By the SightShield team · · 3 min read
The problem with closed rooms
Modern homes and offices are sealed more tightly than older buildings, and with the AC running and windows shut to keep out dust and traffic noise, very little fresh air comes in. People breathe out carbon dioxide (CO2), and in a closed bedroom or meeting room its level can rise well above the roughly 420 parts per million found outdoors. Rooms above about 1000 ppm commonly feel stuffy, and research such as a 2012 Lawrence Berkeley National Laboratory study has linked elevated CO2 with poorer decision-making performance.
Opening windows fixes CO2 but brings in dust, pollen, vehicle pollution, mosquitoes and heat. A fresh air system brings in outside air through filters and removes stale air, with the windows closed.
How heat recovery works
A ventilation unit has two fans: one brings fresh outdoor air in, the other takes stale indoor air out. Both streams pass through a heat exchanger core without mixing. In summer, the cool outgoing air pre-cools the incoming air, so the AC has less work to do. In cooler weather, the outgoing warmth is recovered instead.
HRV vs ERV
For most Indian homes and offices, especially with air-conditioning and a humid monsoon season, an ERV is the usual choice.
| HRV (heat recovery ventilator) | ERV (energy recovery ventilator) | |
|---|---|---|
| Transfers | Heat only | Heat and some moisture |
| Best for | Dry or cold climates | Humid and air-conditioned buildings |
| Effect in monsoon | Brings in humid air | Reduces moisture brought in |
| Typical use in India | Less common | Most homes and offices |
Filters
The filter decides how clean the incoming air is. A coarse pre-filter catches insects and large dust; a fine filter (such as ePM1 / F7–F9 class) captures most fine particles including much of the PM2.5 from traffic and construction; some units add HEPA-grade filters. Filters need cleaning or replacement on a schedule — more often near busy roads and construction sites.
Sizing a system
Ventilation units are sized by airflow — the volume of fresh air delivered per hour — based on the number of occupants and the size and use of each room. Bedrooms need enough fresh air for the people sleeping there; meeting rooms need much more per square metre because more people share them. A designer calculates airflow for each room, then chooses a unit and duct sizes to deliver it quietly.
CO2 in Meeting Rooms and Classrooms →Offices and meeting rooms
Offices with sealed glass façades and split ACs often have no planned fresh air at all, and meeting rooms become stuffy within half an hour. Fresh air units with CO2 control, ducted to meeting rooms and work areas, keep air quality steady through the day, and help staff stay alert during long meetings.
Hybrid Work and Office Security →Fresh air vs air purifier vs AC
- An AC cools and recirculates the room’s air; most split ACs bring in no fresh air.
- An air purifier cleans the air already in the room but does not lower CO2.
- A fresh air system replaces stale air with filtered outdoor air, lowering CO2 and odours, and can work alongside both.
Controls and automation
Units can run on a fixed speed, a schedule, or automatically from a CO2 or air-quality sensor that increases airflow when rooms get crowded. They can be linked to home or office automation so the fresh air boosts when a meeting room is in use or the bedroom is occupied at night.
See our fresh air ventilation service →Quick answers
Does a fresh air system increase the electricity bill?+
The fans use modest power, and heat recovery reduces the extra cooling load from the incoming air, so the overall effect is small compared with the AC itself.
Is ERV or HRV better for India?+
For most homes and offices with AC and a humid monsoon, an ERV is the usual choice.
Does it bring in outdoor pollution?+
Incoming air passes through filters. With fine filters, the air coming in is cleaner than air from an open window.
How often do filters need changing?+
It depends on location and the filter type — check them every few months, more often near main roads and construction.
Can a fresh air system replace exhaust fans?+
In many homes, extraction from bathrooms and kitchens is part of the system, but cooking hoods still need their own exhaust.
Is a fresh air system the same as a ducted AC?+
No. A ducted AC recirculates and cools indoor air; a fresh air system brings in outdoor air and removes stale air.