Why Does CO2 Increase in an Air-Conditioned Room? Temtop C1+ Ventilation Guide Malaysia
Why Does CO2 Increase in an Air-Conditioned Room? Temtop C1+ Ventilation Guide Malaysia
You switch on the air conditioner.
The room becomes cool.
The doors and windows are closed.
Everything feels comfortable.
But after several people remain inside for an hour or two, a CO₂ monitor may show that carbon dioxide concentration has steadily increased.
Why?
The answer is important for Malaysian homes, offices, classrooms and other air-conditioned spaces:
Cooling the air and ventilating the room are not necessarily the same thing.
A CO₂ monitor such as the Temtop C1+ can help users see what is happening inside an occupied room instead of judging indoor conditions from temperature alone.
Why Does CO₂ Rise Indoors?
People produce carbon dioxide through normal respiration.
Every person inside a room continuously exhales air containing CO₂.
If outdoor air is introduced at a sufficient rate, occupant-generated CO₂ is diluted.
If the room has limited outdoor-air ventilation, however, CO₂ can gradually accumulate.
The basic relationship is straightforward:
More occupants + longer occupancy + insufficient outdoor-air ventilation = greater potential for indoor CO₂ accumulation.
This is why a small meeting room containing ten people can behave very differently from the same room when only one person is inside.
Doesn't the Air Conditioner Bring in Fresh Air?
Not necessarily.
This is one of the most common misunderstandings about indoor air conditioning.
Many air-conditioning systems cool indoor air by circulating air through the cooling system and returning conditioned air to the room.
Outdoor-air ventilation depends on the actual building and HVAC design.
Therefore, an air conditioner can be operating normally while the amount of outdoor air entering an occupied space is still a separate question.
This explains a situation many people recognise:
Room temperature: comfortable
Air conditioner: running
Doors/windows: closed
Occupancy: high
CO₂: increasing
There is no contradiction.
The air-conditioning system may be doing its cooling job while CO₂ generated by occupants continues accumulating.
Malaysia Makes This Especially Relevant
Malaysia's climate encourages extensive use of air conditioning.
In places such as Kuala Lumpur, Selangor, Johor and Penang, offices and homes may keep windows closed for reasons including:
Outdoor heat
High humidity
Traffic noise
Rain
Security
Outdoor dust
Haze
This creates an interesting indoor-environment challenge.
Opening windows may improve outdoor-air exchange under suitable conditions, but there may also be times when outdoor conditions make window opening undesirable.
That is why measurement is useful.
Instead of assuming that a room needs more or less ventilation, users can observe the actual CO₂ trend.
Example 1: Office Meeting Room
Consider an enclosed meeting room.
At 9:00 AM, the room is empty.
At 10:00 AM, ten employees enter.
The door closes.
The air conditioner maintains the room at approximately 23–24°C.
After some time, the temperature may still look excellent.
But every occupant has been continuously producing CO₂.
If outdoor-air ventilation does not sufficiently dilute that CO₂, the concentration increases.
A monitor can show:
Before meeting → relatively lower CO₂
Meeting begins → CO₂ starts increasing
Full occupancy continues → CO₂ continues rising
Meeting ends → CO₂ begins recovering
That pattern can be very useful for facility management.
Example 2: Bedroom at Night
Bedrooms provide another good example.
Imagine two people sleeping in an air-conditioned bedroom.
The windows are closed.
The door remains closed for most of the night.
The room may stay at a comfortable 23°C or 24°C.
But temperature alone tells you very little about CO₂.
By monitoring overnight, users can investigate:
What was the CO₂ level before bedtime?
Did it rise throughout the night?
When did it reach its highest point?
What happened after the bedroom door opened in the morning?
This is much more informative than taking one measurement before going to sleep.
Example 3: Classroom
Now consider 30 students and a teacher inside an air-conditioned classroom.
The difference between 31 occupants and an empty classroom is substantial.
If the room remains closed for a long lesson, occupant-generated CO₂ can create a clear trend.
This is why CO₂ monitoring can be particularly useful in:
Schools
Tuition centres
Training centres
Seminar rooms
Computer classrooms
The objective is not simply to display a number.
It is to understand how occupancy interacts with ventilation.
Example 4: Gym or Fitness Studio
A fitness studio can present another high-occupancy scenario.
People exercising may generate more CO₂ than people sitting quietly.
At busy times, a studio may simultaneously have:
High occupancy
Physical activity
Closed doors
Air conditioning
Monitoring CO₂ trends can provide additional information when evaluating how the indoor environment changes between quiet and peak periods.
Why Temperature Cannot Tell You the CO₂ Level
Temperature and CO₂ are fundamentally different measurements.
A room could be:
22°C with relatively low CO₂
or
22°C with substantially higher CO₂.
Both rooms can feel equally cool.
This is why using an air-conditioner thermostat as an indicator of ventilation is unreliable.
If CO₂ is the parameter you want to understand, measure CO₂ directly.
Temtop C1+ for Air-Conditioned Spaces
The Temtop C1+ measures:
CO₂
Temperature
Relative Humidity
Its specified CO₂ measurement range is:
400–5000 ppm
with specified accuracy of:
±(40 ppm + 5%) from 400–2500 ppm.
This combination allows users to observe something particularly useful:
Temperature can remain stable while CO₂ changes significantly.
That makes the C1+ suitable for investigating air-conditioned indoor spaces.
Why Data History Matters
Suppose you check your office at 8:30 AM.
The room has been empty overnight.
The CO₂ reading looks relatively low.
Does that mean ventilation is adequate throughout the working day?
Not necessarily.
The important period may be:
11:00 AM — Meeting room full
or
3:00 PM — Office at peak occupancy
or
3:00 AM — Bedroom occupied for several hours
Historical monitoring helps reveal these patterns.
The Temtop C1+ supports Bluetooth connectivity and data history, allowing users to look beyond the reading displayed at one particular moment.
CO₂ Trend Can Be More Informative Than One Reading
Imagine seeing 1,000 ppm on a display.
Without context, the number tells only part of the story.
Now compare two situations.
Situation A
The reading was higher earlier and is steadily decreasing after ventilation was increased.
Situation B
The reading was much lower 30 minutes ago and is still rapidly increasing.
The current display might temporarily show a similar number, but the direction and history are completely different.
This is why trend monitoring is so valuable.
Ask:
Is CO₂ rising?
How quickly?
When did the increase begin?
What was the occupancy at that time?
What happens when ventilation conditions change?
Does Opening a Window Reduce CO₂?
It can, if opening the window results in effective exchange with outdoor air.
But the result depends on factors including:
Wind
Window size
Room configuration
Indoor-outdoor temperature differences
Cross ventilation
Other openings
A CO₂ monitor provides a simple way to observe the effect.
Take note of the CO₂ trend.
Change the ventilation condition.
Observe what happens.
Measurement lets you evaluate the actual response instead of assuming the result.
But What About Haze?
This is especially relevant in Malaysia.
There may be periods when outdoor particulate pollution is elevated.
Opening windows to increase outdoor-air exchange may then introduce more outdoor particles into the building.
This demonstrates why indoor environmental management cannot be reduced to one parameter.
During haze conditions, users may need to consider both:
CO₂ / ventilation
and
PM2.5 / particulate pollution.
The Temtop C1+ focuses on CO₂, temperature and humidity.
A separate PM2.5-capable instrument is required when particulate monitoring is also necessary.
CO₂ Monitor vs PM2.5 Monitor
These instruments answer different questions.
CO₂ Monitor
Main question:
“What is happening to CO₂ as people occupy this room?”
Useful for understanding occupancy and ventilation patterns.
PM2.5 Monitor
Main question:
“How much fine particulate matter is present?”
Useful for haze, smoke and particulate investigations.
HCHO / TVOC Monitor
Main question:
“Could volatile pollutants associated with materials, furnishings or renovation be present?”
Useful for appropriate VOC/formaldehyde investigations.
Multi-Parameter IAQ Monitor
Useful when several indoor air quality parameters need to be investigated together.
Choose the meter according to the problem you are trying to solve.
Where Should You Put the Temtop C1+?
For general indoor monitoring, choose a location reasonably representative of the occupied space.
Avoid placing the monitor:
directly beside someone's face,
immediately beside an open window,
directly in front of a fresh-air supply,
directly underneath an air-conditioning outlet.
Local conditions at these positions may not represent the wider occupied zone.
Performance Mode for Ventilation Investigation
The C1+ offers a Performance Mode with a 1-minute sampling interval.
This is useful when you want to observe relatively fast changes.
For example:
10:00 — Meeting begins
10:15 — Occupancy increases
10:45 — Door opens
11:00 — Meeting ends
Frequent measurements can make the relationship between these events and the CO₂ trend easier to see.
For longer-term monitoring, ECO Mode uses a 10-minute sampling interval and offers longer specified battery operation.
Frequently Asked Questions
Why does CO₂ rise when my air conditioner is on?
Because air conditioning and outdoor-air ventilation are not necessarily the same process. People continue generating CO₂ while occupying the room.
Does an air conditioner remove CO₂?
Do not assume that normal cooling automatically removes occupant-generated CO₂. Actual ventilation performance depends on the HVAC and building design.
Why is my bedroom CO₂ higher in the morning?
If people occupy a closed bedroom for several hours, their respiration can increase indoor CO₂ when ventilation is limited.
Does a cold room mean good ventilation?
No. Temperature alone cannot establish the CO₂ level or ventilation condition.
Can Temtop C1+ measure PM2.5?
No. The C1+ focuses on CO₂, temperature and humidity.
Can C1+ be used overnight?
It can be used for longer-duration CO₂ trend monitoring, including observing changes in occupied rooms over time.
Is CO₂ the same as carbon monoxide?
No.
CO₂ = carbon dioxide.
CO = carbon monoxide.
They are different gases and require appropriate instruments for the intended measurement.
The Key Point: Cool Air Does Not Automatically Mean Fresh Air
For Malaysian homes, offices, schools and commercial buildings, this is the main takeaway:
Do not judge ventilation only by room temperature.
A room can be cool while occupant-generated CO₂ is increasing.
The Temtop C1+ CO2 Monitor gives users a straightforward way to observe CO₂, temperature and humidity simultaneously, while Bluetooth data history helps reveal how conditions change over time.
For an air-conditioned room, that can answer a much more useful question than:
“Is the room cold enough?”
It can help you investigate:
“What is happening to the indoor CO₂ while people are actually using this room?”
Contact MTM Precision
MTM Precision Sdn. Bhd.
For Temtop C1+ CO₂ Monitor, indoor CO₂ measurement and ventilation monitoring solutions in Malaysia, contact MTM Precision.
Showroom & Service Centre
No. 29-1 & 29-2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia
Website: www.mtmpre.com.my
Email: mtmpre@yahoo.com
WhatsApp: +6016-660 7346
Serving customers across Selangor, Kuala Lumpur, Johor, Penang, Perak, Negeri Sembilan, Melaka, Pahang, Kedah, Perlis, Terengganu, Sarawak and Sabah, Malaysia.
10 Sep 2026
