Air Quality

HVAC & AC Air Purification: MERV Ratings, In-Duct UV and Fresh Air

In most Indian commercial buildings, the air-handling system is the single largest determinant of indoor air quality — and it is usually fitted with a filter chosen to protect the cooling coil rather than the occupants. Upgrading that filter, within what the fan can tolerate, is frequently the cheapest and largest air-quality improvement available to a facility team.

The constraint is static pressure. Denser media captures more particles but resists airflow more, and a fan sized for a coarse filter cannot simply be loaded with a fine one — airflow drops, cooling capacity suffers, and in the worst case the filter is bypassed or collapses. So HVAC air purification is an engineering exercise, not a shopping decision.

Three levers are available: better filtration, microbial control in the air handler, and controlled fresh air. Most buildings need attention on all three.

MERV ratings and what they capture

Rating	PM2.5 capture	Pressure drop	Typical use
MERV 6–8	Poor	Low	Standard coil-protection filter in most Indian AHUs
MERV 11	Moderate	Moderate	A sensible upgrade where the fan allows
MERV 13	Good (~85% of PM2.5)	Moderate–high	The practical target for offices, schools and clinics
MERV 14–16	Very good	High	Needs an AHU designed for it; verify fan capacity first
HEPA (H13)	99.95%	Very high	Cleanrooms, isolation rooms, dedicated recirculation units — rarely retrofittable in general HVAC

Important

Before upgrading filter grade, have the airflow and static pressure checked. Fitting a MERV 14 filter into a system designed for MERV 8 can cut airflow enough to lose cooling capacity — and a filter that leaks around its frame under pressure delivers no benefit at all.

Where central purification helps and where standalone units win

CENTRAL is better for large open floor plates, because it treats all supply air and needs no equipment in the occupied space.
CENTRAL only works when the system is running. Air handlers that shut off outside office hours provide no purification overnight.
STANDALONE units are better for individual cabins, meeting rooms and any space where you need a verifiable local result.
STANDALONE is also the pragmatic answer in leased premises where you cannot modify the landlord's HVAC.
A combination is common: MERV 13 centrally, plus HEPA units in high-occupancy meeting rooms.

In-duct UV — realistic expectations

In-duct UV-C is widely marketed for air disinfection, but its most dependable and least disputed benefit is different: keeping the cooling coil, drain pan and downstream duct surfaces free of microbial growth. Those surfaces are permanently wet during cooling season and are a genuine reservoir for bacteria and mould, which is where much of the musty smell in older buildings originates. Preventing that growth also keeps coil heat transfer efficient.

Air disinfection as air passes the lamp is much more variable, because dwell time in a duct at typical velocities is a fraction of a second. Specify it against airflow and required dose if that is the goal, and be sceptical of blanket claims.

Fresh air and CO2 — the half everyone forgets

Filtration removes particles from air already in the building. It does not remove CO2, and it does not remove all VOCs.
A tightly sealed, well-filtered office with inadequate fresh air will still reach 1,200–2,000 ppm CO2 by afternoon, with documented effects on alertness and decision-making.
Treated Fresh Air Units (TFA) filter and condition outdoor air before introducing it — the correct way to ventilate in a polluted city, rather than simply opening windows.
Demand-controlled ventilation using CO2 sensors modulates fresh air to occupancy, which balances air quality against the energy cost of conditioning outdoor air.
Monitor CO2 alongside PM2.5. A building can be excellent on one and poor on the other.

A practical upgrade sequence for a facility team

1. Measure first — PM2.5 and CO2 at occupied desks, across a working day. Do not specify anything before you have data.
2. Check filter condition and replacement discipline. Many buildings are running filters months overdue, which is a free fix.
3. Have airflow and static pressure assessed, then upgrade filter grade to the highest the system tolerates, commonly MERV 11–13.
4. Address the air handler's own hygiene — coil and drain pan cleaning, and in-duct UV where growth recurs.
5. Review fresh air provision against occupancy, and add CO2-based control if levels are high.
6. Add standalone HEPA units in meeting rooms and any space the central system serves poorly.

Frequently asked questions

Can I put a HEPA filter in my AC or HVAC system?

Usually not directly. HEPA media has a very high pressure drop, and a fan sized for a MERV 8 filter cannot push air through it — you lose airflow and cooling capacity, and the filter may be bypassed at its frame. Either specify a dedicated HEPA recirculation unit designed for the load, or upgrade central filtration to MERV 11–13 after verifying fan capacity.

Does an air conditioner clean the air?

Barely. An AC's filter is a coarse screen intended to protect the coil from dust, not to capture PM2.5. It also removes some humidity, which helps with mould control. For actual particulate removal you need either upgraded central filtration or standalone HEPA purifiers — the AC alone will not do it.

What MERV rating should an office HVAC use in India?

MERV 13 is the practical target for offices, schools and clinics — it captures a useful share of PM2.5 without the pressure drop of higher grades. MERV 11 is a sensible step if the existing fan cannot support 13. Always have airflow and static pressure assessed before upgrading, and pair the change with a disciplined replacement schedule.

Is in-duct UV worth installing?

Its most reliable benefit is keeping cooling coils and drain pans free of microbial growth, which improves both hygiene and heat-transfer efficiency — and that alone often justifies it in humid Indian conditions. Air disinfection as air passes the lamp is far more variable because duct dwell time is very short, so specify it against airflow and dose rather than accepting general claims.