarm.
Why an Alarm Is Only One Layer of Protection
A smoke/CO alarm is not a substitute for:
· Safe cooking practices
· Proper appliance maintenance
· Appropriate ventilation
· A family escape plan
· Emergency preparedness
· Local fire-safety requirements
The product guidance recommends fire-safety practices such as not smoking in bed, keeping matches away from children, maintaining electrical equipment, keeping cooking equipment and chimneys clean, and never leaving cooking unattended.
The Best Choice for Many Homes
For homeowners who want both smoke and CO protection in the same location, a combination alarm can be a practical solution.
The HM-636THS combines these two functions while adding a 10-year battery, weekly test capability, Hush functionality, alarm memory, and warning indications for battery and end-of-life conditions.
But the most important part is not the number of functions inside the housing.
It is coverage.
Final Takeaway
Smoke alarms and CO alarms are not interchangeable because they detect different hazards.
A smoke alarm cannot replace a CO alarm.
A CO alarm cannot replace a smoke alarm.
A 2-in-1 smoke and carbon monoxide alarm brings both detection functions into one residential device, making it easier for homeowners to build comprehensive coverage throughout the house.
The best home-safety strategy is therefore not simply to buy one alarm.
It is to create appropriate coverage across every level, sleeping area, bedroom, and other key locations, test the alarms regularly, keep them clean, and make sure every family member knows what to do when an alarm sounds.